I've read https://github.com/redisson/redisson
And I found out that there are several
Redis Replicated setup (including support of AWS ElastiCache and Azure Redis Cache)
Redis Cluster setup (including support of AWS ElastiCache Cluster and Azure Redis Cache)
Redis Sentinel setup
Redis with Master with Slave only
I am not a big expert in clusters and I don't understand the difference between these setups.
Could you beiefly explain the differences ?
Disclaimer I am an AWS employee.
I do not know how Redis Replicated Setup is different from Redis in Master-Slave mode. Maybe they mean cross-region replication?
In any case, I can try and explain setups I know about:
Redis with Master with Slave only - is a single shard setup where you create a primary replica together with one or more secondary (slave) replicas (let's hope PC police won't arrest me). This setup is used to improve the durability of your in-memory store. It's not advised to use your secondaries for reads because such setup has eventual consistency guarantees and your replica reads may be stale (depending on the replication lag).
Redis Cluster setup - the setup supported by cloud provides such as AWS Elasticache. In this setup your workload can be spread horizontally across multiple shards and each shard may have its own secondary replicas. Your client library must support this setup since it requires maintaining multiple connections to several nodes at a client level. Moreover, there are some locality rules you need to follow in order to use cluster mode efficiently:
Keys with foo{<shard>}bar notation will be routed to their shard according to what is stored inside curly brackets.
You can not use mset, mget and other multi-key commands across shards. You can still use these commands if their keys contain the same {shard} part.
There are additional cluster mode admin commands that are exposed by Redis but they are usually hijacked and hidden from users by cloud providers since cloud provides use them in order to manage redis cluster themselves.
Redis cluster have an ability to migrate part of your workload between shards. However, it still obliged to preserve correctness with respect to {shard} notation. Since your client library is responsible to fetch data from specific shard it must handle "moved" response when a shard might redirect it to another node.
Redis Sentinel setup - using an additional server that provides service discovery functionality for Redis clusters. Not strictly required and I believe is less popular across users. It serves as a single source of truth regarding each node's health and state. It provides monitoring, management, and service discovery functions for managing your Redis cluster. Many Redis client libraries provide the option of connecting to Redis sentinel nodes in order to achieve automatic service discovery and seamless failover flow. One of the reasons why this setup is less popular is because cloud companies like AWS Elasticache provide this service out of the box.
Related
This is regarding the use case where we are trying to use the Redis in PCF (Pivotal Cloud Foundry). In our use case, we will refresh the Redis cache daily once or twice with the required data and then API will query Redis and then provide the response.
One thing of particular concern for us is that we want API queries to happen from Redis only that means Redis to be available at all times. But whenever we are refreshing the Redis DB, Redis would not be able to serve the APIs since it is refreshing the keys. To avoid that we wanted to setup a Redis in cluster mode or master-slave mode so if one instance is being written another can be read from.
How can we setup Redis cluster or master-slave mode in PCF and then fulfil our requirement?
Please provide any other suggestions as well that you may have.
At the time I write this, the Redis for Pivotal Platform product does not support clustering. See Availability, in the docs here -> https://docs.pivotal.io/redis/2-3/erc.html#offerings.
All Redis for Pivotal Platform services are single VMs without clustering capabilities. This means that planned maintenance jobs (e.g., upgrades) can result in 2–10 minutes of downtime, depending on the nature of the upgrade. Unplanned downtime (e.g., VM failure) also affects the Redis service.
Redis for Pivotal Platform has been used successfully in enterprise-ready apps that can tolerate downtime. Pre-existing data is not lost during downtime with the default persistence configuration. Successful apps include those where the downtime is passively handled or where the app handles failover logic.
If you require clustered Redis, you'd need to look at a different offering. Redis Labs has some offerings that integrate with PCF, you could use a Cloud Provider's Redis offering, or you could host your own.
If the solution you use isn't integrated into PCF, you can create a user-provided service with cf cups and provide the Redis credentials to your application that way. It will function just like a Redis service instance created through the marketplace.
