/*global ArangoServerState, ArangoClusterInfo */
'use strict';

////////////////////////////////////////////////////////////////////////////////
/// @brief JavaScript cluster functionality
///
/// @file
///
/// DISCLAIMER
///
/// Copyright 2012 triagens GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is triAGENS GmbH, Cologne, Germany
///
/// @author Jan Steemann
/// @author Copyright 2012, triAGENS GmbH, Cologne, Germany
////////////////////////////////////////////////////////////////////////////////

var console = require("console");
var arangodb = require("@arangodb");
var ArangoCollection = arangodb.ArangoCollection;
var ArangoError = arangodb.ArangoError;
var request = require("@arangodb/request").request;
var wait = require("internal").wait;
var _ = require("lodash");

var endpointToURL = function (endpoint) {
  if (endpoint.substr(0,6) === "ssl://") {
    return "https://" + endpoint.substr(6);
  }
  var pos = endpoint.indexOf("://");
  if (pos === -1) {
    return "http://" + endpoint;
  }
  return "http" + endpoint.substr(pos);
};

////////////////////////////////////////////////////////////////////////////////
/// @brief create and post an AQL query that will fetch a READ lock on a
/// collection and will time out after a number of seconds
////////////////////////////////////////////////////////////////////////////////

function startReadLockOnLeader (endpoint, database, collName, timeout) {
  var url = endpointToURL(endpoint) + "/_db/" + database;
  var r = request({ url: url + "/_api/replication/holdReadLockCollection",
                    method: "GET" });
  if (r.status !== 200) {
    console.error("startReadLockOnLeader: Could not get ID for shard",
                  collName, r);
    return false;
  }
  try {
    r = JSON.parse(r.body);
  }
  catch (err) {
    console.error("startReadLockOnLeader: Bad response body from",
                  "/_api/replication/holdReadLockCollection", r,
                  JSON.stringify(err));
    return false;
  }
  const id = r.id;

  var body = { "id": id, "collection": collName, "ttl": timeout };
  r = request({ url: url + "/_api/replication/holdReadLockCollection",
                body: JSON.stringify(body),
                method: "POST", headers:  {"x-arango-async": "store"} });
  if (r.status !== 202) {
    console.error("startReadLockOnLeader: Could not start read lock for shard",
                  collName, r);
    return false;
  }
  var rr = r;  // keep a copy

  var count = 0;
  while (++count < 20) {   // wait for some time until read lock established:
    // Now check that we hold the read lock:
    r = request({ url: url + "/_api/replication/holdReadLockCollection",
                  body: JSON.stringify(body),
                  method: "PUT" });
    if (r.status === 200) {
      return id;
    }
    console.debug("startReadLockOnLeader: Do not see read lock yet...");
    wait(0.5);
  }
  var asyncJobId = rr.headers["x-arango-async-id"];
  r = request({ url: url + "/_api/job/" + asyncJobId, body: "", method: "PUT"});
  console.error("startReadLockOnLeader: giving up, async result:", r);
  return false;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief cancel such a query, return whether or not the query was found
////////////////////////////////////////////////////////////////////////////////

function cancelReadLockOnLeader (endpoint, database, lockJobId) {
  var url = endpointToURL(endpoint) + "/_db/" + database +
            "/_api/replication/holdReadLockCollection";
  var r;
  var body = {"id":lockJobId};
  try {
    r = request({url, body: JSON.stringify(body), method: "DELETE" });
  }
  catch (e) {
    console.error("cancelReadLockOnLeader: exception caught:", e);
    return false;
  }
  if (r.status !== 200) {
    console.error("cancelReadLockOnLeader: error", r);
    return false;
  }
  console.debug("cancelReadLockOnLeader: success");
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief cancel barrier from sync
////////////////////////////////////////////////////////////////////////////////

function cancelBarrier (endpoint, database, barrierId) {
  var url = endpointToURL(endpoint) + "/_db/" + database +
            "/_api/replication/barrier/" + barrierId;
  var r = request({url, method: "DELETE" });
  if (r.status !== 200 && r.status !== 204) {
    console.error("CancelBarrier: error", r);
    return false;
  }
  console.debug("cancelBarrier: success");
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief tell leader that we are in sync
////////////////////////////////////////////////////////////////////////////////

function addShardFollower(endpoint, database, shard) {
  console.debug("addShardFollower: tell the leader to put us into the follower list...");
  var url = endpointToURL(endpoint) + "/_db/" + database +
            "/_api/replication/addFollower";
  var body = {followerId: ArangoServerState.id(), shard };
  var r = request({url, body: JSON.stringify(body), method: "PUT"});
  if (r.status !== 200) {
    console.error("addShardFollower: could not add us to the leader's follower list.", r);
    return false;
  }
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief tell leader that we are stop following
////////////////////////////////////////////////////////////////////////////////

function removeShardFollower(endpoint, database, shard) {
  console.debug("removeShardFollower: tell the leader to take us off the follower list...");
  var url = endpointToURL(endpoint) + "/_db/" + database +
            "/_api/replication/removeFollower";
  var body = {followerId: ArangoServerState.id(), shard };
  var r = request({url, body: JSON.stringify(body), method: "PUT"});
  if (r.status !== 200) {
    console.error("removeShardFollower: could not remove us from the leader's follower list.", r);
    return false;
  }
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief lookup for 4-dimensional nested dictionary data
////////////////////////////////////////////////////////////////////////////////

function lookup4d (data, a, b, c) {
  if (! data.hasOwnProperty(a)) {
    return undefined;
  }
  if (! data[a].hasOwnProperty(b)) {
    return undefined;
  }
  if (! data[a][b].hasOwnProperty(c)) {
    return undefined;
  }
  return data[a][b][c];
}

////////////////////////////////////////////////////////////////////////////////
/// @brief return a shardId => server map
////////////////////////////////////////////////////////////////////////////////

function getShardMap (plannedCollections) {
  var shardMap = { };

  var database;

  for (database in plannedCollections) {
    if (plannedCollections.hasOwnProperty(database)) {
      var collections = plannedCollections[database];
      var collection;

      for (collection in collections) {
        if (collections.hasOwnProperty(collection)) {
          var shards = collections[collection].shards;
          var shard;

          for (shard in shards) {
            if (shards.hasOwnProperty(shard)) {
              shardMap[shard] = shards[shard];
            }
          }
        }
      }
    }
  }

  return shardMap;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief return the indexes of a collection as a map
////////////////////////////////////////////////////////////////////////////////

function getIndexMap (shard) {
  var indexes = { }, i;
  var idx = arangodb.db._collection(shard).getIndexes();

  for (i = 0; i < idx.length; ++i) {
    // fetch id without collection name
    var id = idx[i].id.replace(/^[a-zA-Z0-9_\-]*?\/([0-9]+)$/, '$1');

    idx[i].id = id;
    indexes[id] = idx[i];
  }

  return indexes;
}


////////////////////////////////////////////////////////////////////////////////
/// @brief return a hash with the local databases
////////////////////////////////////////////////////////////////////////////////

function getLocalDatabases () {
  var result = { };
  var db = require("internal").db;

  db._databases().forEach(function (database) {
    result[database] = { name: database };
  });

  return result;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief return a hash with the local collections
////////////////////////////////////////////////////////////////////////////////

function getLocalCollections () {
  var result = { };
  var db = require("internal").db;

  db._collections().forEach(function (collection) {
    var name = collection.name();

    if (name.substr(0, 1) !== '_') {
      var data = {
        id: collection._id,
        name: name,
        type: collection.type(),
        status: collection.status(),
        planId: collection.planId()
      };

