> For the complete documentation index, see [llms.txt](https://doc.wearepatchworks.com/product-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://doc.wearepatchworks.com/product-documentation/process-flows/managing-process-flows/flow-level-concurrency.md). # Flow Level Concurrency Limiting concurrent runs for a process flow ### Introduction Every company profile has a `concurrent process flows` allowance - the maximum number of process flows that can run at the same time across your whole account. This allowance is determined by your [Patchworks subscription](https://doc.wearepatchworks.com/product-documentation/getting-started/subscription-tier-limits) and is shared by all of your process flows. By default, there's no cap on how much of that allowance a single process flow can use. If several run jobs for the same flow are waiting in [your queue](https://doc.wearepatchworks.com/product-documentation/process-flows/error-reporting-and-exception-handling/run-logs-and-queue/working-with-your-run-queue), they're picked and run in parallel whenever slots are available. The `maximum concurrent runs` setting lets you set a lower limit for an individual process flow, within your account allowance. Once the number of running instances of that flow reaches the limit, any further run jobs wait in your queue until one finishes. {% hint style="info" %} This setting is optional. Process flows with no `maximum concurrent runs` value behave as they always have - they can use as much of your `concurrent process flows` allowance as the queue gives them. {% endhint %} ### When to use this setting | Scenario | Without a limit | With a limit | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- | | **Scheduled flows and backlogs.** A flow runs on a [schedule](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-shapes/standard-shapes/trigger-shape/trigger-shape-schedule) (e.g. every 10 minutes) and normally completes well within that interval. | If a backlog builds up in your queue, new scheduled run jobs keep being added. When the backlog clears, several instances of the same flow can start at the same time. | Set `maximum concurrent runs` to `1` so runs of this flow are always processed one at a time, in the order they were queued. | | **Parent/child fan-out.** A parent flow uses the [run process flow shape](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-shapes/standard-shapes/run-process-flow-shape) to start a child flow for each record - for example, one per order. | Hundreds or thousands of child run jobs can be queued at once, and as many as your account allowance permits will run in parallel - often causing rate-limiting or throttling errors in downstream systems. | Set `maximum concurrent runs` on the **child** flow to control how many instances run at once - for example, `5`. | | **Flows that shouldn't overlap.** A flow depends on the previous run having finished - for example, it works with a shared file, a cache, or data written by the previous run. | Overlapping runs can process the same records twice or interfere with each other. | Set `maximum concurrent runs` to `1`. | ### How it works When a run job for a process flow is due to be picked from your queue, Patchworks checks how many instances of that flow are currently running: * If the number is **below** the flow's `maximum concurrent runs` limit, the run job is picked and started as normal. * If the number is **at** the limit, the run job stays in your queue. It's picked as soon as one of the running instances completes, subject to the usual [queue order and priority](https://doc.wearepatchworks.com/product-documentation/process-flows/error-reporting-and-exception-handling/run-logs-and-queue/working-with-your-run-queue#how-it-works). Points to note: * This is a per-flow limit **within** your account allowance. A process flow can never run more instances than your `concurrent process flows` allowance permits, whatever value is entered here. * Run jobs held by this limit are waiting in your queue, not running - so they don't use a `concurrent process flows` slot, and other flows aren't blocked while they wait. * Held run jobs still count towards your `scheduled process flows` allowance. If a flow is triggered much more often than it can complete, its queued run jobs will keep building up - in this case, consider lengthening the schedule interval, or [remove queued runs for the flow](https://doc.wearepatchworks.com/product-documentation/process-flows/error-reporting-and-exception-handling/run-logs-and-queue/working-with-your-run-queue#removing-all-run-jobs-for-a-selected-process-flow) from the queue tab. * Held run jobs may wait in your queue for longer than usual. If the flow uses relative dates, check the `use queued time` option in [process flow settings](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-settings) to make sure the right base time is used. ### Setting a maximum concurrent runs limit {% stepper %} {% step %} **Access process flow settings** Open the required process flow and click the `settings` (cog) icon in the canvas actions bar. {% endstep %} {% step %} **Expand advanced settings** Click `advanced settings` to expand this section. {% endstep %} {% step %} **Enter a limit** In the `maximum concurrent runs` field, enter the maximum number of instances of this flow that can run at the same time - for example:
The value must be a whole number of `1` or more. To remove an existing limit, clear the field. {% endstep %} {% step %} **Save your changes** Click the `save` button. {% endstep %} {% endstepper %} {% hint style="info" %} Changing the limit doesn't affect instances that are already running. The new value is applied the next time a run job for this flow is picked from your queue. {% endhint %} ### Maximum concurrent runs vs connection pools If you've previously used [connection pools](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/connection-pools) to protect a downstream system from too many simultaneous requests, it's worth understanding how the two options differ: | | Maximum concurrent runs | Connection pools | | --------------------------------------------------------- | -------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------- | | **What is limited** | The number of instances of **one process flow** that can run at the same time | The number of **simultaneous requests** made to a connector instance, across every flow that uses it | | **Where the wait happens** | In your queue, before the run starts | Inside a running flow, at the connector step | | **Uses a `concurrent process flows` slot while waiting?** | No | Yes - the flow is already running, so it holds a slot until a pool slot is released | | **Best for** | Stopping the same flow stacking up in parallel - scheduled backlogs and parent/child fan-out | Enforcing a system's concurrency limit across several different flows that share a connector instance | The two can be used together: set `maximum concurrent runs` to keep the number of running instances of a flow sensible, and use a connection pool to enforce the exact concurrency limit of the target system. ### Related information * [Process flow settings](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-settings) * [Working with your run queue](https://doc.wearepatchworks.com/product-documentation/process-flows/error-reporting-and-exception-handling/run-logs-and-queue/working-with-your-run-queue) * [Run process flow shape](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-shapes/standard-shapes/run-process-flow-shape) * [Connection pools](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/connection-pools) * [Subscription tier limits](https://doc.wearepatchworks.com/product-documentation/getting-started/subscription-tier-limits)