An open, simple, web-scale and decentralized pubsub protocol. Anybody can play.
As opposed to more developed (and more complex) pubsub specs like Jabber Publish-Subscribe [XEP-0060] this spec's base profile (the barrier-to-entry to speak it) is dead simple. The fancy bits required for high-volume publishers and subscribers are optional. The base profile is HTTP-based, as opposed to XMPP (see more on this below).
To dramatically simplify this spec in several places where we had to choose between supporting A or B, we took it upon ourselves to say "only A", rather than making it an implementation decision.
We offer this spec in hopes that it fills a need or at least advances the state of the discussion in the pubsub space. Polling is extremely wasteful and high latency. We think a decentralized pubsub layer is a fundamental, missing layer in the Internet architecture today and its existence, more than just enabling the obvious lower latency feed readers, would enable many cool applications, most of which we can't even imagine. But we're looking forward to decentralized social networking.
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This document is currently an editor's draft. Current bugs and issues are managed in GitHub.
This document was published by the Social Web Working Group as a First Public Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to email@example.com (subscribe, archives). All comments are welcome.
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This document is governed by the 1 September 2015 W3C Process Document.
(This section is non-normative.)
A potential subscriber initiates discovery by retrieving (GET or HEAD request) the topic to which it wants to subscribe. The HTTP [RFC7231] response from the publisher MUST include at least one Link Header [RFC5988] with rel=hub (a hub link header) as well as exactly one Link Header [RFC5988] with rel=self (the self link header). The former MUST indicate the exact URL of a PubSub hub designated by the publisher. If more than one URL is specified, it is expected that the publisher pings each of these URLs, so the subscriber may subscribe to one or more of these. The latter will point to the permanent URL for the resource being polled.
In the absence of HTTP [RFC7231] Link headers, subscribers MAY fall back to other methods to discover the hub(s) and the canonical URI of the topic. If the topic is an XML based feed, it MAY use embedded link elements as described in Appendix B of Web Linking [RFC5988]. Similarly, for HTML pages, it MAY use embedded link elements as described in Appendix A of Web Linking [RFC5988]. Finally, publishers MAY also use the Well-Known Uniform Resource Identifiers [RFC5785] .host-meta to include the <Link> element with rel="hub".
Subscribing to a topic URL consists of four parts that may occur immediately in sequence or have a delay.
Unsubscribing works in the same way, except with a single parameter changed to indicate the desire to unsubscribe. Also, the Hub will not validate unsubscription requests with the publisher.
Subscription is initiated by the subscriber making an HTTPS or HTTP POST [RFC7231] request to the hub URL. This request has a Content-Type of application/x-www-form-urlencoded (described in Section 17.13.4 of [W3C.REC-html401-19991224]) and the following parameters in its body:
Subscribers MAY also include additional HTTP [RFC7230] request parameters, as well as HTTP [RFC7230] Headers if they are required by the hub. In the context of social web applications, it is considered good practice to include a From HTTP [RFC7231] header (as described in section 5.5.1 of Hypertext Transfer Protocol [RFC7231]) to indicate on behalf of which user the subscription is being performed.
Hubs MUST ignore additional request parameters they do not understand.
Hubs MUST allow subscribers to re-request subscriptions that are already activated. Each subsequent request to a hub to subscribe or unsubscribe MUST override the previous subscription state for a specific topic URL and callback URL combination once the action is verified. Any failures to confirm the subscription action MUST leave the subscription state unchanged. This is required so subscribers can renew their subscriptions before the lease seconds period is over without any interruption.
The topic and callback URLs MAY use HTTP or HTTPS [RFC7230] schemes. The topic URL MUST be the one advertised by the publisher in a Self Link Header during the discovery phase. (See Section 4 ). Hubs MAY refuse subscriptions if the topic URL does not correspond to the one advertised by the publisher. The topic URL can otherwise be free-form following the URI spec [RFC3986]. Hubs MUST always decode non-reserved characters for these URL parameters; see section 2.4 on "When to Encode or Decode" in the URI spec [RFC3986].
The callback URL MAY contain arbitrary query string parameters (e.g., ?foo=bar&red=fish). Hubs MUST preserve the query string during subscription verification by appending new parameters to the end of the list using the & (ampersand) character to join. Existing parameters with names that overlap with those used by verification requests will not be overwritten. For event notification, the callback URL will be POSTed to including any query-string parameters in the URL portion of the request, not as POST body parameters.
The hub MUST respond to a subscription request with an HTTP [RFC7231] 202 "Accepted" response to indicate that the request was received and will now be verified (Section 5.3 ) and validated (Section 5.2 ) by the hub. The hub SHOULD perform the verification and validation of intent as soon as possible.
