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A thousand sign-ups in a minute start a thousand runs. If each one calls a partner’s API that accepts five requests at a time, most of them fail. With a concurrency limit, five runs go at once, the others wait their turn, and none is lost.

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Set a limit

Set concurrency when you register the workflow:
At most five runs of importContacts go at once. The limit is a whole number from 1 to 500.

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What counts

A run counts toward the limit from the moment it’s sent to your app until it pauses or ends:
  • A run on its way to your app, or running in it, counts.
  • A sleeping or waiting run doesn’t, since nothing runs while it’s paused. It counts again once it wakes up and goes back to your app.
  • A pending run doesn’t count yet. It’s the one waiting for its turn.
So a limit caps the work your app does at the same time, not the number of runs in progress.

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Who goes next

Runs over the limit stay pending, and take their turn in the order they were created: the oldest goes first. Nelkap hands out free turns every minute, so a run that was waiting starts within a minute after a turn frees up. A run woken from a sleep or a wait waits for a turn too. It keeps its place by creation date, so it goes ahead of runs created after it.

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Your workspace’s limit

Your workspace also has a limit, set by your plan, on the runs going at once across all its workflows. A workflow without concurrency only has that limit. A workflow with one gets whichever is lower. See Limits.

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Change a limit

The limit you register replaces the old one at once. Raising it lets more runs start on the next minute. Lowering it never stops a run already going: the runs over the new limit finish, and no new run starts until the count falls below it. To remove a limit, register the workflow without concurrency.

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One run at a time

With concurrency: 1, the runs of a workflow go one after the other, in the order they were created. That’s useful for work that must never overlap, like a nightly sync that shouldn’t start before the previous one ended:
The limit applies to the whole workflow, not to each customer. Two runs for different customers also wait for each other.

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See the queue

The Metrics page charts the runs waiting to start, the runs going at once, and how long runs wait before they start. A backlog that keeps growing means the limit is lower than the work coming in. See Metrics.