go 42 lines · 8 steps

Streaming a proxied download in Gin

A Gin handler fetches a file from upstream storage and streams it straight back to the client, preserving range requests and key headers.

Explained by highlit
Gin Intermediate reverse-proxy streaming http-headers range-requests error-handling
1func ProxyDownload(c *gin.Context) {
2 objectKey := c.Param("key")
3 upstream := fmt.Sprintf("%s/files/%s", storageBaseURL, objectKey)
4 
5 req, err := http.NewRequestWithContext(c.Request.Context(), http.MethodGet, upstream, nil)
6 if err != nil {
7 c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "failed to build upstream request"})
8 return
9 }
10 
11 if rng := c.GetHeader("Range"); rng != "" {
12 req.Header.Set("Range", rng)
13 }
14 req.Header.Set("Authorization", "Bearer "+storageToken)
15 
16 resp, err := httpClient.Do(req)
17 if err != nil {
18 c.AbortWithStatusJSON(http.StatusBadGateway, gin.H{"error": "upstream unreachable"})
19 return
20 }
21 defer resp.Body.Close()
22 
23 if resp.StatusCode >= http.StatusBadRequest {
24 c.AbortWithStatusJSON(resp.StatusCode, gin.H{"error": "upstream returned an error"})
25 return
26 }
27 
28 extraHeaders := map[string]string{}
29 if cd := resp.Header.Get("Content-Disposition"); cd != "" {
30 extraHeaders["Content-Disposition"] = cd
31 }
32 if ar := resp.Header.Get("Accept-Ranges"); ar != "" {
33 extraHeaders["Accept-Ranges"] = ar
34 }
35 
36 contentType := resp.Header.Get("Content-Type")
37 if contentType == "" {
38 contentType = "application/octet-stream"
39 }
40 
41 c.DataFromReader(resp.StatusCode, resp.ContentLength, contentType, resp.Body, extraHeaders)
42}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Tying the outbound request to the incoming request's context lets client disconnects cancel upstream work automatically.
  2. 2Streaming from the response body with DataFromReader avoids buffering the whole file in memory.
  3. 3A proxy should forward meaningful headers like Range and Content-Disposition so downstream behavior stays intact.

Related explainers

ruby 
class Order
  class InvalidTransition < StandardError; end
 
  TRANSITIONS = {

A state machine for order transitions in Ruby

state-machine data-driven error-handling
Intermediate 8 steps
go 
package theme
 
import (
	"fmt"

Per-tenant HTML templates with Gin's renderer

multi-tenancy concurrency html-templates
Advanced 8 steps
php 
<?php
 
namespace App\Http\Controllers;
 

Streaming a filtered CSV export in Laravel

streaming csv-export query-builder
Intermediate 9 steps
go 
package auth
 
import (
	"net/http"

Setting and reading secure session cookies in Go

cookies session-management security
Intermediate 6 steps
rust 
use std::cmp::Ordering;
use std::str::FromStr;
 
#[derive(Debug, Clone, PartialEq, Eq)]

Parsing and ordering semantic versions in Rust

parsing trait-implementation ordering
Intermediate 8 steps
javascript 
'use client';
 
import { useEffect } from 'react';
import * as Sentry from '@sentry/nextjs';

How a Next.js error boundary recovers

error-boundary error-handling observability
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

Streaming a proxied download in Gin — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code