Within Redis, there are two different ways of persisting data to disk. One is a method called snapshotting that takes the data as it exists at one moment in time and writes it to disk. The other method is called AOF, or append—only file, and it works by copying incoming write commands to disk as they happen. These methods can be used together, separately, or not at all in some circumstances. Which to choose will depend on your data and your application.
One of the primary reasons why you’d want to store in—memory data on disk is so that you have it later, or so that you can back it up to a remote location in the case of failure. Additionally, the data that’s stored in Redis may have taken a long time to compute, or may be in the process of computation, and you may want to have access to it later without having to compute it again. For some Redis uses, “computation” may simply involve an act of copying data from another database into Redis (as was the case in section 2.4), but for others, Redis could be storing aggregate analytics data from billions of log lines.
Two different groups of configuration options control how Redis will write data to disk. All of these configuration options with example configuration values can be seen in the following listing. We’ll talk about them all more specifically in sections 4.1.1 and 4.1.2, but for now, we’ll just look at the options so you can get familiar with them.
As you saw in listing 4.1, the first few options deal with basic snapshotting, like what to name the snapshot on disk, how often to perform an automatic snapshot, whether to compress the snapshot, and whether to keep accepting writes on failure. The second group of options configure the AOF subsystem, telling Redis whether to use it, how often to sync writes to disk, whether to sync during AOF compaction, and how often to compact the AOF. In the next section, we’ll talk about using snapshots to keep our data safe.