Using File-Watchers

Many front-end development frameworks rely on a filesystem watcher that tells the OS when a file is modified and triggers some type of event (usually a browser refresh). Nanobox includes file-watching functionality that can be enabled when running your app locally.

To enable the file-watcher for the duration of run, set fs_watch: true in the run.config section of your boxfile.yml.

  fs_watch: true

How the File-Watcher Works

Nanobox includes two file-watchers - a fast watcher and a slow watcher. If the fast file-watcher panics or fails, it will failover to the slower file-watcher.


The fast file-watcher uses your OS's native file-watcher which holds a file descriptor open for every file in your project directory and gets notified by your OS any time a file changes.

NOTE: The most common cause of the fast file-watcher panicking is exceeding your system's ulimit, which defines how many file can be open at a time. A quick google search will provide instructions on increasing your system's ulimit unique to your OS.


The slow file-watcher is a "polling" solution that loops through all the files in your project directory and checks if they have been updated. This solution gets slower as the number of file in your project increases, but is still relatively fast. In our benchmarks, a codebase with 1000+ files took about a second to register a modified file with minimal system overhead.

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