Streaming XHR with Ruby and Mongrel#

Wed Jan 13 09:56:29 2010

An entirely pun-free post title today, and still it sounds like something you'd see a vet about. Hey ho.

Suppose you are writing a web app in which the server needs to update the client when things change, and you don't want to do it by polling. It turns out there is a technique for this that is probably more than two years old: you make the client do an XMLHttpRequest (aside: that name is almost as bad as its capitalization) to the server, and then the server sends its response v e r y   s l o w l y. The clients XmLhTtPReQuEst object will get an onreadystatechange event every time a new packet arrives, and just has to pull the new data out of xhr.responseText and decide what to do with it.

Well, that's the theory. There are a variety of more-or-less-documented bugs and pitfalls to do with browser compatibility, as there always are (google "Comet", there are lots of resources and none of them I have the personal experience to recommend), but the new wrinkle I observed when doing this yesterday was a quarter-of-a-second lag between the server sending and the client receiving. Odd. Ruby can't be that slow, can it?

Wel, no, it's not. After monkeying with wget and netcat and wireshark and mostly failing to find out what was going on, I did strace -e setsockopt ruby server.rb and connected to it, and lo, what should I find but that something in Ruby or something in Mongrel was setting the TCP_CORK socket option

setsockopt(5, SOL_TCP, TCP_CORK, [1], 4) = 0