Maintaining CQS When Tracking Number Of Queries

Question

In my web app I'm tracking view counts on pages.

Right now, the action in the controller issues a command to the data layer to increment the view count on the model before returning the result of the query.

This action seems to break the rules of Command-Query-Separation, since with the request the user agent is submitting a query and unwittingly issuing a command (to increment the view count)

What architectural decisions need to be taken to maintain the Command-Query-Separation in this action?

Answer 1

You should consider CQS relative to the conceptual level of the operation in question. Here are some examples that all seem to violate CQS, but only do so on a different conceptual level:

• A ReadFile call on a file system object does not modify the file - but it canupdate the last accessed timestamp on the file.
• A FindById call to a repository should not change the database - but it can very well add the queried object to a cache.
• A GET operation on a REST API should not change the model - but it can update statistical data.

These examples have one thing in common: They maintain the state of the model the client works on, but they do modify data outside of that model. And this is not a violation of CQS.

Another way to see this is through the principle of least surprise. A client of the above mentioned REST API would not expect the model to change with a GET request, but he doesn't care if the API updates a statistical counter.