By executing transactions in memory, VoltDB, frees itself from much of the management overhead and I/O costs of traditional database products. However, accidents do happen and it is important that the contents of the database be safeguarded against loss or corruption.
Snapshots provide one mechanism for safeguarding your data, by creating a point-in-time copy of the database contents. But what happens to the transactions that occur between snapshots?
Command logging provides a more complete solution to the durability and availability of your VoltDB database. Command logging keeps a record of every transaction (that is, stored procedure) as it is executed. Then, if the servers fail for any reason, the database can restore the last snapshot and "replay" the subsequent logs to re-establish the database contents in their entirety.
The key to command logging is that it logs the invocations, not the consequences, of the transactions. A single stored procedure can include many individual SQL statements and each SQL statement can modify hundreds or thousands of table rows. By recording only the invocation, the command logs are kept to a bare minimum, limiting the impact the disk I/O will have on performance.
However, any additional processing can impact overall performance, especially when it involves disk I/O. So it is important to understand the tradeoffs concerning different aspects of command logging and how it interacts with the hardware and any other options you are utilizing. The following sections explain how command logging works and how to configure it to meet your specific needs.
When command logging is enabled, VoltDB keeps a log of every transaction (that is, stored procedure) invocation. At first, the log of the invocations are held in memory. Then, at a set interval the logs are physically written to disk. Of course, at a high transaction rate, even limiting the logs to just invocations, the logs begin to fill up. So at a broader interval, the server initiates a snapshot. Once the snapshot is complete, the command logging process is able to free up — or "truncate" — the log keeping only a record of procedure invocations since the last snapshot.
This process can continue indefinitely, using snapshots as a baseline and loading and truncating the command logs for all transactions since the last snapshot.
The frequency with which the transactions are written to the command log is configurable (as described in Section 14.3, “Configuring Command Logging for Optimal Performance”). By adjusting the frequency and type of logging (synchronous or asynchronous) you can balance the performance needs of your application against the level of durability desired.
In reverse, when it is time to "replay" the logs, you start the database and the server nodes establish a quorum, the first thing the database servers do is restore the most recent snapshot. Then they replay all of the transactions in the log since that snapshot.