Portal:Cloud VPS/Admin/Rabbitmq

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Many OpenStack services communicate with one another via rabbitmq. For example:

- Nova services relay messages to nova-conductor via rabbitmq, and nova-conductor marshals database reads and writes - When an instance is created via nova-api, nova-api passes a rabbitmq message to nova-scheduler, who then schedules a VM (again via a rabbit message) on a nova-compute node - nova-scheduler assesses capacity of compute nodes via rabbitmq messages - designate-sink subscribes to rabbitmq notifications in order to detect and respond to VM creation/deletion - etc.

When VM creation is failing, very often the issue is with rabbitmq. Typically rabbit can be restarted with minimal harm, which will prompt all clients to reconnect:

root@cloudcontrol1003:~# service rabbitmq-server restart
root@cloudcontrol1004:~# service rabbitmq-server restart

HA setup

For redundancy we use a cluster of two rabbitmq servers in a primary/secondary relationship. Some documentation about how this is set up can be found [| here]. Most of the pieces of this are puppetized, but when standing up a new pair a couple of manual steps are needed.

On the secondary host (where the primary host is cloudcontrol1003):

 root@cloudcontrol1004:~# rabbitmqctl stop_app
 root@cloudcontrol1004:~# rabbitmqctl join_cluster rabbit@cloudcontrol1003
 root@cloudcontrol1004:~# rabbitmqctl start_app
 root@cloudcontrol1004:~# rabbitmqctl set_policy ha-all '^(?!amq\.).*' '{"ha-mode": "all"}'

Troubleshooting

Lists

 $ sudo rabbitmqctl list_exchanges
 $ sudo rabbitmqctl list_channels
 $ sudo rabbitmqctl list_connections
 $ sudo rabbitmqctl list_consumers
 $ sudo rabbitmqctl list_queues

Logs

 /var/log/rabbitmq/rabbit@<hostname>.log

Check local server health

 $ sudo rabbitmqctl status

Checking cluster health

If cluster_status hangs check for stuck processes.

 $ sudo rabbitmqctl cluster_status
 Cluster status of node rabbit@cloudcontrol1003 ...
 [{nodes,[{disc,[rabbit@cloudcontrol1003,rabbit@cloudcontrol1004]}]},
  {running_nodes,[rabbit@cloudcontrol1004,rabbit@cloudcontrol1003]},
  {cluster_name,<<"rabbit@cloudcontrol1003.wikimedia.org">>},
  {partitions,[]},
  {alarms,[{rabbit@cloudcontrol1004,[]},{rabbit@cloudcontrol1003,[]}]}]

Viewing stuck/suspicious processes

Note: Suspicious processes are not always a problem. However, if you find a large number of suspicious processes that are not decreasing this usually indicates a larger issue.

 $ sudo rabbitmqctl eval 'rabbit_diagnostics:maybe_stuck().'
 2019-07-23 21:34:54 There are 2247 processes.
 2019-07-23 21:34:54 Investigated 0 processes this round, 5000ms to go.
 ...
 2019-07-23 21:34:58 Investigated 0 processes this round, 500ms to go.
 2019-07-23 21:34:59 Found 0 suspicious processes.

Viewing unacknowledged messages

 $ sudo rabbitmqctl list_channels connection messages_unacknowledged

Recovering from split-brain partitioned nodes

When cluster members lose connectivity with each other they can become partitioned (split-brain). You can check for partitioned hosts with the following command:

$ sudo rabbitmqctl cluster_status
Cluster status of node rabbit@cloudcontrol1003 ...
[{nodes,[{disc,[rabbit@cloudcontrol1003,rabbit@cloudcontrol1004]}]},
 {running_nodes,[rabbit@cloudcontrol1003]},                          # this line should have both hosts listed
 {cluster_name,<<"rabbit@cloudcontrol1003.wikimedia.org">>},
 {partitions,[{rabbit@cloudcontrol1003,[rabbit@cloudcontrol1004]}]}, # this line should NOT have any hosts listed
 {alarms,[{rabbit@cloudcontrol1003,[]}]}]

When this happens you will typically see log messages like: File: /var/log/rabbitmq/rabbit@<hostname>.log

=ERROR REPORT==== 2-Dec-2019::00:08:08 ===
Channel error on connection <0.27123.2916> (208.80.154.23:52790 -> 208.80.154.23:5672, vhost: '/', user: 'nova'), channel 1:
operation basic.publish caused a channel exception not_found: "no exchange 'reply_f48d171794a340' in vhost '/'"

To recover the cluster you will need to restart rabbitmq

sudo systemctl restart rabbitmq-server