Creating, Indexing, and Deleting a Documentedit

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, as shown in Figure 9, “Creating, indexing, or deleting a single document”.

Figure 9. Creating, indexing, or deleting a single document

Creating, indexing or deleting a single document

Here is the sequence of steps necessary to successfully create, index, or delete a document on both the primary and any replica shards:

  1. The client sends a create, index, or delete request to Node 1.
  2. The node uses the document’s _id to determine that the document belongs to shard 0. It forwards the request to Node 3, where the primary copy of shard 0 is currently allocated.
  3. Node 3 executes the request on the primary shard. If it is successful, it forwards the request in parallel to the replica shards on Node 1 and Node 2. Once all of the replica shards report success, Node 3 reports success to the coordinating 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 that 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:

replication

The default value for replication is sync. This causes the primary shard to wait for successful responses from available replica shards before returning.

If you set replication to async, 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 whether the replicas succeeded.

This option is mentioned specifically to advise against using it. The default sync replication allows Elasticsearch to exert back pressure on whatever system is feeding it with data. With async replication, it is possible to overload Elasticsearch by sending too many requests without waiting for their completion.

consistency

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 follows:

int( (primary + number_of_replicas) / 2 ) + 1

The allowed values for consistency are one (just the primary shard), all (the primary and all replicas), or the default quorum, or majority, of shard copies.

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 three replicas, a quorum would be as follows:

int( (primary + 3 replicas) / 2 ) + 1 = 3

But if you start only two nodes, there will be insufficient active shard copies to satisfy the quorum, and you will be unable to index or delete any documents.

timeout
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 1 minute. If you need to, you can use the timeout parameter to make it abort sooner: 100 is 100 milliseconds, and 30s is 30 seconds.
Note

A new index has 1 replica 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 enforced only when number_of_replicas is greater than 1.