There is no HA setup which is safe if you don't test from time to time in development environments, or even better if you can, in production environments, if they work.Popular client libraries have Sentinel support, but not all. You need Sentinel support in your clients.However there are ways to deploy Sentinel that make the window to lose writes limited to certain moments, while there are other less secure ways to deploy it. Sentinel + Redis distributed system does not guarantee that acknowledged writes are retained during failures, since Redis uses asynchronous replication.So for example different physical servers or Virtual Machines executed on different availability zones. The three Sentinel instances should be placed into computers or virtual machines that are believed to fail in an independent way.You need at least three Sentinel instances for a robust deployment.Fundamental things to know about Sentinel before deploying Otherwise Sentinels can't talk and can't agree about what to do, so failover Sentinels by default run listening for connections to TCP port 26379, soįor Sentinels to work, port 26379 of your servers must be open to receiveĬonnections from the IP addresses of the other Sentinel instances. Sentinel will simply refuse to start if no configuration file is given or if the configuration file path is not writable. However it is mandatory to use a configuration file when running Sentinel, as this file will be used by the system in order to save the current state that will be reloaded in case of restarts. Sentinel mode: redis-server /path/to/nf -sentinel Otherwise you can use directly the redis-server executable starting it in With the following command line: redis-sentinel /path/to/nf Link with that name to the redis-server executable) you can run Sentinel If you are using the redis-sentinel executable (or if you have a symbolic Redis Sentinel version 1, shipped with Redis 2.6, is deprecated and should not be used. Sometimes are back ported into the latest stable branch as soon as they are New developments are performed in the unstable branch, and new features The initial Sentinel implementation using stronger and simpler-to-predictĪlgorithms (that are explained in this documentation).Ī stable release of Redis Sentinel is shipped since Redis 2.8. The current version of Sentinel is called Sentinel 2. Order to understand how exactly Sentinel works. Properties of Sentinel, to more complex information (that are optional) in
Starting from basic information needed in order to understand the basic In this document concepts will be introduced gradually The sum of Sentinels, Redis instances (masters and replicas) and clientsĬonnecting to Sentinel and Redis, are also a larger distributed system with There is no fun in having a failover system which is itself a single point of failure, after all. Sentinel works even if not all the Sentinel processes are working, making the system robust against failures.This lowers the probability of false positives. Failure detection is performed when multiple Sentinels agree about the fact a given master is no longer available.
The advantage of having multiple Sentinel processes cooperating are the following: Sentinel itself is designed to run in a configuration where there are multiple Sentinel processes cooperating together. If a failover occurs, Sentinels will report the new address. Sentinel acts as a source of authority for clients service discovery: clients connect to Sentinels in order to ask for the address of the current Redis master responsible for a given service. If a master is not working as expected, Sentinel can start a failover process where a replica is promoted to master, the other additional replicas are reconfigured to use the new master, and the applications using the Redis server are informed about the new address to use when connecting. Sentinel can notify the system administrator, or other computer programs, via an API, that something is wrong with one of the monitored Redis instances. Sentinel constantly checks if your master and replica instances are working as expected.
Sentinel events full#
This is the full list of Sentinel capabilities at a macroscopic level (i.e. Notifications and acts as a configuration provider for clients. Redis Sentinel also provides other collateral tasks such as monitoring, Redis Sentinel provides high availability for Redis when not using Redis Cluster. High availability for non-clustered Redis
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