httpcache

httpcache is a Go package that provides a standards-compliant http.RoundTripper for transparent HTTP response caching, following RFC 9111 (HTTP Caching).

Note: This package is intended for use as a private (client-side) cache. It is not a shared or proxy cache. It is designed to be used with an HTTP client to cache responses from origin servers, improving performance and reducing load on those servers.

Features

• Plug-and-Play: Just swap in as your HTTP client's transport; no extra configuration needed. ¹
• RFC 9111 Compliance: Handles validation, expiration, and revalidation (see details).
• Cache Control: Supports all required HTTP cache control directives, as well as extensions like stale-while-revalidate, stale-if-error, and immutable (view details).
• Cache Backends: Built-in support for file system and memory caches, with the ability to implement custom backends (see Cache Backends).
• Cache Maintenance API: Optional REST endpoints for listing, retrieving, and deleting cache entries (see Cache Maintenance API).
• Extensible: Options for logging, transport and timeouts (see Options).
• Debuggable: Adds a cache status header to every response (see Cache Status Headers).
• Zero Dependencies: No external dependencies, pure Go implementation.

Demonstration of HTTP caching in action. See _examples/app for code.

Installation

To install the package, run:

go get github.com/bartventer/httpcache

Quick Start

To get started, create a new HTTP client with the httpcache transport, specifying a cache backend DSN. You'll need to register the desired cache backend before using it. Here's an example using the built-in file system cache:

package main

import (
    "log/slog"
    "net/http"
    "time"

    "github.com/bartventer/httpcache"
    // Register the file system cache backend
    _ "github.com/bartventer/httpcache/store/fscache"
)

func main() {
    // Example DSN for the file system cache backend
    dsn := "fscache://?appname=myapp"
    client := &http.Client{
        Transport: httpcache.NewTransport(
            dsn,
            httpcache.WithSWRTimeout(10*time.Second),
            httpcache.WithLogger(slog.Default()),
        ),
    }
    // ... Use the client as usual
}

Note: The DSN format and options depend on the cache backend you choose. Refer to the Cache Backends section for details on available backends and their DSN formats.

Cache Backends

The following built-in cache backends are available:

Backend	DSN Example	Description
fscache	fscache://?appname=myapp	File system cache, stores responses on disk. Suitable for persistent caching across restarts. Supports context cancellation, as well as optional AES-GCM encryption.
memcache	memcache://	In-memory cache, suitable for ephemeral caching. Does not persist across restarts.

Consult the documentation for each backend for specific configuration options and usage details.

Custom Cache Backends

To implement a custom cache backend, create a type that satisfies the store/driver.Conn interface, then register it using the store.Register function. Refer to the built-in backends for examples of how to implement this interface.

Cache Maintenance API (Debug Only)

A REST API is available for cache inspection and maintenance, intended for debugging and development use only. Do not expose these endpoints in production.

Endpoints:

• GET /debug/httpcache — List cache keys (if supported)
• GET /debug/httpcache/{key} — Retrieve a cache entry
• DELETE /debug/httpcache/{key} — Delete a cache entry

All endpoints require a dsn query parameter to select the cache backend.

Usage Example:

import (
    "net/http"
    "github.com/bartventer/httpcache/store/expapi"
)

func main() {
    expapi.Register()
    http.ListenAndServe(":8080", nil)
}

To use a custom ServeMux, pass expapi.WithServeMux(mux) to expapi.Register().

Options

Option	Description	Default Value
WithUpstream(http.RoundTripper)	Set a custom transport for upstream/origin requests	http.DefaultTransport
WithSWRTimeout(time.Duration)	Set the stale-while-revalidate timeout	5 * time.Second
WithLogger(*slog.Logger)	Set a logger for debug output	slog.New(slog.DiscardHandler)

Cache Status Headers

This package sets a cache status header on every response:

• X-Httpcache-Status: The primary, detailed cache status header (always set).
• X-From-Cache: (Legacy) Provided for compatibility with gregjones/httpcache. Only set for cache hits/stale/revalidated responses.

Header Value Mapping

X-Httpcache-Status	X-From-Cache	Description
HIT	1	Served from cache
STALE	1	Served from cache but stale
REVALIDATED	1	Revalidated with origin
MISS	(not set)	Served from origin
BYPASS	(not set)	Bypassed cache, served from origin

Example: Stale cache hit

HTTP/1.1 200 OK
X-Httpcache-Status: STALE
X-From-Cache: 1
Content-Type: application/json

Limitations

• Range Requests & Partial Content: This cache does not support HTTP range requests or partial/incomplete responses (e.g., status code 206, Range/Content-Range headers). All requests with a Range header are bypassed, and 206 responses are not cached. For example:

  GET /example.txt HTTP/1.1
  Host: example.com
  Range: bytes=0-99

  The above request will bypass the cache and fetch the response directly from the origin server. See RFC 9111 §3.3-3.4 for details.

