Copy Target

When you set the copy_target configuration field, instead of using the target of the run directly, mirrord will create a new pod using the pod spec of the original target, and use that new pod as a target.

This can be useful when you want to run your application with access to the resources and I/O of a target that isn’t reliable, for example because the target pod keeps crashing, or because it is managed by a Job and might terminate before you are done debugging your application with mirrord.

Health Checks #

The new, copied pod will not have any liveness, readiness or startup probes even if the original pod spec does define them. This means you can steal traffic without having to also answer those probes. This might come in handy when debugging with breakpoints with stolen traffic. Without copy_target, if you linger too long on a breakpoint, the application might miss some probes, which could cause a target pod to restart.

Replacing a Whole Deployment Using scale_down #

When the scale_down option is set (only valid when the target is a deployment), mirrord will scale the target deployment down to zero, effectively replacing all existing pods of that deployment by the one new copied pod, that is then used as the target for the mirrord run.

This can be useful e.g. when a deployment reads from a queue. By scaling it down to zero, the application you run with mirrord does not have to compete with the deployment for queue items.

Only one mirrord run can scale down a deployment at each time. If you try to scale down a deployment that is already being scaled down in another mirrord session (by you or by a teammate), mirrord will display an error and exit.

You can see active copied targets by running mirrord operator status. When there are no active copy targets, the relevant part of the output will say “No active copy targets”.

When there are active copy targets, the relevant section of the output will look like this:

Active Copy Targets:
| Original Target               | Namespace | Copy Pod Name                | Scale Down? |
| deployment/py-serv-deployment | default   | mirrord-copy-job-wd8kj-2gvd4 | *           |

With an asterisk marking copy targets that are also scaling down their original target.

Please note however that you don’t necessarily have to check if a target is already being scaled down, as trying to scale it down again will not interrupt the ongoing session, it will just result in your new run exiting with an error.