Mercure: The Specification

Abstract

Mercure provides a common publish-subscribe mechanism for public and private web resources. Mercure enables the pushing of any web content to web browsers and other clients in a fast, reliable and battery-efficient way. It is especially useful for publishing real-time updates of resources served through sites and web APIs to web and mobile apps.

Subscription requests are relayed through hubs, which validate and verify the request. When new or updated content becomes available, hubs check if subscribers are authorized to receive it then distribute it.

Terminology

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in (RFC2119).

Topic: The unit to which one can subscribe to changes. The topic SHOULD be identified by an IRI (RFC3987). Using an HTTPS (RFC7230) or HTTP (RFC7230) URI (RFC3986) is RECOMMENDED.
Update: The message containing the updated version of the topic. An update can be marked as private, consequently, it must be dispatched only to subscribers allowed to receive it.
Topic selector: An expression matching one or several topics.
Publisher: An owner of a topic. Notifies the hub when the topic feed has been updated. As in almost all pubsub systems, the publisher is unaware of the subscribers, if any. Other pubsub systems might call the publisher the "source". Typically a site or a web API, but can also be a web browser.
Subscriber: A client application that subscribes to real-time updates of topics using topic selectors. Typically a web or a mobile application, but can also be a server.
Subscription: A topic selector used by a subscriber to receive updates. A single subscriber can have several subscriptions, when it provides several topic selectors.
Hub: A server that handles subscription requests and distributes the content to subscribers when the corresponding topics have been updated. Any hub MAY implement its own policies on who can use it.

Discovery

The discovery mechanism aims at identifying at least 2 URLs.

The URL of one or more hubs designated by the publisher.
The canonical URL for the topic to which subscribers are expected to use for subscriptions.

The URL of the hub MUST be the "well-known" (RFC5785) fixed path /.well-known/mercure.

If the publisher is a server, it SHOULD advertise the URL of one or more hubs to the subscriber, allowing it to receive live updates when topics are updated. If more than one hub URL is specified, the publisher MUST notifies each hub, so the subscriber MAY subscribe to one or more of them.

Note: Publishers may wish to advertise and publish to more than one hub for fault tolerance and redundancy. If one hub fails to propagate an update to the document, then using multiple independent hub is a way to increase the likelihood of delivery to subscribers. As such, subscribers may subscribe to one or more of the advertised hubs.

The publisher SHOULD include at least one Link Header (RFC5988) with rel=mercure (a hub link header). The target URL of these links MUST be a hub implementing the Mercure protocol.

The publisher MAY provide the following target attributes in the Link Headers:

last-event-id: the identifier of the last event dispatched by the publisher at the time of the generation of this resource. If provided, it MUST be passed to the hub through a query parameter called lastEventID and will be used to ensure that possible updates having been made between the resource generation by the server and the connection to the hub are not lost. See reconciliation.
content-type: the content type of the updates that will be pushed by the hub. If omitted, the subscriber MUST assume that the content type will be the same as that of the original resource. Setting the content-type attribute is especially useful to hint that partial updates will be pushed, using formats such as JSON Patch (RFC6902) or JSON Merge Patch (RFC7386).
key-set: the URL of the key set to use to decrypt updates, encoded in the JWK set format (JSON Web Key Set) (RFC7517). See encryption. As this key set will contain a secret key, the publisher must ensure that only the subscriber can access to this URL. To do so, the authorization mechanism (see authorization) can be reused.

All these attributes are optional.

The publisher MAY also include one Link Header (RFC5988) with rel=self (the self link header). It SHOULD contain the canonical URL for the topic to which subscribers are expected to use for subscriptions. If the Link with rel=self is omitted, the current URL of the resource MUST be used as a fallback.

Minimal example:

GET /books/foo HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"

{"@id": "/books/foo", "foo": "bar"}

Links embedded in HTML or XML documents as defined in the WebSub recommendation (W3C.REC-websub-20180123) MAY also be supported by subscribers. If both a header and an embedded link are provided, the header MUST be preferred.

Content Negotiation

For practical purposes, it is important that the rel=self URL only offers a single representation. As the hub has no way of knowing what Media Type ((RFC6838)) or language may have been requested by the subscriber upon discovery, it would not be able to deliver the content using the appropriate representation of the document.

It is, however, possible to perform content negotiation by returning an appropriate rel=self URL according to the HTTP headers used in the initial discovery request. For example, a request to /books/foo with an Accept header containing application/ld+json could return a rel=self value of /books/foo.jsonld.

The example below illustrates how a topic URL can return different Link headers depending on the Accept header that was sent.

