---
description: FHIR Topic-Based Subscriptions support for standard FHIR R4/R5 subscription interoperability.
---

# FHIR Topic-Based Subscriptions

{% hint style="info" %}
This functionality requires [FHIR Schema](../profiling-and-validation/fhir-schema-validator/) validation engine to be [enabled](../profiling-and-validation/fhir-schema-validator/).
[R4-backported](https://hl7.org/fhir/uv/subscriptions-backport/STU1.1/#topic-based-subscriptions---fhir-r4) subscriptions are available in Aidbox version 2512 or later.

[R5](https://hl7.org/fhir/uv/subscriptions-backport/STU1.1/#topic-based-subscriptions---fhir-r5-and-later) and [R4B-backported](https://hl7.org/fhir/uv/subscriptions-backport/STU1.1/#topic-based-subscriptions---fhir-r4b-and-later) subscriptions require Aidbox version 2601 or later.
{% endhint %}

## Overview

Aidbox provides full support for the [FHIR Topic-Based Subscriptions Framework](https://build.fhir.org/subscriptions.html) according to the specification. This allows external systems to create and manage FHIR `Subscription` resources to receive notifications about data changes.

### When to use FHIR Subscriptions

Use FHIR Topic-Based Subscriptions when you:

- Need conformance to the FHIR subscription specification
- Need to provide the ability for external systems or customers to create their own subscriptions
- Expect many subscribers (tens to thousands) with different filters to the same topic

### When to use Aidbox Topic-Based Subscriptions instead

For simple subscriptions used for internal integrations, [Aidbox Topic-Based Subscriptions](aidbox-topic-based-subscriptions.md#currently-supported-channels) (Kafka, Webhook, GCP Pub/Sub, etc.) may work better as they require less setup and are managed server-side.               |


## Key Components

FHIR Topic-Based Subscriptions build on the [Aidbox Topic-Based Subscriptions](aidbox-topic-based-subscriptions.md) infrastructure, using the same core resources with additional configuration for FHIR-standard subscription support.

[`SubscriptionTopic`](https://build.fhir.org/subscriptiontopic.html) resource is modeled as two Aidbox resources: `AidboxSubscriptionTopic` and `AidboxTopicDestination`.

1. **`AidboxSubscriptionTopic`** - implements the topic's triggers and filters
2. **`AidboxTopicDestination`** - binds it to the canonical FHIR SubscriptionTopic URL

Once configured, clients can create standard FHIR [`Subscription`](https://build.fhir.org/subscription.html) resource instances referencing the topic URL.

### AidboxSubscriptionTopic

[`AidboxSubscriptionTopic`](aidbox-topic-based-subscriptions.md#aidboxsubscriptiontopic) is a custom Aidbox resource that models the FHIR [`SubscriptionTopic`](https://build.fhir.org/subscriptiontopic.html) resource.

**Why a custom resource?** FHIR SubscriptionTopic is not always fully computable — it often uses codes to describe event triggers that require human interpretation. Aidbox uses FHIRPath expressions to make triggers computable.

For example, the [DaVinci PAS SubscriptionTopic](https://build.fhir.org/ig/HL7/davinci-pas/SubscriptionTopic-PASSubscriptionTopic.json.html) defines an event trigger using a code:

```json
"eventTrigger": [{
  "description": "When a new result is made ready by the intermediary system",
  "event": {
    "coding": [{
      "system": "http://hl7.org/fhir/us/davinci-pas/CodeSystem/PASTempCodes",
      "code": "result-available"
    }]
  },
  "resource": "http://hl7.org/fhir/StructureDefinition/ClaimResponse"
}]
```

In Aidbox, you would model this with a computable FHIRPath expression:

```json
{
  "resourceType": "AidboxSubscriptionTopic",
  "url": "http://hl7.org/fhir/us/davinci-pas/SubscriptionTopic/PASSubscriptionTopic",
  "status": "active",
  "trigger": [{
    "resource": "ClaimResponse",
    "fhirPathCriteria": "status = 'active'"
  }]
}
```

{% hint style="info" %}
What "result-available" means may vary from installation to installation. The FHIRPath expression should match your specific business logic.
{% endhint %}

### canFilterBy

The `canFilterBy` element in `AidboxSubscriptionTopic` defines what filter parameters subscribers can use when creating their subscriptions. This allows subscribers to receive only events that match their specific criteria.

| Field | Description |
|-------|-------------|
| `filterParameter` | Name of the filter parameter that subscribers will use |
| `filterDefinitionFhirPathExpression` | FHIRPath expression to extract the filter value from the resource |
| `resource` | Resource type this filter applies to |
| `comparator` | Comparison operators allowed for this filter. Currently only `eq` (equals) is supported. |
| `description` | Human-readable description of the filter |

```json
{
  "resourceType": "AidboxSubscriptionTopic",
  "url": "http://example.org/SubscriptionTopic/claim-response-topic",
  "status": "active",
  "trigger": [{
    "resource": "ClaimResponse",
    "fhirPathCriteria": "status = 'active'"
  }],
  "canFilterBy": [{
    "description": "Filter by insurer organization",
    "resource": "ClaimResponse",
    "filterParameter": "insurer",
    "filterDefinitionFhirPathExpression": "insurer",
    "comparator": ["eq"]
  }]
}
```

Subscribers can then specify filter criteria in their `Subscription` resource to receive only events matching their filters (e.g., only ClaimResponses for a specific insurer).

