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AWX Clustering/HA Overview

Prior to 3.1, the Ansible Tower HA solution was not a true high-availability system. This system has been entirely rewritten in 3.1 with a focus towards a proper highly-available clustered system. This has been extended further in 3.2 to allow grouping of clustered instances into different pools/queues.

  • Each instance should be able to act as an entry point for UI and API Access. This should enable AWX administrators to use load balancers in front of as many instances as they wish and maintain good data visibility.

  • Each instance should be able to join the AWX cluster and expand its ability to execute jobs.

  • Provisioning new instance should be as simple as updating the inventory file and re-running the setup playbook.

  • Instances can be de-provisioned with a simple management command.

  • Instances can be grouped into one or more Instance Groups to share resources for topical purposes.

  • These instance groups should be assignable to certain resources:

    • Organizations
    • Inventories
    • Job Templates

    ...such that execution of jobs under those resources will favor particular queues.

It's important to point out a few existing things:

  • PostgreSQL is still a standalone instance and is not clustered. Replica configuration will not be managed. If the user configures standby replicas, database failover will also not be managed.
  • All instances should be reachable from all other instances and they should be able to reach the database. It's also important for the hosts to have a stable address and/or hostname (depending on how you configure the AWX host).
  • Existing old-style HA deployments will be transitioned automatically to the new HA system during the upgrade process to 3.1.
  • Manual projects will need to be synced to all instances by the customer.

Ansible Tower 3.3 adds support for container-based clusters using Openshift or Kubernetes.

Important Changes

  • There is no concept of primary/secondary in the new AWX system. All systems are primary.
  • The inventory file for AWX deployments should be saved/persisted. If new instances are to be provisioned, the passwords and configuration options as well as host names will need to be available to the installer.

Concepts and Configuration

Installation and the Inventory File

The current standalone instance configuration doesn't change for a 3.1+ 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. The customer may optionally define other groups and group instances in those groups. These groups should be prefixed with instance_group_. One instance must be present in the tower group. Technically tower is a group like any other instance_group_ group, but it must always be present and if a specific group is not associated with a specific resource, then job execution will always fall back to the tower group:
[tower]
hostA
hostB
hostC

[instance_group_east]
hostB
hostC

[instance_group_west]
hostC
hostD

The database group remains in order to specify an external Postgres. If the database host is provisioned separately, this group should be empty.

[tower]
hostA
hostB
hostC

[database]
hostDB

Recommendations and constraints:

  • Do not create a group named instance_group_tower.
  • Do not name any instance the same as a group name.

Provisioning and Deprovisioning Instances and Groups

  • Provisioning - Provisioning instances after installation is supported by updating the inventory file and re-running the setup playbook. It's important that this file contains all passwords and related information used when installing the cluster; if this is not the case, other instances may be reconfigured (this can be done intentionally).

  • Deprovisioning - AWX does not automatically deprovision instances since it cannot distinguish between an instance that was taken offline intentionally or due to failure.

    Starting with AWX version 19.3.0, deprovisioning an instance results in one or more Receptor configurations needing to be updated across one or more nodes, which therefore cannot be done via a manual process; the Automation Mesh Installer needs to deprovision the nodes.

    Adding to and removing from the mesh does not require that every node is listed in the inventory file; in other words, the absence of a node from the inventory file does not indicate that a node should be removed. Instead, a hostvar of node_state: deprovision conveys to the mesh installer that the node should be deprovisioned.

  • Removing/Deprovisioning Instance Groups - AWX does not automatically de-provision or remove instance groups, even though re-provisioning will often cause these to be unused. They may still show up in API endpoints and stats monitoring. These groups can be removed with the following command:

$ awx-manage unregister_queue --queuename=<name>

Configuring Instances and Instance Groups from the API

Instance Groups can be created by posting to /api/v2/instance_groups as a System Administrator.

Once created, Instances can be associated with an Instance Group with:

HTTP POST /api/v2/instance_groups/x/instances/ {'id': y}`

An Instance that is added to an InstanceGroup will automatically reconfigure itself to listen on the group's work queue. See the following section Instance Group Policies for more details.

Instance Group Policies

AWX Instances can be configured to automatically join Instance Groups when they come online by defining a policy. These policies are evaluated for every new Instance that comes online.

