Redis sentinel vs clustering

Question

I understand redis sentinel is a way of configuring HA (high availability) among multiple redis instances. As I see, there is one redis instance actively serving the client requests at any given time. There are two additional servers are on standby (waiting for a failure to happen, so one of them can be in action again).

• Is it waste of resources?
• Is there a better way of using full use of the resources available?
• Is Redis clustering an alternative to Redis sentinel?

I already looked up redis documentation for sentinel and clustering, can somebody having experience explain please.

UPDATE

OK. In my real deployment scenario I have two servers dedicated for redis. I have another server my Jboss server is running. The application running in Jboss is configured to connect to redis master server(M).

Failover scenario

Ideally, I think when Master cache server fails (either Redis process goes down or machine failure) the application in Jboss needs to connect to Slave cache server. How would I configure the redis servers to achieve this?

+--------+          +--------+
| Master  |---------| Slave  |
|         |         |        |
+--------+          +--------+

Configuration: quorum = 1

Fielder

This could also help - fnordig.de/2015/06/01/redis-sentinel-and-redis-cluster – Jocularity

This too could help: alex.dzyoba.com/blog/redis-ha – Aspic

Answer 1

First, lets talk sentinel.

Sentinel manages the failover, it doesn't configure Redis for HA. It is an important distinction. Second, the diagram you posted is actually a bad setup - you don't want to run Sentinel on the same node as the Redis nodes it is managing. When you lose that host you lose both.

As to "Is it waste of resources?" it depends on your use case. You don't need three Redis nodes in that setup, you only need two. Three increases your redundancy, but is not required. If you need the added redundancy then it isn't a waste of resources. If you don't need redundancy then you just run a single Redis instance and call it good - as running more would be "wasted".

Another reason for running two slaves would be to split reads. Again, if you need it then it wouldn't be a waste.

As to "Is there a better way of using full use of the resources available?" we can't answer that as it is far too dependent on your specific scenario and code. That said if the amount of data to store is "small" and the command rate is not exceedingly high, then remember you don't need to dedicate a host to Redis.

Now for "Is Redis clustering an alternative to Redis sentinel?". It really depends entirely on your use case. Redis Cluster is not an HA solution - it is a multiple writer/larger-than-ram solution. If your goal is just HA then it likely won't be suitable for you. Redis Cluster comes with limitations, particularly around multi-key operations, so it isn't necessarily a straightforward "just use cluster" operation.

If you think having three hosts running Redis (and three running sentinel) is wasteful, you'll likely hold Cluster to be even more so as it does require more resources.

The questions you've asked are probably too broad and opinion-based to survive as written. If you have a specific case/problem you are working out please update with that so we can provide specific assistance and information.

Update for specifics:

For proper failover management in your scenario I would go with 3 sentinels, one running on your JBoss server. If you have 3 JBoss nodes then go with one on each. I'd have a Redis pod (master+slave) on separate nodes, and let sentinel manage the failover.

From there it is a matter of wiring up JBoss/Jedis to use Sentinel for it's information and connection management. As I don't use those a quick search turns up that Jedis has the support for it, you just need to configure it correctly. Some examples I found are at Looking for an example of Jedis with Sentinel and https://github.com/xetorthio/jedis/issues/725 which talk about JedisSentinelPool being the route for using a pool.

When Sentinel executes a failover the clients will be disconnected and Jedis will (should?) handle the reconnection by asking the Sentinels who the current master is.

Herculaneum

Hi @The-Real-Bill, could you please elaborate on "Sentinel manages the failover, it doesn't configure Redis for HA." On the official document(redis.io/topics/sentinel), it says "Redis Sentinel provides high availability for Redis." – Shulem

HA Redis requires several pieces to be HA. Sentinel only handles one piece: the failover. It doesn't set up the replication and it doesn't provide and HA endpoint. It provides service discovery so a client can know of where to talk to to get to the master. This it doesn't configure Redis for HA. – Herculaneum

