Response to Netflix's “Node.js in Flames” Blog Post
gist.github.com
gist.github.com
While creating a grammar from the combination of the regular expressions (which is why you can't just have a hash) is definitely a solution (quite possibly THE solution), that's definitely not an MVP-type of solution, and is pretty non-trivial.
The actual problem was them constantly adding handlers by accident.
One of the reasons I love and use HN so much (I give almost no other site so much pull on my personal opinion) is that smart (or very careful and well-read dumb people) leave well-thought-out comments like the one I mentioned.
However, I can't take a stack seriously as an enterprise or production ready piece of software when it does something like iterate a list recursively in performance sensitive code. It's just bizarre. JS doesn't have tail call optimization and for loops are just fine. Being able to blow out the JS stack by having 'too many handlers' is terrifying to me, especially because JS runtimes have catastrophically poor support for handling and debugging stack overflows.
The use of a single array for handlers that can contain multiple items also feels like an architectural red flag to me, but I can understand how that would happen since JS's standard library is so anemic and lacks useful data structures. What you really want is a set (or ordered set, if handler order is super important - it probably is here)
I saw a number of entries in those flame charts that appeared to be V8 compilation internals, which is suspicious to me. But those are probably just debugger/symbol resolution issues, I hope, and not actual JIT recompilation constantly happening during route dispatch? (If v8 were compiling code constantly during dispatch, that would certainly explain multiple-ms pauses...)
Can you explain how you would do asynchronous tasks in series without recursion? For loops successfully handle this when either the tasks are synchronous or can be run in parallel. Neither of those are true here.
It's pretty trivial to implement this given a promises API and any sort of event queue, but you could also just do it with callbacks. So in the browser you'd literally just use setTimeout.
One reason this is so common is that it's trivial to end up blowing out the stack with recursive IO callbacks, because sometimes all your read()s complete instantly because all the data's in cache. Oops!
Maybe this can turn into a chance to write some code (I'm willing to) to improve express?
The 'push ready callbacks onto a queue' model is really common and relatively straightforward to optimize. I've used it in a variety of applications and it can provide very good throughput in both single and multiple threaded scenarios.
1. Create some sort of handler execution queue
2. Push first handler onto queue
3. When a handler runs, which may translate into the firing of another handler/ translation of the route, push a "handle callback" (for lack of a better term) onto the queue
4. Return to #3 and continue forever (which is a possibility) or run out of handlers at some point (be done)
Most robust implementations have a limit on how long they will pump the queue instead of continuing forever - in most of my applications I cap it at somewhere around 1-10ms. You'd want a shorter limit for realtime stuff like games, a higher limit for high-throughput scenarios like servers. When the limit is reached I yield to the event loop and then continue running handlers from the queue after that.
Not personally familiar with Express's internals, but if this is async iteration nothing's pushing onto the call stack in the first place. Unless you're using generators, but in that case you can just use a for loop.
Which is why open source is so beautiful -- someone who is "more serious" can come along, download the source, rewrite that functionality and submit a fork request.
Wouldn't a better solution, in this context, be to aggregate values under the same key?
Of course when you're operating on the scale of Netflix then, yeah, sure.
"This turned out be caused by a periodic (10/hour) function in our code. The main purpose of this was to refresh our route handlers from an external source. This was implemented by deleting old handlers and adding new ones to the array. Unfortunately, it was also inadvertently adding a static route handler with the same path each time it ran."
And express allows for more than one handler even for a static route.
app.get('/', function(req, res, next) { next(); });
app.get('/', function(req, res) { res.send(); });This can be a little confusing, especially given that you'd be specifying static and non-static paths separately, but it could be argued that that is less confusing than some methods. For example, nginx's method allows you to define paths in any order, but checks routes matching regular expressions first in config order, then picks a static route by most specific match; IOW the order of your routes in nginx doesn't matter until you have regular expressions, in which case it is surprisingly critical some of the time and irrelevant the rest of the time.
Breaking it up into 'regular expression routes (checked first)' and 'static routes (checked last)' could simplify things conceptually, but doubtlessly raises more usability issues as well.