Purpose

Build a scalable architecture for news website using below components:

  • load balancer with content switching capability
  • cache server
  • application server

Definition

  • Content switching: the ability to route traffic based on the content of the HTTP request: URI, parameters, headers, etc…
    HAproxy is a good example of OpenSource reverse proxy load-balancer with content switching capability.
  • cache server: a server able to quickly deliver static content.
    Squid, Varnish and Apache Traffic Server are OpenSource cache reverse proxy.
  • application server: the server which build the pages for your news website.
    This can be either Apache+PHP, Tomcat+Java, IIS+asp .net, etc…

Target Network Diagram

Architecture

Context

All the traffic pass through the Aloha load-balancer.
HAproxy, the layer 7 load-balancer included in Aloha, will do the content switching to route request either to cache servers or to application servers.
If cache server misses an object, it will get it from the application servers.

Haproxy configuration

Service configuration:

[sourcecode language=”text”]
frontend public
bind :80
acl DYN path_beg /user
acl DYN path_beg /profile
acl DYN method POST
use_backend APPLICATION if DYN
default_backend CACHE
[/sourcecode]

The content switching is achieved by the few lines beginning with the keyword acl.
If a URI starts with /user or /profile or if the method is a POSTthen, the traffic will be redirected to the APPLICATION server pool, otherwise the CACHE pool will be used.

Application pool configuration:

[sourcecode language=”text”]
backend APPLICATION
balance roundrobin
cookie PHPSESSID prefix
option httpchk /health
http-check expect string GOOD
server APP1 1.1.1.1:80 cookie app1 check
server APP2 1.1.1.2:80 cookie app2 check
[/sourcecode]

We maintain backend server persistence using the cookie sent by the application server, named PHPSESSID in this example. You can change this cookie name to the cookie provided by your application, like JSESSIONIDASP.NET_SessionId or anything else.
Note the health check URL: /health. The script executed on the backend will check server health (database availability, CPU usage, memory usage, etc…) and will return a GOOD if everything looks fine and a WRONG if not. With this, HAproxy will consider the server as ready only if the server returns a GOOD.

Cache pool configuration

[sourcecode language=”text”]
backend CACHE
balance url
hash-type consistent
option httpchk /
http-check expect KEYWORD
reqidel ^Accept-Encoding unless { hdr_sub(Accept-Encoding) gzip }
reqirep ^Accept-Encoding: .*gzip.* Accept-Encoding: gzip
server CACHE1 1.1.1.3:80 check
server CACHE2 1.1.1.4:80 check
[/sourcecode]

Here, we balance requests according to the URL. The purpose of this metric is to “force” a single URL to always be retrieved from the same server.
Main benefits are :

  1. less objects in caches memory
  2. less requests to the application server for static and pseudo-static content

In order to lower the impact of the Vary: header on Content Encoding, we added the two lines reqidel / reqirep to normalize a bit the Accept-Encoding header.

Links