CQRS 和 DDD：文件上传

Question

I'm new to the concept of DDD and CQRS and can't find a final solution how to upload images, or files in general, in a clean way.

Imagine the following scenario: In an online portal there is a support request formular where a file (image in specific) could be attached to. The posted data will raise a CreateSupportRequestCommand . Then the required aggregates will be loaded and changed.

I have three ideas to solve this, but I'm not very satisfied with them.

Way 1:
1. Post all data including the image (multipart) in a single request
2. Create a FileUploadCommand , which is returning the FileUploadId .
3. After that create a CreateSupportRequestCommand and pass the FileUploadId with the root data in the constructor.
Drawback: A single request will trigger two commands. In terms of CQRS one user interaction should be only one command.

Way 2:
1. Post the image to a seperate endpoint, create a temporary file and return the id or a file handle.
2. Post the formular with the attached tempfile id.
3. Invoke the CreateSupportRequestCommand with all root data including a file handle which points to the physical file.
4. Inside the command persist the tempfile into a FileUpload aggregate (by FileUploadRepository ) then
5. Create the SupportRequest aggregate, assign the FileUploadId and persist.
Drawback: I handle 2 aggregates in the same command. Creating a support request is not responsible for uploading the file.

Way 3: 1. Post the image to a seperate endpoint, create a temporary file and return the id or a file handle.
2. Post the formular with the attached tempfile id.
3. Invoke the CreateSupportRequestCommand with all root data including a file handle which points to the physical file.
4. Only persist the root data to the SupportRequest aggregate. Raise a SupportRequestCreatedEvent and attach the file handle.
5. Inside the event process and assign the file handle.
Drawback: The SupportRequestCreatedEvent should not really care about a file handle.

Is there a better way to solve this?

Answer 1

I do not think handling File upload is a Domain Concern. The file metadata like FileContentId may be part of your domain but not the actual file upload. I would perform the file operation before the CommandHandler is executed. Probably in a middleware or perhaps before queing up the Command onto the message bus.

CreateSupportRequestCommandHandler would then only be invoking an operation like CreateSupportRequest on your aggrerate (say SupportRequest ). Within that CreateSupportRequest method you will have all your business rule pretaining to the operation. SupportRequest then eventually would be saved in your repository.