I've two webservices, each service features its own database, the first is master (A) along with other is slave (B). If your call is built to service B, additionally, it calls A to sync A's database.

If for whatever reason A isn't available, B must bring A current using its data at another time.

Any suggestion on which mechanism can be used as from process data synchronization?

You are able to setup an expert-master replication between 2 databases. We all do this with MySQL.

It may sound such as the command pattern might be helpful for you - keep "skipped" transactions and apply them later. You might want to perform some chicanery to sort out which from the last couple of calls you've made to some happened, and which did not.

If Your is up-to-date from another source and also you loose the hyperlink (as opposed to a heading down completely), you might have a fight to deal with to solve any conflicts. I'd recommend a Temporal Database of some kind to assist manage that.

Alternatively, perhaps you have considered utilizing a texting system for example MSMQ?