PSR-7 Accepted!

I'm pleased to announce that as of 22:00 CDT on 18 May 2015, PSR-7 (HTTP Message Interfaces) has been accepted!

The road to PSR-7

The road to PSR-7 was a long and winding one. It started in summer of 2012 as a draft proposal on HTTP clients by Benjamin Eberlei, during which others proposed that perhaps a smaller standard on the HTTP message interfaces themselves — which would also allow targeting server-side applications, as those rely on the messages.

At that point, Chris Wilkinson picked up the ball and created the formal draft that became PSR-7. In it, he developed the original HTTP message interfaces. During his tenure as editor, he also started drafting a related proposal on URIs, but this was never picked up as a PSR. PSR-7 was polished significantly during his tenure, particularly in the aspect of keeping headers as part of the message, and having a MessageInterface describing the commonalities between the two message types, with differentiators being in the request and response descriptions.

At a certain point (I'm not sure when, as I've still not found any formal announcement in the group archives), the baton was handed over to Michael Dowling. His big addition to the proposal was modeling the message body as a stream. This has been fairly controversial from its introduction, and prompted a lengthy blogpost in summer 2014 detailing the why. I feel this was a stellar decision. Every language I've surveyed that has first-class HTTP abstractions has modeled the message bodies as streams: doing so allows for async operations in many languages (though not natively in PHP for a bit longer, alas), but also ensures that large messages do not eat your available memory. PHP itself models message bodies as streams (php://input and the output buffer are examples), so modeling them this way is a natural fit.

Shortly after that blog post was written, I was playing heavily with node.js, and was immediately struck by how uniform various modules were in terms of handling HTTP. This was in large part due to Node having a built-in, well designed (mostly!) HTTP abstraction. The other part was a side effect of that: middleware essentially arises naturally in the language due to that abstraction, meaning that middleware is abundant and works pretty much anywhere you're writing web-facing applications.

I decided to port Connect, the middleware library that gave rise to Express, the arguably dominant node web framework, to PHP. In doing so, I had one huge hurdle to jump immediately: HTTP abstraction.

Sure, every framework has an HTTP abstraction. I even contribute to one in Zend Framework. But my thought process was: I want to expose as many PHP developers to these concepts as possible, but my choice of HTTP abstraction might end up raising tribal flags. And then I remembered seeing Michael's post on PSR-7 streams and thought, "maybe I should target PSR-7!"

The problem was there was no implementation yet.

So, I put together an implementation over a weekend, and went to the list with it. About two days after Michael posted saying he was going to abandon PSR-7 due to time constraints.

After a few weeks of discussion and heavy thinking, I decided to pick it up and try to move it forward. Phil Sturgeon and Beau Simensen agreed to continue as sponsor and coordinator, respectively, and so my tenure as editor began.

This has been an interesting journey for me. When I started, there were still debates about using streams; I had to quickly ramp up on the spec and Michael's arguments, and see if I agreed with them (I did), and, better, if I could defend them (I could).

I discovered there was another aspect I felt needed to be addressed though: the messages worked great for client-side aspects, but fell short for server-side: users were left to parse the URI for query string arguments, and to parse the body manually, and headers for cookies, and… well, this is where the "mostly" came in with Node: you end up having to do a lot of stuff yourself, or rely on a microframework for that stuff. I felt we could do better. Thus, the ServerRequestInterface was born, providing access to the data we take for granted in PHP's superglobals, but also providing some necessary abstraction for populating that data, as PHP sometimes does a poor job of it (e.g., you do not get a populated $_POST for non-POST requests or for POST requests without specific media types).

December 2014 came and went, and Phil resigned from FIG, leaving me in need of an additional sponsor. Paul M Jones graciously stepped up.

The server request additions had a fair bit of back and forth, but gained consensus in the end. Except that a certain amount of feedback concerned the fact that these were value objects, but mutable. So, in January, I took a deep dive into understanding value objects and immutability, and applied the concepts to the specification. We ended up with something that's very robust, without side effects, and which eliminates whole categories of potential problems due to changing state. This, too, was quite controversial, until folks saw actual real-world examples.

I also introduced a UriInterface in order to abstract the URI components. I and others discovered that we often needed to parse the URI to get at things like the scheme, path, host, etc., and that this was tedious, repetitious, and sometimes error-prone. Introducing a URI abstraction solves this. I tried to do so in such a way that we can be forward-compatible with a later, formal URI proposal, and borrowed heavily from work Chris Wilkinson had done earlier.

At this point, we decided to put it up for a vote. Initial results were quite positive, and it looked like we had a shoo-in. Except sometime during the second week, we got a lot of people reviewing the proposal for the first time. There's nothing like imminent acceptance to raise the interest of developers, and a number of flaws were found. Key among them were the fact that we weren't doing a great job of detailing the relationship between the URI and the Host header, nor were file uploads being handled particularly well.

So, we cancelled the vote around 24 hours before acceptance, and went back to draft stage. Bernard Schussek provided some great justifications for an abstraction around file uploads which we ended up incorporating, and we ironed out the URI/host relationships in a much simpler fashion than we had previously.

And that took us to a second vote, which puts us where we are today: with a new PSR now accepted!


Many people helped contribute to this proposal. While I may be the editor and largely the public face for it at this point, it was the result of years of work before I even stepped in, and many folks contributed after I did. In particular, I want to thank:

  • Larry Garfield, who tested PSR-7 out on some sample projects, which enabled me to get concrete feedback from a usage perspective.
  • Evert Pot, who ultimately voted against the proposal, but who provided fantastic feedback and discussion throughout the lifetime of it. His help was invaluable.
  • Phil, Beau, and Paul for enduring my rants, frustrations, and self-doubts the last few months!