Hello I'm a new in Redis. Is it possible to synchronize masters between themselves and if some will drop others will work?
You do not syncronize primaries (masters) between them. You create another redis server with a replica (slave) role. The primary and replica talk to each other and get syncronized.
In case the primary fails, the replica becomes the primary and start serving client request. When your original primary came back to live again, it will be a replica or the promoted replica (now the primary)
This promotion magic can be automatically achieved using Redis Sentinel or Redis Cluster
Some interesting links you should read
Redis replication
Redis sentinel
Redis cluster
Related
is there a way to promote a replica node to primary with cluster mode enabled? I have googled but only found solution with cluster mode disabled.
You can send CLUSTER FAILOVER command to the replica to promote it to be a new master. However, you must ensure that the replica has already been known by majority of the masters in the cluster.
Check the doc on manually failover for detail.
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
We have an application here which uses redis cluster to hold some short lived keys, on the order of 2 seconds. While we want the master to fail over to the slave if something happens to the master, short term loss of this ephemeral data is unimportant. To save bandwidth we'd like to disable replication between the master and slave. Is there any configuration that of master and slave in a cluster that can make this happen?
When running a single instance redis, I can use "slave of" to create a (or as many I like) readonly replica of this one redis node.
When using redis cluster, I split my Data into Partitons (Masters) and can create a slave for each partition.
Is it possible to treat this cluster as a single instance and connect a "slave of" Slave to this cluster which will hold a replica of all Data in the cluster and not just the partition of the connected node?
If not possible with redis cluster, is this might a working solution when using sentinel?
Our current Problem:
We are using the "slave of" feature together with keepalived to failover our redis instance on an outage of the master.
But we have lots of "slave of" slaves connected to the virtual IP of the failover setup, to deliver cached data.
Now everytime the system fails over (for maintenance reasons e.g.) all connected slaves have a timout for up to 30 seconds, when they have to resync their data with the new master.
We allready played with all possible redis config parameters but can't get this syncing time to be shorter (e.g. by relying on the replication-backlog, which isn't available on the new master after the failover).
Anyone any ideas?
a very good doc here : http://redis.io/presentation/Redis_Cluster.pdf and here http://fr.slideshare.net/NoSQLmatters/no-sql-matters-bcn-2014 (slide #9) or better https://www.javacodegeeks.com/2015/09/redis-clustering.html
If you want "slave" in Redis cluster mode, you need use replication of all nodes.
Regards,
Well, I just read this article:
https://seanmcgary.com/posts/how-to-build-a-fault-tolerant-redis-cluster-with-sentinel
The author used a single master with Redis Cluster, with 2 slaves per master, instead of one, and he let Redis Sentinel take care of the election of a slave to a master when the master is down.
You could play with this setup to see if the election of Master occurs quickly. While it's happening, clients would be served by a slave and should experience no downtime.
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.