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

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Every document in ArangoDB has a revision, stored in the system attribute _rev. It is fully managed by the server and read-only for the user.

Its value should be treated as opaque, no guarantees regarding its format and properties are given except that it will be different after a document update. More specifically, _rev values are unique across all documents and all collections in a single server setup. In a cluster setup, 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.

The _rev attribute can be used as a pre-condition for queries, to avoid lost update situations. That is, if a client fetches a document from the server, modifies it locally (but with the _rev attribute untouched) and sends it back to the server to update the document, but meanwhile the document was changed by another operation, then the revisions do not match anymore and the operation is cancelled by the server. Without this mechanism, the client would accidentally overwrite changes made to the document without knowing about it.

When an existing document is updated or replaced, ArangoDB will write a new version of this document to the write-ahead logfile (regardless of the storage engine). When the new version of the document has been written, the old version(s) will still be present, at least on disk. The same is true when an existing document (version) gets removed: the old version of the document plus the removal operation will be on disk for some time.

On disk it is therefore possible that multiple revisions of the same document (as identified by the same _key value) exist at the same time. However, stale revisions are not accessible. Once a document was updated or removed successfully, no query or other data retrieval operation done by the user will be able to see it any more. Furthermore, after some time, old revisions will be removed internally. This is to avoid ever-growing disk usage.

Multiple Documents in a single Command

The document API can 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.