HTTP Cache

1. Simple Meaning

HTTP Cache means storing a copy of the server’s response so that the next time someone requests the same thing, it can be served faster without asking the backend again.

Think of it like:

• You go to a shop and ask for a Coke.

• The shopkeeper gets it from the warehouse (backend server) — takes 5 minutes.

• Next time, he already has Coke in the fridge (cache) — gives it in 5 seconds.

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2. Why Caching is Important

Without caching:

• Every request hits your backend.

• Backend works harder, even for the same repeated request.

• Slow responses under high traffic.

With caching:

• Common requests are served from stored copies.

• Backend is less busy.

• Responses are much faster.

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3. Types of Caching in HTTP

1. Browser Cache → Stored in the user’s browser.

2. Proxy Cache → Stored in a middle server like Nginx.

3. CDN Cache → Stored in geographically distributed servers.

We’re focusing on Proxy Cache in Nginx.

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4. How Nginx HTTP Cache Works

1. First request → Nginx checks if response is cached.

2. If not cached → Nginx asks backend → stores response in cache → sends to client.

3. If cached → Nginx sends stored copy directly to client.

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5. Nginx HTTP Cache Basic Setup

Let’s say you want to cache image files for 1 hour.

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=my_cache:10m inactive=60m;

server {
    listen 80;
    server_name example.com;

    location /images/ {
        proxy_cache my_cache;
        proxy_cache_valid 200 1h;
        proxy_pass http://localhost:3000;
    }
}

Explanation:

• /var/cache/nginx → Folder where cached files are stored.

• keys_zone=my_cache:10m → Name + memory used for cache metadata.

• inactive=60m → If not used for 60 minutes, remove from cache.

• proxy_cache_valid 200 1h → Cache only successful responses (200 OK) for 1 hour.

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6. Example Use Case

Without Cache:

• 1000 users request the same /images/logo.png

• Backend gets 1000 hits.

With Cache:

• First request → Backend

• Next 999 requests → Served from Nginx cache (0 backend hits).

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7. Advanced Cache Controls

• Cache different URLs differently:

location /api/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 10m;
}
location /images/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 1h;
}

• Bypass cache for logged-in users:

location / {
    if ($http_cookie ~* "session_id") {
        proxy_no_cache 1;
        proxy_cache_bypass 1;
    }
}

• Clear cache manually (delete files in /var/cache/nginx).

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8. Benefits of HTTP Cache

• Faster load time

• Less load on backend

• Better scalability

• Lower bandwidth cost