When you need a trigger beyond what the templates in the UI trigger builder provide, you can define a custom trigger in JSON. With custom triggers, you have access to the full capabilities of Prefect’s automation system—allowing you to react to many kinds of events and metrics in your workspace.

Each automation has a single trigger that, when fired, causes all of its associated actions to run. That single trigger may be a reactive or proactive event trigger, a trigger monitoring the value of a metric, or a composite trigger that combines several underlying triggers.

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Event triggers

Event triggers are the most common type of trigger. They are intended to react to the presence or absence of an event. Event triggers are indicated with {"type": "event"}.

This is the schema that defines an event trigger:

NameTypeSupports trailing wildcardsDescription
matchobject:material-check:Labels for resources which this Automation will match.
match_relatedobject:material-check:Labels for related resources which this Automation will match.
posturestring enumN/AThe posture of this Automation, either Reactive or Proactive. Reactive automations respond to the presence of the expected events, while Proactive automations respond to the absence of those expected events.
afterarray of strings:material-check:Event(s), one of which must have first been seen to start this automation.
expectarray of strings:material-check:The event(s) this automation expects to see. If empty, this automation will evaluate any matched event.
for_eacharray of strings:material-close:Evaluate the Automation separately for each distinct value of these labels on the resource. By default, labels refer to the primary resource of the triggering event. You may also refer to labels from related resources by specifying related:<role>:<label>. This will use the value of that label for the first related resource in that role.
thresholdintegerN/AThe number of events required for this Automation to trigger (for Reactive automations), or the number of events expected (for Proactive automations)
withinnumberN/AThe time period over which the events must occur. For Reactive triggers, this may be as low as 0 seconds, but must be at least 10 seconds for Proactive triggers.

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Resource matching

Both the event and metric triggers support matching events for specific resources in your workspace, including most Prefect objects (like flows, deployment, blocks, work pools, tags) as well as resources you have defined in any events you emit yourself.

The match and match_related fields control which events a trigger considers for evaluation by filtering on the contents of their resource and related fields, respectively. Each label added to a match filter is ANDed with the other labels, and can accept a single value or a list of multiple values that are ORed together.

See the resource and related fields on the following prefect.flow-run.Completed event, truncated for this example. Its primary resource is a flow run, and since that flow run was started by a deployment, it is related to both its flow and its deployment:

"resource": {
  "prefect.resource.id": "prefect.flow-run.925eacce-7fe5-4753-8f02-77f1511543db",
  "prefect.resource.name": "cute-kittiwake"
}
"related": [
  {
    "prefect.resource.id": "prefect.flow.cb6126db-d528-402f-b439-96637187a8ca",
    "prefect.resource.role": "flow",
    "prefect.resource.name": "hello"
  },
  {
    "prefect.resource.id": "prefect.deployment.37ca4a08-e2d9-4628-a310-cc15a323378e",
    "prefect.resource.role": "deployment",
    "prefect.resource.name": "example"
  }
]

There are a number of valid ways to select the above event for evaluation, and the approach depends on the purpose of the automation.

The following configuration filters for any events whose primary resource is a flow run, and that flow run has a name starting with cute- or radical-.

"match": {
  "prefect.resource.id": "prefect.flow-run.*",
  "prefect.resource.name": ["cute-*", "radical-*"]
},
"match_related": {},
...

By comparison, this configuration filters for any events this specific deployment is a related resource to:

"match": {},
"match_related": {
  "prefect.resource.id": "prefect.deployment.37ca4a08-e2d9-4628-a310-cc15a323378e"
},
...

Both of the above approaches will select the example prefect.flow-run.Completed event, but will permit additional, possibly undesired events through the filter as well. You can combine match and match_related for more restrictive filtering:

"match": {
  "prefect.resource.id": "prefect.flow-run.*",
  "prefect.resource.name": ["cute-*", "radical-*"]
},
"match_related": {
  "prefect.resource.id": "prefect.deployment.37ca4a08-e2d9-4628-a310-cc15a323378e"
},
...