GET /books/foo HTTP/1.1
Host: example.com
Accept: application/ld+json

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: </books/foo.jsonld>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

{"@id": "/books/foo", "foo": "bar"}

GET /books/foo HTTP/1.1
Host: example.com
Accept: text/html

HTTP/1.1 200 OK
Content-type: text/html
Link: </books/foo.html>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

<!doctype html>
<title>foo: bar</title>

Similarly, the technique can also be used to return a different rel=self URL depending on the language requested by the Accept-Language header.

GET /books/foo HTTP/1.1
Host: example.com
Accept-Language: fr-FR

HTTP/1.1 200 OK
Content-type: application/ld+json
Content-Language: fr-FR
Link: </books/foo-fr-FR.jsonld>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

{"@id": "/books/foo", "foo": "bar", "@context": {"@language": "fr-FR"}}

Topic Selectors

A topic selector is an expression intended to be matched by one or several topics. A topic selector can also be used to match other topic selectors for authorization purposes. See authorization.

A topic selector can be any string including URI Templates (RFC6570) and the reserved string * that matches all topics. It is RECOMMENDED to use URI Templates or the reserved string * as topic selectors.

Note: URLs and IRIs are valid URI templates.

To determine if a string matches a selector, the following steps must be followed:

If the topic selector is * then the string matches the selector.
If the topic selector and the string are exactly the same, the string matches the selector. This characteristic allows to compare a URI Template with another one.
If the topic selector is a valid URI Template, and that the string matches this URI Template, the string matches the selector.
Otherwise the string does not match the selector.

Subscription

The subscriber subscribes to a URL exposed by a hub to receive updates from one or many topics. To subscribe to updates, the client opens an HTTPS connection following the Server-Sent Events specification (W3C.REC-eventsource-20150203) to the hub's subscription URL advertised by the publisher. The GET HTTP method must be used. The connection SHOULD use HTTP version 2 or superior to leverage multiplexing and other performance-oriented related features provided by these versions.

The subscriber specifies the list of topics to get updates from by using one or several query parameters named topic. The topic query parameters MUST contain topic selectors. See topic selectors.

The protocol doesn't specify the maximum number of topic parameters that can be sent, but the hub MAY apply an arbitrary limit. A subscription is created for every provided topic parameter. See subscription events.

The EventSource JavaScript interface MAY be used to establish the connection. Any other appropriate mechanism including, but not limited to, readable streams (W3C.NOTE-streams-api-20161129) and XMLHttpRequest (used by popular polyfills) MAY also be used.

The hub sends to the subscriber updates for topics matching the provided topic selectors.

If an update is marked as private, the hub MUST NOT dispatch it to subscribers not authorized to receive it. See authorization.

The hub MUST send these updates as text/event-stream compliant events [!@W3C.REC-eventsource-20150203].

The data property MUST contain the new version of the topic. It can be the full resource, or a partial update by using formats such as JSON Patch (RFC6902) or JSON Merge Patch (RFC7386).

All other properties defined in the Server-Sent Events specification MAY be used and MUST be supported by hubs.

The resource SHOULD be represented in a format with hypermedia capabilities such as JSON-LD (W3C.REC-json-ld-20140116), Atom (RFC4287), XML (W3C.REC-xml-20081126) or HTML (W3C.REC-html52-20171214).

Web Linking (RFC5988) SHOULD be used to indicate the IRI of the resource sent in the event. When using Atom, XML or HTML as the serialization format for the resource, the document SHOULD contain a link element with a self relation containing the IRI of the resource. When using JSON-LD, the document SHOULD contain an @id property containing the IRI of the resource.

Example:

// The subscriber subscribes to updates
// for the https://example.com/foo topic, the bar topic,
// and to any topic matching https://example.com/books/{name}
const url = new URL('https://example.com/.well-known/mercure');
url.searchParams.append('topic', 'https://example.com/foo');
url.searchParams.append('topic', 'bar');
url.searchParams.append('topic', 'https://example.com/bar/{id}');

const eventSource = new EventSource(url);

// The callback will be called every time an update is published
eventSource.onmessage = function ({data}) {
    console.log(data);
};

The hub MAY require subscribers and publishers to be authenticated, and MAY apply extra authorization rules not defined in this specification.

Publication

The publisher sends updates by issuing POST HTTPS requests on the hub URL. When it receives an update, the hub dispatches it to subscribers using the established server-sent events connections.

The hub MAY also dispatch this update using other protocols such as WebSub (W3C.REC-websub-20180123) or ActivityPub (W3C.REC-activitypub-20180123).

