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# Quorum Behaviors

The persistence services in a cluster exhibit these behaviors as they form and maintain a quorum.

## Cluster Start

When the cluster starts for the first time, all the services are empty. The services wait until all the services in the cluster have started, and are running simultaneously. Then they constitute a quorum for the first time, and can begin interacting with clients to store messages.

## Steady State

After forming that initial quorum, client interaction continues while the quorum exists (that is, a majority of services continue to satisfy all three conditions of Quorum Conditions: General Rule).

## Tail End Loss

When a persistence service has a stored state, but might have suffered the loss of a tail end of the state, the persistence service is considered empty for the purpose of quorum formation.

The persistence service assumes tail end loss in the following circumstances:

- The persistence service was running at a disaster recovery site, and the disaster recovery site was activated.

- The persistence service was restored from a saved state. This applies only to situations where the administrator manually saved the state of the persistence cluster. This does not apply when the disk persistence feature is enabled and the persistence service recovers from its non-backup disk persistence file. See Saving and Loading Persistence State.

- The persistence service was restored from a backup disk persistence file. See Persistence Clusters Status Table, Persistence Clusters Status Table.

The tail-end loss assumption is cleared once the persistence service successfully joins a quorum.

## Rejoining the Quorum

A service can rejoin an existing quorum, for example, after that service restarts, or becomes reachable (after recovery from network segmentation). The rejoining service copies the data state of the quorum leader.

The current leader remains in place, even when the rejoining service has higher weight. That is, weights are relevant only for leader election. Election occurs only when the quorum does not already have a leader.

## Reforming a Quorum

If the services no longer satisfy the quorum conditions, client interaction stops. The quorum must reconstitute through any of the scenarios in the following table.

Reforming a Quorum
Scenario | Description
--- | ---
Complete | The cluster can form a quorum of non-empty services (that is, they satisfy all three conditions of Quorum Conditions: General Rule). For example, this scenario can occur after recovery from network segmentation. Alternatively, it can occur after suspending the cluster, saving the state of its services, and, finally, restarting the services.
Force Quorum | A majority of the defined services are reachable, but the candidates cannot satisfy the non-empty condition. After verifying preconditions, the administrator may force a quorum (using the FTL server monitoring interface). For more details, see Before Forcing a Quorum. Also see Cluster Details Panel, Force Quorum Setting: Force Quorum Delay. The services in the forced quorum copy the most recent message data state.

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