Receive events with webhooks

Use webhooks in your Prefect Cloud workspace to receive, observe, and respond to events from other systems in your ecosystem. Each webhook exposes a unique URL endpoint to receive events from other systems and transforms them into Prefect events for use in automations.

Webhooks are defined by two essential components: a unique URL and a template that translates incoming web requests to a Prefect event.

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Configure webhooks

Set up your webhooks through the Prefect Cloud API, Prefect Cloud UI, or Prefect CLI.

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Through the Prefect Cloud API

Webhooks are managed through the Webhooks API endpoints. This is a Prefect Cloud-only feature. Authenticate API calls using the standard authentication methods you use with Prefect Cloud.

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Through Prefect Cloud

You can create and manage webhooks from the Prefect Cloud UI.

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Through the Prefect CLI

You can manage and interact with webhooks through the prefect cloud webhook command group.

prefect cloud webhook --help

Create your first webhook by invoking create:

prefect cloud webhook create your-webhook-name \
    --description "Receives webhooks from your system" \
    --template '{ "event": "your.event.name", "resource": { "prefect.resource.id": "your.resource.id" } }'

Note the template string, discussed in greater detail below.

You can retrieve details for a specific webhook by ID using get, or optionally query all webhooks in your workspace with ls:

# get webhook by ID
prefect cloud webhook get <webhook-id>

# list all configured webhooks in your workspace
prefect cloud webhook ls

Disable an existing webhook without deleting it using toggle:

prefect cloud webhook toggle <webhook-id>
Webhook is now disabled

prefect cloud webhook toggle <webhook-id>
Webhook is now enabled

Use rotate to generate a new, random endpoint if you’re concerned your webhook endpoint was compromised:

prefect cloud webhook rotate <webhook-url-slug>

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Webhook endpoints

The webhook endpoints have randomly generated opaque URLs that do not divulge any information about your Prefect Cloud workspace. They are rooted at https://api.prefect.cloud/hooks/. For example: https://api.prefect.cloud/hooks/AERylZ_uewzpDx-8fcweHQ. Prefect Cloud assigns this URL when you create a webhook; it cannot be set through the API. You may rotate your webhook URL at any time without losing the associated configuration.

All webhooks may accept requests with the most common HTTP methods:

• Use GET, HEAD, and DELETE for webhooks that define a static event template, or a template that does not depend on the body of the HTTP request. The headers of the request will be available for templates.
• Use POST, PUT, and PATCH when the webhook request includes a body. See How HTTP request components are handled for more details on how the body is parsed.

Prefect Cloud webhooks are deliberately quiet to the outside world, and only return a 204 No Content response when they are successful or a 400 Bad Request error when there is any error interpreting the request. For more visibility when your webhooks fail, see the Troubleshooting section below.

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Webhook templates

The purpose of a webhook is to accept an HTTP request from another system and produce a Prefect event from it. You often have little control over the format of those requests, so Prefect’s webhook system gives you full configuration on how you turn those notifications from other systems into meaningful events in your Prefect Cloud workspace. The template you define for each webhook determines how individual components of the incoming HTTP request become the event name and resource labels of the resulting Prefect event.

As with the templates available in Prefect Cloud Automation for defining notifications and other parameters, you write templates in Jinja2. All of the built-in Jinja2 blocks and filters are available, as well as the filters from the jinja2-humanize-extensions package.

Your goal when defining an event template is to produce a valid JSON object that defines (at minimum) the event name and the resource["prefect.resource.id"], which are required of all events. The simplest template is one in which these are statically defined.

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Static webhook events

Here’s a static webhook template example that notifies Prefect when your recommendations machine learning model has been updated, so you can send a Slack notification to your team and run a few subsequent deployments. Those models are produced on a daily schedule by another team that is using cron for scheduling. They aren’t able to use Prefect for their flows yet, but they are happy to add a curl to the end of their daily script to notify you. Because this webhook is only used for a single event from a single resource, your template can be entirely static:

{
    "event": "model.refreshed",
    "resource": {
        "prefect.resource.id": "product.models.recommendations",
        "prefect.resource.name": "Recommendations [Products]",
        "producing-team": "Data Science"
    }
}

A webhook with this template may be invoked through any of the HTTP methods, including a GET request with no body, so the team you are integrating with can include this line at the end of their daily script:

curl https://api.prefect.cloud/hooks/AERylZ_uewzpDx-8fcweHQ

Each time the script hits the webhook, the webhook produces a single Prefect event with that name and resource in your workspace.

