FSWatcher

A production-ready file watcher for Go with built-in debouncing and filtering.

FSWatcher is a robust, concurrent, and cross-platform file system watcher for Go. It provides a simple and powerful API to monitor directories for file system changes, designed for high-performance applications and development tools. The goal is to abstract away the complexities of platform-specific APIs, offering a unified, easy-to-use, and dependency-free interface.

Table of contents

• Supported platforms
• Why FSWatcher
• Quick start
• What makes it different
• Options
• Methods
• Logging
• Workflow diagram
• Project structure
• Advanced usage
• FAQ

Platforms

FSWatcher uses native OS APIs for efficient, low-overhead monitoring with near-zero CPU usage when idle.

Platform	Native System API Used	Status
✅ macOS	FSEvents framework	Default (requires CGO)
	kqueue	Supported (Pure Go, no CGO)
✅ BSD (FreeBSD, OpenBSD, NetBSD, DragonFly)	kqueue	Fully supported (Pure Go)
✅ Linux	inotify	Fully supported
	fanotify	Partial support (planned for future enhancements)
✅ Windows	ReadDirectoryChangesW	Fully supported

macOS and BSD Support

On macOS, FSWatcher supports two backends:

1. FSEvents (Default): Uses the native macOS FSEvents framework. Recommended for macOS — provides the most efficient and comprehensive monitoring. Requires CGO (CGO_ENABLED=1)
2. kqueue (Pure Go): Uses the BSD kernel queue notification interface. Allows building a static binary without CGO (CGO_ENABLED=0), with a shared backend usable on FreeBSD, OpenBSD, NetBSD, and DragonFly

Why use FSEvents (CGO)? The FSEvents API is designed specifically for file system monitoring and includes OS-level coalescing. This makes it significantly more accurate and CPU-efficient than kqueue for high-volume operations. Benchmarks show FSEvents achieves 100% path detection accuracy where kqueue may miss up to 30% of events for short-lived files under heavy load.

To use the kqueue backend on macOS, disable CGO when building:

CGO_ENABLED=0 go build

The library automatically selects the correct implementation based on the build tags. For BSD systems, kqueue is the default and only backend.

Why FSWatcher

Most Go file watchers give you raw OS events—which means duplicate events and noise from system files. FSWatcher solves this:

• Built-in debouncing - Merge rapid-fire events automatically
• Smart filtering - Regex patterns + automatic system file exclusion (.git, .DS_Store, etc.)
• Zero dependencies - Just standard library + native OS APIs
• Context-based - Modern Go patterns with graceful shutdown

Quick Start

To add FSWatcher to your project, use go get:

go get github.com/sgtdi/fswatcher

package main

import (
	"context"
	"fmt"
	"time"

	"github.com/sgtdi/fswatcher"
)

func main() {
    // Create watcher with debouncing that watches the current working directory
    w, _ := fswatcher.New(
        fswatcher.WithCooldown(200*time.Millisecond),
    )

    ctx := context.Background()
    go w.Watch(ctx)
    fmt.Println("fswatcher started, change a file in watcher dir")

    // Process clean, debounced events
    for event := range w.Events() {
        var types, flags []string
        // Loop through types and flags
        for _, t := range event.Types {
            types = append(types, t.String())
        }
        for _, f := range event.Flags {
            flags = append(flags, f)
        }
        fmt.Printf("File changed: %s %v %v\n", event.Path, types, flags)
    }
}

What makes it different

Feature	FSWatcher	fsnotify / notify
Debouncing	Built-in, configurable	Manual implementation
Path filtering	Regex include/exclude	Manual implementation
System files	Auto-ignored	Manual filtering
API style	Functional options	Imperative
Duplicate events	Handled automatically	Manual deduplication

Options

Customize the watcher's behavior using functional options passed to fswatcher.New().

Option	Description	Default
WithPath(path, ...)	Adds an initial directory to watch. Can be called multiple times. Accepts optional PathOption values like WithDepth, WithEventMask, WithPathIncRegex, etc	Current directory
WithCooldown(d)	Sets the debouncing cooldown period. Events for the same path arriving within this duration will be merged	100ms
WithBufferSize(size)	Sets the size of the main event channel	4096
WithIncRegex(patterns...)	Global include regex patterns — only matching paths are processed. If not set, all non-excluded paths are processed	(none)
WithExcRegex(patterns...)	Global exclude regex patterns — matching paths are ignored. Exclusions take precedence over inclusions	(none)
WithSeverity(level)	Sets the logging verbosity (SeverityDebug, SeverityInfo, SeverityWarn, SeverityError)	SeverityWarn
WithLogFile(path)	Sets a file for logging. Use "stdout" to log to the console or "" to disable	(disabled)
WithLinuxPlatform(p)	Sets a specific backend (PlatformInotify or PlatformFanotify) on Linux	PlatformInotify
WithDepth(depth)	Sets the watch depth for a path (WatchNested or WatchTopLevel). Passed to WithPath	WatchNested
WithEventMask(types...)	Filters event types for a specific path. Only the listed types are forwarded to Events(), everything else is dropped. Passed to WithPath	(all types)
WithPathIncRegex(patterns...)	Include regex patterns scoped to a specific path, independent of the global WithIncRegex. Passed to WithPath	(none)
WithPathExcRegex(patterns...)	Exclude regex patterns scoped to a specific path, independent of the global WithExcRegex. Passed to WithPath	(none)
WithPathFilter(filter)	Custom PathFilter implementation for a specific path. Passed to WithPath	(none)

