although I've much experience writing code. I don't really have much experience deploying things. I am writing a project that uses mongodb for persistence, redis for meta-caching, and play for serving pages. I am deciding whether to buy a dedicated server vs buying multiple small/medium instance from amazon/linode (one for each, mongo, redis, play). I have thought of the trade-offs as below, I wonder if anyone can add to the list or provide further insights. I am leaning toward (b) buying two sets of instances from linode and amazon, so if one of them have an outage it will fail over to the other provider. Also if anyone has any tips for deploying scala/maven cluster or tools to do so, much appreciated.
A. put everything in one instance
Pros:
- faster speed between database and page servlet (same host).
- cheaper.
- less end points to secure.
Cons:
- harder to manage. (in my opinion)
- harder to upgrade a single module. if there are installation issues, it might bring down the whole system.
B. put each module (mongo,redis,play) in different instances
Pros:
- sharding is easier.
- easier to create cluster for a single purpose. (i.e. cluster of redis)
- easier to allocate resources between module.
- less likely everything will fail at once.
Cons:
- bandwidth between modules -> $
- secure each connection and end point.
I can only comment about the technical aspects (not cost, serviceability, etc ...)
It is not mentioned whether the dedicated instance is a physical box, or simply a large VM. If the application generates a lot of roundtrips to MongoDB or Redis, then the difference will be quite significant.
With a VM, the cost of I/Os, OS scheduling and system calls is higher. These elements tend to represent an important part in the performance cost of efficient remote data stores like MongoDB or Redis, and the virtualization toll is higher for them.
From a system point of view, I would not put MongoDB and Redis/Play on the same box if the MongoDB database is expected to be larger than the available memory. MongoDB maps data files in memory, and relies on the OS to perform memory swapping. It is designed for this. The other processes are not. Swapping induced by MongoDB will have catastrophic consequences on Redis and Play response time if they are all on the same box. So I would at least separate MongoDB from Redis/Play.
If you plan to use Redis for caching, it makes sense to keep it on the same box than the Play server. Redis will use memory, but low CPU. Play will use CPU, but not much memory. So it seems a good fit. Also, I'm not sure it is possible from Play, but if you use a unix domain socket to connect to Redis instead of the TCP loopback, you can achieve about 50% more throughput for free.
