JavaScript Interface to Collections

This is an introduction to ArangoDB’s interface for collections and how to handle collections from the JavaScript shell arangosh. For other languages see the corresponding language API.

The most important call is the call to create a new collection.

Address of a Collection

All collections in ArangoDB have a unique identifier and a unique name. The namespace for collections is shared with views, so there cannot exist a collection and a view with the same name in the same database. ArangoDB internally uses the collection’s unique identifier to look up collections. This identifier, however, is managed by ArangoDB and the user has no control over it. In order to allow users to use their own names, each collection also has a unique name which is specified by the user. To access a collection from the user perspective, the collection name should be used, i.e.:

Collection

db._collection(collection-name)

A collection is created by a “db._create” call.

For example: Assume that the collection identifier is 7254820 and the name is demo, then the collection can be accessed as:

db._collection("demo")

If no collection with such a name exists, then null is returned.

There is a short-cut that can be used for non-system collections:

Collection name

db.collection-name

This call will either return the collection named db.collection-name or create a new one with that name and a set of default properties.

Note: Creating a collection on the fly using db.collection-name is not recommend and does not work in arangosh. To create a new collection, please use

Create

db._create(collection-name)

This call will create a new collection called collection-name. This method is a database method and is documented in detail at Database Methods

Synchronous replication

Starting in ArangoDB 3.0, the distributed version offers synchronous replication, which means that there is the option to replicate all data automatically within the ArangoDB cluster. This is configured for sharded collections on a per collection basis by specifying a “replication factor” when the collection is created. A replication factor of k means that altogether k copies of each shard are kept in the cluster on k different servers, and are kept in sync. That is, every write operation is automatically replicated on all copies.

This is organized using a leader/follower model. At all times, one of the servers holding replicas for a shard is “the leader” and all others are “followers”, this configuration is held in the Agency (see Cluster for details of the ArangoDB cluster architecture). Every write operation is sent to the leader by one of the Coordinators, and then replicated to all followers before the operation is reported to have succeeded. The leader keeps a record of which followers are currently in sync. In case of network problems or a failure of a follower, a leader can and will drop a follower temporarily after 3 seconds, such that service can resume. In due course, the follower will automatically resynchronize with the leader to restore resilience.

If a leader fails, the cluster Agency automatically initiates a failover routine after around 15 seconds, promoting one of the followers to leader. The other followers (and the former leader, when it comes back), automatically resynchronize with the new leader to restore resilience. Usually, this whole failover procedure can be handled transparently for the Coordinator, such that the user code does not even see an error message.

Obviously, this fault tolerance comes at a cost of increased latency. Each write operation needs an additional network roundtrip for the synchronous replication of the followers, but all replication operations to all followers happen concurrently. This is, why the default replication factor is 1, which means no replication.

For details on how to switch on synchronous replication for a collection, see the database method db._create(collection-name) in the section about Database Methods.