Lsyncd

Lsyncd (Live Syncing Daemon) is a tool that watches local directory trees for changes using inotify (on Linux) and synchronizes them to remote destinations using rsync or rsync+SSH. It provides near-real-time file replication between servers with minimal delay and resource usage.

The tool works by monitoring the filesystem for file creation, modification, deletion, and move events. When changes are detected, Lsyncd batches them briefly (configurable delay, typically 1-5 seconds) and then triggers an rsync transfer to the destination server. This event-driven approach is far more efficient than periodic rsync cron jobs because only changed files are synchronized and the delay is measured in seconds rather than minutes.

For hosting providers, Lsyncd serves several critical use cases. High availability setups use Lsyncd to replicate website files from a primary server to a standby server in near-real-time. If the primary fails, the standby has an up-to-date copy of all customer files ready to serve traffic.

Content distribution is another common use case. Providers with multiple web servers behind a load balancer use Lsyncd to keep website files synchronized across all nodes. When a customer uploads a file via FTP or their CMS, Lsyncd replicates it to the other servers within seconds.

Lsyncd configuration uses Lua scripting, which provides flexibility for complex synchronization rules. You can exclude specific file patterns, set different sync targets for different directories, and add custom actions triggered by file events. The configuration is more readable than raw inotifywait scripts.

Important limitations: Lsyncd is one-directional (source to destination). For bidirectional sync, tools like Unison or csync2 are more appropriate. Also, for database replication, use proper database replication tools since Lsyncd only handles filesystem changes.