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Testing sync at Dropbox (2020)

…and how we rewrote the heart of sync with confidence. Executing a full rewrite of the Dropbox sync engine was pretty daunting.

Heirloom operates on the same deterministic random seed principle for controlling the client’s execution, but because it necessarily talks to a real Dropbox server over the network, it must trade off some determinism guarantees. We’re currently thinking about how to solve this problem (e.g., by decoupling the global PRNG into several independent ones), but for now, the developer must analyze the scenario themself, inserting new logging and fine-tuning their grep filter to focus on what’s most relevant in the trace. Special thanks to Ben Blum and Sujay Jayakar for contributions and to Geoff Song, John Lai, Liam Whelan, Gautam Gupta, Iulia Tamas, and the whole Sync team for comments and review.

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