I'm setting up a new website in the next month or two. Even though the traffic won't require a scalable HA website, I'm going to start out as if the website needs to support huge traffic so I can get some experience running such a website.
I'd like any feedback on what I am thinking of doing… As for hardware, I am colocating 6 servers at this time and plan to use Amazon S3 to host the static files (which should grow quickly to 1TB or 2TB of mostly images). 2 of the servers are going to be my frontend load balancers running haproxy. The remaining 4 servers with be nginx/varnish servers (nginx for the PHP/MySQL part of the site and varnish to cache the Amazon S3 files to save bandwidth charges by Amazon). I plan on doing DNS load balancing using pairs of A records for each hosted domain that will point to each of my frontend haproxy load balancers. Most traffic will be HTTPS, so I plan on having the frontend load balancers to handle the SSL (using the new haproxy support for SSL). The two load balancers will proxy to the 4 backend servers. These 4 backend servers will run haproxy in front of nginx/varnish with load balancing of "first" and a suitable MAXCONN. Server 1 haproxy will first route to the localhost nginx/varnish and when MAXCONN connections are active to the localhost, will forward the connection to Server 2 haproxy. Server 2 and 3 will be set up similarly to first route requests to localhost and when full, route subsequent requests to the next server. Server 4 will route excess requests to a small Amazon EC2 instance to return a "servers are all busy" page. Hopefully, I will be able to add a 5th backend server at Amazon to handle the overload if it looks like I really do have traffic that will fill all 4 backend servers that I am colo'ing (I don't really expect this to ever be necessary). Nginx will proxy to PHP on localhost and each localhost (of my 4 backend servers) will have 2 MySQL instances - one for the main Read-Only DB and one for a Read-Write SessionDB. PHP will go directly to the main DB (not through HAProxy) and will use HAProxy to select the proper SessionDB to use (each user session must use the same SessionDB so the one a request needs might be on any of the backend servers). Each SessionDB will be the master of one slave SessionDB on a different backend server for handling the failure of the master (haproxy will send requests to the slave SessionDB if the master is down or failing). So, each backend server will have haproxy to "first" balance HTTP to nginx/varnish. The backends also have PHP and 3 instances of MySQL (one for mainDB, one for master sessionDB, and one for another backend's slave sessionDB). Also, the 2 frontend servers will be running separate instances of haproxy. I hope to use keepalived to route the VIPs for one frontend to the other frontend in case of failure. Or, should I use heartbeat? There seems to be two HA solutions here. I know this is a very long description of what I am thinking of doing and I thank you if you have read this far. I'm looking for any comments on this setup. Especially, any comments on using "first" load balancing/MAXCONN on the backend servers so that a request load balanced from the frontend will keep the backend servers from overloading (possibly bouncing a request from server 1 to server 2 to server 3 to server 4 to EC2 "server busy" server) are especially appreciated. Also, any comments on using pairs of master/slave sessionDBs to provide high availability but still have session data saved/retrieved for a given user from the same DB are appreciated. I believe this setup will allow the load to be distributed evenly over the 4 backends and only have the front end load balancers do simple round robin without session stickiness. Kevin
