WebSub cannot be used by browsers because browsers cannot receive POST requests. WebSub is for server to server communications.
Mercure is basically WebSub over SSE.
The draft has been discussed several times on the HTTP WG.
Yes, as it 's "just" SSE, this works pretty well behind Cloudflare and other similar solutions. Mercure also automatically sends the NGINX-specific headers required to allow unbuffered connections.
Native mobile apps are also entirely supported. It's a very common use case. Most mobile languages have SSE client libraries.
Once the connection is upgraded, you loose all metadata included in the HTTP headers (because it’s not HTTP) and all protections relying on it.
Also CORS and SOP can be bypassed: https://dev.to/pssingh21/websockets-bypassing-sop-cors-5ajm
Of course you can reimplement everything by hand (and you must if you use WebSockets), but with SSE/Mercure you don't have to because it's plain old HTTP.
Indeed that's how it works. One of the key point of the solution is that you don't need anything client-side. The native EventSource class is all you need (but you can use more advanced SSE client libraries if wanted).
Reconnection and state reconciliation are achieved automatically. The hub implements all the needed features: it stores sent events and automatically resend them at reconnection time if they have been lost. It's possible to do this transparently because browsers will automatically send the ID of the last received message in a Last-Event-ID header when reconnecting. This feature is just often not implemented by SSE servers (because it’s not trivial to do).
It's also possible to ask events received since a specific ID, matching one or several topics just by passing the query parameters defined in the protocol section.
By the way, we are working on a new website that will make these things more clear.
WebSockets are hard to secure (they totally bypass CORS as well as other browser built-in protections), don't work (yet) with HTTP/3 and for most use cases require to implement many features by yourself: reconnection in case of network failure, refetch of lost messages, authorization, topic mechanism…
Mercure, which is built on top of SSE (it's more an extension to SSE than an alternative to it) fix these issues.
However, SSE (as well as WebSockets) can be hard to use with stacks not designed to handle persistent connections such as PHP, serverless, most web servers proxying Ruby, Python etc apps. Even for languages designed to handle persistent connections, it's often more efficient and easier to manage persistent connections with ah-hoc software running on dedicated hardware.
That's what Mercure allows. Mercure provides a "hub", a server that will maintain the persistent connections with the browsers, store events, and re-send them in case of network issues (or if asked for old messages by a client). To broadcast a message to all connected users, the server app (or even a client) can just send a single POST request to the hub. The hub will also check that clients are authorized to subscribe or publish to a given topic (that's why JWT is used).
The reference implementation is written in Go, as a module for the Caddy web server, and his very efficient/optimized (it can handle thousands of persistent connections on very small machines).
Install a Mercure hub and you have all these features available without having to write any code. Client-side, no SDK is required, you can embrace the built-in EventSource JavaScript class.
A while ago I created Mercure: an open pub-sub protocol built on top of SSE that is a replacement for WebSockets-based solutions such as Pusher. Mercure is now used by hundreds of apps in production.
At the core of Mercure is the hub. It is a standalone component that maintains persistent SSE (HTTP) connections to the clients, and it exposes a very simple HTTP API that server apps and clients can use to publish. POSTed updates are broadcasted to all connected clients using SSE. This makes SSE usable even with technologies not able to maintain persistent connections such as PHP and many serverless providers.
Mercure also adds nice features to SSE such as a JWT-based authorization mechanism, the ability to subscribe to several topics using a single connection, events history, automatic state reconciliation in case of network issue…
I maintain an open-source hub written in Go (technically, a module for the Caddy web server) and a SaaS version is also available.
This script is entirely optional and is only used to create static builds of FrankenPHP or to create self-executable PHP apps. It doesn't require a Docker image but does need some system dependencies.
It is not necessary to create a standard (dynamically linked) build of FrankenPHP and isn't used in the Docker images we provide: https://frankenphp.dev/docs/compile/
Optionally, you can embed the PHP scripts in the final binary using a Go feature similar to `include_bytes`: https://frankenphp.dev/docs/embed/
But for PHP, it's even simpler: PHP is available as a C library, and we just use it. (libphp can be embedded using static compilation, or used as a dynamic library, we support both).
This means that in addition to any PSR-7 implementation, you can run apps using HttpFoundation (Symfony, Laravel, Drupal, etc) and "legacy" code using superglobals without the need for an adapter.
Unlike Laravel and Symfony, WordPress doesn't support the worker mode of FrankenPHP (yet?), so there are not many benefits in terms of performance (except the ability to preload assets using 103 Early Hints, which can reduce the latency of a page load by 30%).
That being said, FrankenPHP makes it easy to enable HTTP cache with WordPress and simplifies the deployment story. There is a dedicated project for WordPress and FrankenPHP, that comes with a built-in HTTP cache tailored for WordPress (using the Souin Go library): https://github.com/StephenMiracle/frankenwp
FrankenPHP author here. We would very much appreciate a reproducer. According to most benchmarks, FrankenPHP is generally much faster (~3 times) than FPM when worker mode is enabled, but there are still some outliers, and we're working with PHP maintainers to fix them.
I maintain the Mercure protocol (built on SSE) and the reference implementation (written in Go, available as a standalone binary and a Caddy module) which does exactly that: https://mercure.rocks
In addition to the free and open source server, we also provide a cloud offering and on-premises versions that support clustering using Redis Streams, Kafka, Pulsar or Postgres LISTEN/NOTIFY as backends.
The solution is used by many big actors in production for years:
The draft has been discussed several times on the HTTP WG.