Optimizing Redis Usage For Caching

27 Jul 2015 by Parker Selbert

If you’re convinced that Redis is the right tool for caching I whole heartedly agree, it’s amazing! Here are four essential optimizations for leveraging Redis as a cache in your infrastructure. Most of these optimizations are still helpful even if you are using a hosted solution.

Use a Dedicated Cache Instance

Unlike Memcached, which is multi-threaded, Redis only runs a single thread per process. Considering the brute speed of Redis a single process seems like plenty for many workloads. That’s until your platform traffic starts rising, background jobs are firing continuously, pub/sub channels are relaying thousands of payloads over the network and the cache is being hit continuously. Each request to Redis is blocking, which can throw off the timing of background jobs or be an outright bottleneck for a set of load balanced servers.

Configure multiple separate instances of Redis to alleviate pressure on a single process. Separate instances by workload: one for background jobs, another for pub/sub and another dedicated to caching. Don’t rely on partitioning data into multiple Redis databases! Each of those databases is still backed by a single process so all of the same caveats apply.

To summarize:

Loosen Persistence

You should care about persistence and replication, two features only available in Redis. Even if your goal is to build a cache it helps that after an upgrade or a reboot your data are [sic] still there.

Antirez

Each Redis instance has its own configuration file and can be tuned according to the use-case. Caching servers, for example, can be configured to use RDB persistence to periodically save a single backup instead of AOF persistence logs. By only taking periodic snapshots of the database RDB maximizes performance at the expense of up-to-the-second consistency. For a hybrid Redis instance that may be storing business critical background jobs data consistency is paramount. With a cache it is alright to lose some data in the event of a disaster, after reboot most of the cache will be warm and intact.

To summarize:

Manage Memory Effectively

Once you have a Redis instance dedicated to caching you can start to optimize memory management in ways that don’t make sense for a hybrid database. When ephemeral and long-lived data is co-mingled it is imperative that ephemeral keys have a TTL and Redis is free to clean up expired keys.

Redis can manage memory in a variety of ways. The management policies vary from never evicting keys (noeviction) to randomly evicting a key when memory is full (allkeys-random). Hybridized databases typically use volatile-* policies, which require the presence of expiration values or they behave identically to noeviction. There is another policy that works better for cache data, allkeys-lru. The allkeys-lru policy attempts to remove the less recently used (LRU) keys first in order to make space for the new data added.

It is also worth to note that setting an expire to a key costs memory, so using a policy like allkeys-lru is more memory efficient since there is no need to set an expire for the key to be evicted under memory pressure.

Redis Documentation

Redis uses an approximated LRU algorithm instead of an exact algorithm. What this means is that you can conserve memory in favor of inaccuracy by tuning the number of samples to check with each eviction. Set maxmemory-samples to a low level, say around 5, for “good enough” eviction with a low memory footprint. Lastly, and most importantly, set a maxmemory limit to a comfortable amount of RAM. Without a limit Redis can’t function properly as a LRU cache and will start replying with errors when memory consuming commands start failing.

To summarize:

Utilize Intelligent Caching

Because of Redis data structures, the usual pattern used with memcached of destroying objects when the cache is invalidated, to recreate it from the DB later, is a primitive way of using Redis.

Antirez

Only storing serialized HTML or JSON as strings, the standard way of caching for web applications, doesn’t fully utilize Redis as a cache. One of the great strengths of Redis over Memcached is the rich set of data structures available. Ordered lists, structured hashes, and sorted sets are particularly useful caching tools only available through Redis. Caching is more than stuffing everything into strings.

Let’s look at the Hash type for a specific example.

Small hashes are encoded in a very small space, so you should try representing your data using hashes every time it is possible. For instance if you have objects representing users in a web application, instead of using different keys for name, surname, email, password, use a single hash with all the required fields.

Redis Documentation

Instead of storing objects as a serialized string you can store the object as fields and values available through a single key. Using a Hash saves web servers the work of fetching an entire serialized value, de-serializing it, updating it, re-serializing it, and finally writing it back to the cache. Eliminating that flow for every minor update pushes the work into Redis and out of your applications, where it is supposed to be.

To Summarize:

Happy optimizing. Go forth and cache!