Implement FileWatcher #301
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This pull request changes Litestream's current polling-based file monitoring approach and uses a file watcher instead. It uses
inotifyon Linux andkqueueon BSD. This provides two benefits. First, it's a step toward real-time read replication as changes will be able to be detected and propagated in milliseconds. Second, it allows a process to scale the number of monitored databases beyond what is practical right now.Currently, change detection is throttled at once per 100ms. This will be reduced to 10ms after another bug is fixed that was detected with the
HighLoadtest case. There's a trade off between change detection frequency and writing a lot of small files to disk. Limiting this to 100 files per second seems like a good threshold while still delivering low latency. This will be adjustable in the future.Originally the plan was to use fsnotify which is a great library but has some limitations due to it providing consistency across all platforms. Litestream needs to have a lot of control as it is being used for low-level replication and that isn't provided through the current
fsnotifyAPI. The library is also transitioning to an unmaintained state so there was some hesitancy around that going forward as well. Ultimately, the total amount of code to implement file watching for Linux & BSD wasn't too bad and it's nice to have more low-level control.Fixes #190