Subscribed Projections

Snapshot projections and ManagedProjections have one thing in common: The application actively controls the frequency and time of updates by actively calling a method. While this gives the user a maximum of control, it also requires synchronicity. Especially when building query models, this is not necessarily a good thing. This is where SubscribedProjections come into play.

Definition

A SubscribedProjection is subscribed once to a Fact-stream and is asynchronously updated as soon as the application receives relevant facts.

Subscribed projections are created by the application and subscribed (once) to factus. As soon as Factus receives matching Facts from the FactCast Server, it updates the projection. The expected latency is obviously dependent on a variety of parameters, but under normal circumstances it is expected to be <100ms, sometimes <10ms.

Its strength however (being updated in the background) is also its weakness: As an application you never know, which state the projection is in (eventual consistency).

While this is a perfect projection type for occasionally connected operations or public query models, the inherent eventual consistency might be confusing to users, for instance in a read-after-write scenario, where the user does not see his own write. This can lead to suboptimal UX und thus should be used cautiously after carefully considering the trade-offs.

A SubscribedProjection is also StateAware and WriterTokenAware. However, the token will not be released as frequently as with ManagedProjections. This may lead to “starving” Models, if the process keeping the lock is non-responsive.

Please keep that in mind when implementing the locking facility.