Rate limiting a web crawler

[Simon Connah]

Hi,

I want to build a simple web crawler. I know how I am going to do it but 
I have one problem.

Obviously I don't want to negatively impact any of the websites that I 
am crawling so I want to implement some form of rate limiting of HTTP 
requests to specific domain names.

What I'd like is some form of timer which calls a piece of code say 
every 5 seconds or something and that code is what goes off and crawls 
the website.

I'm just not sure on the best way to call code based on a timer.

Could anyone offer some advice on the best way to do this? It will be 
running on Linux and using the python-daemon library to run it as a 
service and will be using at least Python 3.6.

Thanks for any help.