Persistence Services and Clusters
If a cluster of FTL servers provide persistence services, those persistence services cooperate in a persistence cluster. A persistence cluster maintains a set of persistence stores which store messages in durables.
Administrators define persistence clusters and services within the realm definition. You can define any number of persistence clusters in the realm. A cluster consists of a set of named persistence services.
Each FTL server can provide at most one named persistence service. You assign each named persistence service to a specific FTL server in the FTL server configuration file.
The persistence services within a cluster communicate among themselves to replicate message data and acknowledgment data. Replication ensures that the services can continue to deliver messages to subscribers, even if some of the individual services become unavailable.
A persistence cluster as a whole maintains a set of persistence stores, replicating their data among the persistence services for fault tolerance.
To ensure the benefits of a persistence cluster, namely, in-memory replication and fault-tolerant service, run each FTL server on a dedicated host computer.
- Quorum and Leader
A persistence cluster can interact with clients only when a quorum of persistence services exists. This requirement ensures correct replication and fault tolerance. - Quorum Conditions: General Rule
A valid quorum ensures correct replication and fault tolerance. As a general rule, a group of candidate services must satisfy all three of the following conditions to form a valid quorum. - Cluster Size
For delivery assurance with fault tolerance, use a minimum cluster size of 3 services, so that a quorum can exist even when one service is unavailable. - Quorum Behaviors
The persistence services in a cluster exhibit these behaviors as they form and maintain a quorum. - Persistence Service Fault Tolerance
Replication of data among a quorum of persistence services ensures that the cluster as whole remains complete and correct, despite temporary outage of some of its services.