10.3. Recovering from System Failures


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10.3. Recovering from System Failures

When running without K-safety (in other words, a K-safety value of zero) any node failure is fatal and will bring down the database (since there are no longer enough partitions to maintain operation). When running with K-safety on, if a node goes down, the remaining nodes of the database cluster log an error indicating that a node has failed.

By default, these error messages are logged to the console terminal. Since the loss of one or more nodes reduces the reliability of the cluster, you may want to increase the urgency of these messages. For example, you can configure a separate Log4J appender (such as the SMTP appender) to report node failure messages. To do this, you should configure the appender to handle messages of class HOST and severity level ERROR or greater. See the chapter on Logging in the VoltDB Administrator's Guide for more information about configuring logging.

When a node fails with K-safety enabled, the database continues to operate. But at the earliest possible convenience, you should repair (or replace) the failed node.

To replace a failed node to a running VoltDB cluster, you restart the VoltDB server process specifying the address of at least one of the remaining nodes of the cluster as the host. For example, to rejoin a node to the VoltDB cluster where server5 is one of the current member nodes, you use the following voltdb start command:

$ voltdb  start --host=server5   

If you started the servers specifying multiple hosts, you can use the same voltdb start command used to start the cluster as a whole since, even if the failed node is in the host list, one of the other nodes in the list can service its rejoin request.

If the failed server cannot be restarted (for example, if hardware problems caused the failure) you can start a replacement server in its place. Note you will need to initialize a root directory on the replacement server before you can start the database process. You can either initialize the root with the original configuration file. Or, if you have changed the configuration, you can download a copy of the current configuration from the VoltDB Management Center and use that file to initialize the root directory before starting:

$ voltdb init --config=latest-config.xml
$ voltdb  start --host=server5   

Note that at least one node you specify in the --host argument must be an active member of the cluster. It does not have to be one of the nodes identified as the host when the cluster was originally started.

10.3.1. What Happens When a Node Rejoins the Cluster

When you use voltdb start to bring back a server to a running cluster, the node first rejoins the cluster, then retrieves a copy of the database schema and the appropriate data for its partitions from other nodes in the cluster. Rejoining the cluster only takes seconds and once this is done and the schema is received, the node can accept and distribute stored procedure requests like any other member.

However, the new node will not actively participate in the work until a full working copy of its partition data is received. While the data is being copied, the cluster separates the rejoin process from the standard transactional workflow, allowing the database to continue operating with a minimal impact to throughput or latency. So the database remains available and responsive to client applications throughout the rejoin procedure.

It is important to remember that the cluster is not fully K-safe until the restoration is complete. For example, if the cluster was established with a K-safety value of two and one node failed, until that node rejoins and is updated, the cluster is operating with a K-safety value of one. Once the node is up to date, the cluster becomes fully operational and the original K-safety is restored.

10.3.2. Where and When Recovery May Fail

It is possible to rejoin any appropriately configured node to the cluster. It does not have to be the same physical machine that failed. This way, if a node fails for hardware reasons, it is possible to replace it in the cluster immediately with a new node, giving you time to diagnose and repair the faulty hardware without endangering the database itself.

There are a few conditions in which the rejoin operation may fail. Those situations include the following:

  • Insufficient K-safety

    If the database is running without K-safety, or more nodes fail simultaneously than the cluster is capable of sustaining, the entire cluster will fail and must be restarted from scratch. (At a minimum, a VoltDB database running with K-safety can withstand at least as many simultaneous failures as the K-safety value. It may be able to withstand more node failures, depending upon the specific situation. But the K-safety value tells you the minimum number of node failures that the cluster can withstand.)

  • Mismatched configuration in the root directory

    If the configuration file that you specify when initializing the root directory does not match the current configuration of the database, the cluster will refuse to let the node rejoin.

  • More nodes attempt to rejoin than have failed

    If one or more nodes fail, the cluster will accept rejoin requests from as many nodes as failed. For example, if one node fails, the first node requesting to rejoin will be accepted. Once the cluster is back to the correct number of nodes, any further requests to rejoin will be rejected. (This is the same behavior as if you try to start more nodes than specified in the --count argument to the voltdb start command when starting the database.)