arangodb/Documentation/Books/Manual/Administration/Sharding

!CHAPTER Sharding

ArangoDB is organizing its collection data in shards. Sharding allows to use multiple machines to run a cluster of ArangoDB
instances that together constitute a single database. This enables
you to store much more data, since ArangoDB distributes the data 
automatically to the different servers. In many situations one can 
also reap a benefit in data throughput, again because the load can
be distributed to multiple machines.

Shards are configured per collection so multiple shards of data form the collection as a whole. To determine in which shard the data is to be stored ArangoDB performs a hash across the values. By default this hash is being created from _key.

To configure the number of shards:

```
127.0.0.1:8529@_system> db._create("sharded_collection", {"numberOfShards": 4});
```

To configure the hashing:

```
127.0.0.1:8529@_system> db._create("sharded_collection", {"numberOfShards": 4, "shardKeys": ["country"]});
```

This would be useful to keep data of every country in one shard which would result in better performance for queries working on a per country base. You can also specify multiple `shardKeys`. Note however that if you change the shard keys from their defailt `["_key"]`, then finding a document in the collection by its primary key involves a request to every single shard. Furthermore, in this case one can no longer prescribe the primary key value of a new document but must use the automatically generated one. This latter restriction comes from the fact that ensuring uniqueness of the primary key would be very inefficient if the user could specify the primary key.