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New MongoDB Drivers for PHP and HHVM: Architecture

We recently released a new version of the MongoDB driver for PHP (the mongodb extension). This release is the result of nearly a year and a half of work to re-engineer and rewrite the original MongoDB driver (mongo). In the previous blog post, I covered the back story of the how and why we undertook this effort. In this new blog post, I will talk about the architecture of the new driver.

Let's go over the goals that we came up with at the end of the previous post (in a different order), and then explain what that means for the new architecture.

Support for Other PHP Engines like HHVM

When we were starting the process of writing the drivers, we noticed a big uptake in HHVM usage, and hence we thought about creating something that would not only work for PHP, but also for HHVM.

PHP 5 has been around for ages, but we wanted to support a reasonably new version only. Some of the older versions (such as PHP 5.3) came with a significant maintenance burden and/or lacked features at the language and extension API levels, so we set a minimum version requirement of PHP 5.4.

On the other side, HHVM is a new PHP engine, developed by Facebook as an iteration on an earlier effort to compile PHP code to C++. HHVM contains a JIT engine, and also supports a PHP language extension called Hack.

HHVM is an engine that we did not have any experience with, but users were asking about solutions for running HHVM with an official MongoDB driver. Its extension API does not share anything with PHP's own; moreover, it is in C++ instead of plain C.

And then, PHP 7 came along. Its internals are very much different than PHP 5, although that's not readily apparent from PHP userland. (Userland is PHP code that people write, unlike internals, which are written in C). With PHP 5 and 7 being so different we were not quite sure whether they would qualify as a different engines or not. In the end, they did not.

Of course, to support three (two) engines meant that we would need to invest three (two) times the amount of development effort, and that is not great. This brings us to the next point.

The Driver Should Be Bare Bones

Because we did not want to do three times the amount of work, we decided that the new driver should be "bare bones". This means that the new drivers would only implement features that are strictly necessary to communicate with MongoDB. That meant no command helpers and no fancy syntactic sugar.

Concentrating on only basic features would make it a lot faster to develop the drivers, and a lot less work to keep them up to date.

Besides that, the old mongo extension also implemented the MongoDB wire protocol: the binary protocol that drivers use to communicate with the server. Every other driver in the MongoDB ecosystem was reimplementing the exact same thing. That is of course not very efficient.

No Reinvention of the Wheel

Luckily, when we began develoment on the new PHP drivers, a project to implement a C driver for MongoDB (including handling of BSON, our data format) was nearly complete. This meant that we also would not have to implement (and maintain) the wire protocol for each of these new drivers. Instead, we choose to use libbson, a "new shared library written in C for developers wanting to work with the BSON serialization format", as well as mongo-c-driver (aka libmongoc), a "client library written in C for MongoDB". We ended up having to use a few "private" APIs in the C driver, as the C driver is mainly focussed on being a driver, and not a client library upon which other drivers are built; however, they are in the process of changing this.

Provide an Easy to Use API

A bare bones driver that implements the most basic things is a great idea from the perspective of a driver author, but it does not necessarily suit our users well. They often prefer an easy to use API for writing their applications. This is why in addition to the drivers we are also developing a higher level PHP library geared towards application developers.

This PHP library is installable through Composer, and implements a nicer and easier to use API. In the future, this is where we well add user management, GridFS support, and various other non-core extensions to the driver. Because these additions are implemented in PHP, maintaining this library is a lot less complex than maintaining them through C or C++. On top of that, there is no need to reimplement such features for both engines (PHP and HHVM); hence, this approach will save us time.

Backwards Compatibility

Neither the new mongodb extension or the PHP library implement the same APIs found in mongo. This means that some effort is required to port existing applications that make use of mongo to either the new APIs. We strongly recommend that folks use the PHP library's API over mongodb, though, as it will be much easier to work with. That said, it is possible to install the old and new drivers alongside each other (from mongodb 1.1.2), but you cannot share resources (i.e. connections, query results) between them. A migration guide for converting applications from the old to new driver and library will follow shortly.


When we tie all the bits and pieces together, the new architecture diagram looks like:

As you can see, in the bottom we have the system libraries libbson and mongo-c-driver. On top of that, we find drivers for PHP 5 and 7, and HHVM. The highest level contains our userland PHP library. As part of this userland layer, we are also working on support for GridFS, helping out on the effort to make Doctrine ODM work with the new driver, and supporting an effort to write a userland library that imitates the mongo extension's API.

What's Next?

In the next installment of this series on the new MongoDB driver, we will focus on a new feature in the new MongoDB driver: ODS - the Object Data Store.


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