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Basics and Terminology

Documents in ArangoDB are JSON objects. These objects can be nested (to any depth) and may contain lists. Each document has a unique primary key which identifies it within its collection. Furthermore, each document is uniquely identified by its document handle across all collections in the same database. Different revisions of the same document (identified by its handle) can be distinguished by their document revision. Any transaction only ever sees a single revision of a document. For example:

  "_id" : "myusers/3456789",
  "_key" : "3456789",
  "_rev" : "14253647",
  "firstName" : "John",
  "lastName" : "Doe",
  "address" : {
    "street" : "Road To Nowhere 1",
    "city" : "Gotham"
  "hobbies" : [
    {"name": "swimming", "howFavorite": 10},
    {"name": "biking", "howFavorite": 6},
    {"name": "programming", "howFavorite": 4}

All documents contain special attributes: the document handle is stored as a string in _id, the document’s primary key in _key and the document revision in _rev. The value of the _key attribute can be specified by the user when creating a document. _id and _key values are immutable once the document has been created. The _rev value is maintained by ArangoDB automatically.

Document Handle

A document handle uniquely identifies a document in the database. It is a string and consists of the collection’s name and the document key (_key attribute) separated by /.

Document Key

A document key uniquely identifies a document in the collection it is stored in. It can and should be used by clients when specific documents are queried. The document key is stored in the _key attribute of each document. The key values are automatically indexed by ArangoDB in a collection’s primary index. Thus looking up a document by its key is a fast operation. The _key value of a document is immutable once the document has been created. By default, ArangoDB will auto-generate a document key if no _key attribute is specified, and use the user-specified _key otherwise. The generated _key is guaranteed to be unique in the collection it was generated for. This also applies to sharded collections in a cluster. It can’t be guaranteed that the _key is unique within a database or across a whole node or instance however.

This behavior can be changed on a per-collection level by creating collections with the keyOptions attribute.

Using keyOptions it is possible to disallow user-specified keys completely, or to force a specific regime for auto-generating the _key values.

Document Revision

As ArangoDB supports MVCC (Multiple Version Concurrency Control), documents can exist in more than one revision. The document revision is the MVCC token used to specify a particular revision of a document (identified by its _id). It is a string value that contained (up to ArangoDB 3.0) an integer number and is unique within the list of document revisions for a single document. In ArangoDB >= 3.1 the _rev strings are in fact time stamps. They use the local clock of the DBserver that actually writes the document and have millisecond accuracy. Actually, a “Hybrid Logical Clock” is used (for this concept see this paper).

Within one shard it is guaranteed that two different document revisions have a different _rev string, even if they are written in the same millisecond, and that these stamps are ascending.

Note however that different servers in your cluster might have a clock skew, and therefore between different shards or even between different collections the time stamps are not guaranteed to be comparable.

The Hybrid Logical Clock feature does one thing to address this issue: Whenever a message is sent from some server A in your cluster to another one B, it is ensured that any timestamp taken on B after the message has arrived is greater than any timestamp taken on A before the message was sent. This ensures that if there is some “causality” between events on different servers, time stamps increase from cause to effect. A direct consequence of this is that sometimes a server has to take timestamps that seem to come from the future of its own clock. It will however still produce ever increasing timestamps. If the clock skew is small, then your timestamps will relatively accurately describe the time when the document revision was actually written.

ArangoDB uses 64bit unsigned integer values to maintain document revisions internally. At this stage we intentionally do not document the exact format of the revision values. When returning document revisions to clients, ArangoDB will put them into a string to ensure the revision is not clipped by clients that do not support big integers. Clients should treat the revision returned by ArangoDB as an opaque string when they store or use it locally. This will allow ArangoDB to change the format of revisions later if this should be required (as has happened with 3.1 with the Hybrid Logical Clock). Clients can use revisions to perform simple equality/non-equality comparisons (e.g. to check whether a document has changed or not), but they should not use revision ids to perform greater/less than comparisons with them to check if a document revision is older than one another, even if this might work for some cases.

Document revisions can be used to conditionally query, update, replace or delete documents in the database. In order to find a particular revision of a document, you need the document handle or key, and the document revision.

Multiple Documents in a single Command

Beginning with ArangoDB 3.0 the basic document API has been extended to handle not only single documents but multiple documents in a single command. This is crucial for performance, in particular in the cluster situation, in which a single request can involve multiple network hops within the cluster. Another advantage is that it reduces the overhead of individual network round trips between the client and the server. The general idea to perform multiple document operations in a single command is to use JSON arrays of objects in the place of a single document. As a consequence, document keys, handles and revisions for preconditions have to be supplied embedded in the individual documents given. Multiple document operations are restricted to a single document or edge collection. See the API descriptions for collection objects for details. Note that the API for database objects do not offer these operations.