Resizing Clusters

The essential steps to changing the topology of an existing IRONdb cluster are as follows:

  1. Create your new topology.
  2. Load the new topology to all nodes that will be part of the new cluster.
  3. Start the rebalance operation on each node, which starts the migration to the new topology.

These procedures utilize the IRONdb REST API which, by default, listens on TCP port 8112. See the Rebalancing APIs reference for details on the endpoints mentioned below.

Caveats

These procedures cannot be used to transform a cluster with no sides into a sided cluster, or vice versa.

Adding Nodes

An existing IRONdb cluster has two nodes with write factor of 2. A new node is prepared by running the installation which creates a standalone node with its own topology. We want to combine these three nodes together to create a three-node cluster with write factor of 3.

  • Combine the XML content of both old topologies to create a new topology XML document with three nodes.

  • Import the new topology to get the new topology hash. Unlike an initial cluster setup, this file does not need to be placed anywhere special, as we are going to load this via the REST API.

    /opt/circonus/bin/snowthimport -f newtopology.xml -t .

    This will generate a canonical topology configuration file named for the new topology hash. Note this hash string for use in later steps.

  • Load the new topology to each of the three nodes via POST to /topology/[new topology hash] with new topology config:

    curl -X POST -d@<hash> http://<node>:<api-port>/topology/<hash>

    This will install the new topology config in the existing topology directory, alongside the current topology for that node.

  • Start migration to the new topology on each of the three nodes via POST to /rebalance/activate/[new topology hash]:

    curl -X POST http://<node>:<api-port>/rebalance/activate/<hash>

    IRONdb will start rebalancing the data in the old topology to the new topology once all the nodes in old and new topology receive the rebalance activation POST with the same new topology hash. Rebalancing involves recalculating the node ownership for each individual metric stream, and then sending that stream to the new node. All metric data remains available during a rebalance, under the old topology. New, incoming metric data is replicated to both old and new topologies.

    After all nodes complete the rebalance, they will switch their active topology from old to new. Each node will then kick off a delete operation of any metrics that no longer belong on that node.

  • To view progress, retrieve the rebalance state via GET of /rebalance/state:

    curl http://<node>:<api-port>/rebalance/state

  • To abort the rebalance, POST to /rebalance/deactivate/[new topology hash]

    curl -X POST http://node:<api-port>/rebalance/deactivate/c790e663badf603ac555e02a07bf195f06932f271ee1f2d7fae104b3a1b232f3

Removing Nodes

Shrinking a cluster is basically the same as adding, above:

  • Create a new topology with the nodes that should remain.
  • Load the new topology to the nodes that should remain.
  • Start rebalance to new topology on the nodes that should remain.

One difference is that nodes that are not in the new topology do not automatically clean up their data, which avoids delaying the cleanup phase unnecessarily. It is up to the operator to do this as part of decommissioning the unused nodes.

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