      // merge properties
      var properties = collection.properties();
      var p;
      for (p in properties) {
        if (properties.hasOwnProperty(p)) {
          data[p] = properties[p];
        }
      }

      result[name] = data;
    }
  });

  return result;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief create databases if they exist in the plan but not locally
////////////////////////////////////////////////////////////////////////////////

function createLocalDatabases (plannedDatabases, currentDatabases, writeLocked) {

  var ourselves = global.ArangoServerState.id();
  var createDatabaseAgency = function (payload) {
    global.ArangoAgency.set("Current/Databases/" + payload.name + "/" + ourselves,
                     payload);
  };

  var db = require("internal").db;
  db._useDatabase("_system");

  var localDatabases = getLocalDatabases();
  var name;

  // check which databases need to be created locally
  for (name in plannedDatabases) {
    if (plannedDatabases.hasOwnProperty(name)) {
      var payload = plannedDatabases[name];
      payload.error = false;
      payload.errorNum = 0;
      payload.errorMessage = "no error";

      if (! localDatabases.hasOwnProperty(name)) {
        // must create database

        // TODO: handle options and user information

        console.info("creating local database '%s'", payload.name);

        try {
          db._createDatabase(payload.name);
          payload.error = false;
          payload.errorNum = 0;
          payload.errorMessage = "no error";
        }
        catch (err) {
          payload.error = true;
          payload.errorNum = err.errorNum;
          payload.errorMessage = err.errorMessage;
        }
        writeLocked({ part: "Current" },
                    createDatabaseAgency,
                    [ payload ]);
      } else if (typeof currentDatabases[name] !== 'object' || !currentDatabases[name].hasOwnProperty(ourselves)) {
        // mop: ok during cluster startup we have this buggy situation where a dbserver
        // has a database but has not yet announced it to the agency :S
        writeLocked({ part: "Current" },
                    createDatabaseAgency,
                    [ payload ]);
      }
    }
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief drop databases if they do exist locally but not in the plan
////////////////////////////////////////////////////////////////////////////////

function dropLocalDatabases (plannedDatabases, writeLocked) {
  var ourselves = global.ArangoServerState.id();

  var dropDatabaseAgency = function (payload) {
    try {
      global.ArangoAgency.remove("Current/Databases/" + payload.name + "/" + ourselves);
    }
    catch (err) {
      // ignore errors
    }
  };

  var db = require("internal").db;
  db._useDatabase("_system");

  var localDatabases = getLocalDatabases();
  var name;

  // check which databases need to be deleted locally
  for (name in localDatabases) {
    if (localDatabases.hasOwnProperty(name)) {
      if (! plannedDatabases.hasOwnProperty(name) && name.substr(0, 1) !== '_') {
        // must drop database

        console.info("dropping local database '%s'", name);

        // Do we have to stop a replication applier first?
        if (ArangoServerState.role() === "SECONDARY") {
          try {
            db._useDatabase(name);
            var rep = require("@arangodb/replication");
            var state = rep.applier.state();
            if (state.state.running === true) {
              console.info("stopping replication applier first");
              rep.applier.stop();
            }
          }
          finally {
            db._useDatabase("_system");
          }
        }
        db._dropDatabase(name);

        writeLocked({ part: "Current" },
                    dropDatabaseAgency,
                    [ { name: name } ]);
      }
    }
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief clean up what's in Current/Databases for ourselves
////////////////////////////////////////////////////////////////////////////////

function cleanupCurrentDatabases (currentDatabases, writeLocked) {
  var ourselves = global.ArangoServerState.id();

  var dropDatabaseAgency = function (payload) {
    try {
      global.ArangoAgency.remove("Current/Databases/" + payload.name + "/" + ourselves);
    }
    catch (err) {
      // ignore errors
    }
  };

  var db = require("internal").db;
  db._useDatabase("_system");

  var localDatabases = getLocalDatabases();
  var name;

  for (name in currentDatabases) {
    if (currentDatabases.hasOwnProperty(name) && name.substr(0, 1) !== '_') {
      if (! localDatabases.hasOwnProperty(name)) {
        // we found a database we don't have locally

        if (currentDatabases[name].hasOwnProperty(ourselves)) {
          // we are entered for a database that we don't have locally
          console.info("cleaning up entry for unknown database '%s'", name);

          writeLocked({ part: "Current" },
                      dropDatabaseAgency,
                      [ { name: name } ]);
        }

      }
    }
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief handle database changes
////////////////////////////////////////////////////////////////////////////////

function handleDatabaseChanges (plan, current, writeLocked) {

  var plannedDatabases = plan.Databases;
  var currentDatabases = current.Databases;

  createLocalDatabases(plannedDatabases, currentDatabases, writeLocked);
  dropLocalDatabases(plannedDatabases, writeLocked);
  cleanupCurrentDatabases(currentDatabases, writeLocked);
}

////////////////////////////////////////////////////////////////////////////////
/// @brief create collections if they exist in the plan but not locally
////////////////////////////////////////////////////////////////////////////////

function createLocalCollections (plannedCollections, planVersion,
                                 currentCollections,
                                 takeOverResponsibility, writeLocked) {
  var ourselves = global.ArangoServerState.id();

  var createCollectionAgency = function (database, shard, collInfo, error) {
    var payload = { error: error.error,
                    errorNum: error.errorNum,
                    errorMessage: error.errorMessage,
                    indexes: collInfo.indexes,
                    servers: [ ourselves ],
                    planVersion: planVersion };

    global.ArangoAgency.set("Current/Collections/" + database + "/" +
                            collInfo.planId + "/" + shard,
                            payload);
  };

  var takeOver = createCollectionAgency;


  var db = require("internal").db;
  db._useDatabase("_system");

  var migrate = writeLocked => {
    var localDatabases = getLocalDatabases();
    var database;
    var i;

    // iterate over all matching databases
    for (database in plannedCollections) {
      if (plannedCollections.hasOwnProperty(database)) {
        if (localDatabases.hasOwnProperty(database)) {
          // switch into other database
          db._useDatabase(database);

          try {
            // iterate over collections of database
            var localCollections = getLocalCollections();

            var collections = plannedCollections[database];

            // diff the collections
            Object.keys(collections).forEach(function(collection) {
              var collInfo = collections[collection];
              var shards = collInfo.shards;
              var shard;

              collInfo.planId = collInfo.id;
              var save = [collInfo.id, collInfo.name];
              delete collInfo.id;  // must not actually set it here
              delete collInfo.name;  // name is now shard

              for (shard in shards) {
                if (shards.hasOwnProperty(shard)) {
                  var didWrite = false;
                  if (shards[shard].indexOf(ourselves) >= 0) {
                    var isLeader = shards[shard][0] === ourselves;
                    var wasLeader = isLeader;
                    try {
                      var currentServers = currentCollections[database][collection][shard].servers;
                      wasLeader = currentServers[0] === ourselves;
                    }
                    catch(err) {
                    }