Instance Group Policies are controlled by three optional fields on an Instance Group:

  • policy_instance_percentage: This is a number between 0 - 100. It guarantees that this percentage of active AWX instances will be added to this Instance Group. As new instances come online, if the number of Instances in this group relative to the total number of instances is fewer than the given percentage, then new ones will be added until the percentage condition is satisfied.
  • policy_instance_minimum: This policy attempts to keep at least this many Instances in the Instance Group. If the number of available instances is lower than this minimum, then all Instances will be placed in this Instance Group.
  • policy_instance_list: This is a fixed list of Instance names to always include in this Instance Group.

NOTES

  • Instances that are assigned directly to Instance Groups by posting to /api/v2/instance_groups/x/instances or /api/v2/instances/x/instance_groups are automatically added to the policy_instance_list. This means they are subject to the normal caveats for policy_instance_list and must be manually managed.

  • policy_instance_percentage and policy_instance_minimum work together. For example, if you have a policy_instance_percentage of 50% and a policy_instance_minimum of 2 and you start 6 Instances, 3 of them would be assigned to the Instance Group. If you reduce the number of Instances to 2, then both of them would be assigned to the Instance Group to satisfy policy_instance_minimum. In this way, you can set a lower bound on the amount of available resources.

  • Policies don't actively prevent Instances from being associated with multiple Instance Groups but this can effectively be achieved by making the percentages sum to 100. If you have 4 Instance Groups, assign each a percentage value of 25 and the Instances will be distributed among them with no overlap.

Manually Pinning Instances to Specific Groups

If you have a special Instance which needs to be exclusively assigned to a specific Instance Group but don't want it to automatically join other groups via "percentage" or "minimum" policies:

  1. Add the Instance to one or more Instance Groups' policy_instance_list.
  2. Update the Instance's managed_by_policy property to be False.

This will prevent the Instance from being automatically added to other groups based on percentage and minimum policy; it will only belong to the groups you've manually assigned it to:

HTTP PATCH /api/v2/instance_groups/N/
{
    "policy_instance_list": ["special-instance"]
}

HTTP PATCH /api/v2/instances/X/
{
    "managed_by_policy": False
}

Status and Monitoring

AWX itself reports as much status as it can via the API at /api/v2/ping in order to provide validation of the health of the Cluster. This includes:

  • The instance servicing the HTTP request.
  • The last heartbeat time of all other instances in the cluster.
  • Instance Groups and Instance membership in those groups.

A more detailed view of Instances and Instance Groups, including running jobs and membership information can be seen at /api/v2/instances/ and /api/v2/instance_groups.

Instance Services and Failure Behavior

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

  • HTTP Services - This includes the AWX application itself as well as external web services.
  • Callback Receiver - Receives job events that result from running Ansible jobs.
  • Celery - The worker queue that processes and runs all jobs.
  • Redis - this is used as a queue for AWX to process ansible playbook callback events.

AWX 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.

Job Runtime Behavior

Ideally a regular user of AWX should not notice any semantic difference to the way jobs are run and reported. Behind the scenes it is worth pointing out the differences in how the system behaves.

When a job is submitted from the API interface, it gets pushed into the dispatcher queue via postgres notify/listen (https://www.postgresql.org/docs/10/sql-notify.html), and the task is handled by the dispatcher process running on that specific AWX node. If an instance fails while executing jobs, then the work is marked as permanently failed.

If a cluster is divided into separate Instance Groups, then the behavior is similar to the cluster as a whole. If two instances are assigned to a group then either one is just as likely to receive a job as any other in the same group.

As AWX instances are brought online, it effectively expands the work capacity of the AWX system. If those instances are also placed into Instance Groups, then they also expand that group's capacity. If an instance is performing work and it is a member of multiple groups, then capacity will be reduced from all groups for which it is a member. De-provisioning an instance will remove capacity from the cluster wherever that instance was assigned.

It's important to note that not all instances are required to be provisioned with an equal capacity.

If an Instance Group is configured but all instances in that group are offline or unavailable, any jobs that are launched targeting only that group will be stuck in a waiting state until instances become available. Fallback or backup resources should be provisioned to handle any work that might encounter this scenario.

Project Synchronization Behavior

It is important that project updates run on the instance which prepares the ansible-runner private data directory. This is accomplished by a project sync which is done by the dispatcher control / launch process. The sync will update the source tree to the correct version on the instance immediately prior to transmitting 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 of "sync" and a job_type of "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. The only exception to this behavior is when the project is in the "never updated" state (meaning that no project updates of any type have been run), in which case a sync should fill in the project's initial revision and status, and subsequent syncs should not make such changes.

