<commit_before>## Runtime Reconfiguration

etcd comes with support for incremental runtime reconfiguration, which allows users to update the membership of the cluster at run time.

## Reconfiguration Use Cases

Let us walk through the four use cases for re-configuring a cluster: replacing a member, increasing or decreasing cluster size, and restarting a cluster from a majority failure.

### Replace a Non-recoverable Member

The most common use case of cluster reconfiguration is to replace a member because of a permanent failure of the existing member: for example, hardware failure or data directory corruption.
It is important to replace failed members as soon as the failure is detected.
If etcd falls below a simple majority of members it can no longer accept writes: e.g. in a 3 member cluster the loss of two members will cause writes to fail and the cluster to stop operating.

If you want to migrate a running member to another machine, please refer [member migration section][member migration].<commit_msg>doc: add note for reconfiguration<commit_after>## Runtime Reconfiguration

etcd comes with support for incremental runtime reconfiguration, which allows users to update the membership of the cluster at run time.

Reconfiguration requests can only be processed when the the majority of the cluster members are functioning. It is **highly recommended** to always have a cluster size greater than two in production. It is unsafe to remove a member from a two member cluster. The majority of a two member cluster is also two. If there is a failure during the removal process, the cluster might not able to make progress and need to [restart from majority failure][majority failure].

[majority failure]: #restart-cluster-from-majority-failure

## Reconfiguration Use Cases

Let us walk through the four use cases for re-configuring a cluster: replacing a member, increasing or decreasing cluster size, and restarting a cluster from a majority failure.

### Replace a Non-recoverable Member

The most common use case of cluster reconfiguration is to replace a member because of a permanent failure of the existing member: for example, hardware failure or data directory corruption.
It is important to replace failed members as soon as the failure is detected.
If etcd falls below a simple majority of members it can no longer accept writes: e.g. in a 3 member cluster the loss of two members will cause writes to fail and the cluster to stop operating.

If you want to migrate a running member to another machine, please refer [member migration section][member migration].
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