                    // found a shard we are responsible for

                    var error = { error: false, errorNum: 0,
                      errorMessage: "no error" };

                    if (! localCollections.hasOwnProperty(shard)) {
                      // must create this shard
                      console.info("creating local shard '%s/%s' for central '%s/%s'",
                          database,
                          shard,
                          database,
                          collInfo.planId);

                      try {
                        if (collInfo.type === ArangoCollection.TYPE_EDGE) {
                          db._createEdgeCollection(shard, collInfo);
                        }
                        else {
                          db._create(shard, collInfo);
                        }
                      }
                      catch (err2) {
                        error = { error: true, errorNum: err2.errorNum,
                          errorMessage: err2.errorMessage };
                        console.error("creating local shard '%s/%s' for central '%s/%s' failed: %s",
                            database,
                            shard,
                            database,
                            collInfo.planId,
                            JSON.stringify(err2));
                      }

                      if (isLeader) {
                        writeLocked({ part: "Current" },
                            createCollectionAgency,
                            [ database, shard, collInfo, error ]);
                        didWrite = true;
                      }
                    }
                    else {
                      if (!isLeader && wasLeader) {
                        db._collection(shard).leaderResign();
                      }

                      if (localCollections[shard].status !== collInfo.status) {
                        console.info("detected status change for local shard '%s/%s'",
                            database,
                            shard);

                        if (collInfo.status === ArangoCollection.STATUS_UNLOADED) {
                          console.info("unloading local shard '%s/%s'",
                              database,
                              shard);
                          db._collection(shard).unload();
                        }
                        else if (collInfo.status === ArangoCollection.STATUS_LOADED) {
                          console.info("loading local shard '%s/%s'",
                              database,
                              shard);
                          db._collection(shard).load();
                        }
                        if (isLeader) {
                          writeLocked({ part: "Current" },
                              createCollectionAgency,
                              [ database, shard, collInfo, error ]);
                          didWrite = true;
                        }
                      }

                      // collection exists, now compare collection properties
                      var properties = { };
                      var cmp = [ "journalSize", "waitForSync", "doCompact",
                      "indexBuckets" ];
                      for (i = 0; i < cmp.length; ++i) {
                        var p = cmp[i];
                        if (localCollections[shard][p] !== collInfo[p]) {
                          // property change
                          properties[p] = collInfo[p];
                        }
                      }

                      if (Object.keys(properties).length > 0) {
                        console.info("updating properties for local shard '%s/%s'",
                            database,
                            shard);

                        try {
                          db._collection(shard).properties(properties);
                        }
                        catch (err3) {
                          error = { error: true, errorNum: err3.errorNum,
                            errorMessage: err3.errorMessage };
                        }
                        if (isLeader) {
                          writeLocked({ part: "Current" },
                              createCollectionAgency,
                              [ database, shard, collInfo, error ]);
                          didWrite = true;
                        }
                      }
                    }

                    if (error.error) {
                      if (takeOverResponsibility && !didWrite) {
                        if (isLeader) {
                          writeLocked({ part: "Current" },
                              takeOver,
                              [ database, shard, collInfo, error ]);
                        }
                      }
                      continue;  // No point to look for properties and
                      // indices, if the creation has not worked
                    }

                    var indexes = getIndexMap(shard);
                    var idx;
                    var index;

                    if (collInfo.hasOwnProperty("indexes")) {
                      for (i = 0; i < collInfo.indexes.length; ++i) {
                        index = collInfo.indexes[i];

                        var changed = false;

                        if (index.type !== "primary" && index.type !== "edge" &&
                            ! indexes.hasOwnProperty(index.id)) {
                          console.info("creating index '%s/%s': %s",
                              database,
                              shard,
                              JSON.stringify(index));

                          try {
                            arangodb.db._collection(shard).ensureIndex(index);
                            index.error = false;
                            index.errorNum = 0;
                            index.errorMessage = "";
                          }
                          catch (err5) {
                            index.error = true;
                            index.errorNum = err5.errorNum;
                            index.errorMessage = err5.errorMessage;
                          }

                          changed = true;
                        }
                        if (changed && isLeader) {
                          writeLocked({ part: "Current" },
                              createCollectionAgency,
                              [ database, shard, collInfo, error ]);
                          didWrite = true;
                        }
                      }

                      var changed2 = false;
                      for (idx in indexes) {
                        if (indexes.hasOwnProperty(idx)) {
                          // found an index in the index map, check if it must be deleted

                          if (indexes[idx].type !== "primary" && indexes[idx].type !== "edge") {
                            var found = false;
                            for (i = 0; i < collInfo.indexes.length; ++i) {
                              if (collInfo.indexes[i].id === idx) {
                                found = true;
                                break;
                              }
                            }

                            if (! found) {
                              // found an index to delete locally
                              changed2 = true;
                              index = indexes[idx];

                              console.info("dropping index '%s/%s': %s",
                                  database,
                                  shard,
                                  JSON.stringify(index));

                              arangodb.db._collection(shard).dropIndex(index);

                              delete indexes[idx];
                              collInfo.indexes.splice(i, i);
                            }
                          }
                        }
                      }
                      if (changed2 && isLeader) {
                        writeLocked({ part: "Current" },
                            createCollectionAgency,
                            [ database, shard, collInfo, error ]);
                        didWrite = true;
                      }
                    }

                    if ((takeOverResponsibility && !didWrite && isLeader) ||
                        (!didWrite && isLeader && !wasLeader)) {
                      writeLocked({ part: "Current" },
                          takeOver,
                          [ database, shard, collInfo, error ]);
                    }
                  }
                }
              }
              collInfo.id = save[0];
              collInfo.name = save[1];

            });
          }
          catch (err) {
            // always return to previous database
            db._useDatabase("_system");
            throw err;
          }

          db._useDatabase("_system");
        }
      }
    }
  };

  if (takeOverResponsibility) {
    // mop: if this is a complete takeover we need a global lock because
    // otherwise the coordinator might fetch results which are only partly
    // migrated
    var fakeLock = (lockInfo, cb, args) => {
      if (!lockInfo || lockInfo.part !== 'Current') {
        throw new Error("Invalid lockInfo " + JSON.stringify(lockInfo));
      }
      return cb(...args);
    };
    writeLocked({ part: "Current" }, migrate, [fakeLock]);
  } else {
    migrate(writeLocked);
  }
}

function leaderResign(database, collId, shardName, ourselves) {
  console.info("trying to withdraw as leader of shard '%s/%s' of '%s/%s'",
               database, shardName, database, collId);
  try {
    var db = require("internal").db;
    db._executeTransaction(
      { "collections": { "write": [shardName] },
        "action": function() {
          var path = "Current/Collections/" + database + "/" + collId + "/" +
                     shardName + "/servers";
          var servers = global.ArangoAgency.get(path).arango.Current
                          .Collections[database][collId][shardName].servers;
          if (servers[0] === ourselves) {
            servers[0] = "_" + ourselves;
            global.ArangoAgency.set(path, servers);
            global.ArangoAgency.increaseVersion("Current/Version");
          }
        } });
  } catch (x) {
    console.error("exception thrown when resigning:", x);
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief drop collections if they exist locally but not in the plan
////////////////////////////////////////////////////////////////////////////////

function dropLocalCollections (plannedCollections, currentCollections,
                               writeLocked) {
  var ourselves = global.ArangoServerState.id();

  var dropCollectionAgency = function (database, shardID, id) {
    try {
      global.ArangoAgency.remove("Current/Collections/" + database + "/" + id + "/" + shardID);
    }
    catch (err) {
      // ignore errors
    }
  };

  var db = require("internal").db;
  db._useDatabase("_system");
  var shardMap = getShardMap(plannedCollections);

  var localDatabases = getLocalDatabases();
  var database;

