Wednesday, October 28, 2015

Adding Custom Metadata to Domain Events with Prooph Event Store

When you record events with the Prooph EventStore only the payload (the event data that was recorded by your aggregate roots) gets recorded. If you use the EventStore together with the Prooph ServiceBus and enable the TransactionManager from the EventStoreBusBridge, some additional metadata gets recorded, too. That is the "causation_id" and the "causation_name" (the command id that triggered this event and the command name).

In a real-word application you might want to record some additional metadata, like "who" issued the command, what was his IP address, which user-agent did he use, and so on. This is especially useful when you want to know, which commands "John Doe" sent to the system. The simplest way to achieve this, is to use an EventStore-Plugin. Here is an example for Zend Framework 2, using the Zend\Authentication component to get the issuer and Zend\Http\PhpEnvironment\RemoteAddress to get the client's IP address.

This will add "issued_by", "ip_address" and "user_agent" to the metadata of all recorded events. Now you still need to enable the plugin. If you use the provided container-factories, you simply add the plugin there:

Keep in mind, that the domain model is unaware of the domain event's metadata. Therefore if you need to know the issuer f.e. in your domain model, you should write this as a CommandBus-Plugin that manipulates the command. This way you can get the issuer from the command in your command handler within the domain. An example:

In this case you need to add the enricher as CommandBus-Plugin instead of an EventStore-Plugin of course:

You noticed perhaps that we need to use a special command class that knows the issuer. That is easy to achieve:

Now you only need to extend your commands from this base-class.

Note: If you work with Prooph EventStore, you don't need to extends the base-classes from Prooph, like Command. You're free to implement them by yourself for your needs.

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