ETag

ETag middleware for HTTP caching in Fiber

ETag middleware for Fiber that helps caches validate responses and saves bandwidth by avoiding full retransmits when content is unchanged.

Signatures

func New(config ...Config) fiber.Handler

Usage

import (
    "github.com/gofiber/fiber/v3"
    "github.com/gofiber/fiber/v3/middleware/etag"
)

Basic Example

// Initialize default config
app.Use(etag.New())

// GET / -> ETag: "13-1831710635"
app.Get("/", func(c fiber.Ctx) error {
    return c.SendString("Hello, World!")
})

Weak ETags

// Or extend your config for customization
app.Use(etag.New(etag.Config{
    Weak: true,
}))

// GET / -> ETag: W/"13-1831710635"
app.Get("/", func(c fiber.Ctx) error {
    return c.SendString("Hello, World!")
})

Entity tags in requests must be quoted per RFC 9110. For example:

If-None-Match: "example-etag"

Configuration

Next · func(fiber.Ctx) bool · default: "nil"

Defines a function to skip this middleware when it returns true.

Weak · bool · default: "false"

Enables weak validators. Weak ETags are easier to generate but less reliable for comparisons. Weak ETags are prefixed with W/.

Default Config

var ConfigDefault = Config{
    Next: nil,
    Weak: false,
}

How It Works

The ETag middleware generates a unique hash based on the response body:

1. Strong ETags (default): Used for byte-for-byte identical content
2. Weak ETags: Indicated by W/ prefix, used when content is semantically equivalent but not byte-identical

When a client sends an If-None-Match header with a matching ETag, the server returns 304 Not Modified instead of the full response body, saving bandwidth.

Common Use Cases

API Response Caching

app.Use(etag.New())

app.Get("/api/users", func(c fiber.Ctx) error {
    users := getUsersFromDB()
    return c.JSON(users)
})

Static Content with Strong ETags

app.Use("/static", etag.New(etag.Config{
    Weak: false, // Strong ETags for exact matching
}))

app.Static("/static", "./public")

Dynamic Content with Weak ETags

app.Use("/pages", etag.New(etag.Config{
    Weak: true, // Content may vary slightly
}))

app.Get("/pages/:id", func(c fiber.Ctx) error {
    page := getPageContent(c.Params("id"))
    return c.SendString(page)
})

Selective ETag Usage

app.Use(etag.New(etag.Config{
    Next: func(c fiber.Ctx) bool {
        // Skip ETag for certain paths
        return c.Path() == "/api/realtime" || 
               c.Path() == "/health"
    },
}))

Combining with Cache Middleware

import (
    "github.com/gofiber/fiber/v3/middleware/cache"
    "github.com/gofiber/fiber/v3/middleware/etag"
)

// Apply both middlewares
app.Use(etag.New())
app.Use(cache.New(cache.Config{
    Expiration: 5 * time.Minute,
}))

Client Usage

Using ETags in HTTP Requests

# First request - get ETag
curl -i http://localhost:3000/api/data
# Response includes: ETag: "abc123"

# Subsequent request - send If-None-Match
curl -i http://localhost:3000/api/data -H 'If-None-Match: "abc123"'
# Response: 304 Not Modified (if content unchanged)

JavaScript Fetch API

let etag = null;

async function fetchData() {
    const headers = {};
    if (etag) {
        headers['If-None-Match'] = etag;
    }

    const response = await fetch('/api/data', { headers });
    
    if (response.status === 304) {
        console.log('Using cached data');
        return cachedData;
    }
    
    etag = response.headers.get('ETag');
    const data = await response.json();
    cachedData = data;
    return data;
}

Performance Benefits

• Reduced Bandwidth: 304 responses have no body, saving transfer costs
• Faster Response Times: Smaller responses mean quicker page loads
• Server Load Reduction: Less data processing and transmission
• CDN Optimization: Better cache hit rates with proper ETag usage