  // iterate over all databases
  for (database in localDatabases) {
    if (localDatabases.hasOwnProperty(database)) {
      var removeAll = ! plannedCollections.hasOwnProperty(database);

      // switch into other database
      db._useDatabase(database);

      try {
        // iterate over collections of database
        var collections = getLocalCollections();
        var collection;

        for (collection in collections) {
          if (collections.hasOwnProperty(collection)) {
            // found a local collection
            // check if it is in the plan and we are responsible for it

            var remove = removeAll ||
                         (! shardMap.hasOwnProperty(collection)) ||
                         (shardMap[collection].indexOf(ourselves) === -1);

            if (remove) {
              var currentServers;
              // May be we have been the leader and are asked to withdraw:
              if (shardMap.hasOwnProperty(collection) &&
                  shardMap[collection][0] === "_" + ourselves) {
                try {
                  currentServers = currentCollections[database]
                      [collections[collection].planId][collection].servers;
                } catch (err2) {
                  currentServers = [];
                }
                if (currentServers[0] === ourselves) {
                  leaderResign(database, collections[collection].planId,
                               collection, ourselves);
                }
              } else {
                // Remove us from the follower list, this is a best effort,
                // we might actually have been the leader ourselves, in which
                // case we try to unfollow the new leader, no problem, we
                // simply ignore any errors. If a proper error occurs, this
                // is also no problem, since the leader will soon notice
                // that the shard here is gone and will drop us automatically:
                var servers = shardMap[collection];
                try {
                  currentServers = currentCollections[database]
                      [collections[collection].planId][collection].servers;
                } catch (err2) {
                  currentServers = [];
                }
                if (servers !== undefined &&
                    currentServers.indexOf(ourselves) >= 0) {
                  var endpoint = ArangoClusterInfo.getServerEndpoint(servers[0]);
                  try {
                    removeShardFollower(endpoint, database, collection);
                  } catch (err) {
                  }
                }
                console.info("dropping local shard '%s/%s' of '%s/%s",
                             database,
                             collection,
                             database,
                             collections[collection].planId);

                db._drop(collection);

                if (removeAll || ! shardMap.hasOwnProperty(collection)) {
                  console.info("cleaning out Current entry for shard %s in",
                               "agency for %s/%s", collection, database,
                               collections[collection].name);
                  writeLocked({ part: "Current" },
                              dropCollectionAgency,
                              [ database, collection, collections[collection].planId ]);
                }
              }
            }
          }
        }
      }
      catch (err) {
        db._useDatabase("_system");
        throw err;
      }
      db._useDatabase("_system");
    }
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief clean up what's in Current/Collections for ourselves
////////////////////////////////////////////////////////////////////////////////

function cleanupCurrentCollections (plannedCollections, currentCollections,
                                    writeLocked) {
  var dropCollectionAgency = function (database, collection, shardID) {
    try {
      global.ArangoAgency.remove("Current/Collections/" + database + "/" + collection + "/" + shardID);
    }
    catch (err) {
      // ignore errors
    }
  };

  var db = require("internal").db;
  db._useDatabase("_system");

  var shardMap = getShardMap(plannedCollections);
  var database;

  for (database in currentCollections) {
    if (currentCollections.hasOwnProperty(database)) {
      var collections = currentCollections[database];
      var collection;

      for (collection in collections) {
        if (collections.hasOwnProperty(collection)) {
          var shards = collections[collection];
          var shard;

          for (shard in shards) {
            if (shards.hasOwnProperty(shard)) {

              if (! shardMap.hasOwnProperty(shard)) {
                // found an entry in current of a shard that is no longer
                // mentioned in the plan
                console.info("cleaning up entry for shard '%s' of '%s/%s",
                             shard,
                             database,
                             collection);

                writeLocked({ part: "Current" },
                            dropCollectionAgency,
                            [ database, collection, shard ]);
              }
            }
          }
        }
      }

    }
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief lock key space
////////////////////////////////////////////////////////////////////////////////

function lockSyncKeyspace() {
  while (!global.KEY_SET_CAS("shardSynchronization", "lock", 1, null)) {
    wait(0.001);
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief unlock key space
////////////////////////////////////////////////////////////////////////////////

function unlockSyncKeyspace() {
  global.KEY_SET("shardSynchronization", "lock", null);
}

////////////////////////////////////////////////////////////////////////////////
/// @brief launch a scheduled job if needed
////////////////////////////////////////////////////////////////////////////////

function tryLaunchJob() {
  const registerTask = require("internal").registerTask;
  var isStopping = require("internal").isStopping;
  if (isStopping()) {
    return;
  }
  lockSyncKeyspace();
  try {
    var jobs = global.KEYSPACE_GET("shardSynchronization");
    if (jobs.running === null) {
      var shards = Object.keys(jobs.scheduled).sort();
      if (shards.length > 0) {
        var jobInfo = jobs.scheduled[shards[0]];
        try {
          registerTask({
            database: jobInfo.database,
            params: {database: jobInfo.database, shard: jobInfo.shard,
                     planId: jobInfo.planId, leader: jobInfo.leader},
            command: function(params) {
              require("@arangodb/cluster").synchronizeOneShard(
                params.database, params.shard, params.planId, params.leader);
            }});
        } catch (err) {
          if (! require("internal").isStopping()) {
            console.error("Could not registerTask for shard synchronization.");
          }
          return;
        }
        global.KEY_SET("shardSynchronization", "running", jobInfo);
        console.info("scheduleOneShardSynchronization: have launched job", jobInfo);
        delete jobs.scheduled[shards[0]];
        global.KEY_SET("shardSynchronization", "scheduled", jobs.scheduled);
      }
    }
  }
  finally {
    unlockSyncKeyspace();
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief synchronize one shard, this is run as a V8 task
////////////////////////////////////////////////////////////////////////////////

function synchronizeOneShard(database, shard, planId, leader) {
  // synchronize this shard from the leader
  // this function will throw if anything goes wrong

  var isStopping = require("internal").isStopping;
  var ourselves = global.ArangoServerState.id();

  function terminateAndStartOther() {
    lockSyncKeyspace();
    try {
      global.KEY_SET("shardSynchronization", "running", null);
    }
    finally {
       unlockSyncKeyspace();
    }
    tryLaunchJob();  // start a new one if needed
  }