RFC 9111 Compliance Matrix

§	Title	Requirement	Implemented	Notes
1.	Introduction	N/A	N/A	Nothing to implement
2.	Overview of Cache Operation	N/A	N/A	Nothing to implement
3.	Storing Responses in Caches	Required	✔️	Details
4.	Constructing Responses from Caches	Required	✔️	Details
5.	Field Definitions	Required	✔️	Details
6.	Relationship to Applications and Other Caches	N/A	N/A	Nothing to implement
7.	Security Considerations	N/A	N/A	Nothing to implement
8.	IANA Considerations	N/A	N/A	Nothing to implement
9.	References	N/A	N/A	Nothing to implement

Legend for Requirements:

Requirement	Description
Required	Must be implemented for RFC compliance
Optional	May be implemented, but not required for compliance
Obsolete	Directive is no longer relevant as per RFC 9111
Deprecated	Directive is deprecated as per RFC 9111, but can still be implemented
N/A	Nothing to implement or not applicable to private caches

§3. Storing Responses in Caches (Details)

§	Title	Requirement	Implemented	Notes
3.1.	Storing Header and Trailer Fields	Required	✔️
3.2.	Updating Stored Header Fields	Required	✔️
3.3.	Storing Incomplete Responses	Optional	❌	See Limitations
3.4.	Combining Partial Content	Optional	❌	See Limitations
3.5.	Storing Responses to Authenticated Requests	N/A	N/A	Not applicable to private client-side caches

§4. Constructing Responses from Caches (Details)

§	Title	Requirement	Implemented	Notes
4.1.	Calculating Cache Keys with the Vary Header Field	Required	✔️
4.2.	Freshness	Required	✔️	Details

§4.2. Freshness (Subsections)

§	Title	Requirement	Implemented	Notes
4.2.1.	Calculating Freshness Lifetime	Required	✔️
4.2.2.	Calculating Heuristic Freshness	Required	✔️
4.2.3.	Calculating Age	Required	✔️
4.2.4.	Serving Stale Responses	Required	✔️

§	Title	Requirement	Implemented	Notes
4.3.	Validation	Required	✔️	Details

§4.3. Validation (Subsections)

§	Title	Requirement	Implemented	Notes
4.3.1.	Sending a Validation Request	Required	✔️
4.3.2.	Handling Received Validation Request	N/A	N/A	Not applicable to private client-side caches
4.3.3.	Handling a Validation Response	Required	✔️
4.3.4.	Freshening Stored Responses upon Validation	Required	✔️
4.3.5.	Freshening Responses with HEAD	Optional	❌	Pointless, rather use conditional GETs; see RFC 9110 §13.2.1 last para

§	Title	Requirement	Implemented	Notes
4.4.	Invalidating Stored Responses	Required	✔️

§5. Field Definitions (Details)

§	Title	Requirement	Implemented	Notes
5.1.	Age	Required	✔️
5.2.	Cache-Control	Required	✔️	Details
5.3.	Expires	Required	✔️
5.4.	Pragma	Deprecated	❌	Deprecated by RFC 9111; not implemented
5.5.	Warning	Obsolete	❌	Obsoleted by RFC 9111; not implemented

§5.2. Cache-Control Directives

§	Title	Requirement	Implemented	Notes
5.2.1.	Request Directives	Optional	✔️	Details

§5.2.1. Request Directives (Details)

§	Title/Directive	Requirement	Implemented	Notes
5.2.1.1.	max-age	Optional	✔️
5.2.1.2.	max-stale	Optional	✔️
5.2.1.3.	min-fresh	Optional	✔️
5.2.1.4.	no-cache	Optional	✔️
5.2.1.5.	no-store	Optional	✔️
5.2.1.6.	no-transform	Optional	✔️	Compliant by default - implementation never transforms content
5.2.1.7.	only-if-cached	Optional	✔️

Section	Requirement	Implemented	Notes
5.2.2. Response Directives	Required	✔️	Details

§5.2.2. Response Directives (Details)

§	Title/Directive	Requirement	Implemented	Notes
5.2.2.1.	max-age	Required	✔️
5.2.2.2.	must-revalidate	Required	✔️
5.2.2.3.	must-understand	Required	✔️
5.2.2.4.	no-cache	Required	✔️	Both qualified and unqualified forms supported
5.2.2.5.	no-store	Required	✔️
5.2.2.6.	no-transform	Required	✔️	Compliant by default - implementation never transforms content
5.2.2.7.	private	N/A	N/A	Intended for shared caches; not applicable to private caches
5.2.2.8.	proxy-revalidate	N/A	N/A	Intended for shared caches; not applicable to private caches
5.2.2.9.	public	Optional	✔️
5.2.2.10.	s-maxage	N/A	N/A	Intended for shared caches; not applicable to private caches

§	Title	Requirement	Implemented	Notes
5.2.3.	Extension Directives	Optional	partially	Details

§5.2.3. Extension Directives (Details)

The following additional cache control directives are supported, as defined in various RFCs:

Reference	Directive	Notes
RFC 5861, §3	stale-while-revalidate	Only applies to responses
RFC 5861, §4	stale-if-error	Applies to both requests and responses
RFC 8246, §2	immutable	Only applies to responses

License

This project is licensed under the Apache License 2.0. See the LICENSE file for details.

Notes

Footnotes

1. No configuration is needed beyond the cache backend DSN. Caching is handled automatically based on HTTP headers and directives. To use a custom upstream transport, pass it with the WithUpstream option. This lets you add httpcache to your existing HTTP client with minimal changes. See Options for details. ↩