Asynchronous replication

Transactions are honored in replication, i.e. transactional write operations will become visible on slaves atomically.

As all write operations will be logged to a master database’s write-ahead log, the replication in ArangoDB currently cannot be used for write-scaling. The main purposes of the replication in current ArangoDB are to provide read-scalability and “hot backups” of specific databases.

It is possible to connect multiple slave databases to the same master database. Slavedatabases should be used as read-only instances, and no user-initiated write operations should be carried out on them. Otherwise data conflicts may occur that cannot be solved automatically, and that will make the replication stop.

In this setup, write operations are applied first in the master database, and applied in the slave database(s) afterwards.

For example, let’s assume a write operation is executed in the master database at point in time t0. To make a slave database apply the same operation, it must first fetch the write operation’s data from master database’s write-ahead log, then parse it and apply it locally. This will happen at some point in time after t0, let’s say t1.

The difference between t1 and t0 is called the replication lag, and it is unavoidable in asynchronous replication. The amount of replication lag depends on many factors, a few of which are:

• the load of the master and the slaves

Replication is configured on a per-database level. If multiple database are to be replicated, the replication must be set up individually per database.

The replication applier on the slave can be used to perform a one-time synchronizationwith the master (and then stop), or to perform an ongoing replication of changes. Toresume replication on slave restart, the autoStart attribute of the replication applier must be set to true.

As the master servers are logging any write operation in the write-ahead-log anyway replication doesn’t cause any extra overhead on the master. However it will of course cause some overhead for the master to serve incoming read requests of the slaves. Returning the requested data is however a trivial task for the master and should not result in a notable performance degration in production.