Troubleshooting Redis

There are a lot of moving parts that needs to be taken care carefully in order for the HA setup to work as expected.

Before proceeding with the troubleshooting below, check your firewall rules:

• Redis machines
  • Accept TCP connection in 6379
  • Connect to the other Redis machines via TCP in 6379
• Sentinel machines
  • Accept TCP connection in 26379
  • Connect to other Sentinel machines via TCP in 26379
  • Connect to the Redis machines via TCP in 6379

Troubleshooting Redis replication

You can check if everything is correct by connecting to each server using redis-cli application, and sending the info replication command as below.

/opt/gitlab/embedded/bin/redis-cli -h <redis-host-or-ip> -a '<redis-password>' info replication

When connected to a Primary Redis, you will see the number of connected replicas, and a list of each with connection details:

# Replication
role:master
connected_replicas:1
replica0:ip=10.133.5.21,port=6379,state=online,offset=208037514,lag=1
master_repl_offset:208037658
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:206989083
repl_backlog_histlen:1048576

When it's a replica, you will see details of the primary connection and if its up or down:

# Replication
role:replica
master_host:10.133.1.58
master_port:6379
master_link_status:up
master_last_io_seconds_ago:1
master_sync_in_progress:0
replica_repl_offset:208096498
replica_priority:100
replica_read_only:1
connected_replicas:0
master_repl_offset:0
repl_backlog_active:0
repl_backlog_size:1048576
repl_backlog_first_byte_offset:0
repl_backlog_histlen:0

Troubleshooting Sentinel

If you get an error like: Redis::CannotConnectError: No sentinels available., there may be something wrong with your configuration files or it can be related to this issue.

You must make sure you are defining the same value in redis['master_name'] and redis['master_pasword'] as you defined for your sentinel node.

The way the Redis connector redis-rb works with sentinel is a bit non-intuitive. We try to hide the complexity in omnibus, but it still requires a few extra configurations.

---

To make sure your configuration is correct:

1. SSH into your GitLab application server

2. Enter the Rails console:

   # For Omnibus installations
   sudo gitlab-rails console
   
   # For source installations
   sudo -u git rails console -e production

3. Run in the console:

   redis = Redis.new(Gitlab::Redis::SharedState.params)
   redis.info

   Keep this screen open and try to simulate a failover below.

4. To simulate a failover on primary Redis, SSH into the Redis server and run:

   # port must match your primary redis port, and the sleep time must be a few seconds bigger than defined one
    redis-cli -h localhost -p 6379 DEBUG sleep 20

5. Then back in the Rails console from the first step, run:

   redis.info

   You should see a different port after a few seconds delay (the failover/reconnect time).

Troubleshooting a non-bundled Redis with an installation from source

If you get an error in GitLab like Redis::CannotConnectError: No sentinels available., there may be something wrong with your configuration files or it can be related to this upstream issue.

You must make sure that resque.yml and sentinel.conf are configured correctly, otherwise redis-rb will not work properly.

The master-group-name (gitlab-redis) defined in (sentinel.conf) must be used as the hostname in GitLab (resque.yml):

# sentinel.conf:
sentinel monitor gitlab-redis 10.0.0.1 6379 2
sentinel down-after-milliseconds gitlab-redis 10000
sentinel config-epoch gitlab-redis 0
sentinel leader-epoch gitlab-redis 0

# resque.yaml
production:
  url: redis://:myredispassword@gitlab-redis/
  sentinels:
    -
      host: 10.0.0.1
      port: 26379 # point to sentinel, not to redis port
    -
      host: 10.0.0.2
      port: 26379 # point to sentinel, not to redis port
    -
      host: 10.0.0.3
      port: 26379 # point to sentinel, not to redis port

When in doubt, read the Redis Sentinel documentation.