Now this trigger will filter only for events whose primary resource is a flow run started by a specific deployment, and that flow run has a name starting with cute- or radical-.

​
Expected events

Once an event has passed through the match filters, you must decide if this event counts toward the trigger’s threshold. That is determined by the event names present in expect.

This configuration informs the trigger to evaluate only prefect.flow-run.Completed events that have passed the match filters.

"expect": [
  "prefect.flow-run.Completed"
],
...

threshold decides the quantity of expected events needed to satisfy the trigger. Increasing the threshold above 1 requires use of within to define a range of time when multiple events are seen. The following configuration expects two occurrences of prefect.flow-run.Completed within 60 seconds:

"expect": [
  "prefect.flow-run.Completed"
],
"threshold": 2,
"within": 60,
...

Use after to handle scenarios that require more complex event reactivity.

For example, this flow emits an event indicating the table it operates on is missing or empty:

from prefect import flow
from prefect.events import emit_event
from myproject.db import Table


@flow
def transform(table_name: str):
  table = Table(table_name)

  if not table.exists():
    emit_event(
        event="table-missing",
        resource={"prefect.resource.id": "etl-events.transform"}
    )
  elif table.is_empty():
    emit_event(
        event="table-empty",
        resource={"prefect.resource.id": "etl-events.transform"}
    )
  else:
    # transform data
    ...

The following configuration uses after to prevent this automation from firing unless either a table-missing or a table-empty event has occurred before a flow run of this deployment completes.

Note how match and match_related ensure the trigger only evaluates events that are relevant to its purpose.

"match": {
  "prefect.resource.id": [
    "prefect.flow-run.*",
    "etl-events.transform"
  ]
},
"match_related": {
  "prefect.resource.id": "prefect.deployment.37ca4a08-e2d9-4628-a310-cc15a323378e"
}
"after": [
  "table-missing",
  "table-empty"
]
"expect": [
  "prefect.flow-run.Completed"
],
...

​
Evaluation strategy

All of the previous examples were designed around a reactive posture; that is, count up events toward the threshold until it is met, then execute actions. To respond to the absence of events, use a proactive posture. A proactive trigger fires when its threshold has not been met by the end of the window of time defined by within. Proactive triggers must have a within value of at least 10 seconds.

The following trigger fires if a prefect.flow-run.Completed event is not seen within 60 seconds after a prefect.flow-run.Running event is seen:

{
  "match": {
    "prefect.resource.id": "prefect.flow-run.*"
  },
  "match_related": {},
  "after": [
    "prefect.flow-run.Running"
  ],
  "expect": [
    "prefect.flow-run.Completed"
  ],
  "for_each": [],
  "posture": "Proactive",
  "threshold": 1,
  "within": 60
}

However, without for_each, a prefect.flow-run.Completed event from a different flow run than the one that started this trigger with its prefect.flow-run.Running event could satisfy the condition. Adding a for_each of prefect.resource.id causes this trigger to be evaluated separately for each flow run id associated with these events.

{
  "match": {
    "prefect.resource.id": "prefect.flow-run.*"
  },
  "match_related": {},
  "after": [
    "prefect.flow-run.Running"
  ],
  "expect": [
    "prefect.flow-run.Completed"
  ],
  "for_each": [
    "prefect.resource.id"
  ],
  "posture": "Proactive",
  "threshold": 1,
  "within": 60
}

​
Metric triggers

Metric triggers ({"type": "metric"}) fire when the value of a metric in your workspace crosses a threshold you defined. For example, you can trigger an automation when the success rate of flows in your workspace drops below 95% over the course of an hour.

Prefect’s metrics are all derived by examining your workspace’s events, and if applicable, use the occurred times of those events as the basis for their calculations.

