Clusters

A cluster is elastic capacity: instead of fixed machines that sit there between tests, units are created when a test starts and destroyed when it ends.

You pay for what a test uses, and you're not limited by how many machines you happened to set up in advance.

When to use one

Use a cluster when your load varies a lot between tests, or when you want a large run occasionally without keeping that capacity around. Use fixed servers when you want predictable, always-available machines — or when your test needs to come from a specific network location.

Setting one up

A cluster runs on a Docker host you provide. Atlas talks to it through a small controller you install on that host, which connects outbound — so the host needs no inbound access, exactly like a load generator.

Go to Fleet → Clusters, create a cluster, and run the install command it gives you on your Docker host. When the controller connects, the cluster shows as ready.

Running on a cluster

On the Run Test tab choose Cluster instead of Servers, and say how many units you want:

atlas run -p SHOP --cluster my-cluster --units 10

Each unit is one JMeter instance, equivalent to one load generator. Ten units running a 100-thread plan is 1,000 concurrent users.

While it runs

Units register themselves as they come up, then the test deploys and starts as usual. When the test finishes, every unit is destroyed.

If a run ends unexpectedly, check the Clusters tab to confirm nothing was left behind.

TIP

Clusters are newer than fixed servers. If a run needs to be reliable — a scheduled regression test, a customer demo — fixed machines are the safer choice today.