When doing joins in an ArangoDB cluster data has to be exchanged between different servers.
Joins will be executed on a Coordinator. It will prepare an execution planand execute it. When executing, the Coordinator will contact all shards of thestarting point of the join and ask for their data. The DB-Servers carryingout this operation will load all their local data and then ask the cluster forthe other part of the join. This again will be distributed to all involved shardsof this join part.
In sum this results in much network traffic and slow results depending of theamount of data that has to be sent throughout the cluster.
Satellite collections are collections that are intended to address this issue.
They will facilitate the synchronous replication and replicate all its datato all DB-Servers that are part of the cluster.
This greatly improves performance for such joins at the costs of increasedstorage requirements and poorer write performance on this data.
To create a satellite collection set the replicationFactor of this collectionto “satellite”.
Let’s analyse a normal join not involving satellite collections:
All shards involved querying the collection will fan out via theCoordinator to the shards of . In sum 8 shards will open 8 connectionsto the Coordinator asking for the results of the join. The Coordinatorwill fan out to the 8 shards of . So there will be quite somenetwork traffic.
In this scenario all shards of nonsatellite will be contacted. Howeveras the join is a satellite join all shards can do the join locallyas the data is replicated to all servers reducing the network overheaddramatically.
The cluster will automatically keep all satellite collections on all servers in syncby facilitating the synchronous replication. This means that write will be executedon the leader only and this server will coordinate replication to the followers.If a follower doesn’t answer in time (due to network problems, temporary shutdown etc.)it may be removed as a follower. This is being reported to the Agency.
The follower (once back in business) will then periodically check the Agency and knowthat it is out of sync. It will then automatically catch up. This may take a whiledepending on how much data has to be synced. When doing a join involving the satelliteyou can specify how long the DB-Server is allowed to wait for sync until the queryis being aborted.
Check for details.
During network failure there is also a minimal chance that a query was properlydistributed to the DB-Servers but that a previous satellite write could not bereplicated to a follower and the leader dropped the follower. The follower howeveronly checks every few seconds if it is really in sync so it might indeed deliverstale results.