An application CAN send events directly to subscribers without using an external hub server, if it is able to do so. In this case, it MAY NOT implement the endpoint to publish updates.

The request MUST be encoded using the application/x-www-form-urlencoded format (W3C.REC-html52-20171214) and contains the following name-value tuples:

topic: The identifiers of the updated topic. It is RECOMMENDED to use an IRI as identifier. If this name is present several times, the first occurrence is considered to be the canonical IRI of the topic, and other ones are considered to be alternate IRIs. The hub MUST dispatch this update to subscribers that are subscribed to both canonical or alternate IRIs.
data (optional): the content of the new version of this topic.
private (optional): if this name is set, the update MUST NOT be dispatched to subscribers not authorized to receive it. See authorization. It is recommended to set the value to on but it CAN contain any value including an empty string.
id (optional): the topic's revision identifier: it will be used as the SSE's id property. The provided ID MUST NOT start with the # character. The provided ID SHOULD be a valid IRI. If omitted, the hub MUST generate a valid IRI (RFC3987). An UUID (RFC4122) or a DID MAY be used. Alternatively the hub MAY generate a relative URI composed of a fragment (starting with #). This is convenient to return an offset or a sequence that is unique for this hub. Even if provided, the hub MAY ignore the ID provided by the client and generate its own ID.
type (optional): the SSE's event property (a specific event type).
retry (optional): the SSE's retry property (the reconnection time).

In the event of success, the HTTP response's body MUST be the id associated to this update generated by the hub and a success HTTP status code MUST be returned. The publisher MUST be authorized to publish updates. See authorization.

Example:

POST /.well-known/mercure HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Bearer [snip]

topic=https://example.com/foo&data=the%20content

HTTP/1.1 200 OK
Content-type: text/plain

urn:uuid:e1ee88e2-532a-4d6f-ba70-f0f8bd584022

Authorization

To ensure that they are authorized, both publishers and subscribers must present a valid JWS (RFC7515) in compact serialization to the hub. This JWS SHOULD be short-lived, especially if the subscriber is a web browser. A different key MAY be used to sign subscribers' and publishers' tokens.

Three mechanisms are defined to present the JWS to the hub:

using an Authorization HTTP header
using a cookie
using an authorization URI query parameter

When using any authorization mechanism, the connection MUST use an encryption layer such as HTTPS.

If an Authorization HTTP header is presented by the client, the JWS it contains MUST be used. The content of the authorization query parameter and of the cookie MUST be ignored.

If an authorization query parameter is set by the client and no Authorization HTTP header is presented, the content of the query parameter MUST be used, the content of the cookie must be ignored.

If the client tries to execute an operation it is not allowed to, a 403 HTTP status code SHOULD be returned.

Authorization HTTP Header

If the publisher or the subscriber is not a web browser, it SHOULD use an Authorization HTTP header. This Authorization header MUST contain the string Bearer followed by a space character and by the JWS. The hub will check that the JWS conforms to the rules (defined later) ensuring that the client is authorized to publish or subscribe to updates.

By the EventSource specification (W3C.REC-eventsource-20150203), web browsers can not set custom HTTP headers for such connections, and they can only be established using the GET HTTP method. However, cookies are supported and can be included even in cross-domain requests if the CORS credentials are set:

If the publisher or the subscriber is a web browser, it SHOULD, whenever possible, send a cookie containing the JWS when connecting to the hub. It is RECOMMENDED to name the cookie mercureAuthorization, but it may be necessary to use a different name to prevent conflicts when using multiple hubs on the same domain.

The cookie SHOULD be set during discovery (see discovery) to improve the overall security. Consequently, if the cookie is set during discovery, both the publisher and the hub have to share the same second level domain. The Domain attribute MAY be used to allow the publisher and the hub to use different subdomains. See discovery.

The cookie SHOULD have the Secure, HttpOnly and SameSite attributes set. The cookie's Path attribute SHOULD also be set to the hub's URL. See security considerations.

URI Query Parameter

If it's not possible for the client to use an Authorization HTTP header nor a cookie, the JWS can be passed as a request URI query component as defined by "Uniform Resource Identifier (URI): Generic Syntax" (RFC3986), using the authorization parameter.

The authorization query parameter MUST be properly separated from the topic parameter and from other request-specific parameters using & character(s) (ASCII code 38).

For example, the client makes the following HTTP request using transport-layer security:

GET /.well-known/mercure?topic=https://example.com/books/foo&authorization=<JWS> HTTP/1.1
Host: hub.example.com

Clients using the URI Query Parameter method SHOULD also send a Cache-Control header containing the no-store option. Server success (2XX status) responses to these requests SHOULD contain a Cache-Control header with the private option.

