When to use Singleton vs Transient vs Request using Ninject and MongoDB

Question

I'm not quite sure when I should use SingletonScope() vs TransientScope() vs RequestScope() when I do my binding in my global.cs file.

I have for example my call to MongoSession (using NoRM and the mvcStarter project http://mvcstarter.codeplex.com/) which is set to SingletonScope but I created a repository that use this MongoSession object to make calls to Mongo easier, e.g., I have a NewsRepository which uses MongoSession to fetch my News items from the data. As an example I have a call that fetches News items that has DisplayOnHome set to true and get the latest by CreationDate. Should such a repository be SingletonScope or would RequestScope would be more appropriate?

When should I use each of it and why?

Answer 1

In general in a web app, you want state to be request scope as much as possible.

Only in the case of very low level optimisations are you ever likely to run into a case where its appropriate to create singleton objects (and the chances even then are that you'll pull such caching / sharing logic out into another class which gets pulled in as a dependency on your other [request scope] objects and make that singleton scope). Remember that a singleton in the context of a web app means multiple threads using the same objects. This is rarely good news.

On the same basis, transient scope is the most straightforward default (and that's why Ninject 2 makes it so) - request scope should only come into the equation when something needs to be shared for performance reasons, etc. (or because that's simply the context of the sharing [as mentioned in the other answer]).

Answer 2

I guess the answer would depend on whether your MongoSession represents a unit of work or not. Most database related classes that I've worked with (mostly in the context of ORM, such as NHibernate or EF4) revolve around a context, entities, and tracked state that represent a unit of work. A unit of work should never be kept around longer than the length of time required to perform the given unit of work, after which the unit should be committed or rolled back. That would mean you should use RequestScope.

If your MongoSession is not a unit of work, you could keep it around for the lifetime of an MVC session, in which case SessionScope would then be appropriate.