Create, index and delete requests are write operations, which must be successfully completed on the primary shard before they can be copied to any associated replica shards.
[[img-distrib-write]] .Creating, indexing or deleting a single document image::images/04-02_write.png["Creating, indexing or deleting a single document"]
Below we list the sequence of steps necessary to successfully create, index or
delete a document on both the primary and any replica shards, as depicted in
The client sends a create, index or delete request to
The node uses the document's
_id to determine that the document
belongs to shard
0. It forwards the request to
where the primary copy of shard
0 is currently allocated.
Node 3executes the request on the primary shard. If it is successful, it forwards the request in parallel to the replica shards on
Node 2. Once all of the replica shards report success,
Node 3reports success to the requesting node, which reports success to the client.
By the time the client receives a successful response, the document change has been executed on the primary shard and on all replica shards. Your change is safe.
There are a number of optional request parameters which allow you to influence this process, possibly increasing performance at the cost of data security. These options are seldom used because Elasticsearch is already fast, but they are explained here for the sake of completeness.
The default value for replication is
sync. This causes the primary shard to
wait for successful responses from the replica shards before returning.
If you set
async, then it will return success to the client
as soon as the request has been executed on the primary shard. It will still
forward the request to the replicas, but you will not know if the replicas
succeeded or not.
syncreplication as it is possible to overload Elasticsearch by sending too many requests without waiting for their completion.
By default, the primary shard requires a quorum or majority of shard copies (where a shard copy can be a primary or a replica shard) to be available before even attempting a write operation. This is to prevent writing data to the ``wrong side'' of a network partition. A quorum is defined as:
int( (primary + number_of_replicas) / 2 ) + 1
The allowed values for
one (just the primary shard),
(the primary and all replicas) or the default
quorum or majority of shard
Note that the
number_of_replicas is the number of replicas specified in
the index settings, not the number of replicas that are currently active. If
you have specified that an index should have 3 replicas then a quorum would
int( (primary + 3 replicas) / 2 ) + 1 = 3
But if you only start 2 nodes, then there will be insufficient active shard copies to satisfy the quorum and you will be unable to index or delete any documents.
What happens if insufficient shard copies are available? Elasticsearch waits,
in the hope that more shards will appear. By default it will wait up to one
minute. If you need to, you can use the
timeout parameter to make it abort
100 is 100 milliseconds,
30s is 30 seconds.
1replica by default, which means that two active shard copies should be required in order to satisfy the need for a
quorum. However, these default settings would prevent us from doing anything useful with a single-node cluster. To avoid this problem, the requirement for a quorum is only enforced when
number_of_replicasis greater than