Because of the security weaknesses associated with the URI method (see security considerations), including the high likelihood that the URL containing the access token will be logged, it SHOULD NOT be used unless it is impossible to transport the access token in the Authorization request header field or in a secure cookie. Hubs MAY support this method.

This method is not recommended due to its security deficiencies.

Publishers

Publishers MUST be authorized to dispatch updates to the hub, and MUST prove that they are authorized to send updates for the specified topics.

To be allowed to publish an update, the JWS presented by the publisher MUST contain a claim called mercure, and this claim MUST contain a publish key. mercure.publish contains an array of topic selectors. See topic selectors.

If mercure.publish is not defined, or contains an empty array, then the publisher MUST NOT be authorized to dispatch any update. Otherwise, the hub MUST check that every topics of the update to dispatch matches at least one of the topic selectors contained in mercure.publish.

If the publisher is not authorized for all the topics of an update, the hub MUST NOT dispatch the update (even if some topics in the list are allowed) and MUST return a 403 HTTP status code.

Subscribers

To receive updates marked as private, a subscriber MUST prove that it is authorized for at least one of the topics of this update. If the subscriber is not authorized to receive an update marked as private, it MUST NOT receive it.

If the presented JWS contains an expiration time in the standard exp claim defined in (RFC7519), the connection MUST be closed by the hub at that time.

To receive updates marked as private, the JWS presented by the subscriber MUST have a claim named mercure with a key named subscribe that contains an array of topic selectors. See topic selectors.

The hub MUST check that at least one topic of the update to dispatch (canonical or alternate) matches at least one topic selector provided in mercure.subscribe.

This behavior makes it possible to subscribe to several topics using URI templates while guaranteeing that only authorized subscribers will receive updates marked as private (even if their canonical topics are matched by these templates).

Let's say that a subscriber wants to receive updates concerning all book resources it has access to. The subscriber can use the topic selector https://example.com/books/{id} as value of the topic query parameter. Adding this same URI template to the mercure.subscribe claim of the JWS presented by the subscriber to the hub would allow this subscriber to receive all updates for all book resources. It is not what we want here: this subscriber is only authorized to access some of these resources.

To solve this problem, the mercure.subscribe claim could contain a topic selector such as: https://example.com/users/foo/{?topic}.

The publisher could then take advantage of the previously described behavior by publishing a private update having https://example.com/books/1 as canonical topic and https://example.com/users/foo/?topic=https%3A%2F%2Fexample.com%2Fbooks%2F1 as alternate topic:

POST /.well-known/mercure HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Bearer [snip]

topic=https://example.com/books/1&topic=https://example.com/users/foo/?topic=https%3A%2F%2Fexample.com%2Fbooks%2F1&private=on

The subscriber is subscribed to https://example.com/books/{id} that is matched by the canonical topic of the update. This canonical topic isn't matched by the topic selector provided in its JWS claim mercure.subscribe. However, an alternate topic of the update, https://example.com/users/foo/?topic=https%3A%2F%2Fexample.com%2Fa-random-topic, is matched by it. Consequently, this private update will be received by this subscriber, while other updates having a canonical topic matched by the selector provided in a topic query parameter but not matched by selectors in the mercure.subscribe claim will not.

Payload

The mercure claim of the JWS CAN also contain user-defined values under the payload key. This JSON document will be attached to the subscription and made available in subscription events. See subscription events.

For instance, mercure.payload can contain the user ID of the subscriber, a list of groups it belongs to, or its IP address. Storing data in mercure.payload is a convenient way to share data related to one subscriber to other subscribers.

Reconnection, State Reconciliation and Event Sourcing

The protocol allows to reconciliate states after a reconnection. It can also be used to implement an Event store.

To allow re-establishment in case of connection lost, events dispatched by the hub MUST include an id property. The value contained in this id property SHOULD be an IRI (RFC3987). An UUID (RFC4122) or a DID MAY be used.

According to the server-sent events specification, in case of connection lost the subscriber will try to automatically re-connect. During the re-connection, the subscriber MUST send the last received event ID in a Last-Event-ID HTTP header.

In order to fetch any update dispatched between the initial resource generation by the publisher and the connection to the hub, the subscriber MUST send the event ID provided during the discovery as a Last-Event-ID header or a lastEventID query parameter. See discovery.

EventSource implementations may not allow to set HTTP headers during the first connection (before a reconnection) and implementations in web browsers don't allow to set it.

