HA (active/active) considerations

CloudBees CI High Availability provides tools to troubleshoot possible problems on controller replicas running in HA mode. To enable them, navigate to Manage Jenkins  CloudBees CI High Availability.

When a controller running in HA mode starts for the first time, one of the controller replicas acquires a lock in the shared JENKINS_HOME. This replica is the only one available, and the lock remains until the Setup wizard is ended by a user.

When the Setup wizard ends, the remaining replicas continue the startup process. During this process the remaining replicas, one by one and automatically, acquire the lock, start, and release the lock until all of them are available.

However, if the controller is created using a CasC bundle, the Setup wizard is not displayed and all the replicas automatically follow the same process described above without any human confirmation. One by one, they acquire the lock, start, and release the lock until all of them are up and running.

CloudBees CI supports a range of agent connection modes in High Availability, but each agent must have only one executor. As an agent can connect to only a single replica at a time, agents with multiple executors cannot be properly shared and are not supported by CloudBees CI High Availability (HA).

A single executor requirement is not a problem for cloud agents as they normally define only one executor. They are provisioned, use this executor, and are deprovisioned later.

Permanent or static agents, using additional JVMs, can be configured as multiple-executor agents for HA.

In the permanent agent configuration screen:

The controller clones the original agent to create the executors. These read-only clones are managed by the controller itself, which adds or removes clones and reconfigures them in response to updates to the original permanent agent.

All the permanent agent clones have one executor and use different folders within the executors folder inside the original permanent agent root directory. Outbound agents automatically have the workDir settings for all the clones.

CloudBees CI displays the clones as individual one-executor agents in the Build executor status.

To use Groovy libraries, CloudBees recommends that you set them to a new “clone” mode and configure Git to use a shallow clone.

For concurrent access, if you update library checkouts in a common directory (such as $JENKINS_HOME/workspace/) or use the caching system, it can cause problems. An administrative monitor guides you to make these changes.

Problems can arise in High Availability mode with non-Pipeline project types, such as Freestyle, Matrix, or Maven. When these project types are run, other replicas can load completed build metadata, but cannot take over the build successfully. As a result, if a replica terminates, any builds running on it are immediately aborted.

The benefit of having multiple replicas should always be balanced against the associated cost according to your business case. Scaling horizontally with many replicas will have diminishing returns as the number of replicas increases.

Plugins can be managed and installed from the Manage Jenkins  Plugins screen. When using HA with multiple replicas, dynamic loading of plugins (plugin installation without restarting CloudBees CI) is not supported. Therefore, you must restart each replica of the controller to install or upgrade plugins.

In a CloudBees CI on modern cloud platforms with a managed controller running in HA mode, when selecting Restart Jenkins when installation is complete and no jobs are running, a rolling restart is performed, and when completed, new plugin versions are available in all replicas.

In a CloudBees CI on traditional platforms running in HA mode with multiple replicas, you must restart all controller replicas either manually or using your own automation.

When running a controller in HA mode, requests to API pull-based endpoints may return information about the controller replica that responds to the API request instead of aggregated information about all the controller replicas part of the HA cluster.

Examples of these endpoints are:

For example, when using those plugins, if you make an HTTP API query for JVM heap usage, the returned value would only correspond to the replica that processed the request and not provide insight into other replicas. However, other information, like the number of projects, is accurate because it is automatically synchronized among all the controller replicas.

In general, responses are accurate and display aggregated replica information for:

However, with limited exceptions endpoints display information only about the replica responding to the requests for:

CloudBees CI overrides the following Jenkins core endpoints to provide aggregated information about running builds and agents:

You can also use and configure third-party monitoring solutions like Prometheus using the CloudBees Prometheus Metrics Plugin, to provide aggregated information from all the controller replicas.

The Pipeline: Stage View and Blue Ocean plugins may not accurately display running builds that are owned by other replicas.