Memcached

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Memcached is a simple volatile cache server. It allows you to store key/value pairs where the value is limited to being a string up to 1MB.

It's good at this, but that's all it does. You can access those values by their key at extremely high speed, often saturating available network or even memory bandwidth.

When you restart memcached your data is gone. This is fine for a cache. You shouldn't store anything important there.

If you need high performance or high availability there are 3rd party tools, products, and services available.

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Redis

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Redis can do the same jobs as memcached can, and can do them better.

Redis can act as a cache as well. It can store key/value pairs too. In redis they can even be up to 512MB.

You can turn off persistence and it will happily lose your data on restart too. If you want your cache to survive restarts it lets you do that as well. In fact, that's the default.

It's super fast too, often limited by network or memory bandwidth.

If one instance of redis/memcached isn't enough performance for your workload, redis is the clear choice. Redis includes cluster support and comes with high availability tools (redis-sentinel) right "in the box". Over the past few years redis has also emerged as the clear leader in 3rd party tooling. Companies like Redis Labs, Amazon, and others offer many useful redis tools and services. The ecosystem around redis is much larger. The number of large scale deployments is now likely greater than for memcached.