Actually I'm using a configuration of Redis Master-Slaves with HAProxy for Wordpress to have High Avaibility. This configuration is nice and works perfect (I'm able to remove any server to maintenance without downtime). The problem of this configuration is that only one Redis server is getting all the traffic and the others are just waiting if that server dies, so in a very high load webpage can be a problem, and add more servers is not a solution because always only one will be master.
With this in mind, I'm thinking if maybe I can just use a Redis Cluster to allow to read/write on all nodes but I'm not really sure if it will works on my setup.
My setup is limited to three nodes the most of times, and I've read in some places that Redis cluster minimal setup is three nodes, but six is recommended. This is rational because this setup allow to have Slaves nodes that will become Masters if her Master dies, and then all data will be kept, but what happend if data don't cares?. I mean, on my setups the data is just cached objects, so if don't exists it just create it again so:
The data will be lost (don't care), and the other nodes will get the objects from clients again, to serve it on later requests (like happen if a Flush the data).
The nodes will answer that data doesn't exists and will reject to cache because the object would have to be on other node that is dead.
Someone know it?
Thanks!!
When a master dies, the Redis cluster goes to a down state and any command involving a key served by the failed instance will fail.
This may differ from some other distributed software because Redis Cluster is not the kind of program that every master holds all data. In fact, the key space is horizontally partitioned and each key is served by only one master.
This is mentioned in the specification:
The key space is split into 16384 slots...
a single hash slot will be served by a single node...
The base algorithm used to map keys to hash slots is the following:
HASH_SLOT = CRC16(key) mod 16384
When you setup a cluster, you certainly ask each node to serve a set of slots, and each slot can only be served by one node. If one node dies, you lose the slots on this node unless you have a slave failover to serve them, so that any command involving keys mapped to these slots will fail.
I have multiple redis instances. I made a cluster using different port. Now I want to transfer the data from pre-existing redis instances to the cluster. I know how to transfer data from one instance to the cluster but when the instances are greater than one, I am not able to do it.
You need to define some sort of sharding strategy for your redis cluster. Database Sharding So basically you need to have a certain consistent hashing strategy which will decide given a key, the shard or the redis instance in your cluster the key will go to. You need to have a certain script for this data migration that will have an array of all the redis instances in your cluster.
Then for a given key which you read from the standalone redis, you will use the hashing mechanism to find out the sharding index or the redis instance from the list you maintained earlier to use and accordingly you will write the data in that cluster node. My assumption in all this is that you have an in house redis cluster setup as opposed to the one which Redis Labs provide.
Mirroring is replicating data between Kafka cluster, while Replication is for replicating nodes within a Kafka cluster.
Is there any specific use of Replication, if Mirroring has already been setup?
They are used for different use cases. Let's try to clarify.
As described in the documentation,
The purpose of adding replication in Kafka is for stronger durability and higher availability. We want to guarantee that any successfully published message will not be lost and can be consumed, even when there are server failures. Such failures can be caused by machine error, program error, or more commonly, software upgrades. We have the following high-level goals:
Inside a cluster there might be network partitions (a single server fails, and so forth), therefore we want to provide replication between the nodes. Given a setup of three nodes and one cluster, if server1 fails, there are two replicas Kafka can choose from. Same cluster implies same response times (ok, it also depends on how these servers are configured, sure, but in a normal scenario they should not differ so much).
Mirroring, on the other hand, seems to be very valuable, for example, when you are migrating a data center, or when you have multiple data centers (e.g., AWS in the US and AWS in Ireland). Of course, these are just a couple of use cases. So what you do here is to give applications belonging to the same data center a faster and better way to access data - data locality in some contexts is everything.
If you have one node in each cluster, in case of failure, you might have way higher response times to go, let's say, from AWS located in Ireland to AWS in the US.
You might claim that in order to achieve data locality (services in cluster one read from kafka in cluster one) one still needs to copy the data from one cluster to the other. That's definitely true, but the advantages you might get with mirroring could be higher than those you would get by reading directly (via an SSH tunnel?) from Kafka located in another data center, for example single connections down, clients connection/session times longer (depending on the location of the data center), legislation (some data can be collected in a country while some other data shouldn't).
Replication is the basis of higher availability. You shouldn't use Mirroring to handle high availability in a context where data locality matters. At the same time, you should not use just Replication where you need to duplicate data across data centers (I don't even know if you can without Mirroring/an ssh tunnel).
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
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.
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.
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.
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
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.
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.
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)
Since the redis cluster is still a work in progress, I want to build a simplied one by myselfin the current stage. The system should support data sharding,load balance and master-slave backup. A preliminary plan is as follows:
Master-slave: use multiple master-slave pairs in different locations to enhance the data security. Matsters are responsible for the write operation, while both masters and slaves can provide the read service. Datas are sent to all the masters during one write operation. Use Keepalived between the master and the slave to detect failures and switch master-slave automatically.
Data sharding: write a consistant hash on the client side to support data sharding during write/read in case the memory is not enougth in single machine.
Load balance: use LVS to redirect the read request to the corresponding server for the load balance.
My question is how to combine the LVS and the data sharding together?
For example, because of data sharding, all keys are splited and stored in server A,B and C without overlap. Considering the slave backup and other master-slave pairs, the system will contain 1(A,B,C), 2(A,B,C) , 3(A,B,C) and so on, where each one has three servers. How to configure the LVS to support the redirection in such a situation when a read request comes? Or is there other approachs in redis to achieve the same goal?
Thanks:)
You can get really close to what you need by using:
twemproxy shard data across multiple redis nodes (it also supports node ejection and connection pooling)
redis slave master/slave replication
redis sentinel to handle master failover
depending on your needs you probably need some script listening to fail overs (see sentinel docs) and clean things up when a master goes down