I'm planning to create a high available Redis Cluster. After reading many articles about building Redis cluster i'm confused. So what exactly are
the advantages of a Redis Sentinel Master1 Slave1 Slave2 Cluster? Is it more reliable as a Redis Multinode Sharded Cluster?
the advantages of a Redis Multinode Sharded Cluster? Is it more reliable as a Redis Sentinel Master1 Slave1 Slave2 Cluster?
Further questions to the Redis Sentinel Master1 Slave1 Slave2 Cluster:
when i have 1 Master and the two Slaves and traffic is getting higher and higher so this cluster will be to small how can i make the cluster bigger?
Further questions to the Redis Multinode Sharded Cluster:
why are there so many demos with running a cluster on a single instance but on different ports? That makes no sense to me.
when i have a cluster with 4 masters and 4 replicas, how can an application or a client be sure to write to the cluster? When Master1 and Slave1 are dying but my application is writing always to the IP of Master1 then it will not work anymore. Which solutions are out there to implement a sharded cluster well to make it available for applications to find it with a single ip and port? Keepalived? HAproxy?
when i juse for a 4 master setup with e.g. Keepalived - doesn't that cancel out the different masters?
furthermore i need to understand why the multinode cluster is only for solutions where more data will need to be written as memory is available. Why? For me a multi master setup sounds good to be scaleable.
is it right that the the sharded cluster setup does not support multikey operations when the cluster is not in caching mode?
I'm unsure if these two solutions are the only ones. Hopefully you guys can help me to understand the architectures of Redis. Sorry for so many questions.
I will try to answer some of your questions but first let me describe the different deployment options of Redis.
Redis has three basic deployments: single node, sentinel and cluster.
Single node - The basic solution where you run single process running Redis.
It is not scalable and not highly available.
Redis Sentinel - Deployment that consist of multiple nodes where one is elected as master and the rest are slaves.
It adds high availability since in case of master failure one of the slaves will be automatically promoted to master.
It is not scalable since the master node is the only node that can write data.
You can configure the clients to direct read requests to the slaves, which will take some of the load from the master. However, in this case slaves might return stale data since they replicate the master asynchronously.
Redis Cluster - Deployment that consist of at least 6 nodes (3 masters and 3 slaves). where data is sharded between the masters. It is highly available since in case of master failure, one of his slaves will automatically be promoted to master. It is scalable since you can add more nodes and reshard the data so that the new nodes will take some of the load.
So to answer your questions:
The advantages of Sentinel over Redis Cluster are:
Hardware - You can setup fully working Sentinel deployment with three nodes. Redis Cluster requires at least six nodes.
Simplicity - usually it is easier to maintain and configure.
The advantages of Redis Cluster over Sentinel is that it is scalable.
The decision between that two deployment should be based on your expected load.
If your write load can be managed with a single Redis master node, you can go with Sentinel deployment.
If one node cannot handle your expected load, you must go with Cluster deployment.
Redis Sentinel deployment is not scalable so making the cluster bigger will not improve your performance. The only exception is that adding slaves can improve your read performance (in case you direct read requests to the slaves).
Redis Cluster running on a single node with multiple ports is only for development and demo purposes. In production it is useless.
In Redis Cluster deployment clients should have network access to all nodes (and node only Master1). This is because data is sharded between the masters.
In case client try to write data to Master1 but Master2 is the owner of the data, Master1 will return a MOVE message to the client, guiding it to send the request to Master2.
You cannot have a single HAProxy in front of all Redis nodes.
Same answer as in 5, in the cluster deployment clients should have direct connection to all masters and slaves not through LB or Keepalived.
Not sure I totally understood your question but Redis Cluster is the only solution for Redis that is scalable.
Redis Cluster deployment support multikey operations only when all keys are in the same node. You can use "hash tags" to force multiple keys to be handled by the same master.
Some good links that can help you understand it better:
Description on the different Redis deployment options: https://blog.octo.com/en/what-redis-deployment-do-you-need
Detailed explanation on the architecture of Redis Cluster: https://blog.usejournal.com/first-step-to-redis-cluster-7712e1c31847
Newbie to redis here so please bear with me.
