Question

I’m writing a visit counter for products on a website which uses MongoDB as its’ DB-Engine.

Here it says that Mongo keeps frequently accessed stuff in memory and has an integrated in-memory caching engine.

So can I just relay on this integrated caching system and dumbly set the counters up on every visit or does one still need another caching layer on a high-traffic environment?

Answer 1

They’re two seperate things. MongoDB uses a simple paged memory management system that, by design, keeps the most accessed parts of the memory mapped disk space in memory.

As a result, this will help you most for counters that are requested frequently but do not change often. Unfortunately for website counters these two things are mutually exclusive. Because increasing counters will generally not cause MongoDB to move the document holding the counter on disk the read caching will still be fairly effective.

The main issue is your writes, basically doing an increase per visit is not going to be very cost effective. I suggest a strategy where your counter webapp caches incoming visits and only pushes counter updates every X visits or every Y seconds, whichever comes first. Your main goal here is to reduce writes per second so you definitely do not want a db write per counter visit.