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Subscription Extensions

Use the following FHIR extensions to customize the Subscription behavior. The behavior is non-standard, and will not necessarily work in other FHIR systems.

Adding Extensions​

Here is an example FHIR Subscription Object:

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "Patient",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  }
}

An subscription extension contains an array of objects that have url and value* in them. To add an extension, use one of medplum's url below that contains the value to be passed.

The extension will look like this:

{
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-max-attempts",
      "valueInteger": 3
    }
  ]
}

And your final Subscription object will be:

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "Patient",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-max-attempts",
      "valueInteger": 3
    }
  ]
}

Below are explanations of the different extensions Medplum Provides

Interactions​

Note

By default, FHIR Subscriptions will execute on "create" and "update" operations.

You can use extensions as follows for more fine-grained control over when Subscriptions execute. To confirm if your Subscriptions are executing, navigate to https://app.medplum.com/Subscription/<id>/event to view related AuditEvents.

Subscriptions for "create"-only or "update"-only events​

To restrict the FHIR Subscription to only execute on "create", use the https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction extension with valueCode of create:

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "Patient",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction",
      "valueCode": "create"
    }
  ]
}

You can also restrict the FHIR Subscription to only execute on "update", using the https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction extension with valueCode of update:

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "Patient",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction",
      "valueCode": "update"
    }
  ]
}

Subscriptions for "delete" events​

Note

The delete interaction will contain a different response where configuration will be needed on the incoming data.

Use the https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction extension with valueCode of delete. For example:

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "Patient",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-supported-interaction",
      "valueCode": "delete"
    }
  ]
}

The response for a deleted resource will contain:

{
  "method": "POST",
  "body": "{}",
  "headers": {
    "Content-Type": "application/fhir+json",
    "X-Medplum-Deleted-Resource": "${resource.resourceType}/${resource.id}"
  }
}

Few things to note:

X-Medplum-Deleted-Resource: Will contain the resource type and resource id that was deleted.

body: Will be an empty object in the response {}

Signatures​

When a consumer receives a webhook request, you may want to verify that the request came from the expected sender.

Webhooks can optionally use a FHIR extension to enable an HMAC signature. To enable HMAC signatures, use the extension https://www.medplum.com/fhir/StructureDefinition/subscription-secret and valueString of a cryptographically secure secret.

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "DiagnosticReport?status=completed",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://www.medplum.com/fhir/StructureDefinition/subscription-secret",
      "valueString": "abc"
    }
  ]
}

The valueString will be used to generate a signature. The signature is the payload encoded using SHA-256 (otherwise known as an HMAC). The key for the hash will be the valueString from the FHIR extension. API consumers are encouraged to encode the payload with the secret key and compare the signatures.

Example: TypeScript / Express

app.post('/webhook', (req, res) => {
  const secret = '...'; // Created separately
  const signature = crypto.createHmac('sha256', secret).update(JSON.stringify(req.body)).digest('hex');
  console.log('Signature:', req.headers['x-signature']);
  console.log('Expected:', signature);
  console.log('Received:', req.body);
  res.sendStatus(200);
});

Example: Python / Flask

@app.route("/webhook", methods=["POST"])
def handle_webhook():
    secret = b'...' # Created separately
    message = flask.request.get_data()
    signature = hmac.new(secret, message, hashlib.sha256).hexdigest()
    log('Expected: ' + signature)
    log('Received: ' + flask.request.headers.get('x-signature'))
    return {"ok":True}

Retry Policy​

If your subscription failed or threw an error, you can configure it to attempt to execute the operation multiple times.

To add an attempt number, use the https://medplum.com/fhir/StructureDefinition/subscription-max-attempts extension with the valueInteger set to a number between 1-18.

The default number of attempts is 3.

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "DiagnosticReport?status=completed",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-max-attempts",
      "valueInteger": 3
    }
  ]
}

Custom Status Codes​

HTTP status codes can be customized to determine the success of the subscription operation.

To add custom codes, use the https://medplum.com/fhir/StructureDefinition/subscription-success-codes extension with the valueString having a comma separated list of HTTP status codes for success (i.e., "200,201"). We also allow ranges (i.e., "200-399,404")

Note

If you use custom success codes, you will need to implement ALL of the HTTP status codes that are determined to be successful

{
  "resourceType": "Subscription",
  "reason": "test",
  "status": "active",
  "criteria": "DiagnosticReport?status=completed",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/subscription-success-codes",
      "valueString": "200-399,404"
    }
  ]
}

Expression based criteria​

Medplum offers an extension (fhir-path-criteria-expression) for triggering subscriptions based on more complex conditional logic using a FHIRPath expression. This expression takes in two variables:

  • %previous: The state of the resource before the triggering event
  • %current: The state of the resource after the triggering event.

The expression should return either true or false.

Here is an example Subscription resource with a fhir-path-criteria-expression expression that fires when a Task changes its status:

{
  "resourceType": "Subscription",
  "reason": "Task Status Change",
  "status": "active",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "criteria": "Task",
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/fhir-path-criteria-expression",
      "valueString": "%previous.status != %current.status"
    }
  ]
}
Note

Upon the creation of a resource, there won't be a previous version of the resource: %previous will be empty.

FHIRPath generally treats empty values as "null" and most operators and functions — including != — evaluate to be empty if any of their operands are empty. This means that when a resource is created, the above expression will always be falsy, and the subscription will not fire.

If resource creations should also be included, the FHIRPath expression must account for that case specifically: %previous.exists() implies %previous.status != %current.status. With this expression, if %previous is empty, the overall expression will still evaluate to true and trigger the subscription.

Handling array fields​

Some resource fields, e.g. Patient.name, can contain multiple values: these are formatted in FHIR as JSON arrays. The normal FHIRPath operators like = and != handle equality of these values recursively: they must have exactly the same members or fields to be considered equal. Consider the following subscription:

{
  "resourceType": "Subscription",
  "reason": "Patient Name Change",
  "status": "active",
  "channel": {
    "type": "rest-hook",
    "endpoint": "https://example.com/webhook"
  },
  "criteria": "Patient",
  "extension": [
    {
      "url": "https://medplum.com/fhir/StructureDefinition/fhir-path-criteria-expression",
      "valueString": "%previous.name != %current.name"
    }
  ]
}

Adding a new name to the Patient and leaving the old one(s) unchanged would trigger this subscription, because the two name arrays are not exactly the same. If the subscription should only fire when an existing value is altered or removed, the individual values in the field must be compared instead of the field as a whole:

%previous.name.where( ($this in %current.name).not() ).exists()