Where do I put the logic?

Nov 16, 2011 archive

I could use some advice regarding a project that I’m currently working on, where people can sign up and join various groups (did I hear “Facebook is already doing that”?). I’m now torn on some implementation details and would love some discussions regarding where to put certain pieces of logic.

The purpose of the web site is not important, nor is the big domain model. Let’s ignore all complexity and focus on the Member and Group entities. Also, when this post mentions interfaces, it corresponds to protocols in Objective-C and Swift and is a way to design for the what’s and not the how’s.


In this project, I have a repository interface for each entity. The repositories main implementations have the sole responsibility to hide Entity Framework from the rest of the system. I also have other repository implementations, e.g. fake ones that just create fake data, cache decorators etc.

I then have service interfaces for more sophisticated functionality. Each service uses one or several repositories, which means that a repository can be simple and just handle data from a certain data source, while a service can do sophisticated things with this data, without having to know or care about from where it comes. It also dramatically simplifies unit testing.

The application controllers will use services to do stuff. Controllers can talk to services. Services can talk to each other and repositories. Repositories can not talk to anything else.

The architecture can be illustrated like this:

A simplified view of the architecture

It’s a clean architecture, but leaves me with some problems regarding where to put certain functionality.

The problem

Here’s my dilemma - where do I put additional logic? For instance, say that I want to find all members that are in the same groups as a certain member.

There are a couple of alternatives:

  • Add a GetGroupFriends method to Member. The method could then iterate over all the groups that a member is a member of, but that means that the Member must be fully populated (Member -> GroupMember -> Group), which means that it must use Entity Framework.

  • Add a GetGroupFriendsForMember method to MemberService and have the service do the same thing as above, but with the difference that it uses a repository to retrieve data instead of drilling down through the member object.

  • Add a GetGroupFriendsForMember method to GroupService and have the service do the same thing as above, but with the difference that it uses a repository to retrieve data instead of drilling down through the member object.

  • Add a GetGroupFriends method to Member and a GetGroupFriendsForMember to one of the service classes, and make the service call the entity method.

There are pros and cons will all the approaches above, for instance:

  • Placing the logic in ` Member and using it is simple, but causes coupling to EF. This ruins the abstract system design. Also, Member` isn’t abstract, which means that we can’t mock it easily. This is not an alternative.

  • Placing the logic in a service is also simple, especially since the service is abstract, but which service should own this logic? I would go for the group service, but I’m really not sure.

I started placing the methods in the entities, trying to be a good OO programmer, but the responsibilities of the entities kept growing. I now have all my relation (is X a member of Y, does X know Z) and permission (can X edit Y, can X invite Y to Z) logic in the various services. It’s quite nice, but in some cases, I find that the services have to evaluate data in non-optimal ways.

There are tons of solutions to this problem. I would just really like to discuss it here. If you have found a way that works for you, please share.