The claims here are simply not true - redis with sentinel DOES manage replication from the primary to standby nodes. On failover, the master is changed and replication moves to any remaining nodes from the new master. A recovered node becomes a secondary site as a target for replication. The piece that is missing is that the CLIENT needs to talk to sentinel to receive information about any changes to state. So Sentinel IS a High Availability solution. – Vav

I said it doesn't set up replication and this is true. You configure Redis replication by setting up slaves. Then sentinel will discover it and manage failovers. Sentinel can not set up replication as it only manages an existing replication setup. Try it. Turn on two independent Redis servers and get sentinel to make one slave to another without using slave of directly. It wont work. Nor can it add new slaves. Thus it does. It set up replication. – Herculaneum

@TheRealBill You mentioned: "Redis Cluster is not an HA solution - it is a multiple writer/larger-than-ram solution." But the replicas associated with each master do effectively make Redis cluster HA. Isn't it? If you say that milliseconds lag between master and replica is a problem, then there are no distributed systems that are really HA. – Hutner

Answer 2

This is not direct answer to your question, but think, it's helpful information for Redis newbies, like me. Also this question appears as the first link in google when searching the "Redis cluster vs sentinel".

Redis Sentinel is the name of the Redis high availability solution... It has nothing to do with Redis Cluster and is intended to be used by people that don't need Redis Cluster, but simply a way to perform automatic fail over when a master instance is not functioning correctly.

Taken from the Redis Sentinel design draft 1.3

It's not obviuos when you are new to Redis and implementing failover solution. Official documentations about sentinel and clustering doens't compare to each other, so it's hard to choose the right way without reading tons of documentations.

Jospeh

Answer 3

The recommendation, everywhere, is to start with an odd number of instances, not using two or a multiple of two. That was corrected, but lets correct some other points.

First, to say that Sentinel provides failover without HA is false. When you have failover, you have HA with the additional benefit of application state being replicated. The distinction is that you can have HA in a system without replication (it's HA but it's not fault tolerant).

Second, running a sentinel on the same machine as its target redis instance is not a "bad setup": if you lose your sentinel, or your redis instance, or the whole machine, the results are the same. That's probably why every example of such configurations shows both running on the same machine.

Deliver

Actually there appears to be bug where Sentinel will fail to initiate an election in the "sentinel-on-the-instance" setup, and I've seen it many times here, on the ML, and in individual consulting. Moving the sentinels off of the Redis server fixed it every single time. Therefore, yes doing it that way is a bad setup in that it will fail you at the very moment you need it. Example show it that way because they aren't written by people with extensive operational experience and it is easier. – Herculaneum

Which bug is this, is there a bug report? Do you know if it still exists? – Overarch

Are there any similar issues putting the sentinels on the application machines? – Chateau

Answer 4

Additional info to above answers

Redis Cluster

• One main purpose of the Redis cluster is to equally/uniformly distribute your data load by sharding

• Redis Cluster does not use consistent hashing, but a different form of sharding where every key is conceptually part of what is called as hash slot

• There are 16384 hash slots in Redis Cluster, Every node in a Redis Cluster is responsible for a subset of the hash slots, so, for example, you may have a cluster with 3 nodes, where:

  Node A contains hash slots from 0 to 5500, Node B contains hash slots from 5501 to 11000, Node C contains hash slots from 11001 to 16383

This allows us to add and remove nodes in the cluster easily. For example, if we want to add a new node D, we need to move some hash slot from nodes A, B, C to D

• Redis cluster supports the master-slave structure, you can create slaves A1,B1, C2 along with master A, B, C when creating a cluster, so when master B goes down slave B1 gets promoted as 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.

So in practical terms, what do you get with Redis Cluster?

1.The ability to automatically split your dataset among multiple nodes.

2.The ability to continue operations when a subset of the nodes are experiencing failures or are unable to communicate with the rest of the cluster.

Redis Sentinel

• Redis supports multiple slaves replicating data from a master node.
• This provides a backup for data in master node.
• Redis Sentinel is a system designed to manage master and slave. It runs as separate program. The minimum number of sentinels required in an ideal system is 3. They communicate among themselves and make sure that the Master is alive, if not alive they will promote one of the slaves as master, so later when the dead node spins up it will be acting as a slave for the new master
• Quorum is configurable. Basically it is the number of sentinels that need to agree as the master is down. N/2 +1 should agree. N is the number of nodes in the Pod (note this setup is called a pod and is not a cluster)

So in practical terms, what do you get with Redis Sentinel?