### AidboxTopicDestination

[`AidboxTopicDestination`](aidbox-topic-based-subscriptions.md#aidboxtopicdestination) enables an `AidboxSubscriptionTopic` to be available for FHIR Subscriptions. It acts as a bridge between the topic definition and the FHIR Subscription API.

For FHIR Subscriptions, use the `fhir-native-topic-based-subscription` kind with the following parameters:

| Parameter | Type | Description |
|-----------|------|-------------|
| `fhir-topic` | valueCanonical | Canonical URL of the FHIR SubscriptionTopic this destination implements. |
| `subscription-specification-version` | valueString | FHIR version of the Subscription spec to support. This value determines the format of notification bundles, `$status` and `$events` operation responses. Supported values: `R5`, `R4-backported`, `R4B-backported`. |
| `keep-events-for-period` | valueUnsignedInt | How long events remain available for the `$events` operation (in seconds). If not specified, events are stored indefinitely. |
| `number-of-deliverer` | valueUnsignedInt | Number of parallel delivery workers (default: 4). |

```json
{
  "resourceType": "AidboxTopicDestination",
  "meta": {
    "profile": ["http://aidbox.app/StructureDefinition/aidboxtopicdestination-fhir-native-topic-based-subscription"]
  },
  "kind": "fhir-native-topic-based-subscription",
  "topic": "http://example.org/SubscriptionTopic/claim-response-topic",
  "parameter": [
    {
      "name": "fhir-topic",
      "valueUrl": "http://hl7.org/fhir/us/davinci-pas/SubscriptionTopic/PASSubscriptionTopic"
    },
    {
      "name": "subscription-specification-version",
      "valueString": "R4-backported"
    },
    {
      "name": "keep-events-for-period",
      "valueInteger": 86400
    },
    {
      "name": "number-of-deliverer",
      "valueInteger": 2
    }
  ]
}
```

### FHIR Subscription

Once the topic and destination are configured, clients can create standard FHIR `Subscription` resources to subscribe to events.

#### Supported Fields

| Field | R4 Backport | R5 | Description |
|-------|-------------|-----|-------------|
| Topic | [`criteria`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.criteria) | [`topic`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.topic) | Canonical URL of the SubscriptionTopic |
| Filter criteria | [`backport-filter-criteria`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.criteria.extension:filterCriteria) | [`filterBy`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.filterBy) | Filter events using parameters defined in `canFilterBy` |
| Expiration | [`end`](https://hl7.org/fhir/R4/subscription-definitions.html#Subscription.end) | [`end`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.end) | DateTime when subscription expires and stops receiving events |
| Channel type | [`channel.type`](https://hl7.org/fhir/R4/subscription-definitions.html#Subscription.channel.type) | [`channelType`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.channelType) | Only `rest-hook` is supported |
| Endpoint | [`channel.endpoint`](https://hl7.org/fhir/R4/subscription-definitions.html#Subscription.channel.endpoint) | [`endpoint`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.endpoint) | URL to receive notifications |
| Content type | [`channel.payload`](https://hl7.org/fhir/R4/subscription-definitions.html#Subscription.channel.payload) | [`contentType`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.contentType) | MIME type for notification payload (e.g., `application/fhir+json`) |
| Content level | [`backport-payload-content`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.channel.payload.extension:content) | [`content`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.content) | Payload detail: `empty`, `id-only`, or `full-resource` |
| Headers | [`channel.header`](https://hl7.org/fhir/R4/subscription-definitions.html#Subscription.channel.header) | [`parameter`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.parameter) | Custom HTTP headers sent with notifications |
| Heartbeat | [`backport-heartbeat-period`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.channel.extension:heartbeatPeriod) | [`heartbeatPeriod`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.heartbeatPeriod) | Interval in seconds for keep-alive notifications during inactivity |
| Max count | [`backport-max-count`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.channel.extension:maxCount) | [`maxCount`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.maxCount) | Max events per notification bundle before sending |
| Timeout | [`backport-timeout`](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/StructureDefinition-backport-subscription-definitions.html#diff_Subscription.channel.extension:timeout) | [`timeout`](https://hl7.org/fhir/R5/subscription-definitions.html#Subscription.timeout) | Max seconds to wait for endpoint response before failing delivery |

## Subscription Lifecycle

1. **Requested**: Client creates a Subscription resource with `status: requested`. The subscription is not yet active.

2. **Handshake**: Server immediately sends a handshake notification to the configured endpoint to validate it can receive notifications. The handshake bundle contains a `SubscriptionStatus` with `type: handshake`.

