I have a bunch of questions with respect to ssl, local sessions, and load balancing which seem to be interconnected, so I apologize in advance for the length of this question.
I have a website which uses file-based sessions. The nature of the site is that most of it is http, but some sections are ssl. Currently, because of the file based sessions, it is necessary for any ssl requests to hit the same server as any previous http requests.
Because of time constraints, I want to do the easiest possible thing to load balance increased http and ssl traffic.
There seems to be 2 options for sticky load balancing algorithms:
- ip based
- cookie based
The ip based solution will probably work, but the hashing algorithm will potentially change the server a user goes to when a server goes down or is added which is undesirable with the current file-based session setup. I also suppose that it is technically possible for a user to legitimately change ips while browsing a website.
The cookie based algorithm seems better, but the inability to inspect the cookie when encrypted by ssl seemingly presents its own problems.
I have been googling for examples on how to load balance ssl, and I cannot seem to find any explicit examples of setups which can do cookie based load balancing AND which can deal with increased ssl load by adding another ssl decoder.
Most of the explicit examples I've seen have the ssl decoder (usually hardware, apache_mod_ssl, or nginx) sitting between the browser client and the load balancer. The examples usually seem to have something like this (modified from http://haproxy.1wt.eu/download/1.3/doc/architecture.txt):
192.168.1.1 192.168.1.11-192.168.1.14 -------+-----------+-----+-----+-----+ | | | | | +--+--+ +-+-+ +-+-+ +-+-+ +-+-+ | LB1 | | A | | B | | C | | D | +-----+ +---+ +---+ +---+ +---+ apache 4 cheap web servers mod_ssl haproxy
The ssl decoding part in the above example seems to be a potential bottleneck that is not horizontally scalable.
I've looked at haproxy, and it seems to have a 'mode tcp' option that would allow something like this, which would allow you to have multiple ssl decoders:
haproxy | ------------- | | ssl-decoder-1 ssl-decoder2 | | ------------------- | | | web1 web2 web3
However, in such a setup, it appears you would lose the client ip because haproxy is not decoding the ssl: https://cloud-support.engineyard.com/discussions/problems/335-haproxy-not-passing-x-forwarded-for
I've also looked at nginx, and I also do not see any explicit examples of horizontally scalable ssl-decoders. There seem to be many examples of people having nginx as a potential bottleneck. And at least this link seems to suggest that nginx doesn't even have the option of the haproxy-like setup where you would lose the ip by saying that nginx "doesn't support transparently passing TCP connections to a backend" http://serverfault.com/questions/66875/how-to-pass-apache-ssl-traffic-trough-nginx-proxy .
- Why don't there seem to be more examples of setups adding more ssl decoders to deal with increased traffic?
- Is it because the ssl decoding step is only a theoretical bottleneck, and practically speaking, one decoder will essentially be enough except for sites with ridiculous traffic?
- Another possible solution that comes to mind is perhaps anybody with such increased ssl needs also has a centralized session store, so it doesn't matter which webserver the client hits on sequential requests. Then you could enable mod_ssl or equivalent on every webserver.
- The haproxy solution cited above seems to work (besides the client IP problem), but has anyone come across a sticky cookie based software load balancer solution that would work by increasing the number of decoders while keeping the client IP, or is that perhaps technically not possible (because you have to decode the request to get the client IP, in which case, we have a decoder bottleneck).
Assuming that everything I've said is true, these appear to be my options:
- use ip hashing (bad for users who potentially legitimately change ips, and for server adding and dropping scenarios)
- use nginx or mod_ssl as the 1st program touching the ssl request, this will be a potential ssl decoding bottleneck
- use haproxy as the 1st program touching the ssl request, gaining horizontal ssl scalability, but live with no ips logged at the webserver level for ssl requests (probably temporarily ok)
- over the longer term, move towards a mobile or centralized session store, making sticky sessions unnecessary