EventSoucring gateways (syncronize with external systems)

[vlad...@gmail.com]

Are there best practices for implementation of eventsourcing gateways? The gateway is meant as infrastructure or service which allows to generate a set of events, proceeding from the status returned by some external service.

Even if application based on eventsourcing, some external uncontrollable entitles can still be present. For example, you want to synchronize users list from Azure AD, and perform prompt to service, which return users list. Then you get users list from projection, make difference with external state, and produce events to fill this difference.

Or your application is online-shop, and you should import actual USD/EUR/bitcoin ranks for showing prices. Gateway can poll some currencies provider and produce event. In simple case it's very easy, but if projection state is more complex structure, trivial import is not obvious.

Maybe is there common approach for this case?