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Look for file changes and kick off actions with Watchman

On Guard

Article from ADMIN 26/2015

By Alessio Ciregia

Watchman watches files and directories for changes and triggers actions when specific changes are noted.

Watchman is an open source tool developed by Facebook and released under the terms of Apache License 2.0. The Watchman website [1] states: "Watchman exists to watch files and record when they change. It can also trigger actions (such as rebuilding assets) when matching files change."

Written in C, Watchman is multiplatform: It works on Linux, Mac OS X, BSD, and Illumos/Solaris. Windows is not listed as a supported operating system. Although the main goal of Watchman might not be as I describe in this article, its job is to trigger actions when a file changes. This functionality perfectly fits various needs.

Watchman might seem complex at first glance, with its options and configuration directives (yes, it is), but I'll start with some simple examples; then, you can delve deeper into the program on your own.

Replicating Files

Working with distributed or replicated systems is a common task these days. The magic word nowadays – the cloud – is an unclear term, but if you want to simplify the concept, the cloud is what years ago was called a "cluster." If you want to replicate a directory on a cluster of servers, a lot of software, besides shared storage like NFS, is at your disposal. However, often the learning curve is steep, yet the goal is so simple.

The basic scenario is as follows: You have a directory on a server and you want to replicate its content on another server. The simple way to achieve this is with a copy command (maybe using the rsync tool) triggered by a job scheduled in a crontab. Obviously this solution is not optimal, because the scheduled job will be executed even if the directory has not changed, the time between the execution of a task and a following one could be too long or too short, or the copy starts while a file is in a transitional stage, leading to some sort of

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