Re: [libmicrohttpd] Support for zero-downtime deploys

[Scott J. Goldman]

Rad! I'll try it out and get back to you.

On Apr 25, 2013, at 4:16 AM, Christian Grothoff <address@hidden> wrote:

> Dear Scott,
> 
> I don't like the limitations of your patch, and I think I've come up with a 
> much cleaner solution.  The attached UNTESTED diff should apply cleanly 
> against SVN HEAD.  Please let me know if it works, and I would
> really appreciate a test case (or a few, for the different threading
> modes), as this does seem a bit convoluted and there might be corner
> cases I overlooked.  And after all, the last thing you want for a 
> zero-downtime deployment is crashing code ;-).
> 
> Especially with a test, I can see this going into MHD relatively quickly ;-).
> 
> 
> Happy hacking!
> 
> Christian
> 
> On 04/25/2013 10:31 AM, Scott J. Goldman wrote:
>> Hi.
>> 
>> Where I work, we do continuous deployment of services, so it's important to 
>> us to be able to be able to deploy new code and restart our services in a 
>> way that doesn't cause downtime. One of the ways we do that is by deploying 
>> daemons which can fork-exec themselves and inherit the listening socket. 
>> With a slightly modified libmicrohttpd, we implemented this functionality. 
>> It works like this:
>> 1. send SIGUSR1 to the daemon
>> 2. daemon fork-exec's a new version of itself, passing the fd# of the 
>> listening socket, and a status pipe via command-line args
>> 3. new daemon initializes, starts accepting connections on the socket (with 
>> MHD_OPTION_LISTEN_SOCKET), and sends a message via the status pipe
>> 4. old daemon operates normally until it gets the status message, and then 
>> stops accept()ing new connections, but continues processing old ones
>> 5. old daemon finishes processing all remaining clients, and terminates
>> 
>> libmicrohttpd had nearly all the functionality we needed, except for having 
>> the "quiesce" functionality, to stop accept()'ing new connections, while 
>> continuing to process existing clients. I'm wondering if you would consider 
>> this feature for inclusion in your mainline version of the library. I've 
>> included a link to our patch below that is our rough attempt to hack in this 
>> functionality. I would be happy to revise it with your feedback.
>> 
>> https://gist.github.com/scottjg/5458005
>> https://gist.github.com/scottjg/5458005/raw/4ad505101f80485e24a6a2f1092f173706749df3/quiesce-api.patch
>> 
>> The main issue with the patch is that calling shutdown() on the listening 
>> socket is no longer acceptable in this mode. On Linux, calling shutdown() in 
>> the old daemon will prevent the new daemon from being able to accept() new 
>> clients. For our purposes we used the alternate pipe method to wake up the 
>> event loop, which appears to works fine.
>> 
>> Cheers,
>> -sjg
> <quiesce.diff>