Setup an AEM author cluster with MongoDB
With the introduction of AEM 6 and its following releases, creation of clusters of author instances has been rethought. The typical persistance model which supports clustering in AEM uses MongoDB.
Adobe suggests to define a cluster of author AEM instances when one of the following criteria is met:
1000+ unique users per day
100+ concurrent users
high volumes of page edits
large rollouts or activations
These guidelines do not take in consideration scenarios where, even if these criteria are not met, it is vital to guarantee continuity and fault tolerance. For those scenarios it is available the Author Cold StandBy approach which grants fault tolerance but not work continuity which the cluster grants.
Once the cluster of author AEM instances is defined, it is important to avoid the use of MongoDB as a single point of failure. Therefore the database has to be configured as a ReplicaSet in order to grand robustness.