from Wiktionary, Creative Commons Attribution/Share-Alike License.
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There is something called idempotency that applies to any API in general and comes in handy in distributed systems in particular.
POST requests don't get cached (since they don't offer what's called idempotency, or the ability to get the same result from two identical requests).
Of course another important goal of the lab was to help you wrap your head about how to handle multitenancy (by managing the subscriptions info accordingly) and think a bit about what a real system may need to cope with (for example avoiding to delete all tenant data upon preprovisioning messages, or handle idempotency in case the same message needs to be re-sent).
Scenarios, robust message processing & idempotency, handling poison messages
For example, multi-tenancy, eventual consistency (concurrency model), de-normalized and horizontally partitioned and shared-nothing data, asynchronous and parallel distributed processing, process redundancy and idempotency, service-oriented composition, etc.; these lead to horizontally scaling architectures that are consisted of a larger number of smaller and loosely coupled distributed components/services.
The onus is on the WS publisher to clearly document in the interface what the idempotency criteria is.
Out of this atomicity you will have to build idempotency.