Linux Exercise 11a - optional - Caching

Optional Extension: Enable Caching in Nginx (Reverse Proxy Lab)

1. Goal

In this extension you will:

• Configure Nginx to cache responses from the backend pool
• Verify that repeated requests are served from cache
• Observe cache behavior when you change backend responses

You should add this on top of your existing loadbalance.local setup.

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2. Step 1 – Define a Cache Path

Edit the main Nginx config file:

sudo nano /etc/nginx/nginx.conf

Inside the http { ... } block, add:

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=mycache:10m
                 max_size=100m inactive=60m use_temp_path=off;

Explanation:

• mycache:10m – name of cache zone + metadata size
• max_size=100m – limit cache size
• inactive=60m – cached items not accessed for 60 minutes are removed

Save and exit.

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3. Step 2 – Enable Cache in Your Server Block

Edit your site config:

sudo nano /etc/nginx/sites-available/loadbalance.local

Inside the server { ... } block, modify your location /:

location / {
    proxy_cache mycache;
    proxy_cache_valid 200 1m;
    add_header X-Proxy-Cache $upstream_cache_status;

    proxy_pass http://backend_pool;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}

Explanation:

• proxy_cache mycache; – use the cache zone configured earlier
• proxy_cache_valid 200 1m; – cache HTTP 200 OK responses for 1 minute
• X-Proxy-Cache – custom header with cache status (MISS, HIT, BYPASS, …)

Test and reload:

sudo nginx -t
sudo systemctl reload nginx

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4. Step 3 – Test the Cache

4.1 Initial Request

Run:

curl -I http://loadbalance.local/

Look for the header:

X-Proxy-Cache: MISS

The first request typically results in MISS (nothing cached yet).

4.2 Subsequent Requests

Run:

for i in {1..5}; do curl -I http://loadbalance.local/; done

You should now see:

X-Proxy-Cache: HIT

This indicates the response is being served from cache.

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5. Step 4 – Verify Cache Duration

1. Wait at least 1 minute (based on proxy_cache_valid 200 1m).

2. Call again:

   curl -I http://loadbalance.local/

You should see the status go back to MISS, then HIT again on later requests.

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6. Step 5 – Observe Caching Effects

Try changing one backend to respond differently (e.g. backend 1 returns {"backend":"1-changed"}) while the cache is active.

• You may still see the old data until cache expires.
• This shows that clients are no longer hitting backends directly for each request.

Describe your observations in your protocol:

• How did the X-Proxy-Cache header change?
• When did you see updated backend responses?