Connecting to a ZooKeeper Cluster

Connection State

State of the current connection (do not confuse it with session!) is described by ConnectionState property. ConnectionState can be one of these: Connected, ConnectedReadonly, Disconnected, Expired, Died, and AuthFailed.

Subscribing to Connection State Changes

One can subscribe to the changes of ConnectionState using IObservable OnConnectionStateChanged. When subscribed to, this sequence immediately produces event with current ConnectionState. It may produce an IObserver<T>.OnCompleted notification when client gets disposed, and never produces IObserver<T>.OnError notifications.

Never call any client methods in observer delegate as it would lead to a deadlock

Connection State Transitions

Initially, client is created in Disconnected state. It will try to establish a working connection upon the first request. To establish a connection immediately, use ConnectAsync()/Connect() extensions.

Once successfully connected, ConnectionState will become either Connected or ConnectedReadonly. Then, if for some reason client loses connection, ConnectionState once again becomes Disconnected. ConnectionState becomes Dead only when client instance is disposed and therefore cannot be used anymore. Client is always connected to a single server node. If that node falls out of quorum, client would try to connect to another node unless CanBeReadOnly was set to true. In the latter case, state will become ConnectedReadonly.

ConnectionState.Expired is emitted by server-side when client tries to reconnect to a timed-out session and is not strictly guaranteed to appear every time when session is lost. Given that, a correct way to ensure that ephemeral nodes still exist would be to subscribe onto ConnectionState changes and manually probe nodes with ExistsAsync every time that ConnectionState event is emitted.

Connection-Related Properties

SessionId	Returns an int id of an established session when client is in connected state and 0 otherwise.
SessionTimeout	Returns negotiated session timeout which may differ from Timeout set in ZooKeeperClientSettings upon client creation: ZooKeeper cluster may have configured minSessionTimeout and maxSessionTimeout that limit the possible range of timeout values.