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arangodb/Documentation/DocuBlocks/Rest/Replication/get_api_wal_access_tail.md

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@startDocuBlock get_api_wal_access_tail @brief Fetch recent operations

@RESTHEADER{GET /_api/wal/tail, Tail recent server operations, handleCommandTail}

@RESTQUERYPARAMETERS

@RESTQUERYPARAM{from,number,optional} Exclusive lower bound tick value for results. On successive calls to this API you should set this to the value returned with the x-arango-replication-lastincluded header (Unless that header contains 0).

@RESTQUERYPARAM{to,number,optional} Inclusive upper bound tick value for results.

@RESTQUERYPARAM{lastScanned,number,optional} Should be set to the value of the x-arango-replication-lastscanned header or alternatively 0 on first try. This allows the rocksdb engine to break up large transactions over multiple responses.

@RESTQUERYPARAM{global,bool,optional} Whether operations for all databases should be included. When set to false only the operations for the current database are included. The value true is only valid on the _system database. The default is false.

@RESTQUERYPARAM{chunkSize,number,optional} Approximate maximum size of the returned result.

@RESTQUERYPARAM{syncerId,number,optional} Id of the client used to tail results. The server will use this to keep operations until the client has fetched them. Must be a positive integer. Note this or serverId is required to have a chance at fetching reading all operations with the rocksdb storage engine.

@RESTQUERYPARAM{serverId,number,optional} Id of the client machine. If syncerId is unset, the server will use this to keep operations until the client has fetched them. Must be a positive integer. Note this or syncerId is required to have a chance at fetching reading all operations with the rocksdb storage engine.

@RESTQUERYPARAM{clientInfo,string,optional} Short description of the client, used for informative purposes only.

@RESTQUERYPARAM{barrierId,number,optional} Id of barrier used to keep WAL entries around. Note this is only required for the MMFiles storage engine

@RESTDESCRIPTION Returns data from the server's write-ahead log (also named replication log). This method can be called by replication clients after an initial synchronization of data. The method will return all "recent" logged operations from the server. Clients can replay and apply these operations locally so they get to the same data state as the server.

Clients can call this method repeatedly to incrementally fetch all changes from the server. In this case, they should provide the from value so they will only get returned the log events since their last fetch.

When the from query parameter is not used, the server will return log entries starting at the beginning of its replication log. When the from parameter is used, the server will only return log entries which have higher tick values than the specified from value (note: the log entry with a tick value equal to from will be excluded). Use the from value when incrementally fetching log data.

The to query parameter can be used to optionally restrict the upper bound of the result to a certain tick value. If used, the result will contain only log events with tick values up to (including) to. In incremental fetching, there is no need to use the to parameter. It only makes sense in special situations, when only parts of the change log are required.

The chunkSize query parameter can be used to control the size of the result. It must be specified in bytes. The chunkSize value will only be honored approximately. Otherwise a too low chunkSize value could cause the server to not be able to put just one log entry into the result and return it. Therefore, the chunkSize value will only be consulted after a log entry has been written into the result. If the result size is then bigger than chunkSize, the server will respond with as many log entries as there are in the response already. If the result size is still smaller than chunkSize, the server will try to return more data if there's more data left to return.

If chunkSize is not specified, some server-side default value will be used.

The Content-Type of the result is application/x-arango-dump. This is an easy-to-process format, with all log events going onto separate lines in the response body. Each log event itself is a JSON object, with at least the following attributes:

  • tick: the log event tick value

  • type: the log event type

Individual log events will also have additional attributes, depending on the event type. A few common attributes which are used for multiple events types are:

  • cuid: globally unique id of the view or collection the event was for

  • db: the database name the event was for

  • tid: id of the transaction the event was contained in

  • data: the original document data

A more detailed description of the individual replication event types and their data structures can be found in Operation Types.

The response will also contain the following HTTP headers:

  • x-arango-replication-active: whether or not the logger is active. Clients can use this flag as an indication for their polling frequency. If the logger is not active and there are no more replication events available, it might be sensible for a client to abort, or to go to sleep for a long time and try again later to check whether the logger has been activated.