To work around this problem, the hub MUST also allow to pass the last event ID in a query parameter named lastEventID.

If both the Last-Event-ID HTTP header and the lastEventID query parameter are present, the HTTP header MUST take precedence.

If the Last-Event-ID HTTP header or the lastEventID query parameter exists, the hub SHOULD send all events published following the one bearing this identifier to the subscriber.

The reserved value earliest can be used to hint the hub to send all updates it has for the subscribed topics. According to its own policy, the hub MAY or MAY NOT fulfil this request.

The hub MAY discard some events for operational reasons. When the request contains a Last-Event-ID HTTP header or a lastEventID query parameter the hub MUST set a Last-Event-ID header on the HTTP response. The value of the Last-Event-ID response header MUST be the ID of the event preceding the first one sent to the subscriber, or the reserved value earliest if there is no preceding event (it happens when the hub history is empty, when the subscriber requests the earliest event or when the subscriber requests an event that doesn't exist).

The subscriber SHOULD NOT assume that no events will be lost (it may happen, for example if the hub stores only a limited number of events in its history). In some cases (for instance when sending partial updates in the JSON Patch (RFC6902) format, or when using the hub as an event store), updates lost can cause data lost.

To detect if a data lost ocurred, the subscriber CAN compare the value of the Last-Event-ID response HTTP header with the last event ID it requested. In case of data lost, the subscriber SHOULD re-fetch the original topic.

Note: Native EventSource implementations don't give access to headers associated with the HTTP response, however polyfills and server-sent events clients in most programming languages allow it.

The hub CAN also specify the reconnection time using the retry key, as specified in the server-sent events format.

Active Subscriptions

Mercure provides a mechanism to track active subscriptions. If the hub support this optional set of features, updates will be published when a subscription is created, or terminated, and a web API exposes the list of active subscriptions.

Variables are templated and expanded in accordance with (RFC6570).

Subscription Events

If the hub supports the active subscriptions feature, it MUST publish an update when a subscription is created or terminated. If this feature is implemented by the hub, an update MUST be dispatched every time a subscription is created or terminated.

The topic of these updates MUST be an expansion of /.well-known/mercure/subscriptions/{topic}/{subscriber}. {topic} is the topic selector used for this subscription and {subscriber} is an unique identifier for the subscriber.

Note: Because it is recommended to use URI Templates and IRIs for the {topic} and {subscriber} variables, values will usually contain the :, /, { and } characters. Per (RFC6570), these characters are reserved. They MUST be percent encoded during the expansion process.

If a subscriber has several subscriptions, it SHOULD be identified by the same identifier. {subscriber} SHOULD be an IRI (RFC3987). An UUID (RFC4122) or a DID MAY be used.

The content of the update MUST be a JSON-LD (W3C.REC-json-ld-20140116) document containing at least the following properties:

@context: the fixed value https://mercure.rocks/. @context can be omitted if already defined in a parent node. See json ld context.
id: the identifier of this update, it MUST be the same value as the subscription update's topic
type: the fixed value Subscription
topic: the topic selector used of this subscription
subscriber: the topic identifier of the subscriber. It SHOULD be an IRI.
active: true when the subscription is active, and false when it is terminated
payload (optional): the content of mercure.payload in the subscriber's JWS (see authorization)

The JSON-LD document MAY contain other properties.

In order to only allow authorized subscribers to receive subscription events, the subscription update MUST be marked as private.

Example:

{
   "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "type": "Subscription",
   "topic": "https://example.com/{selector}",
   "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "active": true,
   "payload": {"foo": "bar"}
}

Subscription API

If the hub supports subscription events (see subscription events), it SHOULD also expose active subscriptions through a web API.

For instance, subscribers interested in maintaining a list of active subscriptions can call the web API to retrieve them, and then use subscription events (see subscription events) to keep it up to date.

The web API MUST expose endpoints following these patterns:

/.well-known/mercure/subscriptions: the collection of subscriptions
/.well-known/mercure/subscriptions/{topic}: the collection of subscriptions for the given topic selector
/.well-known/mercure/subscriptions/{topic}/{subscriber}: a specific subscription

To access to the URLs exposed by the web API, clients MUST be authorized according to the rules defined in authorization. The requested URL MUST match at least one of the topic selectors provided in the mercure.subscribe key of the JWS.

The web API MUST set the Content-Type HTTP header to application/ld+json.

URLs returning a single subscription (following the pattern /.well-known/mercure/subscriptions/{topic}/{subscriber}) MUST expose the same JSON-LD document as described in subscription events. If the requested subscription does not exist, a 404 status code MUST be returned.