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Event fields that Prefect Cloud populates for you

You only had to provide the event and resource definition, which is not a completely fleshed out event. Prefect Cloud sets default values for any missing fields, such as occurred and id, so you don’t need to set them in your template. Additionally, Prefect Cloud adds the webhook itself as a related resource on all of the events it produces.

If your template does not produce a payload field, the payload defaults to a standard set of debugging information, including the HTTP method, headers, and body.

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Dynamic webhook events

Let’s say that after a few days you and the Data Science team are getting a lot of value from the automations you have set up with the static webhook. You’ve agreed to upgrade this webhook to handle all of the various models that the team produces. It’s time to add some dynamic information to your webhook template.

Your colleagues on the team have adjusted their daily cron scripts to POST a small body that includes the ID and name of the model that was updated:

curl \
    -d "model=recommendations" \
    -d "friendly_name=Recommendations%20[Products]" \
    -X POST https://api.prefect.cloud/hooks/AERylZ_uewzpDx-8fcweHQ

This script sends a POST request and the body will include a traditional URL-encoded form with two fields describing the model that was updated: model and friendly_name. Here’s the webhook code that uses Jinja to receive these values in your template and produce different events for the different models:

{
    "event": "model.refreshed",
    "resource": {
        "prefect.resource.id": "product.models.{{ body.model }}",
        "prefect.resource.name": "{{ body.friendly_name }}",
        "producing-team": "Data Science"
    }
}

All subsequent POST requests will produce events with those variable resource IDs and names. The other statically defined parts, such as event or the producing-team label you included earlier, will still be used.

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How HTTP request components are handled

The Jinja2 template context includes the three parts of the incoming HTTP request:

• method is the uppercased string of the HTTP method, like GET or POST.
• headers is a case-insensitive dictionary of the HTTP headers included with the request. To prevent accidental disclosures, the Authorization header is removed.
• body represents the body that was posted to the webhook, with a best-effort approach to parse it into an object you can access.

HTTP headers are available without any alteration as a dict-like object, but you may access them with header names in any case. For example, these template expressions all return the value of the Content-Length header:

{{ headers['Content-Length'] }}

{{ headers['content-length'] }}

{{ headers['CoNtEnt-LeNgTh'] }}

The HTTP request body goes through some light preprocessing to make it more useful in templates. If the Content-Type of the request is application/json, the body will be parsed as a JSON object and made available to the webhook templates. If the Content-Type is application/x-www-form-urlencoded (as in our example above), the body is parsed into a flat dict-like object of key-value pairs. Jinja2 supports both index and attribute access to the fields of these objects, so the following two expressions are equivalent:

{{ body['friendly_name'] }}

{{ body.friendly_name }}

Prefect Cloud will attempt to parse any other content type (such as text/plain) as if it were JSON first. In any case where the body cannot be transformed into JSON, it is made available to your templates as a Python str.

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Accept Prefect events directly

In cases where you have more control over the client, your webhook can accept Prefect events directly with a simple pass-through template:

{{ body|tojson }}

This template accepts the incoming body (assuming it was in JSON format) and passes it through unmodified. This allows a POST of a partial Prefect event as in this example:

POST /hooks/AERylZ_uewzpDx-8fcweHQ HTTP/1.1
Host: api.prefect.cloud
Content-Type: application/json
Content-Length: 228

{
    "event": "model.refreshed",
    "resource": {
        "prefect.resource.id": "product.models.recommendations",
        "prefect.resource.name": "Recommendations [Products]",
        "producing-team": "Data Science"
    }
}

The resulting event is filled out with the default values for occurred, id, and other fields as described above.

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Accepting CloudEvents

The Cloud Native Computing Foundation has standardized CloudEvents for use by systems to exchange event information in a common format. These events are supported by major cloud providers and a growing number of cloud-native systems. Prefect Cloud can interpret a webhook containing a CloudEvent natively with the following template:

{{ body|from_cloud_event(headers) }}

The resulting event uses the CloudEvent’s subject as the resource (or the source if no subject is available). The CloudEvent’s data attribute becomes the Prefect event’s payload['data'], and the other CloudEvent metadata will be at payload['cloudevents']. To handle CloudEvents in a more specific way tailored to your use case, use a dynamic template to interpret the incoming body.

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Troubleshooting

The initial configuration of your webhook may require some trial and error as you get the sender and your receiving webhook speaking a compatible language. While you are in this phase, lean upon the event feed in the UI to see events as they happen.

When Prefect Cloud encounters an error during receipt of a webhook, it produces a prefect-cloud.webhook.failed event in your workspace. This event includes critical information about the HTTP method, headers, and body it received, as well as what the template rendered. Keep an eye out for these events when something goes wrong.