How to add persistent connection support to a load-balanced HTTP webservice

Question

We are working on an HTTP webservice load-balanced using haproxy. The webservice is accessed via SSL. It is a RESTful HTTP service and simply accepts JSON, does some work, and returns JSON. There is no notion of a session.

We have redundant load-balancers set up in front of a pair of redundant webservice servers. Each server sits behind Apache, where Apache is used as a proxy in order to handle SSL and logging. If it matters, our webservice is a Clojure (java) application using compojure (jetty) to handle HTTP.

This is a brief diagram showing the path of a client request through our existing system.

client request -> haproxy (load balancing) -> apache (ssl, logging) -> webservice

We would like any connection to the load-balancer to establish a persistent connection and then be served by the same server for all subsequent requests sent through that persistent connection. In other words, we don't want a persistent connection to haproxy making requests to more than one webservice server.

How would you recommend that we get this working? How can we "pin" a given connection to the load-balancer to a specific webservice server? How could we prevent accidentally loading down a specific webservice server with multiple intensive requests?

Answer 1

Using balance source in the defaults block, along with removing option httpclose entries did the trick.

Answer 2

In our HAProxy configuration we do this at the backend level, using the cookie option. This is because he have a number of sites, some of which we do want persistance for - others we do not.

In those that we do the backend looks like this in haproxy.cfg:

backend examplesite
cookie STK insert indirect nocache maxidle 30m maxlife 8h
server server1 192.168.0.1:80 cookie n1
server server2 192.168.0.2:80 cookie n2

This will set a cookie with the name STK on the first request. Haproxy will automatically assign a value to this cookie that it will then use to send subsequent requests to the same node.

We decided to also add the n1 and n2 cookie prefixes... this means that the cookie value will be prefixed with either n1 if the requests are going to node 1 or n2 if they are going to node 2. This is very helpful when debugging.

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Either way I'd suggest taking a look at the configuration documentation around cookie options.

You might also want to look at the appsession option. This allows HAProxy to use an existing cookie (such as ASPNetSessionId or PHPSESSIONID) for the same purpose.

I had problems with it before, but recently had an answer to a question of Server Fault which should resolve this. You could give it a go as it saves using an extra cookie in your requests. Can't get appsession setting in HAProxy to work.