Cache Collision

Memcached vs Redis - More Different Than You Would Expect

Important Note

Everything that follows should be only considered accurate at time of publishing. Both projects are are not considered finished, and so changes that follow publication could invalidate some or all of the information below.

Redis vs Memcached

Redis vs Memcached. Another entry to the list of software holy wars with Emacs vs. Vi, Tabs vs. Spaces, IDE vs. Editor. I have heard a lot of people saying why they use one over the other, and a lot of wrong statements while doing it.

I kept getting questioned when I asked why people were making these choices, and as such started reading in depth into how they worked under the hood. The idea being to be able to intelligently talk in depth about them. I found it really interesting and so I kept reading and learning.

What follows is what I learnt so far. I turned it into a brownbag presentation some time ago, then took the core ideas of that into this blog post. From how they use memory to how they expire items, the differences are not as subtle as you would expect.

Obvious Differences

Let's start by discussing the obvious differences between Redis and memcached. The first and most obvious is that Redis supports multiple data types. Those being,

• strings

• lists

• sets

• hashes

• sorted sets

• bitmaps

• hyperloglogs

  Redis also has some other functionality such as,

• disk persistence

• message broker

• lua scripting

• transactions

• pub/sub

• geospacial support

None of the the above are going to be covered in any detail. If you do need any of the above functionality, then the answer is use Redis, don't read any further and go for a walk outside.

So you want to use Redis primarily as a cache?

However if you want to use Redis primarially as a cache, then the differences are less obvious.

Both Redis and memcached provide atomic get/set/delete operations, automatic expiration of old items, sub millisecond latency, scale to available RAM and are clusterable. So are they the same?

Lets consider their history.

Memcached was created in 2003 and written in perl before being rewritten in C. Orginally created for livejournal it became one of the goto stack enhancements of the Web 2.0 era. It's still in use by very large web properties such as Youtube, Reddit, Facebook, Pinterest, Twitter, Wikipedia and more.

Its original design goal was to leverage unused RAM on web servers, pooling them together to provide a large high speed in-memory cache. It was especially useful for PHP websites allowing you to store state in a stateless programming envrionment without having to write to the database.

Redis was created in 2009, prototyped in TCL and then also rewritten in C. Originally it was written to speed up Salvatore's (the creator's) startup. It is also in use by very large web properties including GitHub, Instagram, Stackoverflow and Craigslist.

It's orginal design goal was to solve Salvatores problems and be a persistent in-memory datastructure store, hence why it supports so many other data types. It became an especially popular addition to the Ruby on Rails stack, probably due to excellent library support and integration.

Neither of the above really differenciates them, and neither does the AWS product page for ElastiCache.