Overview

Cross-cluster replication is done on an index-by-index basis. Replication is configured at the index level. For each configured replication there is a replication source index called the leader index and a replication target index called the follower index.

Replication is active-passive. This means that while the leader index can directly be written into, the follower index can not directly receive writes.

Replication is pull-based. This means that replication is driven by the follower index. This simplifies state management on the leader index and means that cross-cluster replication does not interfere with indexing on the leader index.

Replication can be configured in two ways:

While replication is managed at the index level, replication is performed at the shard level. When a follower index is created, it is automatically configured to have an identical number of shards as the leader index. A follower shard task in the follower index pulls from the corresponding leader shard in the leader index by sending read requests for new operations. These read requests can be served from any copy of the leader shard (primary or replicas).

For each read request sent by the follower shard task, if there are new operations available on the leader shard, the leader shard responds with operations limited by the read parameters that you established when you configured the follower index. If there are no new operations available on the leader shard, the leader shard waits up to a configured timeout for new operations. If new operations occur within that timeout, the leader shard immediately responds with those new operations. Otherwise, if the timeout elapses, the follower shard replies that there are no new operations. The follower shard task updates some statistics and immediately sends another read request to the leader shard. This ensures that the network connections between the remote cluster and the local cluster are continually being used so as to avoid forceful termination by an external source (such as a firewall).

If a read request fails, the cause of the failure is inspected. If the cause of the failure is deemed to be a failure that can be recovered from (for example, a network failure), the follower shard task enters into a retry loop. Otherwise, the follower shard task is paused and requires user intervention before the it can be resumed with the resume follower API.

When operations are received by the follower shard task, they are placed in a write buffer. The follower shard task manages this write buffer and submits bulk write requests from this write buffer to the follower shard. The write buffer and these write requests are managed by the write parameters that you established when you configured the follower index. The write buffer serves as back-pressure against read requests. If the write buffer exceeds its configured limits, no additional read requests are sent by the follower shard task. The follower shard task resumes sending read requests when the write buffer no longer exceeds its configured limits.

Mapping updates applied to the leader index are automatically retrieved as-needed by the follower index.

Settings updates applied to the leader index that are needed by the follower index are automatically retried as-needed by the follower index. Not all settings updates are needed by the follower index. For example, changing the number of replicas on the leader index is not replicated by the follower index.

You can inspect the progress of replication at the shard level with the get follower stats API. This API gives you insight into the read and writes managed by the follower shard task. It also reports read exceptions that can be retried and fatal exceptions that require user intervention.

You can pause replication with the pause follower API and then later resume replication with the resume follower API. Using these APIs in tandem enables you to adjust the read and write parameters on the follower shard task if your initial configuration is not suitable for your use case.

You can terminate replication with the unfollow API. This API converts a follower index to a regular (non-follower) index.