Projection

A projection in Chronicle is a specialized observer. It is a declarative approach to describing events you're interested in and how you want Chronicle to project it into something that you can display in your application or provide as data for an API. Every projection can be very specialized for the purpose it is being used in, in fact it is encouraged to create specialized projections rather than reuse a projection for multiple scenarios. The reason for this is that you then don't run the risk of serving multiple conflicting scenarios, which could for instance impact performance and also general maintainability of your code.

Chronicle projections support relationships, one-to-one and one-to-many. It also supports functions that allow you to count, add or subtract values. The identifier of the projected document can also be configured. By default it will use the event source identifier, but you can point it to any other property or create a composite of multiple properties.

Projections support using the metadata properties of an event as well. This lets you create composite keys that could for instance leverage the occurred date/time value and effectively create group by scenarios. This would effectively give you an easy way to do simple histograms, with count, add and/or subtract of values in the histogram.