arangodb/Documentation/DocuBlocks/collectionDatabaseCreate.md

@brief creates a new document or edge collection db._create(collection-name)

Creates a new document collection named collection-name. If the collection name already exists or if the name format is invalid, an error is thrown. For more information on valid collection names please refer to the naming conventions.

db._create(collection-name, properties)

properties must be an object with the following attributes:

• waitForSync (optional, default false): If true creating a document will only return after the data was synced to disk.

• journalSize (optional, default is a global config parameter, mmfiles-only): The maximal size of a journal or datafile. Note that this also limits the maximal size of a single object. Must be at least 1MB. This option is meaningful for the MMFiles storage engine only.

• isSystem (optional, default is false): If true, create a system collection. In this case collection-name should start with an underscore. End users should normally create non-system collections only. API implementors may be required to create system collections in very special occasions, but normally a regular collection will do.

• keyOptions (optional): additional options for key generation. If specified, then keyOptions should be a JSON array containing the following attributes (note: some of them are optional):

  • type: specifies the type of the key generator. The currently available generators are traditional, autoincrement, uuid and padded.

    The traditional key generator generates numerical keys in ascending order. The autoincrement key generator generates numerical keys in ascending order, the inital offset and the spacing can be configured (note: autoincrement is currently only supported for non-sharded collections). The padded key generator generates keys of a fixed length (16 bytes) in ascending lexicographical sort order. This is ideal for usage with the RocksDB engine, which will slightly benefit keys that are inserted in lexicographically ascending order. The key generator can be used in a single-server or cluster. The uuid key generator generates universally unique 128 bit keys, which are stored in hexadecimal human-readable format. This key generator can be used in a single-server or cluster to generate "seemingly random" keys. The keys produced by this key generator are not lexicographically sorted.

  • allowUserKeys: if set to true, then it is allowed to supply own key values in the _key attribute of a document. If set to false, then the key generator will solely be responsible for generating keys and supplying own key values in the _key attribute of documents is considered an error.

  • increment: increment value for autoincrement key generator. Not used for other key generator types.

  • offset: initial offset value for autoincrement key generator. Not used for other key generator types.

• numberOfShards (optional, default is 1): in a cluster, this value determines the number of shards to create for the collection. In a single server setup, this option is meaningless.

• shardKeys (optional, default is [ "_key" ]): in a cluster, this attribute determines which document attributes are used to determine the target shard for documents. Documents are sent to shards based on the values they have in their shard key attributes. The values of all shard key attributes in a document are hashed, and the hash value is used to determine the target shard. Note that values of shard key attributes cannot be changed once set. This option is meaningless in a single server setup.

  When choosing the shard keys, one must be aware of the following rules and limitations: In a sharded collection with more than one shard it is not possible to set up a unique constraint on an attribute that is not the one and only shard key given in shardKeys. This is because enforcing a unique constraint would otherwise make a global index necessary or need extensive communication for every single write operation. Furthermore, if _key is not the one and only shard key, then it is not possible to set the _key attribute when inserting a document, provided the collection has more than one shard. Again, this is because the database has to enforce the unique constraint on the _key attribute and this can only be done efficiently if this is the only shard key by delegating to the individual shards.

cacheEnabled (optional, default is false, rocksdb-only, from v.3.4): Enable in-memory caching for documents and primary index entries. This can potentially speed up point-lookups significantly, especially if your collection has a subset of frequently accessed keys. Please test this feature carefully to ensure that it does not adversely affect the performance of your system.

• isVolatile (optional, default is false, mmfiles-only): If true then the collection data is kept in-memory only and not made persistent. Unloading the collection will cause the collection data to be discarded. Stopping or re-starting the server will also cause full loss of data in the collection. Setting this option will make the resulting collection be slightly faster than regular collections because ArangoDB does not enforce any synchronization to disk and does not calculate any CRC checksums for datafiles (as there are no datafiles). This option is meaningful for the MMFiles storage engine only.

• indexBuckets (optional, default is 16, mmfiles-only): The number of buckets into which indexes using a hash table are split. The default is 16 and this number has to be a power of 2 and less than or equal to 1024.

  For very large collections one should increase this to avoid long pauses when the hash table has to be initially built or resized, since buckets are resized individually and can be initially built in parallel. For example, 64 might be a sensible value for a collection with 100 000 000 documents. Currently, only the edge index respects this value, but other index types might follow in future ArangoDB versions. Changes (see below) are applied when the collection is loaded the next time. This option is meaningful for the MMFiles storage engine only.

db._create(collection-name, properties, type)

Specifies the optional type of the collection, it can either be document or edge. On default it is document. Instead of giving a type you can also use db._createEdgeCollection or db._createDocumentCollection.

@EXAMPLES

With defaults:

@EXAMPLE_ARANGOSH_OUTPUT{collectionDatabaseCreate} c = db._create("users"); c.properties(); ~ db._drop("users"); @END_EXAMPLE_ARANGOSH_OUTPUT

With properties:

@EXAMPLE_ARANGOSH_OUTPUT{collectionDatabaseCreateProperties} |c = db._create("users", { waitForSync : true, journalSize : 1024 * 1204}); c.properties(); ~ db._drop("users"); @END_EXAMPLE_ARANGOSH_OUTPUT

With a key generator:

@EXAMPLE_ARANGOSH_OUTPUT{collectionDatabaseCreateKey} | db._create("users", { keyOptions: { type: "autoincrement", offset: 10, increment: 5 } }); db.users.save({ name: "user 1" }); db.users.save({ name: "user 2" }); db.users.save({ name: "user 3" }); ~ db._drop("users"); @END_EXAMPLE_ARANGOSH_OUTPUT

With a special key option:

@EXAMPLE_ARANGOSH_OUTPUT{collectionDatabaseCreateSpecialKey} db._create("users", { keyOptions: { allowUserKeys: false } }); db.users.save({ name: "user 1" }); | db.users.save({ name: "user 2", _key: "myuser" }); ~ // xpError(ERROR_ARANGO_DOCUMENT_KEY_UNEXPECTED) db.users.save({ name: "user 3" }); ~ db._drop("users"); @END_EXAMPLE_ARANGOSH_OUTPUT