Clarified CQRS

@UdiDahan has just clarified CQRS for me:

Why CQRS

• Collaboration - multiple actors using/modifying the same set of data
• Staleness - once data has been shown to a user, that same data may have been changed by another actor

Queries

• Use of an additional data store whose data can be a bit out of sync with the master database (if the data we're showing to the user is stale anyway)
• Can be structured just like the ViewModel - no need to transform objects

Query Data Storage

• Does not have to be a relational database!

Scaling Queries

• Assuming that the data being served does not need to 100% up to date we can easily add more store instances without worrying about that they contain the exact same data

Commands and Validation

• Clients send commands to the server - not returning errors to the client, but rather confirming/notifying the user of the outcome
• Validation is different from business rules in that it states a context-independent fact about a command
• Even though a command may be valid, there may still be reasons to reject it
• The server still validates all commands, not trusting clients to do the validation

Commands and Autonomy

• Commands don't need to be processed immediately - they can be queued
• Shouldn't need to access the query store to process commands - any state that is needed should be managed by the autonomous component
• Failure messages due to the database being down or hitting a deadlock should not be returned to the client - we can just rollback and try again by processing all messages in the queue
• Each command could be processed by a different AC, each with its own queue

Service Layers

• Each command is independent of the other - so why should we allow the objects which handle them (domain objects) to depend on each other? Dependencies are things which should be avoided, unless there is a good reason for them

Persistence for Command Processing

• Do we really need to have a column for every single domain object property? What if we just serialized the domain entity and put it into a single column and had another column containing the ID i.e. akin to the key-value storage available from various cloud providers

See: http://www.udidahan.com/2009/12/09/clarified-cqrs