Prefect defines three metrics:

  • Successes ({"name": "successes"}), defined as the number of flow runs that went Pending and then the latest state we saw was not a failure (Failed or Crashed). This metric accounts for retries if the ultimate state was successful.
  • Duration ({"name": "duration"}), defined as the length of time that a flow remains in a Running state before transitioning to a terminal state such as Completed, Failed, or Crashed. Because this time is derived in terms of flow run state change events, it may be greater than the runtime of your function.
  • Lateness ({"name": "lateness"}), defined as the length of time that a Scheduled flow remains in a Late state before transitioning to a Running and/or Crashed state. Only flow runs that the system marks Late are included.

This is the schema of a metric trigger:

NameTypeSupports trailing wildcardsDescription
matchobject:material-check:Labels for resources which this Automation will match.
match_relatedobject:material-check:Labels for related resources which this Automation will match.
metricMetricTriggerQueryN/AThe definition of the metric query to run.

And the MetricTriggerQuery query is defined as:

NameTypeDescription
namestringThe name of the Prefect metric to evaluate (see above).
thresholdnumberThe threshold the current metric value is compared to.
operatorstring ("<", "<=", ">", ">=")The comparison operator to use to decide if the threshold value is met.
rangeduration in secondsHow far back to evaluate the metric.
firing_forduration in secondsHow long the value must exceed the threshold before this trigger fires.

For example, to fire when flow runs tagged production in your workspace are failing at a rate of 10% or worse for the last hour (in other words, your success rate is below 90%), create this trigger:

{
  "type": "metric",
  "match": {
    "prefect.resource.id": "prefect.flow-run.*"
  },
  "match_related": {
    "prefect.resource.id": "prefect.tag.production",
    "prefect.resource.role": "tag"
  },
  "metric": {
    "name": "successes",
    "threshold": 0.9,
    "operator": "<",
    "range": 3600,
    "firing_for": 0
  }
}

To detect when the average lateness of your Kubernetes workloads (running on a work pool named kubernetes) in the last day exceeds five minutes late—and that number hasn’t gotten better for the last 10 minutes—use a trigger like this:

{
  "type": "metric",
  "match": {
    "prefect.resource.id": "prefect.flow-run.*"
  },
  "match_related": {
    "prefect.resource.id": "prefect.work-pool.kubernetes",
    "prefect.resource.role": "work-pool"
  },
  "metric": {
    "name": "lateness",
    "threshold": 300,
    "operator": ">",
    "range": 86400,
    "firing_for": 600
  }
}

​
Composite triggers

To create a trigger from multiple kinds of events and metrics, use a compound or sequence trigger. These higher-order triggers are composed from a set of underlying event and metric triggers.

For example, if you want to run a deployment only after three different flows in your workspace have written their results to a remote filesystem, combine them with a ‘compound’ trigger:

{
  "type": "compound",
  "require": "all",
  "within": 3600,
  "triggers": [
    {
      "type": "event",
      "posture": "Reactive",
      "expect": ["prefect.block.remote-file-system.write_path.called"],
      "match_related": {
        "prefect.resource.name": "daily-customer-export",
        "prefect.resource.role": "flow"
      }
    },
    {
      "type": "event",
      "posture": "Reactive",
      "expect": ["prefect.block.remote-file-system.write_path.called"],
      "match_related": {
        "prefect.resource.name": "daily-revenue-export",
        "prefect.resource.role": "flow"
      }
    },
    {
      "type": "event",
      "posture": "Reactive",
      "expect": ["prefect.block.remote-file-system.write_path.called"],
      "match_related": {
        "prefect.resource.name": "daily-expenses-export",
        "prefect.resource.role": "flow"
      }
    }
  ]
}

This trigger fires once it sees at least one of each of the underlying event triggers fire within the time frame specified. Then the trigger resets its state and fires the next time these three events all happen. The order the events occur doesn’t matter, just that all of the events occur within one hour.