  • x-arango-replication-lastincluded: the tick value of the last included value in the result. In incremental log fetching, this value can be used as the from value for the following request. Note that if the result is empty, the value will be 0. This value should not be used as from value by clients in the next request (otherwise the server would return the log events from the start of the log again).

  • x-arango-replication-lastscanned: the last tick the server scanned while computing the operation log. This might include operations the server did not returned to you due to various reasons (i.e. the value was filtered or skipped). You may use this value in the lastScanned header to allow the rocksdb engine to break up requests over multiple responses.

  • x-arango-replication-lasttick: the last tick value the server has logged in its write ahead log (not necessarily included in the result). By comparing the the last tick and last included tick values, clients have an approximate indication of how many events there are still left to fetch.

  • x-arango-replication-frompresent: is set to true if server returned all tick values starting from the specified tick in the from parameter. Should this be set to false the server did not have these operations anymore and the client might have missed operations.

  • x-arango-replication-checkmore: whether or not there already exists more log data which the client could fetch immediately. If there is more log data available, the client could call logger-follow again with an adjusted from value to fetch remaining log entries until there are no more.

    If there isn't any more log data to fetch, the client might decide to go to sleep for a while before calling the logger again.

Note: this method is not supported on a coordinator in a cluster.

@RESTRETURNCODES

@RESTRETURNCODE{200} is returned if the request was executed successfully, and there are log events available for the requested range. The response body will not be empty in this case.

@RESTRETURNCODE{204} is returned if the request was executed successfully, but there are no log events available for the requested range. The response body will be empty in this case.

@RESTRETURNCODE{400} is returned if either the from or to values are invalid.

@RESTRETURNCODE{405} is returned when an invalid HTTP method is used.

@RESTRETURNCODE{500} is returned if an error occurred while assembling the response.

@RESTRETURNCODE{501} is returned when this operation is called on a coordinator in a cluster.

@EXAMPLES

No log events available

@EXAMPLE_ARANGOSH_RUN{RestWalAccessTailingEmpty} var re = require("@arangodb/replication"); var lastTick = re.logger.state().state.lastLogTick;

var url = "/_api/wal/tail?from=" + lastTick;
var response = logCurlRequest('GET', url);

assert(response.code === 204);

logRawResponse(response);

@END_EXAMPLE_ARANGOSH_RUN

A few log events (One JSON document per line)

@EXAMPLE_ARANGOSH_RUN{RestWalAccessTailingSome} var re = require("@arangodb/replication"); db._drop("products");

var lastTick = re.logger.state().state.lastLogTick;

db._create("products");
db.products.save({ "_key": "p1", "name" : "flux compensator" });
db.products.save({ "_key": "p2", "name" : "hybrid hovercraft", "hp" : 5100 });
db.products.remove("p1");
db.products.update("p2", { "name" : "broken hovercraft" });
db.products.drop();

require("internal").wait(1);
var url = "/_api/wal/tail?from=" + lastTick;
var response = logCurlRequest('GET', url);

assert(response.code === 200);

logJsonLResponse(response);

@END_EXAMPLE_ARANGOSH_RUN

More events than would fit into the response

@EXAMPLE_ARANGOSH_RUN{RestWalAccessTailingBufferLimit} var re = require("@arangodb/replication"); db._drop("products");

var lastTick = re.logger.state().state.lastLogTick;

db._create("products");
db.products.save({ "_key": "p1", "name" : "flux compensator" });
db.products.save({ "_key": "p2", "name" : "hybrid hovercraft", "hp" : 5100 });
db.products.remove("p1");
db.products.update("p2", { "name" : "broken hovercraft" });
db.products.drop();

require("internal").wait(1);
var url = "/_api/wal/tail?from=" + lastTick + "&chunkSize=400";
var response = logCurlRequest('GET', url);

assert(response.code === 200);

logJsonResponse(response);

@END_EXAMPLE_ARANGOSH_RUN @endDocuBlock