If the requested subscription isn't active anymore, the hub can either return the JSON-LD document with the active property set to false or return a 404 status code. Accordingly, collection endpoints CAN return terminated connections with the active property set to false or omit them.

Collection endpoints MUST return JSON-LD documents containing at least the following properties:

@context: the fixed value https://mercure.rocks/. @context can be omitted if already defined in a parent node. See json ld context.
id: the URL used to retrieve the document
type: the fixed value Subscriptions
subscriptions: an array of subscription documents as described in subscription events

In addition, all endpoints MUST set the lastEventID property at the root of the returned JSON-LD document:

lastEventID: the identifier of the last event dispatched by the hub at the time of this request (see reconciliation). The value MUST be earliest if no events have been dispatched yet. The value of this property SHOULD be passed back to the hub when subscribing to subscription events to prevent data loss.

As data returned by this web API is volatile, clients SHOULD validate that a response coming from cache is still valid before using it.

Examples:

GET /.well-known/mercure/subscriptions HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/mercure/subscriptions",
   "type": "Subscriptions",
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb",
   "subscriptions": [
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "active": true,
         "payload": {"foo": "bar"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2Fa-topic/urn%3Auuid%3A1e0cba4c-4bcd-44f0-ae8a-7b76f7ef1280",
         "type": "Subscription",
         "topic": "https://example.com/a-topic",
         "subscriber": "urn:uuid:1e0cba4c-4bcd-44f0-ae8a-7b76f7ef1280",
         "active": true,
         "payload": {"baz": "bat"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Aa6c49794-5f74-4723-999c-3a7e33e51d49",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:a6c49794-5f74-4723-999c-3a7e33e51d49",
         "active": true,
         "payload": {"foo": "bap"}
      }
   ]
}

GET /.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D",
   "type": "Subscriptions",
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb",
   "subscriptions": [
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "active": true,
         "payload": {"foo": "bar"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Aa6c49794-5f74-4723-999c-3a7e33e51d49",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:a6c49794-5f74-4723-999c-3a7e33e51d49",
         "active": true,
         "payload": {"foo": "bap"}
      }
   ]
}

GET /.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6 HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "type": "Subscription",
   "topic": "https://example.com/{selector}",
   "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "active": true,
   "payload": {"foo": "bar"},
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb"
}

JSON-LD Context

The JSON-LD context available at https://mercure.rocks is the following:

{
"@context": {
   "@vocab": "_:",
   "mercure": "https://mercure.rocks/",
   "id": "@id",
   "type": "@type",
   "Subscription": "mercure:Subscription",
   "Subscriptions": "mercure:Subscriptions",
   "subscriptions": "mercure:subscriptions",
   "topic": "mercure:topic",
   "subscriber": "mercure:subscriber",
   "active": "mercure:active",
   "payload": "mercure:payload",
   "lastEventID": "mercure:lastEventID"
}

Encryption

Using HTTPS does not prevent the hub from accessing the update's content. Depending of the intended privacy of information contained in the update, it MAY be necessary to prevent eavesdropping by the hub.

To make sure that the message content can not be read by the hub, the publisher MAY encrypt the message before sending it to the hub. The publisher SHOULD use JSON Web Encryption (RFC7516) to encrypt the update content. The publisher MAY provide the URL pointing to the relevant encryption key(s) in the key-set attribute of the Link HTTP header during the discovery. See discovery. The key-set attribute MUST link to a key encoded using the JSON Web Key Set (RFC7517) format. Any other out-of-band mechanism MAY be used instead to share the key between the publisher and the subscriber.

Update encryption is considered a best practice to prevent mass surveillance. This is especially relevant if the hub is managed by an external provider.

IANA Considerations

Well-Known URIs Registry

A new "well-known" URI as described in discovery has been registered in the "Well-Known URIs" registry as described below:

URI Suffix: mercure
Change Controller: IETF
Specification document(s): This specification, discovery
Related information: N/A

Link Relation Types Registry

A new "Link Relation Type" as described in discovery has been registered in the "Link Relation Type" registry with the following entry:

Relation Name: mercure
Description: The Mercure Hub to use to subscribe to updates of this resource.
Reference: This specification, discovery

JSON Web Token (JWT) Registry

A new "JSON Web Token Claim" as described in authorization will be registered in the "JSON Web Token (JWT)" with the following entry:

Claim Name: mercure
Description: Mercure data.
Reference: This specification, authorization

Security Considerations

The confidentiality of the secret key(s) used to generate the JWSs is a primary concern. The secret key(s) MUST be stored securely. They MUST be revoked immediately in the event of compromission.

