The Leader/Follower setup in ArangoDB has a few limitations. Some of these limitations may be removed in later versions of ArangoDB:
- there is no feedback from the Followers to the Leader. If a Follower cannot apply an event it got from the Leader, the Leader will have a different state of data. In this case, the replication applier on the Follower will stop and report an error. Administrators can then either “fix” the problem or re-sync the data from the Leader to the Follower and start the applier again.
- at the moment it is assumed that only the replication applier executes write operations on a Follower. ArangoDB currently does not prevent users from carrying out their own write operations on Followers, though this might lead to undefined behavior and the replication applier stopping.
- when a replication Follower asks a Leader for log events, the replication Leader will return all write operations for user-defined collections, but it will exclude write operations for certain system collections. The following collections are excluded intentionally from replication: _apps, _trx, _replication, _configuration, _jobs, _queues, _sessions, _foxxlog and all statistics collections. Write operations for the following system collections can be queried from a Leader: _aqlfunctions, _graphs, _users.
- Foxx applications consist of database entries and application scripts in the file system. The file system parts of Foxx applications are not tracked anywhere and thus not replicated in current versions of ArangoDB. To replicate a Foxx application, it is required to copy the application to the remote server and install it there using the foxx-manager utility.
- Leader servers do not know which Followers are or will be connected to them. All servers in a replication setup are currently only loosely coupled. There currently is no way for a client to query which servers are present in a replication.
- failover must be handled by clients or client APIs.
- the replication applier is single-threaded, but write operations on the Leader may be executed in parallel if they affect different collections. Thus the replication applier might not be able to catch up with a very powerful and loaded Leader.
- replication is only supported between the two ArangoDB servers running the same ArangoDB version. It is currently not possible to replicate between different ArangoDB versions.
- a replication applier cannot apply data from itself.