In the previous post I talked about how to use a map reduce index to aggregate events into a final model. You can see this on the right. This is an interesting use case of indexing, and it can consolidate a lot of complexity into a single place, at which point you can utilize additional tooling available inside of RavenDB.
Starting from the previous index, all we need to do is edit the index definition and set the Output Collection, like so:
What does this do? This tell RavenDB that in addition to indexing the data, it should also take the output of the index and create new documents from it in the ShoppingCarts collection. Here is what these documents look like:
You can see at the bottom that this document is flagged as artificial and coming from an index. The document id is a hash of the reduce key, so changes to the same cart will always go to this document.
What is important about this feature is that once the result of the index is a document, we can operate it using all the usual tools for indexes. For example, we might want to create another index on top of the shopping carts, like the following example:
In this case, we are building another aggregation. Taking all the paid shopping carts and computing the total sales per product from these. Note that we are now operating on top of our event streams but are able to extract second level aggregation from the data.
Of course, normal indexes on top of the artificial ShoppingCarts allow you to do things like: “Show me my previous orders”. In essence, you are using the events for your writes, define the aggregation to the final model in an index and then RavenDB take care of the read model.
Some other options to pay attention to is the not doing the read model and the full work on the same database instance as your events. Instead, you can output the documents to a collection and then use RavenDB’s native ETL capabilities to push them to another database (which can be another RavenDB instance or a relational database) for further processing.
The end result is a system that is built on dynamic data flow. Add an event to the system, the index will go through it, aggregate it with other events on the same root and output it to a document, at which point more indexes will pick it up and do further work, ETL will push it to other databases, subscriptions can start operation on it, etc.