When using an in-memory database like Redis, one of the first questions that’s asked is “What happens when my server gets turned off?” Redis has two different forms of persistence available for writing in-memory data to disk in a compact format. The first method is a point-in-time dump either when certain conditions are met (a number of writes in a given period) or when one of the two dump-to-disk commands is called. The other method uses an append-only file that writes every command that alters data in Redis to disk as it happens. Depending on how careful you want to be with your data, append-only writing can be configured to never sync, sync once per second, or sync at the completion of every operation. We’ll discuss these persistence options in more depth in chapter 4.
Even though Redis is able to perform well, due to its in-memory design there are situations where you may need Redis to process more read queries than a single Redis server can handle. To support higher rates of read performance (along with handling failover if the server that Redis is running on crashes), Redis supports master/slave replication where slaves connect to the master and receive an initial copy of the full database. As writes are performed on the master, they’re sent to all connected slaves for updating the slave datasets in real time. With continuously updated data on the slaves, clients can then connect to any slave for reads instead of making requests to the master. We’ll discuss Redis slaves more thoroughly in chapter 4.
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