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The Socialtext platform has both behind-the-firewall and in-the-cloud deployment options, imposing unique constraints on Ether Calc's resource and performance requirements.

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After every 100 commands sent to a room, the server will poll the states from each active client, and save the latest snapshot it receives next to the backlog.

A freshly joined client receives the snapshot along with new commands entered after the snapshot was taken, so it only needs to replay 99 commands at most.

We also experimented with embedded JS engines (V8, Spider Monkey), but they imposed their own performance penalties when running inside Feersum's event loop.

Finally, by August 2011, we resolved to rewrite the server in

The initial rewrite went smoothly, because both Feersum and are based on the same libev event model, and's API matches closely.

It took us only an afternoon to code up a functionally equivalent server in just 80 lines of code, thanks to the concise API offered by Zappa JS.

Deploying on intranets means that we can't simply throw hardware at the problem in the same way multi-tenant, hosted-only systems did (e.g., Doc Verse, which later became part of Google Docs); we can assume only a modest amount of server capacity.

Compared to intranet deployments, cloud-hosted instances offer better capacity and on-demand extension, but network connections from browsers are usually slower and fraught with frequent disconnections and reconnections.

To recap, constraints on these resources shaped Ether Calc's architecture directions: We started with a Web Socket server implemented in Perl 5, backed by Feersum, a libev-based non-blocking web server developed at Socialtext.

Feersum is very fast, capable of handling over 10k requests per second on a single CPU.

When the prototype was put to field testing in June 2011, we quickly discovered a performance problem with long-running sessions.

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