http://blog.exceliance.fr/2014/01/02/haproxy-advanced-redis-health-check/
HAProxy advanced Redis health check
Introduction
Redis is an opensource nosql database working on a
key/value model.
One interesting feature
in Redis is that it is able to write data to disk as
well as a master can synchronize many slaves.
HAProxy can
load-balance Redis servers with no issues at
all.
There is even a built-in health check
for redis in HAProxy.
Unfortunately,
there was no easy way for HAProxy to detect the
status of a redis server: master or slave node. Hence
people usually hacks this part of the architecture.
As written in the title of this post, we’ll learn today how to make a
simple Redisinfrastructure thanks to
newest HAProxy advanced send/expect health
checks.
This feature is available in HAProxy 1.5-dev20 and
above.
Purpose is to make the redis infrastructure as simple as possible and ease fail over for the web servers. HAProxy will have to detect which node is MASTER and route all the connection to it.
Redis high availability diagram with HAProxy
Below, an ascii art diagram of HAProxy load-balancing Redis servers:
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+----+ +----+ +----+ +----+| W1 | | W2 | | W3 | | W4 | Web application servers+----+ +----+ +----+ +----+ \ | | /
\ | | /
\ | | /
+---------+
| HAProxy |
+---------+
/ \
+----+ +----+
| R1 | | R2 | Redis servers
+----+ +----+
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The scenario is simple:
* 4 web application servers need to
store and retrieve data to/from
a Redis database
* one (better using
2) HAProxy servers which
load-balance redis connections
* 2 (at
least) redis servers in an active/standby mode with
replication
Configuration
Below, is the HAProxy configuration for the
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defaults REDIS mode tcp
timeout connect 4s
timeout server 30s
timeout client 30s
frontend ft_redis bind 10.0.0.1:6379 name redis
default_backend bk_redis
backend bk_redis option tcp-check
tcp-check send PING\r\n
tcp-check expect string +PONG
tcp-check send info\ replication\r\n
tcp-check expect string role:master
tcp-check send QUIT\r\n
tcp-check expect string +OK
server R1 10.0.0.11:6379 check inter 1s
server R2 10.0.0.12:6379 check inter 1s
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The HAProxy health check sequence above allows to
consider the Redis master server as UP in the farm
and redirect connections to it.
When
the Redis master server fails, the remaining nodes
elect a new one. HAProxy will detect it thanks to its
health check sequence.
It does not require third party tools and make fail over transparent.