When you are ready to extend the cluster by adding one or more nodes, you simply initialize and start the VoltDB database process on the new nodes using the voltdb init command to initialize and the voltdb start command to start with the --add argument, specifying the name of one or more of the existing cluster nodes as the hosts. For example, if you are adding node ServerX to a cluster where ServerA is already a member, you can execute the following commands on ServerX:

$ voltdb init --config=deployment.xml
$ voltdb start --add --host=ServerA 

Once the elastic add action is initiated, the cluster performs the following tasks:

There are some important notes to consider when expanding the cluster using elastic scaling:

When you want to reduce the size of your cluster, you use the voltadmin resize command to start the resizing process. First, as with any significant maintenance activity, it is a good idea to take a snapshot of the database contents before you begin, just in case you need to restore it later. The next step is to test to make sure the cluster can be reduced. You do this using the voltadmin resize --test command:

$ voltadmin resize --test

The --test qualifier verifies that there are sufficient nodes and partitions to reduce the cluster while maintaining the K-safety and sitesperhost settings. If not, the command will report that the cluster cannot be reduced in size. If resizing is possible, the command reports which nodes will be removed when resizing begins.

Once you are ready to begin the resizing process, you use the voltadmin resize command:

$ voltadmin resize

The command repeats the test phase, reports which nodes will be removed and starts the resizing process.

Once resizing begins, the process cannot be canceled. Even if the cluster stops, resizing will continue once the cluster restarts (and you must restart all of the original nodes so the resize operation can complete). So be sure you want to reduce the cluster size before you issue the voltadmin resize command.

The length of time it takes for resizing to complete depends on the amount of data in the database and the current workload. You can adjust parameters that affect resizing (as described in Section 9.3.3, “Configuring How VoltDB Rebalances Nodes During Elastic Scaling”). However, increasing the duration or throughput for resizing will likely have a corresponding inverse impact on the performance of ongoing database activities. Use the voltadmin status to check on the current status of the resizing operation, or use the @Statistics system procedure with the REBALANCE selector for details.

Finally, if an unexpected event causes the resize process to fail — which will be reported in the server logs — you can restart the resize operation using the voltadmin resize --restart command.

As you add or remove nodes using elastic scaling, VoltDB rebalances the cluster by rearranging data within the partitions. During elastic expansion, as soon as you add the necessary number of nodes (based on the K-safety value), VoltDB rebalances the cluster, moving data from existing partitions to partitions on the new nodes. During elastic contraction, before the nodes are removed, VoltDB rebalances the cluster by moving data from partitions that are being removed to partitions that will remain.

During the rebalance phase, the database remains available and actively processing client requests. How long the rebalance operation takes is dependent on two factors: how often rebalance tasks are processed and how much data each transaction moves.

Rebalance tasks are fully transactional, meaning they operate within the database's ACID-compliant transactional model. Because they involve moving data between two or more partitions, they are also multi-partition transactions. This means that each rebalance work unit can incrementally add to the latency of pending client transactions.

You can control how quickly the rebalance operation completes versus how much rebalance work impacts ongoing client transactions using two attributes of the <elastic> element in the configuration file:

When you change the target duration, VoltDB adjusts the amount of data that is moved in each transaction to reach the target execution time. If you increase the duration, the volume of data moved per transaction increases. Similarly, if you reduce the duration, the volume per transaction decreases.

When you change the target throughput, VoltDB adjusts the frequency of rebalance transactions to achieve the desired volume of data moved per second. If you increase the target throughout, the number of rebalance transactions per second increases. Similarly, if you decrease the target throughout, the number of transactions decreases.

The <elastic> element is a child of the <systemsettings> element. For example, the following configuration file sets the target duration to 15 milliseconds and the target throughput to 1 megabyte per second before starting the database:

<deployment>
   . . .
   <systemsettings>
       <elastic duration="15" throughput="1"/>
   </systemsettings>
</deployment>