  // First wait until the leader has created the shard (visible in
  // Current in the Agency) or we or the shard have vanished from
  // the plan:
  while (true) {
    if (isStopping()) {
      terminateAndStartOther();
      return;
    }
    var planned = [];
    try {
      planned = global.ArangoClusterInfo.getCollectionInfo(database, planId)
                  .shards[shard];
    } catch (e) { }
    if (!Array.isArray(planned) ||
       planned.indexOf(ourselves) <= 0 ||
       planned[0] !== leader) {
      // Things have changed again, simply terminate:
      terminateAndStartOther();
      console.info("synchronizeOneShard: cancelled, %s/%s, %s/%s",
                   database, shard, database, planId);
      return;
    }
    var current = [];
    try {
      current = global.ArangoClusterInfo.getCollectionInfoCurrent(
        database, planId, shard).servers;
    } catch (e2) { }
    if (current[0] === leader) {
      if (current.indexOf(ourselves) === -1) {
        break;   // start synchronization work
      }
      // We are already there, this is rather strange, but never mind:
      terminateAndStartOther();
      console.info("synchronizeOneShard: already done, %s/%s, %s/%s",
                   database, shard, database, planId);
      return;
    }
    console.info("synchronizeOneShard: waiting for leader, %s/%s, %s/%s",
                 database, shard, database, planId);
    wait(1.0);
  }

  // Once we get here, we know that the leader is ready for sync, so
  // we give it a try:
  var ok = false;
  const rep = require("@arangodb/replication");

  console.info("synchronizeOneShard: trying to synchronize local shard",
               "'%s/%s' for central '%s/%s'",
               database, shard, database, planId);
  try {
    var ep = ArangoClusterInfo.getServerEndpoint(leader);
    // First once without a read transaction:
    var sy;
    if (isStopping()) {
      throw "server is shutting down";
    }
    sy = rep.syncCollection(shard,
        { endpoint: ep, incremental: true,
          keepBarrier: true, useCollectionId: false });
    if (sy.error) {
      console.error("synchronizeOneShard: could not initially synchronize",
                    "shard ", shard, sy);
      throw "Initial sync for shard " + shard + " failed";
    } else {
      if (sy.collections.length === 0 ||
          sy.collections[0].name !== shard) {
        cancelBarrier(ep, database, sy.barrierId);
        throw "Shard " + shard + " seems to be gone from leader!";
      } else {
        // Now start a read transaction to stop writes:
        var lockJobId = false;
        try {
          lockJobId = startReadLockOnLeader(ep, database,
                                            shard, 300);
          console.debug("lockJobId:", lockJobId);
        }
        catch (err1) {
          console.error("synchronizeOneShard: exception in startReadLockOnLeader:", err1);
        }
        finally {
          cancelBarrier(ep, database, sy.barrierId);
        }
        if (lockJobId !== false) {
          try {
            var sy2 = rep.syncCollectionFinalize(
              database, shard, sy.lastLogTick, { endpoint: ep });
            if (sy2.error) {
              console.error("synchronizeOneShard: Could not finalize shard synchronization",
                            shard, sy2);
              ok = false;
            } else {
              ok = addShardFollower(ep, database, shard);
            }
          }
          catch (err3) {
            console.error("synchronizeOneshard: exception in",
                          "syncCollectionFinalize:", err3);
          }
          finally {
            if (!cancelReadLockOnLeader(ep, database,
                                        lockJobId)) {
              console.error("synchronizeOneShard: read lock has timed out",
                            "for shard", shard);
              ok = false;
            }
          }
        } else {
          console.error("synchronizeOneShard: lockJobId was false for shard",
                        shard);
        }
        if (ok) {
          console.info("synchronizeOneShard: synchronization worked for shard",
                       shard);
        } else {
          throw "Did not work for shard " + shard + ".";
          // just to log below in catch
        }
      }
    }
  }
  catch (err2) {
    if (!isStopping()) {
      console.error("synchronization of local shard '%s/%s' for central '%s/%s' failed: %s",
                    database, shard, database, planId, JSON.stringify(err2));
    }
  }
  // Tell others that we are done:
  terminateAndStartOther();
  console.info("synchronizeOneShard: done, %s/%s, %s/%s",
               database, shard, database, planId);
}

////////////////////////////////////////////////////////////////////////////////
/// @brief schedule a shard synchronization
////////////////////////////////////////////////////////////////////////////////

function scheduleOneShardSynchronization(database, shard, planId, leader) {
  console.debug("scheduleOneShardSynchronization:", database, shard, planId,
                leader);
  try {
    global.KEY_GET("shardSynchronization", "lock");
  }
  catch (e) {
    global.KEYSPACE_CREATE("shardSynchronization");
    global.KEY_SET("shardSynchronization", "scheduled", {});
    global.KEY_SET("shardSynchronization", "running", null);
    global.KEY_SET("shardSynchronization", "lock", null);
  }

  lockSyncKeyspace();
  try {
    var jobs = global.KEYSPACE_GET("shardSynchronization");
    if ((jobs.running !== null && jobs.running.shard === shard) ||
        jobs.scheduled.hasOwnProperty(shard)) {
      console.debug("task is already running or scheduled,",
                    "ignoring scheduling request");
      return false;
    }

    // If we reach this, we actually have to schedule a new task:
    var jobInfo = { database, shard, planId, leader };
    jobs.scheduled[shard] = jobInfo;
    global.KEY_SET("shardSynchronization", "scheduled", jobs.scheduled);
    console.debug("scheduleOneShardSynchronization: have scheduled job", jobInfo);
  }
  finally {
    unlockSyncKeyspace();
  }
  tryLaunchJob();
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief synchronize collections for which we are followers (synchronously
/// replicated shards)
////////////////////////////////////////////////////////////////////////////////

function synchronizeLocalFollowerCollections (plannedCollections,
                                              currentCollections) {
  var ourselves = global.ArangoServerState.id();

  var db = require("internal").db;
  db._useDatabase("_system");
  var localDatabases = getLocalDatabases();
  var database;

  // iterate over all matching databases
  for (database in plannedCollections) {
    if (plannedCollections.hasOwnProperty(database)) {
      if (localDatabases.hasOwnProperty(database)) {
        // switch into other database
        db._useDatabase(database);

        try {
          // iterate over collections of database
          var collections = plannedCollections[database];
          var collection;

          // diff the collections
          for (collection in collections) {
            if (collections.hasOwnProperty(collection)) {
              var collInfo = collections[collection];
              var shards = collInfo.shards;   // this is the Plan
              var shard;

              collInfo.planId = collInfo.id;

              for (shard in shards) {
                if (shards.hasOwnProperty(shard)) {
                  var pos = shards[shard].indexOf(ourselves);
                  if (pos > 0) {   // found and not in position 0
                    // found a shard we have to replicate synchronously
                    // now see whether we are in sync by looking at the
                    // current entry in the agency:
                    var inCurrent = lookup4d(currentCollections, database,
                                             collection, shard);
                    // If inCurrent is not in order in any way, we schedule
                    // a synchronization job:
                    if (inCurrent === undefined ||
                        ! inCurrent.hasOwnProperty("servers") ||
                        typeof inCurrent.servers !== "object" ||
                        !Array.isArray(inCurrent.servers) ||
                        inCurrent.servers.indexOf(ourselves) === -1 ||
                        inCurrent.servers[0].substr(0, 1) !== "_" ||
                        inCurrent.servers[0] === shards[shard][0]) {
                      scheduleOneShardSynchronization(
                          database, shard, collInfo.planId,
                          inCurrent.servers[0]);
                    }
                  }
                }
              }
            }
          }
        }
        catch (err) {
          // always return to previous database
          db._useDatabase("_system");
          throw err;
        }

