Are Multiple DataContext classes ever appropriate?

Question

In order to fully use LinqToSql in an ASP.net 3.5 application, it is necessary to create DataContext classes (which is usually done using the designer in VS 2008). From the UI perspective, the DataContext is a design of the sections of your database that you would like to expose to through LinqToSql and is integral in setting up the ORM features of LinqToSql.

My question is: I am setting up a project that uses a large database where all tables are interconnected in some way through Foreign Keys. My first inclination is to make one huge DataContext class that models the entire database. That way I could in theory (though I don't know if this would be needed in practice) use the Foreign Key connections that are generated through LinqToSql to easily go between related objects in my code, insert related objects, etc.

However, after giving it some thought, I am now thinking that it may make more sense to create multiple DataContext classes, each one relating to a specific namespace or logical interrelated section within my database. My main concern is that instantiating and disposing one huge DataContext class all the time for individual operations that relate to specific areas of the Database would be impose an unnecessary imposition on application resources. Additionally, it is easier to create and manage smaller DataContext files than one big one. The thing that I would lose is that there would be some distant sections of the database that would not be navigable through LinqToSql (even though a chain of relationships connects them in the actual database). Additionally, there would be some table classes that would exist in more than one DataContext.

Any thoughts or experience on whether multiple DataContexts (corresponding to DB namespaces) are appropriate in place of (or in addition to) one very large DataContext class (corresponding to the whole DB)?

Answer 1

In my experience with LINQ to SQL and LINQ to Entities a DataContext is synonymous to a connection to the database. So if you were to use multiple data stores you would need to use multiple DataContexts. My gut reaction is you wouldn't notice to much of a slow down with a DataContext that encompasses a large number of tables. If you did however you could always split the database logically at points where you can isolate tables that don't have any relationship to other sets of tables and create multiple contexts.

Answer 2

Yaakov,

I'd been wrangling over the same question whilst retro fitting LINQ to SQL over a legacy DB. Our database is a bit of a whopper (150 tables) and after some thought and experimentation I elected to use multiple DataContexts. Whether this is considered an anti-pattern remains to be seen, but for now it makes life manageable.

Answer 3

I disagree with John's answer. The DataContext (or Linq to Entities ObjectContext) is more of a "unit of work" than a connection. It manages change tracking, etc. See this blog post for a description:

Lifetime of a LINQ to SQL DataContext

The four main points of this blog post are that DataContext:

1. Is ideally suited for a "unit of work" approach
2. Is also designed for "stateless" server operation
3. Is not designed for Long-lived usage
4. Should be used very carefully after any SumbitChanges() operation.

Considering that, I don't think using more than one DataContext would do any harm- in fact, creating different DataContexts for different types of work would help make your LinqToSql impelmentation more usuable and organized. The only downside is you wouldn't be able to use sqlmetal to auto-generate your dmbl.

Answer 4

I think John is correct.

"My main concern is that instantiating and disposing one huge DataContext class all the time for individual operations that relate to specific areas of the Database would be impose an unnecessary imposition on application resources"

How do you support that statement? What is your experiment that shows that a large DataContext is a performance bottleneck? Having multiple datacontexts is a lot like having multiple databases and makes sense in similar scenarios, that is, hardly ever. If you are working with multiple datacontexts you need to keep track of which objects belong to which datacontext and you can't relate objects that are not in the same data context. That is a costly design smell for no real benefit.

@Evan "The DataContext (or Linq to Entities ObjectContext) is more of a "unit of work" than a connection" That is precisely why you should not have more than one datacontext. Why would you want more that one "unit of work" at a time?