Continuations in Servers

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Hiroshi Yamauchi, Google


We are looking at a problem in servers about scalability vs. productivity. There are two types of architectures: synchronous (aka blocking, 'thread-per-request') and asynchronous (aka non-blocking, 'select-server' style, event-driven) servers. The key distinction lies in how threads are mapped to requests and whether to block or not in network I/O (eg RPC or database access). The former is better in terms of productivity because the code gets simpler. The latter is better in scalability because a thread does not have to be tied to each request. We are investigating a continuation-based solution that gives us the best of both worlds: simpler code and better scalability. A synthetic rpc server has been 'converted' to use continuations and showed good performance. An experiment with a production server has been ongoing. We've been using a modified version of the old MLVM callcc patches.