        db._useDatabase("_system");
      }
    }
  }
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief handle collection changes
////////////////////////////////////////////////////////////////////////////////

function handleCollectionChanges (plan, current, takeOverResponsibility,
                                  writeLocked) {
  var plannedCollections = plan.Collections;
  var currentCollections = current.Collections;

  var ok = true;

  try {
    createLocalCollections(plannedCollections, plan.Version, currentCollections,
                           takeOverResponsibility, writeLocked);
    dropLocalCollections(plannedCollections, currentCollections, writeLocked);
    cleanupCurrentCollections(plannedCollections, currentCollections,
                              writeLocked);
    if (!synchronizeLocalFollowerCollections(plannedCollections,
                                             currentCollections)) {
      // If not all needed jobs have been scheduled, then work is still
      // ongoing, therefore we want to revisit this soon.
      ok = false;
    }
  }
  catch (err) {
    console.error("Caught error in handleCollectionChanges: " +
                  JSON.stringify(err), JSON.stringify(err.stack));
    ok = false;
  }
  return ok;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief make sure that replication is set up for all databases
////////////////////////////////////////////////////////////////////////////////

function setupReplication () {
  console.debug("Setting up replication...");

  var db = require("internal").db;
  var rep = require("@arangodb/replication");
  var dbs = db._databases();
  var i;
  var ok = true;
  for (i = 0; i < dbs.length; i++) {
    var database = dbs[i];
    try {
      console.debug("Checking replication of database "+database);
      db._useDatabase(database);

      var state = rep.applier.state();
      if (state.state.running === false) {
        var endpoint = ArangoClusterInfo.getServerEndpoint(
                              ArangoServerState.idOfPrimary());
        var config = { "endpoint": endpoint, "includeSystem": false,
                       "incremental": false, "autoStart": true,
                       "requireFromPresent": true};
        console.info("Starting synchronization...");
        var res = rep.sync(config);
        console.info("Last log tick: "+res.lastLogTick+
                    ", starting replication...");
        rep.applier.properties(config);
        var res2 = rep.applier.start(res.lastLogTick);
        console.info("Result of replication start: "+res2);
      }
    }
    catch (err) {
      console.error("Could not set up replication for database ", database, JSON.stringify(err));
      ok = false;
    }
  }
  db._useDatabase("_system");
  return ok;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief role change from secondary to primary
////////////////////////////////////////////////////////////////////////////////

function secondaryToPrimary () {
  console.info("Switching role from secondary to primary...");
  var db = require("internal").db;
  var rep = require("@arangodb/replication");
  var dbs = db._databases();
  var i;
  try {
    for (i = 0; i < dbs.length; i++) {
      var database = dbs[i];
      console.info("Stopping asynchronous replication for db " +
                   database + "...");
      db._useDatabase(database);
      var state = rep.applier.state();
      if (state.state.running === true) {
        try {
          rep.applier.stop();
        }
        catch (err) {
          console.info("Exception caught whilst stopping replication!");
        }
      }
      rep.applier.forget();
    }
  }
  finally {
    db._useDatabase("_system");
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief role change from primary to secondary
////////////////////////////////////////////////////////////////////////////////

function primaryToSecondary () {
  console.info("Switching role from primary to secondary...");
}

////////////////////////////////////////////////////////////////////////////////
/// @brief change handling trampoline function
////////////////////////////////////////////////////////////////////////////////

function handleChanges (plan, current, writeLocked) {
  var changed = false;
  var role = ArangoServerState.role();
  if (role === "PRIMARY" || role === "SECONDARY") {
    // Need to check role change for automatic failover:
    var myId = ArangoServerState.id();
    if (role === "PRIMARY") {
      if (! plan.DBServers[myId]) {
        // Ooops! We do not seem to be a primary any more!
        changed = ArangoServerState.redetermineRole();
      }
    } else { // role === "SECONDARY"
      if (plan.DBServers[myId]) {
        changed = ArangoServerState.redetermineRole();
        if (!changed) {
          // mop: oops...changing role has failed. retry next time.
          return false;
        }
      } else {
        var found = null;
        var p;
        for (p in plan) {
          if (plan.hasOwnProperty(p) && plan[p] === myId) {
            found = p;
            break;
          }
        }
        if (found !== ArangoServerState.idOfPrimary()) {
          // Note this includes the case that we are not found at all!
          changed = ArangoServerState.redetermineRole();
        }
      }
    }
  }
  var oldRole = role;
  if (changed) {
    role = ArangoServerState.role();
    console.log("Our role has changed to " + role);
    if (oldRole === "SECONDARY" && role === "PRIMARY") {
      secondaryToPrimary();
    }
    else if (oldRole === "PRIMARY" && role === "SECONDARY") {
      primaryToSecondary();
    }
  }

  handleDatabaseChanges(plan, current, writeLocked);
  var success;
  if (role === "PRIMARY" || role === "COORDINATOR") {
    // Note: This is only ever called for DBservers (primary and secondary),
    // we keep the coordinator case here just in case...
    success = handleCollectionChanges(plan, current, changed, writeLocked);
  }
  else {
    success = setupReplication();
  }

  return success;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief throw an ArangoError
////////////////////////////////////////////////////////////////////////////////

var raiseError = function (code, msg) {
  var err = new ArangoError();
  err.errorNum = code;
  err.errorMessage = msg;

  throw err;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief retrieve a list of shards for a collection
////////////////////////////////////////////////////////////////////////////////

var shardList = function (dbName, collectionName) {
  var ci = global.ArangoClusterInfo.getCollectionInfo(dbName, collectionName);

  if (ci === undefined || typeof ci !== 'object') {
    throw "unable to determine shard list for '" + dbName + "/" + collectionName + "'";
  }

  var shards = [ ], shard;
  for (shard in ci.shards) {
    if (ci.shards.hasOwnProperty(shard)) {
      shards.push(shard);
    }
  }

  if (shards.length === 0) {
    raiseError(arangodb.errors.ERROR_ARANGO_COLLECTION_NOT_FOUND.code,
               arangodb.errors.ERROR_ARANGO_COLLECTION_NOT_FOUND.message);
  }

  return shards;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief wait for a distributed response
////////////////////////////////////////////////////////////////////////////////

var waitForDistributedResponse = function (data, numberOfRequests) {
  var received = [ ];
  try {

    while (received.length < numberOfRequests) {
      var result = global.ArangoClusterComm.wait(data);
      var status = result.status;

      if (status === "ERROR") {
        raiseError(arangodb.errors.ERROR_INTERNAL.code,
                   "received an error from a DB server: " + JSON.stringify(result));
      }
      else if (status === "TIMEOUT") {
        raiseError(arangodb.errors.ERROR_CLUSTER_TIMEOUT.code,
                   arangodb.errors.ERROR_CLUSTER_TIMEOUT.message);
      }
      else if (status === "DROPPED") {
        raiseError(arangodb.errors.ERROR_INTERNAL.code,
                   "the operation was dropped");
      }
      else if (status === "RECEIVED") {
        received.push(result);

        if (result.headers && result.headers.hasOwnProperty('x-arango-response-code')) {
          var code = parseInt(result.headers['x-arango-response-code'].substr(0, 3), 10);

          if (code >= 400) {
            var body;

            try {
              body = JSON.parse(result.body);
            }
            catch (err) {
              raiseError(arangodb.errors.ERROR_INTERNAL.code,
                         "error parsing JSON received from a DB server: " + err.message);
            }

            raiseError(body.errorNum,
                       body.errorMessage);
          }
        }
      }
      else {
        // something else... wait without GC
        require("internal").wait(0.1, false);
      }
    }
  } finally {
    global.ArangoClusterComm.drop(data);
  }
  return received;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief whether or not clustering is enabled
////////////////////////////////////////////////////////////////////////////////

var isCluster = function () {
  var role = global.ArangoServerState.role();

  return (role !== undefined && role !== "SINGLE");
};