If a hub finds any errors in the subscription request, an appropriate HTTP [RFC7231] error response code (4xx or 5xx) MUST be returned. In the event of an error, hubs SHOULD return a description of the error in the response body as plain text. Hubs MAY decide to reject some callback URLs or topic URLs based on their own policies (e.g., domain authorization, topic URL port numbers).
Subscriptions MAY be validated by the Hubs who may require more details to accept or refuse a subscription. The Hub MAY also check with the publisher whether the subscription should be accepted.
If (and when), the subscription is accepted, the hub MUST perform the verification of intent of the subscriber.
If (and when), the subscription is denied, the hub MUST inform the subscriber by sending an HTTP [RFC7231] GET request to the subscriber's callback URL as given in the subscription request. This request has the following query string arguments appended (format described in Section 17.13.4 of [W3C.REC-html401-19991224]):
Hubs may provide an additional HTTP [RFC7231] Location header (as described in section 7.1.2 of Hypertext Transfer Protocol [RFC7231]) to indicate that the subscriber may retry subscribing to a different hub.topic. This allows for limited distribution to specific groups or users in the context of social web applications.
The subscription MAY be denied by the hub at any point (even if it was previously accepted). The Subscriber SHOULD then consider that the subscription is not possible anymore.
In order to prevent an attacker from creating unwanted subscriptions on behalf of a subscriber (or unsubscribing desired ones), a hub must ensure that the subscriber did indeed send the subscription request.
The hub verifies a subscription request by sending an HTTP [RFC7231] GET request to the subscriber's callback URL as given in the subscription request. This request has the following query string arguments appended (format described in Section 17.13.4 of [W3C.REC-html401-19991224]):
The subscriber MUST confirm that the hub.topic corresponds to a pending subscription or unsubscription that it wishes to carry out. If so, the subscriber MUST respond with an HTTP success (2xx) code with a response body equal to the hub.challenge parameter. If the subscriber does not agree with the action, the subscriber MUST respond with a 404 "Not Found" response.
The hub MUST consider other server response codes (3xx, 4xx, 5xx) to mean that the verification request has failed. If the subscriber returns an HTTP [RFC7231] success (2xx) but the content body does not match the hub.challenge parameter, the hub MUST also consider verification to have failed.
Hubs MAY make the hub.lease_seconds equal to the value the subscriber passed in their subscription request but MAY change the value depending on the hub's policies. To sustain a subscription, the subscriber MUST re-request the subscription on the hub before hub.lease_seconds seconds has elapsed.
The publisher MUST inform the hubs it previously designated when a topic has been updated. The hub and the publisher can agree on any mechanism, as long as the hub is eventually able send the updated payload to the subscribers.
A content distribution request is an HTTP [RFC7231] POST request from hub to the subscriber's callback URL with the payload of the notification. This request MUST have a Content-Type corresponding to the type of the topic. The hub MAY reduce the payload to a diff between two consecutive versions if its format allows it.
The request MUST include a Link Header [RFC5988] with rel=hub pointing to the Hub as well as a Link Header [RFC5988] with rel=self set to the topic that's being updated. The Hub SHOULD combine both headers into a single Link Header [RFC5988].
The successful response from the subscriber's callback URL MUST be an HTTP [RFC7231] success (2xx) code. The hub MUST consider all other subscriber response codes as failures; that means subscribers MUST NOT use HTTP redirects for moving subscriptions. The response body from the subscriber MUST be ignored by the hub. Hubs SHOULD retry notifications repeatedly until successful (up to some reasonable maximum over a reasonable time period). Subscribers SHOULD respond to notifications as quickly as possible; their success response code SHOULD only indicate receipt of the message, not acknowledgment that it was successfully processed by the subscriber.
If the subscriber supplied a value for hub.secret in their subscription request, the hub MUST generate an HMAC signature of the payload and include that signature in the request headers of the content distribution request. The X-Hub-Signature header's value MUST be in the form method=signature where method is one of the recognized algorithm names and signature is the, hexadecimal representation of the signature. The signature MUST be computed using the HMAC algorithm [RFC6151] with the request body as the data and the hub.secret as the key.
The following algorithms are added as recognized algorithm names, as specified by [FIPS PUB 180-4]:
When subscribers receive a content distribution request with theX-Hub-Signature header specified, they SHOULD recompute the signature with the shared secret using the same method (provided in the X-Hub-Signature header) as the hub. If the signature does not match, subscribers MUST still return a 2xx success response to acknowledge receipt, but locally ignore the message as invalid. Using this technique along with HTTPS [RFC2818] for subscription requests enables simple subscribers to receive authenticated notifications from hubs without the need for subscribers to run an HTTPS [RFC2818] server.
Please note however that this signature only ensures that the payload was not forged. Since the notification also includes headers, these should not be considered as safe by the subscriber, unless of course the subscriber uses HTTPS [RFC2818] callbacks.