Errors

Background

ZooKeeper is a very low level system that requires users to do a lot of housekeeping. See:

• http://wiki.apache.org/hadoop/ZooKeeper/FAQ

The Curator Framework is designed to hide as much of the details/tedium of this housekeeping as is possible.

Connection Guarantees

The Curator Framework constantly monitors the connection to the ZooKeeper ensemble. Further every operation is wrapped in a retry mechanism. Thus, the following guarantees can be made:

• Every Curator operation properly waits until the ZooKeeper connection is established
• Every Curator Framework operation (create, getData, etc.) is guaranteed to manage connection loss and/or session expiration per the currently set retry policy
• If the connection is temporarily lost, Curator will attempt to retry the operation until it succeeds per the currently set retry policy
• All Curator recipes attempt to deal with connection issues in an appropriate way

Notifications

Curator exposes several listenable interfaces for clients to monitor the state of the ZooKeeper connection.

ConnectionStateListener is called when there are connection disruptions. Clients can monitor these changes and take appropriate action. There are three possible state changes:

SUSPENDED	There has been a loss of connection. Leaders, locks, etc. should suspend until the connection is re-established. If the connection times-out you will receive a LOST notice
RECONNECTED	A suspended connection has been re-established
LOST	The connection is confirmed to be lost. Close any locks, leaders, etc. and attempt to re-create them. NOTE: it is possible to get a RECONNECTED state after this but you should still consider any locks, etc. as dirty/unstable
READ_ONLY	The connection has gone into read-only mode. This can only happen if you pass true for CuratorFrameworkFactory.Builder.canBeReadOnly(). See the ZooKeeper doc regarding read only connections: http://wiki.apache.org/hadoop/ZooKeeper/GSoCReadOnlyMode. The connection will remain in read only mode until another state change is sent.

UnhandledErrorListener is called when a background task, etc. catch an exception. In general, Curator users shouldn't care about these as they are logged. However, you can listen for them if you choose.

Recipes

In general, the recipes attempt to deal with errors and connection issues. See the doc for each recipe for details on how it deals with errors.