Monday, February 23, 2015

Mighty Warp

  • Outperforms nginx.

  • Under 1300 lines of source.

  • Clear control flow: handles one request per thread using blocking calls.

  • Slowloris DoS protection.

The secret is GHC’s runtime system (RTS). Every Haskell program must spend time in the RTS, and maybe this does hurt performance in certain cases, but for web servers it is a huge win: the RTS automatically transforms code that seems to handle one request per thread into a server with multiple event-driven processes. This saves many a context switch while keeping the source simple.

Best of all, this magic technology is widely available. To start a webserver on port 3000 using the Warp library, run these commands on Ubuntu [original inspiration]:

sudo apt-get install cabal-install
cabal update
cabal install warp
cat > server.hs << EOF
#!/usr/bin/env runghc
{-# LANGUAGE OverloadedStrings #-}
import Network.Wai (responseLBS)
import Network.Wai.Handler.Warp (run)
import Network.HTTP.Types (status200)
import Network.HTTP.Types.Header (hContentType)

main = run 3000 $ \_ f -> f $ responseLBS
  status200 [(hContentType, "text/plain")] "Hello, world!\n"
chmod +x server.hs

Eliminating context switches is the best part of the story, but there’s more. Copying data can be avoided with a simple but clever trick the authors call splicing. Using conduits instead of lazy I/O solves the non-deterministic resource finalization problem. And a few judiciously placed lockless atomic operations can work wonders: in particular, for basic Slowloris protection and for a robust file descriptor cache.


Those who fear straying too far from a C-like language can still reap the benefits in Go:

  • Goroutines are like green threads.

  • Channels are like conduits.

  • Array slices are like splices.

If I didn’t know better, I would say the designers of Go emulated the inventor of the California roll: they took some of the best features of languages like Haskell and made them palatable to a wider audience.

I wonder how Go’s RTS compares. One innate advantage GHC may have is Haskell’s type system, which leads to largely non-destructive computation, which ultimately leads to a cheap and effective scheduling scheme (namely, context switching on memory allocation). Still, I expect a well-written Go web server could achieve similar results.