Mainly to get off HPHP and relieve both that team and ours of the constant coordination and toe-mashing. We thought “modernizing” the code would be a better investment than jerry-rigging the Zend engine back into our infrastructure. I think it was the right call.
Also, with Python we could interact with other services using basic Thrift bindings, and even provide our own endpoints. With PHP, all RPCs and queries were prebuilt into our standard libraries with custom glue code that increasingly made non-Zend assumptions. Again, we could have made it work, but we would be swimming against the current.
All the rest of the database management code ended up being moved to Python, and benefitting greatly, so it eventually worked out.
Did you consider "moving back" to PHP instead of emigrating to Python? Given that the code was originally written for PHP that sounds like the easiest way out of any problems caused by HPHP. Was it politics which led to the decision that HPHP was the only allowable PHP engine?
If we wanted to continue to run Zend we would have had to build out the entire supporting toolchain, from system package up. FB servers at the time ran on an ancient Linux distribution (I don’t remember if it was ever made public so I won’t say) with security patches and a modern kernel, so the system PHP was 4.2 or something similarly unusable. I think our server team eventually disabled the package because it kept causing problems when it was inadvertently installed.
And if we went that route, we would have had to build Thrift bindings and write all the glue code for any other internal service we wanted to talk to. They all had official client libraries, but they weren’t built for Zend anymore, and since they were frequent targets of micro-optimizations, they would no longer run on it. We would have been the sole maintainers of this alternate distribution and libraries, as well. And THEN we would have had to talk to the server team and convince them that it’s cool install this custom package that directly conflicts with some of the most important code we run that’s literally maintained by a team of PhDs and industry luminaries, but we pinky swear it won’t mess up. (Ever built a .deb or .rpm?) That would have been politics, but it never got that far.
Another team had already done the work for Python to “compile” your script and all its dependencies into a self-executing compressed file. All you had to do to talk to another service was import the client libraries, write your logic, run the packager, and ship the binary. (Very Go-like, in retrospect.) It was already widely used and well-supported, as a group effort. I even fixed a few bugs. So, for this particular niche at this particular company, PHP was a victim of its success elsewhere.
Also, with Python we could interact with other services using basic Thrift bindings, and even provide our own endpoints. With PHP, all RPCs and queries were prebuilt into our standard libraries with custom glue code that increasingly made non-Zend assumptions. Again, we could have made it work, but we would be swimming against the current.
All the rest of the database management code ended up being moved to Python, and benefitting greatly, so it eventually worked out.
In summary, organizational concerns.