Possessing valid JWSs allows any client to subscribe, or to publish to the hub. Their confidentiality MUST therefore be ensured. To do so, JWSs MUST only be transmitted over secure connections.

Also, when the client is a web browser, the JWS SHOULD not be made accessible to JavaScript scripts for resilience against Cross-site Scripting (XSS) attacks. It's the main reason why, when the client is a web browser, using HttpOnly cookies as the authorization mechanism SHOULD always be preferred.

In the event of compromission, revoking JWSs before their expiration is often difficult. To that end, using short-lived tokens is strongly RECOMMENDED.

The publish endpoint of the hub may be targeted by Cross-Site Request Forgery (CSRF) attacks when the cookie-based authorization mechanism is used. Therefore, implementations supporting this mechanism MUST mitigate such attacks.

The first prevention method to implement is to set the mercureAuthorization cookie's SameSite attribute. However, some web browsers still not support this attribute and will remain vulnerable. Additionally, hub implementations SHOULD use the Origin and Referer HTTP headers set by web browsers to verify that the source origin matches the target origin. If none of these headers are available, the hub SHOULD discard the request.

CSRF prevention techniques, including those previously mentioned, are described in depth in OWASP's Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet.

JWSs SHOULD NOT be passed in page URLs (for example, using the authorization query string parameter). Browsers, web servers, and other software may not adequately secure URLs in the browser history, web server logs, and other data structures. If JWS are passed in page URLs, attackers might be able to steal them from the history data, logs, or other unsecured locations.

Implementation Status

[RFC Editor Note: Please remove this entire section prior to publication as an RFC.]

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in (RFC6982). The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to RFC 6982, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit."

Mercure.rocks Hub

Organization responsible for the implementation:

Dunglas Services SAS Les-Tilleuls.coop

Implementation Name and Details:

Mercure.rocks, available at https://mercure.rocks

Brief Description:

This is the reference implementation of the Mercure hub. It is written in Go and is optimized for performance.

Level of Maturity:

Widely used.

Coverage:

All the features of the protocol.

Version compatibility:

The implementation follows the latest draft.

Licensing:

All code is covered under the GNU Affero Public License version 3 or later.

Implementation Experience:

Used in production.

Contact Information:

Kévin Dunglas, contact@mercure.rocks https://mercure.rocks

Interoperability:

Reported compatible with all major browsers and server-side tools.

Freddie

Implementation Name and Details:

Freddie, https://github.com/bpolaszek/freddie

Brief Description:

Freddie is a PHP implementation of the Mercure Hub Specification.

Level of Maturity:

Stable.

Coverage:

All the features of the protocol except the subscription events.

Version compatibility:

The implementation follows the latest draft.

Licensing:

All code is covered under the GNU General Public License v3.0.

Contact Information:

https://github.com/bpolaszek/freddie

Interoperability:

Reported compatible with all major browsers and server-side tools.

Ilshidur/node-mercure

Implementation Name and Details:

Ilshidur/node-mercure, https://github.com/Ilshidur/node-mercure

Brief Description:

Hub and Publisher implemented in Node.

Level of Maturity:

Beta, not suitable for production.

Coverage:

All the features of the protocol except the subscription events.

Version compatibility:

The implementation currently follows the revision 5 of the draft.

Licensing:

All code is covered under the GNU Public License version 3 or later.

Contact Information:

https://github.com/Ilshidur/node-mercure

Interoperability:

Reported compatible with all major browsers and server-side tools.

Symfony

Implementation Name and Details:

Symfony Mercure Component, available at https://symfony.com/doc/current/components/mercure.html

Brief Description:

This a publisher library written in PHP. It also provides support for Mercure in the Symfony web framework.

Level of Maturity:

Widely used.

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation follows the latest draft.

Licensing:

All code is covered under the MIT license.

Implementation Experience:

Used in production.

Contact Information:

https://symfony.com

Interoperability:

Reported compatible with the Mercure.rocks Hub.

API Platform

Implementation Name and Details:

API Platform, available at https://api-platform.com/docs/core/mercure/

Brief Description:

The API Platform framework, allows to create async APIs implementing the Mercure protocol and to generate clients for these APIs.

Level of Maturity:

Widely used.

Coverage:

All the publisher and consumer features of the protocol.

Version compatibility:

The implementation follows the latest draft.

Licensing:

All code is covered under the MIT license.

Implementation Experience:

Used in production.

