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The flow-level concurrency property allows you to control the number of concurrent executions of a given flow by setting the limit key.

You can treat that concurrency as a global concurrency limit for that specific flow. The concurrency limit and behavior is then applied to all executions of that flow, regardless of whether those executions have been started automatically via a trigger, webhook, an API call or an ad-hoc run created from the UI.

For example, if you set the concurrency limit to 2, then only two executions of that flow will be allowed to run at the same time. If you try to trigger a third execution, it will be, by default, queued until one of the two running executions is completed.

It's also possible to CANCEL or FAIL an execution if the concurrency limit is reached. You can customize that behavior by setting the behavior, which is an Enum-type property with three possible values: QUEUE, CANCEL or FAIL. When concurrency is set to 2, and using the CANCEL or FAIL behavior, the third execution's state will be immediately set to CANCELLED or FAILED status respectively without running any task.

Here is an example flow that uses the concurrency property to limit the number of concurrent executions to 2. The bash task will sleep for 10 seconds, so you can trigger multiple executions of that flow and see how the concurrency property behaves.

id: concurrency_limited_flow
namespace: dev

  behavior: QUEUE # or CANCEL or FAIL
  limit: 2 # can be any integer >= 1

  - id: bash
    type: io.kestra.plugin.scripts.shell.Commands
      - sleep 10