CODEWITHSUNDEEP
domain-driven-designcqrs
kind_robot
kind_robot

Reputation: 2523

Meta data in event and command messages

I am curious to find out how you guys are incorporating meta data about command/event messages in a cqrs solution. For example, I want to know who, when, which host, etc. generated the command. I don't want to put these into message itself.

Say in a web app, user created a shopping cart CreateShoppingCart { CartID, UserID }. Then added items to it, AddItem { CartID, ItemID, Amount, etc }. I want to record exacty when the used clicked the "Add To Cart" button.

  1. I can add this into some Dictionary<string, object> Headers { get; set; } property. That property could be in a BaseMessage class.
  2. I can rely on the messaging framework (something like NServiceBus) and add this data into the message header in the message context.
  3. Send seperate command for this info. Something like LogCommandDetails { CommandID: 'id of AddItem command', DateTime, Some other meta data }. When this comamnd is handled, I can update the projection of ItemAdded event and add this data into the projection.

What are your thoughts?

Thanks

Upvotes: 2

Views: 1002

Answers (1)

eulerfx
eulerfx

Reputation: 37719

Typically this information is stored in message headers, which is option 2. This is exactly what message headers are for. Note, there is a subtle difference between a message from perspective of a messaging framework and a message in your domain which is the body of the message in a messaging framework.

However, it can be difficult to discern what is data and what is metadata. I run into this issue with dates, among other things. For example, is a timestamp associated with an event metadata or proper domain data? What if the timestamp is required for execution of certain business logic? In your example, do you need to record the date for reporting or audit purposes, or is the date needed for the domain to function? In the former case, use headers, in the latter, place the date in the message body.

Upvotes: 2

Related Questions