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Arseny Zinchenko
Arseny Zinchenko

Posted on • Originally published at rtfm.co.ua on

Redis: replication, part 3 - redis-py and work with Redis Sentinel from Python

#database #python #redis #replication

Still on the subject about Redis replication and Redis Sentinel – a couple of examples using the redis-py library for Python.

Previous series posts:

  1. Redis: replication, part 1 – an overview. Replication vs Sharding. Sentinel vs Cluster. Redis topology
  2. Redis: replication, part 2 – Master-Slave replication, and Redis Sentinel

All Redis clients for Python can be found here – redis.io/clients.

At first, will use redis-py without Sentinel, just to check how it’s working.

Then – will spin up Sentinel and will check masters and slaves discovery.

A working environment aka Redis replication and Redis Sentinel are already configured in the previous post – here is just Python examples.

redis-py and Redis

Install redis-py on Debian:

 root@redis-0:/home/admin# apt install python-redis
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Also, pip can be used but I’d prefer to use normal repositories.

Check it:

 root@redis-0:/home/admin# python Python 2.7.13 (default, Sep 26 2018, 18:42:22) [GCC 6.3.0 20170516] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import redis >>> r = redis.Redis(host='localhost', port=6379, db=0) >>> r.get('test') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python2.7/dist-packages/redis/client.py", line 880, in get return self.execute_command('GET', name) File "/usr/lib/python2.7/dist-packages/redis/client.py", line 573, in execute_command return self.parse_response(connection, command_name, **options) File "/usr/lib/python2.7/dist-packages/redis/client.py", line 585, in parse_response response = connection.read_response() File "/usr/lib/python2.7/dist-packages/redis/connection.py", line 582, in read_response raise response redis.exceptions.ResponseError: NOAUTH Authentication required.
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Okay, works.

Find authorization in documentation:

 >>> help(redis.Redis)
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Add the password:

 >>> r = redis.Redis(host='localhost', password='foobared', port=6379, db=0) >>> r.get('test') 'test'
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Okay.

redis-py and Redis Sentinel

Now run Sentinel instances if they are not started yet and let’s try Redis via Redis Sentinel cluster client.

Sentinel’s configs /etc/redis/sentinel.conf on hosts now are:

 sentinel monitor redis-test 52.58.69.184 6379 2 sentinel down-after-milliseconds redis-test 6001 sentinel failover-timeout redis-test 60000 bind 0.0.0.0 sentinel auth-pass redis-test foobared
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After Sentinel instance start – it will update config, so now on the redis-1 (which is the first slave here) config looks like:

 admin@redis-1:~$ cat /etc/redis/sentinel.conf sentinel myid e3e62e6577aa975f93346dad3d4f8e25833fd8f1 sentinel monitor redis-test 52.29.101.118 6379 2 sentinel down-after-milliseconds redis-test 6001 bind 0.0.0.0 sentinel failover-timeout redis-test 60000 # Generated by CONFIG REWRITE port 26379 dir "/home/admin" sentinel auth-pass redis-test foobared sentinel config-epoch redis-test 1 sentinel leader-epoch redis-test 1 sentinel known-slave redis-test 52.58.69.184 6379 sentinel known-slave redis-test 35.159.18.26 6379 sentinel known-sentinel redis-test 172.31.46.202 26379 e0fe655c59aa3cc32eab1c0858c52418700abe79 sentinel known-sentinel redis-test 172.31.41.39 26379 07a450af0d2f178410b78ee0f5ae99ce1cd0ac62 sentinel current-epoch 1
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Check Sentinel cluster status:

 root@redis-0:/home/admin# redis-cli -p 26379 info sentinel Sentinel sentinel_masters:1 sentinel_tilt:0 sentinel_running_scripts:0 sentinel_scripts_queue_length:0 sentinel_simulate_failure_flags:0 master0:name=redis-test,status=ok,address=52.58.69.184:6379,slaves=2,sentinels=3
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And use redis-py:

 >>> from redis.sentinel import Sentinel >>> sentinel = Sentinel([('localhost', 26379)], socket_timeout=0.1)
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You can get a master’s IP using the discover_master() method:

 >>> sentinel.discover_master('redis-test') ('52.58.69.184', 6379)
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And slaves:

 >>> sentinel.discover_slaves('redis-test') [('52.29.101.118', 6379), ('35.159.18.26', 6379)]
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To work with a master use the master_for() method:

| master_for(self, service_name, redis_class=, connection_pool_class=, **kwargs)

| Returns a redis client instance for the “service_name“ master.

