Sunday, July 20, 2014

Changing the Endpoint Address of an RHQ Storage Node

There is very limited support for changing the endpoint address of a storage node. In fact the only way to do so is by undeploying and redeploying the node with the new address. And in some cases, like when there is only a single storage node, this is not even an option. BZ 1103841 was opened to address this, and the changes will go into RHQ 4.13.

Changing the endpoint address of a Cassandra node is a routine maintenance operation. I am referring specifically to the address on which Cassandra uses for gossip. This address is specified by the listen_address property in cassandra.yaml. The key thing when changing the address is to ensure that the node's token assignments do not change. Rob Coli's post on changing a node's address provides a nice summary of the configuration changes involved.

With CASSANDRA-7356 however, things are even easier. Change the value of listen_address and restart Cassandra with the following system properties defined in cassandra-env.sh,

  • -Dcassandra.replace_address=<new_address>
  • -Dcassandra.replace_address_first_boot=true 
The seeds property in cassandra.yaml might need to be updated as well. Note that there is no need to worry about the auto_bootstrap, initial_token, or num_tokens properties.

For the RHQ Storage Node, these system properties will be set in cassandra-jvm.properties. Users will be able to update a node's address either through the storage node admin UI or through the RHQ CLI. One interesting to note is that the RHQ Storage Node resource type uses the node's endpoint address as its resource key. This is not good. When the address changes, the agent will think it has discovered a new Storage Node resource. To prevent this we can add resource upgrade support in the rhq-storage plugin, and change the resource key to use the node's host ID which is a UUID that does not change. The host ID is exposed through the StorageServiceMBean.getLocalHostId JMX attribute.

If you interested in learning more about the work involved with adding support for changing a storage node's endpoint address, check out the wiki design doc that I will be updating over the next several days.

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