////////////////////////////////////////////////////////////////////////////////
/// @brief whether or not we are a coordinator
////////////////////////////////////////////////////////////////////////////////

var isCoordinator = function () {
  return global.ArangoServerState.isCoordinator();
};

////////////////////////////////////////////////////////////////////////////////
/// @brief role
////////////////////////////////////////////////////////////////////////////////

var role = function () {
  var role = global.ArangoServerState.role();

  if (role !== "SINGLE") {
    return role;
  }
  return undefined;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief status
////////////////////////////////////////////////////////////////////////////////

var status = function () {
  if (! isCluster() || ! global.ArangoServerState.initialized()) {
    return undefined;
  }

  return global.ArangoServerState.status();
};

////////////////////////////////////////////////////////////////////////////////
/// @brief isCoordinatorRequest
////////////////////////////////////////////////////////////////////////////////

var isCoordinatorRequest = function (req) {
  if (! req || ! req.hasOwnProperty("headers")) {
    return false;
  }

  return req.headers.hasOwnProperty("x-arango-coordinator");
};

////////////////////////////////////////////////////////////////////////////////
/// @brief handlePlanChange
////////////////////////////////////////////////////////////////////////////////

var handlePlanChange = function (plan, current) {
  if (! isCluster() || isCoordinator() || ! global.ArangoServerState.initialized()) {
    return true;
  }

  let versions = {
    plan: plan.Version,
    current: current.Version,
  };

  //////////////////////////////////////////////////////////////////////////////
  /// @brief execute an action under a write-lock
  //////////////////////////////////////////////////////////////////////////////

  function writeLocked (lockInfo, cb, args) {
    var timeout = lockInfo.timeout;
    if (timeout === undefined) {
      timeout = 60;
    }

    var ttl = lockInfo.ttl;
    if (ttl === undefined) {
      ttl = 120;
    }
    if (require("internal").coverage || require("internal").valgrind) {
      ttl *= 10;
      timeout *= 10;
    }

    global.ArangoAgency.lockWrite(lockInfo.part, ttl, timeout);

    try {
      cb.apply(null, args);
      global.ArangoAgency.increaseVersion(lockInfo.part + "/Version");

      let version = global.ArangoAgency.get(lockInfo.part + "/Version");
      versions[lockInfo.part.toLowerCase()] = version.arango[lockInfo.part].Version;

      global.ArangoAgency.unlockWrite(lockInfo.part, timeout);
    }
    catch (err) {
      global.ArangoAgency.unlockWrite(lockInfo.part, timeout);
      throw err;
    }
  }

  try {
    versions.success = handleChanges(plan, current, writeLocked);

    console.info("plan change handling successful");
  } catch (err) {
    console.error("error details: %s", JSON.stringify(err));
    console.error("error stack: %s", err.stack);
    console.error("plan change handling failed");
    versions.success = false;
  }
  return versions;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief coordinatorId
////////////////////////////////////////////////////////////////////////////////

var coordinatorId = function () {
  if (! isCoordinator()) {
    console.error("not a coordinator");
  }
  return global.ArangoServerState.id();
};

////////////////////////////////////////////////////////////////////////////////
/// @brief bootstrap db servers
////////////////////////////////////////////////////////////////////////////////

var bootstrapDbServers = function (isRelaunch) {
  global.ArangoClusterInfo.reloadDBServers();

  var dbServers = global.ArangoClusterInfo.getDBServers();
  var ops = [];
  var i;

  var options = {
    coordTransactionID: global.ArangoClusterComm.getId(),
    timeout: 90
  };

  for (i = 0;  i < dbServers.length;  ++i) {
    var server = dbServers[i];

    var op = global.ArangoClusterComm.asyncRequest(
      "POST",
      "server:" + server,
      "_system",
      "/_admin/cluster/bootstrapDbServer",
      '{"isRelaunch": ' + (isRelaunch ? "true" : "false") + '}',
      {},
      options);

    ops.push(op);
  }

  var result = true;

  for (i = 0;  i < ops.length;  ++i) {
    var r = global.ArangoClusterComm.wait(ops[i]);

    if (r.status === "RECEIVED") {
      console.info("bootstraped DB server %s", dbServers[i]);
    }
    else if (r.status === "TIMEOUT") {
      console.error("cannot bootstrap DB server %s: operation timed out", dbServers[i]);
      result = false;
    }
    else {
      console.error("cannot bootstrap DB server %s: %s", dbServers[i], JSON.stringify(r));
      result = false;
    }
  }

  return result;
};

////////////////////////////////////////////////////////////////////////////////
/// @brief shard distribution
////////////////////////////////////////////////////////////////////////////////

function format(x) {
  var r = {};
  var keys = Object.keys(x);
  for (var i = 0; i < keys.length; ++i) {
    var y = x[keys[i]];
    r[keys[i]] = { leader: y[0], followers: y.slice(1) };
  }
  return r;
}

function shardDistribution() {
  var db = require("internal").db;
  var dbName = db._name();
  var colls = db._collections();
  var result = {};
  for (var i = 0; i < colls.length; ++i) {
    var collName = colls[i].name();
    var collInfo = global.ArangoClusterInfo.getCollectionInfo(dbName,
                                                              collName);
    var shards = collInfo.shards;
    var collInfoCurrent = {};
    var shardNames = Object.keys(shards);
    for (var j = 0; j < shardNames.length; ++j) {
      collInfoCurrent[shardNames[j]] =
        global.ArangoClusterInfo.getCollectionInfoCurrent(
          dbName, collName, shardNames[j]).servers;
    }
    result[collName] = {Plan: format(collInfo.shards),
                        Current: format(collInfoCurrent)};
  }

  return {
    results: result
  };
}

////////////////////////////////////////////////////////////////////////////////
/// @brief move shard
////////////////////////////////////////////////////////////////////////////////

function moveShard(info) {
  var isLeader;
  var collInfo;
  try {
    collInfo = global.ArangoClusterInfo.getCollectionInfo(info.database,
                                                          info.collection);
    var shards = collInfo.shards;
    var shard = shards[info.shard];
    var pos = shard.indexOf(info.fromServer);
    if (pos === -1) {
      throw "Banana";
    } else if (pos === 0) {
      isLeader = true;
    } else {
      isLeader = false;
    }
  } catch (e2) {
    return "Combination of database, collection, shard and fromServer does not make sense.";
  }

