Limit number of CFHTTP requests sent every x seconds

Question

I'm making an application that will continually send CFHTTP requests to a server to search for items, as well as sending further CFHTTP requests to perform actions on any returned results.

The issue I'm having is that the server has a maximum threshold of 3 requests per second and even when I try to implement a sleep call every 4 milliseconds it doesn't work properly as, although it delays, the CFHTTP requests can queue up if it takes them a couple of seconds to return so that it then tries to send multiple in the same second triggering the threshold to be exceeded.

Is there a way I can ensure that there are never more than 3 active CFHTTP requests?

Answer 1

If you're limiting concurrent requests, then first part of this answer applies. If you're looking to limit the number of requests per second, then the bit at the end applies. The question kind of asks both things.

If I understand correctly, you've got a number of threads (either as requests CF is processing or threads CF has created itself) which all need to make calls to the same rate-limited domain. What you need is a central way of co-ordinating access, combined with a nice way of controlling program execution.

I don't know of any native limits that CF might support (I'd be happy to be proven wrong) so you're likely to have to implement your own. The cheap'n'nasty way to do this is to increment and decrement a allowed_conenctions variable in a long-lived scope such as appliation. The downsides are that you have to implement checking all over the place and that if there are no spare connections, you'll have to wait somehow.

Really what you have is a resource pool (of allowed HTTP connections) and I'm guessing that you want your code to wait until a connection is free. CF does this kind of thing already for database connections.

In your case, there isn't really a need to keep anything in a pool (as HTTP connections aren't long-lived), other than a permit to use the resource. Java provides a class which ought to provide what you're after, the Semaphore.

I've not tried it but in theory, something like the snippet below ought to work:

//Application.cfc:onApplicationStart()
application.http_pool = CreateObject("java","java.util.concurrent.Semaphore").init(3)

//Meanwhile, elsewhere in your code
application.http_pool.acquire()
//Make my HTTP call
application.http_pool.release()

You could even wrap the HTTP object to provide this functionality without having to use the acquire/release each time, which would make it more reliable.

EDIT It you're looking to limit rates, look at guava's RateLimiter which has the same general interface as Semaphore above, but implements rate limiting for you. You'd need to add guava to ColdFusion's classpath, or use JavaLoader or use CF10 which has classloading facilities built-in.

Answer 2

I think you are going to need to implement some sort of logging widget as part of your process. The log will keep track of request frequency. If the threshold is not met, then you would just skip over that iteration of your CFHTTP call. I don't mean a file log or a database log, but something implemented in the application or even request scope depending on your implementation. There is no way to throttle CFHTTP itself. It is basically a very simplistic wrapper around a Java HTTP library which then goes straight to the underlying operating system.