Wondering how to perform NGINX passive cache invalidation? Our NGINX Support team is here to lend a hand with your queries and issues.

NGINX passive cache invalidation

Basically, the application on the backend can add a header to the response asking nginx to invalidate a cached URL.

To test this concept your nginx will need Lua support enable.

If you are using Debian/Ubuntu the package nginx-extras already has support enabled.

As you will see on the following configuration, the queue is stored in a share dictionary, so take care of the size use by it.

server {
listen 80;
server_name backend;
root /usr/share/nginx/www;
index index.html index.html;
location / {
header_filter_by_lua ‘
ngx.header[“X-Expire-Content”] = “/random”
‘;
content_by_lua ‘
ngx.say(“Adding /random URL to the invalidation queue.”)
‘;
}
location = /random {
content_by_lua ‘
ngx.say(math.random(100))
‘;
}
}


lua_shared_dict expire_queue 10m;
proxy_cache_path /dev/shm/nginx/ levels=1:2 keys_zone=default:10m;
server {
listen 80;
root /usr/share/nginx/www;
index index.html index.htm;
server_name frontend;
location / {
header_filter_by_lua ‘
if ngx.header[“X-Expire-Content”] then
ngx.log(ngx.INFO, “Backend sent invalidation header, adding to the queue ” .. ngx.header[“X-Expire-Content”])
ngx.shared.expire_queue:add(ngx.header[“X-Expire-Content”],1)
end
‘;
set_by_lua $http_cache_purge ‘
local expire = ngx.shared.expire_queue:get(ngx.var.uri)
if expire then
ngx.log(ngx.INFO, “Removing URL from queue and invalidating, ” .. ngx.var.uri)
ngx.shared.expire_queue:delete(ngx.var.uri)
end
return expire
‘;
add_header X-Cached $upstream_cache_status;
proxy_cache_bypass $http_cache_purge;
proxy_cache default;
proxy_cache_valid 200 302 72h;
proxy_pass http://backend/;
}
}

root@precise32:~# wget -S -q -O – frontend/random # Missed request
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Fri, 29 Nov 2013 05:24:21 GMT
Content-Type: application/octet-stream
Content-Length: 3
Connection: keep-alive
X-Cached: MISS
54
root@precise32:~# wget -S -q -O – frontend/random # Cached request
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Fri, 29 Nov 2013 05:24:23 GMT
Content-Type: application/octet-stream
Content-Length: 3
Connection: keep-alive
X-Cached: HIT
54
root@precise32:~# wget -S -q -O – frontend/ # Invalidation queue access
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Fri, 29 Nov 2013 05:24:27 GMT
Content-Type: application/octet-stream
Content-Length: 44
Connection: keep-alive
X-Expire-Content: /random
X-Cached: MISS
Adding /random URL to the invalidation queue.
root@precise32:~# wget -S -q -O – frontend/random # Cache invalidation request using bypass
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Fri, 29 Nov 2013 05:24:31 GMT
Content-Type: application/octet-stream
Content-Length: 2
Connection: keep-alive
X-Cached: BYPASS
1
root@precise32:~# wget -S -q -O – frontend/random # Cached request
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Fri, 29 Nov 2013 05:24:32 GMT
Content-Type: application/octet-stream
Content-Length: 2
Connection: keep-alive
X-Cached: HIT
1

The backend.conf configuration is just need if you want to setup a lab to simulate a frontend and a backend,

Then, just add the hostnames backend and frontend to your /etc/hosts and it’s working.

Usually, the invalidation is handle on the frontend.conf.

the header_filter_by_lua adds new invalidation requests receive from the backend to the queue and set_by_lua set the variable forcing a bypass on the cache and remove the URL from the queue.

Finally, as you see on the tests, the next request after the backend has for invalidation, makes the client receive a BYPASS as expected.

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Conclusion

In conclusion, our Support Engineers demonstrated how to NGINX passive cache invalidation.