Methods

Once you have a Watcher instance from New(), you can use the following methods to control it:

Method	Description
Watch(ctx)	Starts the watcher. Blocking — runs until the context is canceled. Run in a goroutine. A watcher instance is single-use: after Watch() returns, create a new watcher with New()
Events()	Returns a read-only channel (<-chan WatchEvent) for receiving file system events
AddPath(path)	Adds a new directory to monitor at runtime. Safe to call concurrently with DropPath()
DropPath(path)	Stops monitoring a directory at runtime. Safe to call concurrently with AddPath()
Close()	Initiates a graceful shutdown — alternative to canceling the context
IsRunning()	Returns true if Watch() is currently running
Stats()	Returns a WatcherStats struct with runtime statistics (uptime, events processed, etc.)
Dropped()	Returns a read-only channel for events dropped because the main Events() channel was full

Logging

FSWatcher includes a built-in structured logger to help with debugging and monitoring. You can control the verbosity and output destination using WatcherOpt functions.

Log Severity

Level	Description
SeverityNone	No messages logged
SeverityError	🚨 Only critical errors (e.g., platform failures)
SeverityWarn	⚠️ Errors and warnings (e.g., event queue overflows) — default
SeverityInfo	ℹ️ Errors, warnings, and informational messages (e.g., watcher start/stop, paths added/removed)
SeverityDebug	🐛 Everything, including detailed event processing steps (raw events, filtering, debouncing)

Workflow

The watcher operates in a clear, multi-stage pipeline. Each raw OS event goes through:

Stage	Description
OS API	The OS (FSEvents, inotify, ReadDirectoryChangesW) captures a raw file system event
Filtering	The path is checked against system file rules and regex patterns. Excluded paths are dropped
Debouncing	The event is held for the cooldown period. Subsequent events for the same path are merged. The timer resets on each new event
User channel	The final, clean WatchEvent is sent to Events()

Event Aggregation

FSWatcher uses an EventAggregator to handle the noise of many OS events. For example, when you save a file, the OS might emit multiple "Edit" and "Chmod" events in rapid succession.

The EventAggregator works by:

1. Deduplication — multiple events for the same path within the cooldown window are merged
2. Type collection — the final WatchEvent contains all unique EventType values (e.g., both Create and Edit if both occurred)
3. Flag collection — all platform-specific flags are collected and merged
4. Deterministic ordering — aggregated Types are sorted by enum order and Flags are sorted lexicographically
5. Timer-based flushing — an event is only sent after no new events for that path have arrived within the cooldown window

Project structure

.
├── watcher.go                 # Core watcher logic and public API
├── options.go                 # Configuration options (functional pattern)
├── event.go                   # Event definitions and aggregation logic
├── filters.go                 # Path filtering logic
├── logs.go                    # Logging helpers
├── errors.go                  # Custom error types
├── watcher_darwin.go          # macOS (Pure Go bridge)
├── watcher_darwin_fsevents.go # macOS (FSEvents CGO) implementation
├── watcher_kqueue.go          # kqueue implementation (macOS no-cgo & BSD)
├── watcher_bsd.go             # BSD-specific initialization
├── watcher_linux.go           # Linux platform loader
├── watcher_linux_inotify.go   # Linux (inotify) implementation
├── watcher_linux_fanotify.go  # Linux (fanotify) placeholder
├── watcher_windows.go         # Windows (ReadDirectoryChangesW) implementation
├── go.mod                     # Go module definition
└── examples/
    └── main.go                # Example usage

Advanced usage

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/sgtdi/fswatcher"
)

func main() {

	fsw, err := fswatcher.New(
		fswatcher.WithPath("./"),
		fswatcher.WithSeverity(fswatcher.SeverityDebug),
	)
	if err != nil {
		log.Fatalf("Failed to create watcher: %v", err)
	}

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	go func() {
		if err := fsw.Watch(ctx); err != nil && err != context.Canceled {
			log.Printf("Watcher error: %v", err)
		}
	}()

	for event := range fsw.Events() {
		fmt.Printf("Received event:\n%s", event.String())
	}
}

Per-path event filtering

Use WithEventMask to receive only specific event types on a given path, and WithPathIncRegex / WithPathExcRegex to scope regex filters to individual directories:

fsw, err := fswatcher.New(
    // Watch uploads — only care about new files and deletions, ignore edits
    fswatcher.WithPath("/var/uploads",
        fswatcher.WithEventMask(fswatcher.EventCreate, fswatcher.EventRemove),
    ),
    // Watch config — only .yaml files, all event types
    fswatcher.WithPath("/etc/myapp",
        fswatcher.WithPathIncRegex(`\.ya?ml$`),
    ),
)

FAQ

1. Why create another file watcher?

FSWatcher was built to provide features like built-in debouncing and powerful filtering out-of-the-box, which often require manual implementation in other libraries. It also uses a modern Go API with functional options and context-based lifecycle management

2. How does it handle a large number of files?

It uses native OS APIs, which are highly efficient and do not rely on polling. This allows it to watch directories with hundreds of thousands of files without significant performance degradation, limited only by available system memory and OS-specific limits on file handles

3. What happens if the event buffer is full?

If the main event channel is full, the watcher drops the oldest event and records it in the Dropped() channel. This prevents blocking the event processing pipeline under heavy load

4. Can I watch files, or only directories?

FSWatcher watches directories only. This ensures consistent, predictable behavior across all platforms. Be aware that the Linux inotify backend can struggle with very large or deep directory trees