3. **Active**: If the endpoint responds with HTTP 2xx, the subscription status changes to `active` and starts receiving events. If handshake fails, status changes to `error`.

4. **Event notifications**: When resources matching the topic trigger criteria are created/updated/deleted, notification bundles are sent to the endpoint containing the `SubscriptionStatus` and triggered resources. Events are batched up to `max-count` before sending, or sent earlier if heartbeat period is reached.

5. **Heartbeat**: If configured via `backport-heartbeat-period`, periodic empty notifications are sent during inactivity to confirm the connection is alive.

6. **Expiration/Off**: Subscription stops receiving events when:
   - The `end` date is reached
   - The subscription is deleted

## Operations

Only instance-level operations are supported (not the batch/server-level variants).

### $status

Returns the current status of a subscription including event counts. Response format depends on the `subscription-specification-version` configured in `AidboxTopicDestination`.

**URL:** `GET|POST [base]/Subscription/[id]/$status`

**Parameters:** None

**Response:** Bundle with `SubscriptionStatus` containing:
- `status` - current subscription status (`active`, `error`)
- `events-since-subscription-start` - total event count
- `error` - error details if delivery failed

| Version | Specification |
|---------|---------------|
| `R5` | [Subscription $status](https://hl7.org/fhir/R5/subscription-operation-status.html) |
| `R4-backported` | [Subscription Status Operation](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/OperationDefinition-backport-subscription-status.html) |

### $events

Allows clients to query past events for recovery purposes. Response format depends on the `subscription-specification-version` configured in `AidboxTopicDestination`.

**URL:** `POST [base]/Subscription/[id]/$events`

**Parameters:**

| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `eventsSinceNumber` | string | `eventsUntilNumber - 100` | Start of event range (inclusive) |
| `eventsUntilNumber` | string | Last event number | End of event range (inclusive) |
| `content` | code | Subscription's content level | `full-resource`, `id-only`, or `empty` |

**Defaults:** If no parameters provided, returns up to 100 most recent events.

| Version | Specification |
|---------|---------------|
| `R5` | [Subscription $events](https://hl7.org/fhir/R5/subscription-operation-events.html) |
| `R4-backported` | [Subscription Events Operation](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/OperationDefinition-backport-subscription-events.html) |

## Notification Bundle

The notification bundle format depends on the `subscription-specification-version` configured in `AidboxTopicDestination`.

| Version | Specification |
|---------|---------------|
| `R5` | [SubscriptionStatus](https://hl7.org/fhir/R5/subscriptionstatus.html), [Notification Bundle](https://hl7.org/fhir/R5/bundle.html#subscription-notification) |
| `R4B-backported` | [SubscriptionStatus](https://hl7.org/fhir/R4B/subscriptionstatus.html) |
| `R4-backported` | [Notification Bundle Example](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/Bundle-r4-notification-full-resource.json.html), [All Artifacts](https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/artifacts.html) |

## Internals

This section describes internal implementation details useful for debugging and operations.

### Database Tables

Events and delivery statuses are stored in the `tds` schema:

| Table | Description |
|-------|-------------|
| `tds.fhir_subscription_topic_event` | Stores triggered events. Events are inserted within the same database transaction that processes the FHIR request, ensuring consistency. |
| `tds.fhir_subscription_delivery_status` | Stores delivery facts for each event-subscription pair. A record is created after delivery attempt with `error_code` set to NULL on success or error type on failure. |

### Event Processing

1. When a FHIR resource is created/updated/deleted and matches a topic's trigger criteria, an event is inserted into `fhir_subscription_topic_event` within the same transaction.

2. A background enqueuer process dispatches undelivered events to subscription processors. Events are filtered by each subscription's `filterBy` criteria — only matching events are delivered to that subscription.

3. Each subscription has a lightweight processor that batches events according to `max-count` and `heartbeat-period` settings.

4. Deliverer workers (controlled by `number-of-deliverer`) perform the actual HTTP delivery to subscription endpoints.

### Event Cleanup

If `keep-events-for-period` is specified, a periodic cleanup process deletes events older than the retention period from both `fhir_subscription_topic_event` and `fhir_subscription_delivery_status` tables.

### Multi-Instance Coordination

In multi-instance deployments, PostgreSQL advisory locks ensure only one Aidbox instance runs the delivery process for each `AidboxTopicDestination`. Other instances wait and take over if the lock becomes available.

## Tutorials

For a step-by-step guide to setting up FHIR Topic-Based Subscriptions, follow one of the tutorials below based on your FHIR version:

| Version          | Tutorial                                                                                                          |
|------------------|-------------------------------------------------------------------------------------------------------------------|
| `R4`  | [FHIR R4 Backport Subscription Setup](../../tutorials/subscriptions-tutorials/fhir-subscription-r4-backport.md)   |
| `R4B` | [FHIR R4B Backport Subscription Setup](../../tutorials/subscriptions-tutorials/fhir-subscription-r4b-backport.md) |
| `R5`             | [FHIR R5 Subscription Setup](../../tutorials/subscriptions-tutorials/fhir-subscription-r5.md)