RavenDB Subscriptions allows you to create a query and subscribe to documents that match the query. They are very useful in many scenarios, including backend processing, queues and more.
Subscriptions allow you to define a query on a document, and get all the documents that match this query. The key here is that all documents don’t refer to just the documents that exists now, but also future documents that match the query. That is what the subscription part is all about.
The subscription query operate on a single document at a time, which leads to open questions when we have complex object graphs. Let’s assume that we want to handle via subscriptions all Orders that are managed by an employee residing in London. There isn’t a straightforward of doing this. One option would be to add EmployeeCity to the Orders document, but that is a decidedly inelegant solution. Another option is to use the full capabilities of RavenDB. For Subscription queries, we actually allow you to ask question on other documents, like so:
Now we’ll only get the Orders who employee is in London. Simple and quite elegant.
It does have a caveat, though. We will only evaluate this condition whenever the order changes, not when the employee changed. So if the employee moves, old orders will not be matched against the subscription, but new ones will.