It will make sure that Master is always available (if master goes down, the slave will be promoted as master)

Reference :

https://fnordig.de/2015/06/01/redis-sentinel-and-redis-cluster/

https://redis.io/topics/cluster-tutorial

Blame

Answer 5

This is my understanding after banging my head throughout the documentation.

Sentinel is a kind of hot standby solution where the slaves are kept replicated and ready to be promoted at any time. However, it won't support any multi-node writes. Slaves can be configured for read operations. It's NOT true that Sentinel won't provide HA, it has all the features of a typical active-passive cluster ( though that's not the right term to use here ).

Redis cluster is more or less a distributed solution, working on top of shards. Each chunk of data is being distributed among masters and slaves nodes. A minimum replication factor of 2 ensures that you have two active shards available across master and slaves. If you know the sharding in Mongo or Elasticsearch, it will be easy to catch up.

Eats

Answer 6

Redis can operate in partitioned cluster (with many masters and slaves of those masters) or a single instance mode (single master with replica slaves).
The link here says:

When using Redis in single instance mode, in which a single Redis server manages the entire unpartitioned database, Redis Sentinel is used to manage its availability

It also says:

A Redis cluster, in which data is partitioned among multiple primary instances, manages availability by itself and requires no extra components.

So HA can be ensured in the 2 mentioned scenarios. Hope this clears the doubts. Redis cluster and sentinels are not alternative to each other. They are just used to ensure HA in different cases of partitioned or non-partitioned master.

Standin

Answer 7

Redis Sentinel performs the failover promoting replicas when they see a master is down. You typically want an odd number of sentinel nodes. For the example of one master and one replica, 3 sentinels should be used so there can be a consensus on the decision. Ideally the 3rd sentinel is on a 3rd server so the decision is not skewed (depending on failure). Sentinel takes care of changing the master/replica config settings on your nodes so that promotion and syncing occurs in the correct order and you don’t overwrite data by bringing on an old failed master that now contains older data.

Once you have your sentinel nodes set up to perform failovers, you need to ensure you are pointing to the correct instance. See an example of HAProxy configuration for this. HAProxy performs health checks and will point to the new master if a failure occurs.

Clustering will allow you to scale horizontally and can help handle high loads. It does take a bit of work to set up and configure up front.

There is an open source fork of Redis, “KeyDB” that has eliminated the need for sentinel nodes with an active-replica option. This allows the replica node to accept reads and writes. When a failover occurs HAProxy stops reads/writes with the failed node and just uses the remaining active node which is already sync’d. Timestamping enables the failed nodes to rejoin automatically and resync without losing data when they come back online. Setup is simple and for higher traffic you don’t need special upfront setup to direct reads to the replica node and read/writes to the master. See example of active replication here. KeyDB is also multi-threaded which for some applications might be an alternative to clustering, but really depends on what your needs are.

There is also an example of setting up clustering manually and with the create-cluster tool. These are the same steps if you are using Redis (replace 'keydb' with 'redis' in instruction)

Roux

Answer 8

Except for @Kamarey's answer, I find the redis author's article "Redis cluster, no longer vaporware." (in 2015) has helpful information for Redis newbies like me to understand the difference between sentinel and cluster, especially this paragraph,

people were doing again and again, with their efforts, and many times in a totally broken way, two things:

1. Sharding the dataset among N nodes.
2. A responsive failover procedure in order to survive certain failures.

Problem “2” was so bad that at some point I decided to start the Redis Sentinel project before Cluster was finished in order to provide an HA system ASAP, and one that was more suitable than Redis Cluster for the majority of use cases that required just “2” and not “1”.

One thing confused me first was that the cluster concept always associate with High Availability, but here redis cluster is for data sharding while Sentinel for HA (with master-slave pattern)

Fahland