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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", "update", "delete" operations.

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"
    }
  ]
}

Handling Delete Interaction​

Note

The delete interaction will contain a different response where configuration will be needed on the incoming data. Unless the subscription intended is only to be executed upon 'create' or 'update', it will by default be triggered up on it.

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 {}

Executing only on delete

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

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.

{
  "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"
    }
  ]
}

fhirPathCritera​

Medplum offers an extension for triggering subscriptions based on a string expression. This string expression takes in two variables: %previous and %current. These two variables are the same resource and are compared to one another. The expression should return either true or false.

Here is an example subscription resource with a fhirPathCriteria expression that compares the Patient resource:

{
  "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/fhir-path-criteria-expression",
      "valueString": "%previous.name!=%current.name"
    }
  ]
}
Note

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