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PHP Internals News: Episode 68: Observer API

syndicated from planet-php.net on September 17, 2020

PHP Internals News: Episode 68: Observer API

In this episode of "PHP Internals News" I chat with Levi Morrison (Twitter, GitHub) and Sammy Kaye Powers (Twitter, GitHub, Website) about the new Observer API.

The RSS feed for this podcast is https://derickrethans.nl/feed-phpinternalsnews.xml, you can download this episode's MP3 file, and it's available on Spotify and iTunes. There is a dedicated website: https://phpinternals.news

Transcript

Derick Rethans 0:15

Hi, I'm Derick, and this is PHP internals news, a weekly podcast dedicated to demystifying the development of the PHP language. This is Episode 68. Today I'm talking with Levi Morrison, and Sammy Powers, about something called the observer API, which is something that is new in PHP eight zero. Now, we've already passed feature freeze, of course, but this snuck in at the last possible moment. What this is observer API going to solve?

Levi Morrison 0:44

the observer API is primarily aimed at recording function calls in some way so it can also handle include, and require, and eval, and potentially in the future other things, but this is important because it allows you to write tools that automatically observe, hence the name, when a function begins or ends, or both.

Derick Rethans 1:12

What would you use that for?

Levi Morrison 1:13

So as an example, Xdebug can use this to know when functions are entered or or left, and other tools such as application performance monitoring, or APM tools like data dog, New Relic, tideways, instana so on, they can use these hooks too.

Derick Rethans 1:38

From what I understand that is the point you're coming in from, because we haven't actually done a proper introduction, which I forgot about. I've been out of this for doing this for a while. So both you and Sammy you work for data dog and work on their APM tool, which made you start doing this, I suppose.

Sammy Kaye Powers 1:54

Yeah, absolutely. One of the pain points of tying into the engine to to monitor things is that the hooks are insufficient in a number of different ways. The primary way that you would do function call interception is with a little hook called zend_execute_ex and this will hook all userland function calls. The problem is, it has an inherent stack bomb in it where if, depending on your stack size settings you, you're going to blow up your stack. At some point if you have a very very deeply deep call stack in PHP, PHP, technically has a virtually unlimited call stack. But when you use zend_execute_ex, it actually does limit your stack size to whatever your settings are your ulimit set stack size. One of the issues that this solves is that stack overflow issue that you can run into when intercepting userland calls but the other thing that it solves is the potential JIT issues that are coming with PHP eight, where the optimizations that it does could potentially optimize out a call to zend_execute_ex where a profiling or APM tracing kind of extension would not be able to enter set that call, because of the JIT. The Observer API enables to solve multiple issues with this. Not only that, there's more. there's more features to this thing, because zend_execute_ex by default will intercept all userland function calls, and you have no choice but to intercept every single call, whereas, this API is designed to also allow you to choose which function calls specifically you want to intercept, so there is on the very first call of a function call. And it'll basically send in the zend function. This is a little bit of a point we've been kind of going back and forth on what we actually send in on the initialis

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