Suppose I have [Slave IP Address] which is the slave of [Master IP Address].
Now my master server has been shut down, and I need to set this slave to be master MANUALLY (WITHOUT using sentinel automatic failover, WITH redis command).
Is it possible doing this without restarting the redis service ? (and losing all the cached data)
use SLAVEOF NO ONE to promote a slave to master
http://redis.io/commands/slaveof
it depends, if you are in a cluster you will be better using the fail over. You will need to use the force option in the command
http://redis.io/commands/cluster-failover
Is it possible doing this without restarting the redis service? (and
losing all the cached data)
yes that's possible, you can use
SLAVEOF NO ONE (without sentinel)
But it is recommended to use sentinel to avoid data loss.
sentinel failover master-name(with sentinel)
This will force the sentinel to switch master.
The new master will have all the data that was synchronized before the old-master shutdown.
Redis will automatically choose the best slave with max. data, that will reduce the amount of data we lose when switching master.
Below 2 options in step 3 have helped me to recover the cluster once a master node is down, compute was replaced or other not recoverable state.
1 .- First you need to connect to the slave node, use redis-cli, here a link how to do that: How to connect to remote Redis server?
2 .- Once connected to the slave node run the command cluster nodes to validate master node is in fail state, also run cluster info to see the overall state of your cluster(this is always a good idea)
3 .- Inside the slave node to be promoted run command: cluster failover,
in rare cases when there is some serious issues with redis this
command could fail, and you will need to use cluster failover force
or cluster failover takeover, here more info abut the implications
of those options: https://redis.io/commands/cluster-failover
4 .- Run cluster forged $old_master_id in all your cluster nodes
5 .- Add a new node with cluster meet $new_node_IP $new_node_PORT
6 .- Subscribe your new node to your brand new master, login in to the new bode and run cluster replicate $master_node_id
Steps 1-3 are required for the slave-master promotion and 4-5 are required to left all cluster in a healthy master-slave equilibrium.
As of Redis version 5.0.0 the SLAVEOF command is regarded as deprecated.
If a Redis server is already acting as replica, the command REPLICAOF NO ONE will turn off the replication, turning the Redis server into a MASTER.
Related
I found that Sentinel is mainly used for promoting slave to master automatically when master failed.
I also found that redis-4.0.11's cluster mode seemly also have this function itself.
So when I use redis-4.0.11's in cluster mode, do I need a Sentinel ?
NO, you don't need sentinels in cluster mode.
When a master is down, the cluster will promote one of its slaves to be the new master automatically.
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.
Hello stack community,
I have a question about Redis sentinel for a specific problem case. I use AWS with Multi AZ to create a sensu cluster.
On eu-central-1a I have a sensu+redis(M), a RBMQ+Sentinel and 2 others Sentinels. Same on eu-central-1b but the redis is my slave on this AZ.
What happen if there is a problem and eu-central-1a can not communicate with eu-central-1b ? What I think is that Sentinel on eu-central-1b should promote my redis slave to master, because he can not contact my redis master. So I should have 2 redis masters running together on 2 different AZ.
But when the link is retrieved between AZ, I will still have 2 masters, with 2 different datas. What will happen in this case ? One master will become a slave and data will be replicated without loss ? Do we need to restart a master and he will be a slave ?
Sentinel detects changes to the master for example
If the master goes down and is unreachable a new slave is elected. This is based on the quorum where multiple sentinels agree that the master has gone down. The failover then occurs.
Once the sentinel detects the master come back online it is then a slave I believe thus the new master continues I believe. You will loose data in the switchover from master to new master that in inevitable.
If you loose connection then yes sentinel wont work correctly as it relies on multiple sentinels to agree the master redis is down. You shouldn't use sentinel in a 2 sentinel system.
Basic solution would be for you to put a extra sentinel on another server maybe the client/application server that isn't running redis/sentinel this way you can make use of the quorum and sentinels agreeing the master is down.
We have a setup with a number of redis (2.8) servers (lets say 4) and as many redis sentinels. On startup of each machine, we set a pre-select machine as master through the command line and all the rest as slaves of that. and the sentinels all monitor these machines. The clients first connect to the local sentinel and retrieve the master's IP address and then connect there.
This setup is trouble free most of the time but sometimes the sentinels go out of sync with servers. if I name the machines A,B,C and D - sentinels will think B is master while redis servers are all connected to A as the master. bringing down redis server on B doesnt help either. I had to bring it down and manually "Sentinel failover" on A to fix the issue. Question is
1. What causes this to happen and whats the easiest and quickest way to fix this ?
2. What is best configuration - is there something better than this ?
The only time you should set a master is the first time. Once sentinel has taken over management of replication you should let it do it. This includes on restarts. Don't use the command line to set replication. Let sentinel and redis manage it. This is why you're getting issues - you've told sentinel it is authoritative, but you are telling the Redis servers to ignore sentinel.
Sentinel stores the status in its Config file, so when it restarts it can resume the last configuration. So even on restart, let sentinel do it's job.
Also, if you have 4 servers (be specific, not "let's say") you should be running a quorum of three on your monitor statement in sentinel. With a quorum of two you can wind up with two masters
We have a redis cluster with a master and a slave managed by three sentinel processes, and an additional remote slave, hosted in a different datacenter, for transparent failover and data preservation in the case that something bad happens to the master and slave machines.
It may happen that a transient error takes down the master redis process only, and in this situation we would like to see the slave process promoted to master, and the remote slave reslaved to it. However, it seems that sentinel could just as easily promote the remote slave to master, and we have not found any way to prevent this.
Is there any way to mark a particular slave machine as unpromotable, so that sentinel will not try to make it the master in the event of a failover?
Yes. In the slave's config file set the slave-priority setting to zero (the number not the word).