Question

I am attempting to learn and apply the CQRS design approach (pattern and architecture) to a new project but seem to be missing a key piece.

My client application executes a query and retrieves a list of light-weight, read-only DTOs from the read model. The user selects an item and clicks a button to initiate some action. The action is performed by creating and sending the corresponding command object to the write model (where the command handler carries out the action, updates the data store, etc.) At some point, however, I need to update the UI to reflect changes to the state of the application resulting from the action.

How does the UI know when it is time to refresh the original list?

Additional Info

I have noticed that most articles/blogs discussing CQRS use MVC client apps in their examples. I am working on a Silverlight client right now and am beginning to wonder if the pattern simply doesn’t work in that case.

Follow-Up Question

After thinking more about Bartlomiej’s response and subsequent discussion, I am wondering about error handling in CQRS. Given that commands are basically fire-and-forget asynchronous operations, how do we report an error condition to the UI?

I see ‘refreshing the UI’ to take one of two forms:

1. The operation succeeds, data has changed and the UI should be updated to reflect these changes
2. The operation fails, data has not changed but the user should be notified of the failure and potential corrective actions.

Even with a Post-Redirect-Get pattern in an MVC, you can’t really Redirect until you know the outcome of the operation. None of the examples I’ve seen thus far address these real-world concerns.

Answer 1

I’ve been struggling with similar issues for a WPF client. The re-query trigger for any data is dependent on the data your updating, commands tend to fall into categories:

1. The command is a true fire and forget method, it informs the back-end of a state change but this change does not need to be reflected in the UI, or the change simply isn’t important to the UI.

2. The command will alter the result of a single query

3. The command will alter the result of multiple queries, usually (in my domain at least) in a cascading fashion, that is, changing the state of a single “high level” piece of data will likely affect many “low level” caches.

My first trigger is the page load, very few items are exempt from this as most pages must assume data has been updated since it was last visited. Though some systems may be able to escape with only updating financial and other critical data in this way.

For short commands I also update data when ‘success’ is returned from a command. Though this is mostly laziness as IMHO all CQRS commands should be fired asynchronously. It’s still an option I couldn’t live without but one you may have to if your implementation expects high latency between command and query.

One pattern I’m starting to make use of is the mediator (most MVVM frameworks come with one). When I fire a command, I also fire a message to the mediator specifying which command was launched. Each Cache (A view model property Retriever<T>) listens for commands which affect it and then updates appropriately. I try to minimise the number of messages while still minimising the number of caches that update unnecessary from a single message so I’ll (hopefully) eventually end up with a shortlist of update reasons, with each ‘reason’ updating a list of caches.

Another approach is simple honesty, I find that by exposing graphically how the system updates itself makes users more willing to be patient with it. On firing a command show some UI indicating you’re waiting for the successful response, on error you could offer to retry / show the error, on success you start the update of the relevant fields. Baring in mind that this command could have been fired from another terminal (of which you have no knowledge) so data will need to timeout eventually to avoid missing state changes invoked by other machines also.

Noting the irony that the only efficient method of updating cache’s and values on a client is to un-separate the commands and queries again, be it through hardcoding or something like a hashmap.