If you want a flow run to complete prior to starting to watch for those three events, you can combine the entire previous trigger as the second part of a sequence of two triggers:

{
  // the outer trigger is now a "sequence" trigger
  "type": "sequence",
  "within": 7200,
  "triggers": [
    // with the first child trigger expecting a Completed event
    {
      "type": "event",
      "posture": "Reactive",
      "expect": ["prefect.flow-run.Completed"],
      "match_related": {
        "prefect.resource.name": "daily-export-initiator",
        "prefect.resource.role": "flow"
      }
    },
    // and the second child trigger being the compound trigger from the prior example
    {
      "type": "compound",
      "require": "all",
      "within": 3600,
      "triggers": [
        {
          "type": "event",
          "posture": "Reactive",
          "expect": ["prefect.block.remote-file-system.write_path.called"],
          "match_related": {
            "prefect.resource.name": "daily-customer-export",
            "prefect.resource.role": "flow"
          }
        },
        {
          "type": "event",
          "posture": "Reactive",
          "expect": ["prefect.block.remote-file-system.write_path.called"],
          "match_related": {
            "prefect.resource.name": "daily-revenue-export",
            "prefect.resource.role": "flow"
          }
        },
        {
          "type": "event",
          "posture": "Reactive",
          "expect": ["prefect.block.remote-file-system.write_path.called"],
          "match_related": {
            "prefect.resource.name": "daily-expenses-export",
            "prefect.resource.role": "flow"
          }
        }
      ]
    }
  ]
}

In this case, the trigger only fires if it sees the daily-export-initiator flow complete, and then the three files written by the other flows.

The within parameter for compound and sequence triggers restricts how close in time (in seconds) the child triggers must fire to satisfy the composite trigger. For example, if the daily-export-initiator flow runs, but the other three flows don’t write their result files until three hours later, this trigger won’t fire. Placing these time constraints on the triggers can prevent a misfire if you know that the events will generally happen within a specific timeframe— and you don’t want a stray older event included in the evaluation of the trigger.

If this isn’t a concern for you, you may omit the within period, in which case there is no limit to how far apart in time the child triggers occur.

You can compose any type of trigger into higher-order composite triggers, including proactive event triggers and metric triggers. In the following example, the compound trigger fires if any of the following events occur: a flow run stuck in Pending, a work pool becoming unready, or the average amount of Late work in your workspace going over 10 minutes:

{
  "type": "compound",
  "require": "any",
  "triggers": [
    {
      "type": "event",
      "posture": "Proactive",
      "after": ["prefect.flow-run.Pending"],
      "expect": ["prefect.flow-run.Running", "prefect.flow-run.Crashed"],
      "for_each": ["prefect.resource.id"],
      "match_related": {
        "prefect.resource.name": "daily-customer-export",
        "prefect.resource.role": "flow"
      }
    },
    {
      "type": "event",
      "posture": "Reactive",
      "expect": ["prefect.work-pool.not-ready"],
      "match": {
        "prefect.resource.name": "kubernetes-workers",
      }
    },
    {
      "type": "metric",
      "metric": {
        "name": "lateness",
        "operator": ">",
        "threshold": 600,
        "range": 3600,
        "firing_for": 300
      }
    }
  ]
}

For compound triggers, the require parameter may be "any", "all", or a number between 1 and the number of child triggers. In the example above, if you feel that you are receiving too many spurious notifications for issues that resolve on their own, you can specify {"require": 2} to express that any two of the triggers must fire in order for the compound trigger to fire. Sequence triggers, on the other hand, always require all of their child triggers to fire before they fire.

Compound triggers are defined as:

NameTypeDescription
requirenumber, "any", or "all"The number of child triggers that must fire for this trigger to fire
withintime, in secondsHow close in time the child triggers must fire for this trigger to fire
triggersarray of other triggers

Sequence triggers are defined as:

NameTypeDescription
withintime, in secondsHow close in time the child triggers must fire for this trigger to fire
triggersarray of other triggers

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