Contact Information:

https://api-platform.com

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

Laravel Mercure Broadcaster

Implementation Name and Details:

Laravel Mercure Broadcaster, available at https://github.com/mvanduijker/laravel-mercure-broadcaster

Brief Description:

Laravel broadcaster for Mercure. Use the Mercure protocol as transport for Laravel Broadcast.

Level of Maturity:

Production

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation currently follows the revision 5 of the draft. An open Pull Request adds support for the latest version of the draft.

Licensing:

All code is covered under the MIT license.

Implementation Experience:

Used in production.

Contact Information:

https://github.com/mvanduijker/laravel-mercure-broadcaster

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

dart_mercure

Implementation Name and Details:

dart*mercure, available at <https://github.com/wallforfry/dart*mercure>

Brief Description:

Publisher and Subscriber library for Dart / Flutter.

Level of Maturity:

Stable

Coverage:

All the publisher and subscriber features of the protocol.

Version compatibility:

The implementation follows the latest draft.

Licensing:

All code is covered under the BSD 2-Clause "Simplified" License.

Contact Information:

https://github.com/wallforfry/dart_mercure

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

pymercure

Implementation Name and Details:

pymercure, available at https://github.com/vitorluis/python-mercure

Brief Description:

Publisher and Subscriber library for Python.

Level of Maturity:

Alpha

Coverage:

All the publisher and subscriber features of the protocol.

Version compatibility:

The implementation currently follows the revision 5 of the draft. An open Pull Request adds support for the latest version of the draft.

Licensing:

All code is covered under the BSD 2-Clause "Simplified" License.

Contact Information:

https://github.com/vitorluis/python-mercure

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

Amphp Mercure Publisher

Implementation Name and Details:

Amphp Mercure Publisher, available at https://github.com/eislambey/amp-mercure-publisher

Brief Description:

Async Mercure publisher based on Amphp.

Level of Maturity:

Stable

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation currently follows the revision 5 of the draft. An open Pull Request adds support for the latest version of the draft.

Licensing:

All code is covered under the MIT license.

Contact Information:

https://github.com/eislambey/amp-mercure-publisher

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

Java Library for Mercure

Implementation Name and Details:

Java Library for Mercure, available at https://github.com/vitorluis/java-mercure

Brief Description:

Java library to publish messages to a Mercure Hub!

Level of Maturity:

Alpha

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation currently follows the revision 5 of the draft. An open Pull Request adds support for the latest version of the draft.

Licensing:

All code is covered under the MIT license.

Contact Information:

https://github.com/vitorluis/java-mercure

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

Yii 2 Mercure behavior

Implementation Name and Details:

Yii 2 Mercure behavior, available at https://github.com/bizley/mercure-behavior

Brief Description:

Yii 2 behavior to automatically publish updates to a Mercure hub.

Level of Maturity:

Stable

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation currently follows the revision 5 of the draft.

Licensing:

All code is covered under the Apache License 2.0.

Contact Information:

https://github.com/bizley/mercure-behavior

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

GitHub Action for Mercure

Implementation Name and Details:

GitHub Action for Mercure, available at https://github.com/marketplace/actions/github-action-for-mercure

Brief Description:

Send a Mercure update when a GitHub event occurs.

Level of Maturity:

Stable

Coverage:

All the publisher features of the protocol.

Version compatibility:

The implementation currently follows the latest version of the draft.

Licensing:

All code is covered under the GNU Public License version 3 or later.

Contact Information:

https://github.com/Ilshidur/action-mercure

Interoperability:

Reported compatible with the reference implementation of the Mercure Hub.

Other Implementations

Other implementations can be found on GitHub: https://github.com/topics/mercure

Acknowledgements

Parts of this specification, especially discovery have been adapted from the WebSub recommendation (W3C.REC-websub-20180123). The editor wish to thanks all the authors of this specification.

Mercure: The Specification

Abstract

Terminology

Discovery

Content Negotiation

Topic Selectors

Subscription

Publication

Authorization

Authorization HTTP Header

Cookie

URI Query Parameter

Publishers

Subscribers

Payload

Reconnection, State Reconciliation and Event Sourcing

Active Subscriptions

Subscription Events

Subscription API

JSON-LD Context

Encryption

IANA Considerations

Well-Known URIs Registry

Link Relation Types Registry

JSON Web Token (JWT) Registry

Security Considerations

Implementation Status

Mercure.rocks Hub

Freddie

Ilshidur/node-mercure

Symfony

API Platform

Laravel Mercure Broadcaster

dart_mercure

pymercure

Amphp Mercure Publisher

Java Library for Mercure

Yii 2 Mercure behavior

GitHub Action for Mercure

Other Implementations

Acknowledgements