7. Clustering¶

7.1. Setup Considerations¶

This section covers initial setup of clusters only. For upgrading an existing cluster, refer to Upgrade Planning of the Ansible Tower Upgrade and Migration Guide.

Important considerations to note in the new clustering environment:

• PostgreSQL is still a standalone instance and is not clustered. Tower does not manage replica configuration or database failover (if the user configures standby replicas).

• When spinning up a cluster, the database node should be a standalone server, and PostgreSQL should not be installed on one of the Tower nodes.

• It is strongly recommended that a minimum of three Tower instances be in a cluster and the supported maximum is 20.

• All instances should be reachable from all other instances and they should be able to reach the database. It is also important for the hosts to have a stable address and/or hostname (depending on how the Tower host is configured).

• All instances must be geographically collocated, with reliable low-latency connections between instances.

• For purposes of upgrading to a clustered environment, your primary instance must be part of the tower group in the inventory AND it needs to be the first host listed in the tower group.

• Manual projects must be manually synced to all instances by the customer, and updated on all instances at once.

• There is no concept of primary/secondary in the new Tower system. All systems are primary.

• The inventory file for Tower deployments should be saved/persisted. If new instances are to be provisioned, the passwords and configuration options, as well as host names, must be made available to the installer.

7.2. Install and Configure¶

Provisioning new instances involves updating the inventory file and re-running the setup playbook. It is important that the inventory file contains all passwords and information used when installing the cluster or other instances may be reconfigured. The current standalone instance configuration does not change for a 3.1 or later deployment. The inventory file does change in some important ways:

• Since there is no primary/secondary configuration, those inventory groups go away and are replaced with a single inventory group, tower.

[tower]
hostA
hostB
hostC

[instance_group_east]
hostB
hostC

[instance_group_west]
hostC
hostD

The database group remains for specifying an external PostgreSQL. If the database host is provisioned separately, this group should be empty:

[tower]
hostA
hostB
hostC

[database]
hostDB

When a playbook runs on an individual Tower instance in a cluster, the output of that playbook is broadcast to all of the other nodes as part of Tower’s websocket-based streaming output functionality. It is best to handle this data broadcast using internal addressing by specifying a private routable address for each node in your inventory:

[tower]
hostA routable_hostname=10.1.0.2
hostB routable_hostname=10.1.0.3
hostC routable_hostname=10.1.0.4

HTTP PATCH /api/v2/settings/jobs/ {'AWX_ISOLATED_PRIVATE_KEY': '', 'AWX_ISOLATED_PUBLIC_KEY': ''}

7.3. Status and Monitoring via Browser API¶

Tower itself reports as much status as it can via the Browsable API at /api/v2/ping in order to provide validation of the health of the cluster, including:

• The instance servicing the HTTP request

• The timestamps of the last heartbeat of all other instances in the cluster

• Instance Groups and Instance membership in those groups

View more details about Instances and Instance Groups, including running jobs and membership information at /api/v2/instances/ and /api/v2/instance_groups/.

7.4. Instance Services and Failure Behavior¶

Each Tower instance is made up of several different services working collaboratively:

• HTTP Services - This includes the Tower application itself as well as external web services.

• Callback Receiver - Receives job events from running Ansible jobs.

• Dispatcher - The worker queue that processes and runs all jobs.

• Redis - This key value store is used as a queue for event data propagated from ansible-playbook to the application.

• Memcached - local caching service for the instance it lives on.

• Rsyslog - log processing service used to deliver logs to various external logging services.

Tower is configured in such a way that if any of these services or their components fail, then all services are restarted. If these fail sufficiently often in a short span of time, then the entire instance will be placed offline in an automated fashion in order to allow remediation without causing unexpected behavior.

For backing up and restoring a clustered environment, refer to Backup and Restore for Clustered Environments section.

7.5. Job Runtime Behavior¶

The way jobs are run and reported to a ‘normal’ user of Tower does not change. On the system side, some differences are worth noting:

• When a job is submitted from the API interface it gets pushed into the dispatcher queue. Each Tower instance will connect to and receive jobs from that queue using a particular scheduling algorithm. Any instance in the cluster is just as likely to receive the work and execute the task. If a instance fails while executing jobs, then the work is marked as permanently failed.

• Project updates run successfully on any instance that could potentially run a job. Projects will sync themselves to the correct version on the instance immediately prior to running the job. If the needed revision is already locally checked out and Galaxy or Collections updates are not needed, then a sync may not be performed.

• When the sync happens, it is recorded in the database as a project update with a launch_type = sync and job_type =  run. Project syncs will not change the status or version of the project; instead, they will update the source tree only on the instance where they run.

• If updates are needed from Galaxy or Collections, a sync is performed that downloads the required roles, consuming that much more space in your /tmp file. In cases where you have a big project (around 10 GB), disk space on /tmp may be an issue.

7.6. Deprovision Instances¶

Re-running the setup playbook does not automatically deprovision instances since clusters do not currently distinguish between an instance that was taken offline intentionally or due to failure. Instead, shut down all services on the Tower instance and then run the deprovisioning tool from any other instance:

1. Shut down the instance or stop the service with the command, ansible-tower-service stop.

2. Run the deprovision command $ awx-manage deprovision_instance --hostname=<name used in inventory file> from another instance to remove it from the Tower cluster.

   Example: awx-manage deprovision_instance --hostname=hostB

Similarly, deprovisioning instance groups in Tower does not automatically deprovision or remove instance groups. For more information, refer to the Deprovision Instance Groups section.