I am looking for a method to have dual datacenter in active-active configuration. My need here is that:
if a datacenter goes down the other datacenter should not need any intervention to carry on working
Networking issues, if they occur, should not prevent one or the other datacenter to fail on set/get redis calls.
Restarting failed datacenter should have a means mirror the working redis' data back before going active
I have been reading up on replication abilities of redis, what I understand is there is
master-slave(s) replication
A cluster can have masters that are sharded
But what I haven't seen is a cluster replicating to another cluster.
Question:
Is there a architecture design where all redis in one dc replicate with the other? (Looks like couchbase appears to have this)
I do see a keyspace notifier which can be used in a pub/sub, what i want to know is whether I can use it to pub/sub redis to redis from one dc to the other to act as replication.
I have a question. Suppose I am using a Redis cluster with 3 shards (with master and slave). I came to know that if a master and its slave fails at the same time Redis Cluster is not able to continue to operate. What happen after that.
Would Redis cluster delete all the other keys from other 2 nodes as well? (When it comes back)
Do we need to manually restart this cluster and can we somehow retain the other keys values (on other nodes)?
How will it behave if I use Azure Redis Cache?
Thanks In Advance
1. Would Redis cluster delete all the other keys from other 2 nodes as well? (When it comes back)
First of all only the operations are blocked not the cluster activity and nothing is done with the data so says the documentation
Redis Cluster failure detection is used to recognize when a master or slave node is no longer reachable by the majority of nodes and then respond by promoting a slave to the role of master. When slave promotion is not possible the cluster is put in an error state to stop receiving queries from clients.
Next regarding if the data gets deleted or not (Under Replication document)
In setups where Redis replication is used, it is strongly advised to have persistence turned on in the master
Which means that only if the persistence was turned off and the master server pair went down then you will loose the data. When the pair comes back up, you will not be able to recover the data. So keep Redis persistence turned on.
2. Do we need to manually restart this cluster and can we somehow retain the other keys values (on other nodes)?
I think the above answer covers it up.
3. How will it behave if I use Azure Redis Cache?
From Azure Redis Cache FAQ
High Availability/SLA: Azure Redis Cache guarantees that a Standard/Premium cache will be available at least 99.9% of the time. To learn more about our SLA, see Azure Redis Cache Pricing. The SLA only covers connectivity to the Cache endpoints. The SLA does not cover protection from data loss. We recommend using the Redis data persistence feature in the Premium tier to increase resiliency against data loss.
So it's kinda their headache
OR
Redis Cluster: If you want to create caches larger than 53 GB or want to shard data across multiple Redis nodes, you can use Redis clustering which is available in the Premium tier. Each node consists of a primary/replica cache pair for high availability. For more information, see How to configure clustering for a Premium Azure Redis Cache.
I am planning on adding Redis to our application as a session and cache store. I have been looking at how to make Redis highly available on an on-premise hosted solution.
The standard approach appears to be to set up Redis as a 3 node replica and use Sentinel for the monitoring and automatic failover.
Redis 2.8 introduces Redis cluster. Does that mean it brings in automatic failover etc and we no longer need to use Sentinel?
No, Cluster and Failover are different scenarios. Also Cluster is in 3.0, not 2.8.
The standard (and minimum) setup for HA is a master and one slave (aka "a pod"), with a separate set of three nodes which run Sentinel and monitor the pod.
This is to ensure failover of the server. However, either your client library has to support using Sentinel to discover master and reconnect on failure, you implement it in your code, or you set up a TCP load balancer and a sentinel monitoring daemon to update your load balancer configuration when a failover occurs at which point the client code doesn't know or care about sentinel.
Cluster isn't there to provide HA, it is there for server-side sharding of data. For Cluster you're looking at 6-7 nodes minimum (3 master, 3 slave, 1 spare) as well as Cluster support in the client and restrictions about commands and Lua script which need to access multiple keys.