Faraway replicas

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Faraway replicas are read-only replicas of BigAnimal 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 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 BigAnimal cluster, making it capable of accepting writes. See Managing replicas.

In BigAnimal, 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 logging 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, BigAnimal doesn't automatically promote faraway replicas to the source cluster in case of a failure.

  • Replication lag — Promoting a faraway replica to a BigAnimal cluster can result in loss of data. See Log shipping.

  • South East Asia region - The South East Asia Azure region does not support the creation of faraway replicas.

Examples

Fig 1: A three-node high availability cluster with two faraway replicas.

*Generic faraway cluster*

Fig 2: Faraway replica in Region B is promoted to a new cluster.

*Faraway cluster after promotion*

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