  var id;
  try {
    id = global.ArangoClusterInfo.uniqid();
    var todo = { "type": "moveShard",
                 "database": info.database,
                 "collection": collInfo.id,
                 "shard": info.shard,
                 "fromServer": info.fromServer,
                 "toServer": info.toServer,
                 "jobId": id,
                 "timeCreated": (new Date()).toISOString(),
                 "creator": ArangoServerState.id() };
    global.ArangoAgency.set("Target/ToDo/" + id, todo);
  } catch (e1) {
    return {error: true, errorMessage: "Cannot write to agency."};
  }

  return {error: false, id: id};
}

////////////////////////////////////////////////////////////////////////////////
/// @brief rebalance shards
////////////////////////////////////////////////////////////////////////////////

function rebalanceShards() {
  var dbServers = global.ArangoClusterInfo.getDBServers();
  var dbTab = {};
  var i, j, k, l;
  for (i = 0; i < dbServers.length; ++i) {
    dbTab[dbServers[i]] = [];
  }
  var shardMap = {};

  // First count and collect:
  var db = require("internal").db;
  var databases = db._databases();
  for (i = 0; i < databases.length; ++i) {
    db._useDatabase(databases[i]);
    try {
      var colls = db._collections();
      for (j = 0; j < colls.length; ++j) {
        var collName = colls[j].name();
        if (collName.substr(0, 1) === "_") {
          continue;
        }
        var collInfo = global.ArangoClusterInfo.getCollectionInfo(
            databases[i], collName);
        var shardNames = Object.keys(collInfo.shards);
        for (k = 0; k < shardNames.length; k++) {
          var shardName = shardNames[k];
          shardMap[shardName] = { database: databases[i], collection: collName,
                                  servers: collInfo.shards[shardName],
                                  weight: 1 };
          dbTab[collInfo.shards[shardName][0]].push(
            { shard: shardName, leader: true,
              weight: shardMap[shardName].weight });
          for (l = 1; l < collInfo.shards[shardName].length; ++l) {
            dbTab[collInfo.shards[shardName][l]].push(
            { shard: shardName, leader: false,
              weight: shardMap[shardName].weight });
          }
        }
      }
    } finally {
      db._useDatabase("_system");
    }
  }

  console.info(shardMap);
  console.info(dbTab);

  // Compute total weight for each DBServer:
  var totalWeight = [];
  for (i = 0; i < dbServers.length; ++i) {
    totalWeight.push( {"server": dbServers[i],
                       "weight": _.reduce(dbTab[dbServers[i]],
                                          (sum, x) => sum + x.weight, 0)} );
  }
  totalWeight = _.sortBy(totalWeight, x => x.weight);

  var shardList = Object.keys(shardMap);
  var countMoved = 0;

  for (i = 0; i < shardList.length; i++) {
    var last = totalWeight.length-1;
    var fullest = totalWeight[last].server;
    var emptiest = totalWeight[0].server;
    var weightDiff = totalWeight[last].weight - totalWeight[0].weight;
    if (weightDiff < 1.0) {
      console.log("rebalanceShards: cluster is balanced");
      return true;
    }
    var shard = shardList[i];
    console.log("rebalanceShards: considering shard", shard,
                "totalWeight=", totalWeight);
    if (shardMap[shard].servers.indexOf(fullest) >= 0 &&
        shardMap[shard].servers.indexOf(emptiest) === -1 &&
        shardMap[shard].weight < 0.9 * weightDiff) {
      var shardInfo = shardMap[shard];
      var todo = { database: shardInfo.database,
                   collection: shardInfo.collection,
                   shard: shard,
                   fromServer: fullest,
                   toServer: emptiest };
      var msg = moveShard(todo);
      if (msg === "") {
        console.info("rebalanceShards: moveShard(", todo, ")");
        totalWeight[last].weight -= shardInfo.weight;
        totalWeight[0].weight += shardInfo.weight;
        totalWeight = _.sortBy(totalWeight, x => x.weight);
        countMoved += 1;
        if (countMoved >= 10) {
          break;
        }
      } else {
        console.error("rebalanceShards: moveShard(", todo, ") produced:", msg);
      }
    }
  }
  if (countMoved === 0) {
    console.log("rebalanceShards: no sensible moves found");
    return true;
  }
  console.log("rebalanceShards: scheduled", countMoved, " shard moves.");
  return true;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief supervision state
////////////////////////////////////////////////////////////////////////////////

function supervisionState() {
  try {
    var result = global.ArangoAgency.get("Target");
    result = result.arango.Target;
    var proj = { ToDo: result.ToDo, Pending: result.Pending,
                 Failed: result.Failed, Finished: result.Finished,
                 error: false };
    return proj;
  }
  catch (err) {
    return { error: true, errorMsg: "could not read /Target in agency",
             exception: err };
  }
}

////////////////////////////////////////////////////////////////////////////////
/// @brief wait for synchronous replication to settle
////////////////////////////////////////////////////////////////////////////////

function waitForSyncReplOneCollection(dbName, collName) {
  console.debug("waitForSyncRepl:", dbName, collName);
  var cinfo = global.ArangoClusterInfo.getCollectionInfo(dbName, collName);
  var count = 120;
  while (--count > 0) {
    var shards = Object.keys(cinfo.shards);
    var ccinfo = shards.map(function(s) {
      return global.ArangoClusterInfo.getCollectionInfoCurrent(dbName,
                collName, s).servers;
    });
    console.debug("waitForSyncRepl", dbName, collName, shards, cinfo.shards, ccinfo);
    var ok = true;
    for (var i = 0; i < shards.length; ++i) {
      if (cinfo.shards[shards[i]].length !== ccinfo[i].length) {
        ok = false;
      }
    }
    if (ok) {
      console.debug("waitForSyncRepl: OK:", dbName, collName, shards);
      return true;
    }
    require("internal").wait(0.5);
  }
  console.warn("waitForSyncRepl:", dbName, collName, ": BAD");
  return false;
}

function waitForSyncRepl(dbName, collList) {
  if (! isCoordinator()) {
    return true;
  }
  var ok = true;
  for (var i = 0; i < collList.length; ++i) {
    ok = waitForSyncReplOneCollection(dbName, collList[i].name()) && ok;
  }
  return ok;
}

exports.bootstrapDbServers            = bootstrapDbServers;
exports.coordinatorId                 = coordinatorId;
exports.handlePlanChange              = handlePlanChange;
exports.isCluster                     = isCluster;
exports.isCoordinator                 = isCoordinator;
exports.isCoordinatorRequest          = isCoordinatorRequest;
exports.role                          = role;
exports.shardList                     = shardList;
exports.status                        = status;
exports.wait                          = waitForDistributedResponse;
exports.endpointToURL                 = endpointToURL;
exports.synchronizeOneShard           = synchronizeOneShard;
exports.shardDistribution             = shardDistribution;
exports.rebalanceShards               = rebalanceShards;
exports.moveShard                     = moveShard;
exports.supervisionState              = supervisionState;
exports.waitForSyncRepl               = waitForSyncRepl;