Faraway replicas

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Notice: EDB will no longer offer the Hosted Cloud Service option for new customers. However, support will remain available for existing customers of the EDB Hosted plan. The associated documentation topics will soon be removed, and updates regarding the removal process will be shared as they become available. The 'Your Cloud Account' option will continue to be available.

Faraway replicas are read-only replicas of Cloud Service clusters that you can provision in most supported regions. You can create faraway replicas of your single-node and high-availability clusters in different regions of your cloud. Database users and applications can read from a nearby faraway replica instead of the source cluster. This ability relieves some of the workload on the source cluster and frees it up to handle write traffic.

In case of a failure, you can manually promote the faraway replica to a full-fledged Cloud Service cluster, making it capable of accepting writes. See Managing replicas.

In Cloud Service, faraway replicas use log shipping to replicate from their source clusters. This means that a replica cluster accesses the write-ahead log (WAL) of its source cluster and “replays” the changes described in it.

A WAL is a file that logs any changes made to a database. Databases write the change to the WAL before actually making the change. This way, if the database goes down before the change can be applied, the WAL has a record of the intended changes. The replica pulls in the changes from the WAL every time a new WAL file is closed, so replication is asynchronous.

Faraway replicas are priced the same as single-node clusters.

Advantages

Disaster recovery — You can create faraway replicas in different regions in your cloud. Deploying faraway replicas across regions can help you build a more solid disaster recovery (DR) plan.
Improved read-query performance — For an improved read-query performance, set up the faraway replica in the same region as your application. For example, applications can read the writes made to a cluster in the us-east region from a us-west replica.
Flexibility — Options you might not find anywhere else:
- Your choice of instance type and size
- Your choice of storage volume and properties
- No limit to the number of replicas you can create

Limitations

Manual intervention — Unlike standby replicas, Cloud Service doesn't automatically promote faraway replicas to the source cluster in case of a failure.
Replication lag — Promoting a faraway replica to a Cloud Service cluster can result in loss of data. See Replication lag with faraway replicas.