Supporting plural and singular parameters

This problem relates to all APIs I would say, but I’ll give an example. You have a service layer, a business layer, and a data access layer. An operation on the service layer for getting instances of a class by Ids can either support passing in a single Id, or passing in plural Ids (in an array, IList, ICollection etc). The same choice is also available at the business layer, and again at the data access layer. So what do you choose at each layer?

If you choose just the singular then at some point someone is going to want to query for multiple Ids, and instead of creating a plural version of the method they may just call the singular version multiple times (very chatty).

If you treat all invocations as plural operations then the client is forced to create a plural parameter even though they may only have one value.

If you provide both at the service layer then should you have 2 versions of the operation all the way down the stack? Which will almost certainly lead to duplication, or do you convert all calls to multiple singular stacks, or a single plural stack? If you treat all calls as either option then you end up either missing performance improvements available by batch processing, or you add the complication of batch processing even when 100% of the calls are actually singular.

I’m not sure there is a ‘correct’ answer.

  • I think you should only create the operation at the service layer when it is needed, so if you only need singular operation start with this. As soon as someone needs the batch version then the additional method signature should be created.
  • If you do need both I think I would be tempted to only have both versions at the service layer (where we want to reduce network traffic). At lower levels I would begin by treating batch calls as multiple calls to the singular stack. The justification for this is that singular versions are almost always more simple, and that means more to me than some premature optimization. As soon as there is a performance bottle neck treating batch calls this way I would look at either having 2 routes down the stack or treating all as batch calls, with the added complexity of the batch operations having to consider the singular case.

About Alex McMahon

I am a software developer, interested in .net, agile, I've previously specialised with .net 3.0 technologies like WCF, whereas now I am trying to specialise in agile development and best practice and patterns. I am obsessed with looking at the latest technologies, tools, and methods, and trying them out. I am currently employed by Rockwell Collins in the UK.
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