 >>> master = sentinel.master_for('redis-test', socket_timeout=0.1)
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But if call master now – it will tell us we aren’t authorized:

 >>> master.set('test-key', 'test-value') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python2.7/dist-packages/redis/client.py", line 1072, in set return self.execute_command('SET', *pieces) File "/usr/lib/python2.7/dist-packages/redis/client.py", line 573, in execute_command return self.parse_response(connection, command_name, **options) File "/usr/lib/python2.7/dist-packages/redis/client.py", line 585, in parse_response response = connection.read_response() File "/usr/lib/python2.7/dist-packages/redis/sentinel.py", line 55, in read_response return super(SentinelManagedConnection, self).read_response() File "/usr/lib/python2.7/dist-packages/redis/connection.py", line 582, in read_response raise response redis.exceptions.ResponseError: NOAUTH Authentication required.
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So during creating the master object – add password:

 >>> master = sentinel.master_for('redis-test', socket_timeout=0.1, password='foobared')
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Check now:

 >>> master.set('test-key', 'test-value') True
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Also, you can get a whole master’s configuration:

 >>> master.config_get() {'appendonly': 'no', ... 'slave-announce-port': '0'}
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Or in a more readable view:

 >>> for i in master.config_get(): ... print(i) ... appendonly requirepass daemonize protected-mode zset-max-ziplist-entries zset-max-ziplist-value dir slave-serve-stale-data cluster-require-full-coverage slowlog-log-slower-than masterauth rdbchecksum ...
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On a salve – check the key we added previously:

 admin@redis-1:~$ redis-cli -a foobared get test-key "test-value"
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Work with slaves is similar to master:

 >>> slave = sentinel.slave_for('redis-test', socket_timeout=0.1, password='foobared') >>> slave.get('test-key') 'test-value'
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Master change and Sentinel failover

Let’s check out the master at this moment:

 >>> sentinel.discover_master('redis-test') ('52.58.69.184', 6379)
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And eventually let’s try to stop Redis master on the redis-0 host:

 root@redis-0:/home/admin# systemctl stop redis-server
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Or by:

 root@redis-0:/home/admin# redis-cli -a foobared DEBUG sleep 600
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(if you’ll just kill the redis-server process with the kill – Sentinel will restart it again).

Check Sentinel’s logs:

 10633:X 01 Apr 13:46:10.430 # +sdown master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.486 # +odown master redis-test 52.58.69.184 6379 #quorum 2/2 10633:X 01 Apr 13:46:10.486 # +new-epoch 1 10633:X 01 Apr 13:46:10.486 # +try-failover master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.488 # +vote-for-leader e3e62e6577aa975f93346dad3d4f8e25833fd8f1 1 10633:X 01 Apr 13:46:10.492 # 07a450af0d2f178410b78ee0f5ae99ce1cd0ac62 voted for e3e62e6577aa975f93346dad3d4f8e25833fd8f1 1 10633:X 01 Apr 13:46:10.492 # e0fe655c59aa3cc32eab1c0858c52418700abe79 voted for e3e62e6577aa975f93346dad3d4f8e25833fd8f1 1 10633:X 01 Apr 13:46:10.559 # +elected-leader master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.559 # +failover-state-select-slave master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.611 # +selected-slave slave 52.29.101.118:6379 52.29.101.118 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.612 * +failover-state-send-slaveof-noone slave 52.29.101.118:6379 52.29.101.118 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:10.664 * +failover-state-wait-promotion slave 52.29.101.118:6379 52.29.101.118 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:11.498 # +promoted-slave slave 52.29.101.118:6379 52.29.101.118 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:11.498 # +failover-state-reconf-slaves master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:11.557 * +slave-reconf-sent slave 35.159.18.26:6379 35.159.18.26 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:12.550 * +slave-reconf-inprog slave 35.159.18.26:6379 35.159.18.26 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:12.550 * +slave-reconf-done slave 35.159.18.26:6379 35.159.18.26 6379 @ redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:12.632 # -odown master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:12.632 # +failover-end master redis-test 52.58.69.184 6379 10633:X 01 Apr 13:46:12.632 # +switch-master redis-test 52.58.69.184 6379 52.29.101.118 6379 10633:X 01 Apr 13:46:12.632 * +slave slave 35.159.18.26:6379 35.159.18.26 6379 @ redis-test 52.29.101.118 6379 10633:X 01 Apr 13:46:12.632 * +slave slave 52.58.69.184:6379 52.58.69.184 6379 @ redis-test 52.29.101.118 6379 10633:X 01 Apr 13:46:18.647 # +sdown slave 52.58.69.184:6379 52.58.69.184 6379 @ redis-test 52.29.101.118 6379
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Check from Python:

 >>> sentinel.discover_master('redis-test') ('52.29.101.118', 6379)
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Master was changed – 52.29.101.118 – it’s the redis-1 host.

Check status from the redis-1:

 admin@redis-1:~$ redis-cli -a foobared info replication Replication role:master connected_slaves:1 slave0:ip=35.159.18.26,port=6379,state=online,offset=76847,lag=0 ...
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Check the Redis node status on the redis-0 host – it must become a slave now:

 root@redis-0:/home/admin# redis-cli -a foobared info replication Replication role:slave ...
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And try to add a new key from Python on the redis-0 host using the same master object we created before:

 >>> master.set('test-key2', 'test-value2') True
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Check:

 admin@redis-2:~$ redis-cli -a foobared get test-key2 "test-value2"
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Done.

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