All project updates run with container isolation (like jobs) and volume mount to the persistent projects folder.

Controlling Where a Particular Job Runs

By default, a job will be submitted to the default queue (formerly the tower queue). To see the name of the queue, view the setting DEFAULT_EXECUTION_QUEUE_NAME. Administrative actions, like project updates, will run in the control plane queue. The name of the control plane queue is surfaced in the setting DEFAULT_CONTROL_PLANE_QUEUE_NAME.

How to Restrict the Instances a Job Will Run On

If the Job Template, Inventory, or Organization have instance groups associated with them, a job run from that Job Template will not be eligible for the default behavior. This means that if all of the instance associated with these three resources are out of capacity, the job will remain in the pending state until capacity frees up.

How to Set Up a Preferred Instance Group

The order of preference in determining which instance group the job gets submitted to is as follows:

  1. Job Template
  2. Inventory
  3. Organization (by way of Inventory)

To expand further: If instance groups are associated with the Job Template and all of them are at capacity, then the job will be submitted to instance groups specified on Inventory, and then Organization.

The global tower group can still be associated with a resource, just like any of the custom instance groups defined in the playbook. This can be used to specify a preferred instance group on the job template or inventory, but still allow the job to be submitted to any instance if those are out of capacity.

Instance Enable / Disable

In order to support temporarily taking an Instance offline, there is a boolean property enabled defined on each instance.

When this property is disabled, no jobs will be assigned to that Instance. Existing jobs will finish but no new work will be assigned.

Acceptance Criteria

When verifying acceptance, we should ensure that the following statements are true:

  • AWX should install as a standalone Instance
  • AWX should install in a Clustered fashion
  • Instances should, optionally, be able to be grouped arbitrarily into different Instance Groups
  • Capacity should be tracked at the group level and capacity impact should make sense relative to what instance a job is running on and what groups that instance is a member of
  • Provisioning should be supported via the setup playbook
  • De-provisioning should be supported via a management command
  • All jobs, inventory updates, and project updates should run successfully
  • Jobs should be able to run on hosts for which they are targeted; if assigned implicitly or directly to groups, then they should only run on instances in those Instance Groups
  • Project updates should manifest their data on the host that will run the job immediately prior to the job running
  • AWX should be able to reasonably survive the removal of all instances in the cluster
  • AWX should behave in a predictable fashion during network partitioning

Testing Considerations

  • Basic testing should be able to demonstrate parity with a standalone instance for all integration testing.
  • Basic playbook testing to verify routing differences, including:
    • Basic FQDN
    • Short-name name resolution
    • IP addresses
    • /etc/hosts static routing information
  • We should test behavior of large and small clusters; small clusters usually consist of 2 - 3 instances and large clusters have 10 - 15 instances.
  • Failure testing should involve killing single instances and killing multiple instances while the cluster is performing work. Job failures during the time period should be predictable and not catastrophic.
  • Instance downtime testing should also include recoverability testing (killing single services and ensuring the system can return itself to a working state).
  • Persistent failure should be tested by killing single services in such a way that the cluster instance cannot be recovered and ensuring that the instance is properly taken offline.
  • Network partitioning failures will also be important. In order to test this:
    • Disallow a single instance from communicating with the other instances but allow it to communicate with the database
    • Break the link between instances such that it forms two or more groups where Group A and Group B can't communicate but all instances can communicate with the database.
  • Crucially, when network partitioning is resolved, all instances should recover into a consistent state.
  • Upgrade Testing - verify behavior before and after are the same for the end user.
  • Project Updates should be thoroughly tested for all SCM types (git, svn, archive) and for manual projects.
  • Setting up instance groups in two scenarios: a) instances are shared between groups b) instances are isolated to particular groups Organizations, Inventories, and Job Templates should be variously assigned to one or many groups and jobs should execute in those groups in preferential order as resources are available.

Performance Testing

Performance testing should be twofold:

  • A large volume of simultaneous jobs
  • Jobs that generate a large amount of output

These should also be benchmarked against the same playbooks using the 3.0.X Tower release and a stable Ansible version. For a large volume playbook (e.g., against 100+ hosts), something like the following is recommended:

https://gist.github.com/michelleperz/fe3a0eb4eda888221229730e34b28b89