Working with Metadata
All Events may contain additional Metadata. This is especially useful for storing information that are not required for Command to be handled, yet are important for auditing and projecting purposes.
Metadata
In Ecotone any communication happens via Messages, and Messages contains of Payload and Headers (Metadata).
So far we've discussed only the Payload part, for example ProductWasCreated Event Class is actually an Payload.
What we actually store in the Event Stream is Message, so Payload and Metadata.
Ecotone Framework use the Metadata for Framework related details, which can be used for identifying messages, correlating, and targeting (which Aggregate it's related too). However we can also use the Metadata for additional information in our Application too.
Metadata Propagation
Ecotone provides Metadata propagation, which take cares of passing Metadata between Command and Events without the need for us to do it manually. This way we can keep our business code clean, yet still be able to access the Metadata later.
Even so, the Metadata is not used in our Ticket Aggregate, when the Event will be stored in the Event Stream, it will be stored along side with our provided Metadata. Therefore we will be able to access it in any Event Handlers:
Manual Propagation
We can also manually add Metadata directly from Command Handler, by packing the our data into Event class.
and then access it from any subflows:
Accessing Metadata in Command Handler
We may access metadata sent from Command Bus in Command Handler when needed:
[Enterprise] Accessing Metadata during Event Application
This feature is available as part of Ecotone Enterprise.
We may also access Metadata inside our Event Sourcing Handler. This may be useful when we need to protect business invariants based on the data, that is stored as part of Metadata.
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