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+---
+layout: post
+title: "Redis Sentinel & Redis Cluster - what?"
+date: 01.06.2015 23:20
+---
+
+In the last week there were several questions regarding Redis Sentinel and Redis Cluster, if one or the other will go away or if they need to be used in combination.
+This post tries to give a _short_ and _precise_ info about both and what they are used for.
+
+## Redis Sentinel
+
+Redis Sentinel was [born in 2012](https://github.com/antirez/redis/commit/6b5daa2df2a0711a25746cb025927dc3deb7717e) and first released when Redis 2.4 was stable.
+It is a system designed to help managing Redis instances.
+
+It will **monitor** your master & slave instances, **notify** you about changed
+behaviour, handle **automatic failover** in case a master is down and act as a
+**configuration provider**, so your clients can find the current master
+instance.
+
+Redis Sentinel runs as a seperate program.
+You should have atleast 3 Sentinel instances monitoring a master instance and its slaves.
+Sentinel instances try to find consensus when doing a failover and only an odd number of instances will prevent most problems, 3 being the minimum.
+In this case one of the Sentinel instances can go down and a failover will still work as (hopefully) the other two instances reach consensus which slave to promote.
+
+One thing about the configurable quorum: this is only the number of Sentinel who have to agree a master is down.
+You still need `N/2 + 1` Sentinels to vote for a slave to be promoted (that `N` is the total number of all Sentinels ever seen for this pod).
+
+A pod of Sentinels can monitor multiple Redis master & slave nodes. Just make sure you don't mix up names, add slaves to the right master and so on.
+
+[Full documentation for Sentinel](http://redis.io/topics/sentinel).
+
+## Redis Cluster
+
+If we go by [first commit](https://github.com/antirez/redis/commit/ecc9109434002d4667cd01a3b7c067a508c876eb),
+then Cluster is even older than Sentinel, dating back to 2011.
+There's a bit more info in [antirez' blog](http://antirez.com/news/79).
+It's released as stable with version 3.0 as of April 1st, 2015.
+
+Redis Cluster is a **data sharding** solution with **automatic management**, **handling failover** and **replication**.
+
+With Redis Cluster your data is split across multiple nodes, each one holding a subset of the full data.
+Slave instances replicate a single master and act as fallback instances.
+In case a master instance will become unavailable due to network splits or software/hardware crashes,
+the remaining Master nodes in the Cluster will register this and will reach a state triggering a failover.
+A suitable Slave of the unavailable Master node will then step up and will be promoted to takeover as a new Master.
+
+You don't need additional failover handling when using Redis Cluster and you should definitely not point Sentinel instances at any of the Cluster nodes.
+You also want to use a *smart* client library that knows about Redis Cluster, so it can automatically redirect you to the right nodes when accessing data.
+
+[Redis Cluster specification][spec] and [Redis Cluster Tutorial][tutorial].  
+I gave a talk about Redis Cluster at the [PHPUGDUS meeting][phpugdus] last month, my slides are [on slidr.io][slides].
+
+---
+
+Want to hear more about Redis, Redis Sentinel or Redis Cluster? [Just invite me!](mailto:janerik@fnordig.de)
+
+
+[spec]: http://redis.io/topics/cluster-spec
+[tutorial]: http://redis.io/topics/cluster-tutorial
+[slides]: http://slidr.io/badboy/redis-cluster
+[phpugdus]: http://www.meetup.com/PHP-Usergroup-Duesseldorf/