Use asynch invocations for create/update/delete operations

[jwalcorn]

Today all of our calls are synchronous - that is, we make a call and sit and wait on the work to happen before returning. For some calls, that's unavoidable; if someone asked to see the details for a particular portfolio, we do in fact have to wait on all of those details (like the details for each stock owned) to return from the downstream microservices before returning the result back to the client. As an aside, there are things we can do, like using a CQRS pattern, to speed up queries, so that the GET calls return faster. But putting that aside for now, there's no reason we need to wait on the other calls that request some work happen in the system, such as creating a new portfolio, or deleting one, or updating one (like to buy or sell stock).

Those could be made using the asynch REST call pattern (as my co-author explains in chapter 4 of our book: https://www.amazon.com/Practical-Cloud-Native-Java-Development-MicroProfile/dp/1801078807, or at this blog: https://openliberty.io/blog/2019/01/24/async-rest-jaxrs-microprofile.html). The only caveat is that if the client immediately attempts to view stuff - like if it shows all portfolios in the system immediately after creating a new one - the list might not show the newly created one (it would be a race condition as to whether it was in place in time to show up in the quickly issued query). Same with buying stock; if we make that asynch, but the client immediately shows details of the given portfolio, the newly bought stock might not show up. We could potentially code in a sleep for 100 milliseconds or whatever, or other approaches to not make it be the query is happening the very next millisecond after the create/update/delete.

[jwalcorn]

Note that issue #5 tracks making the get all query faster via pagination, for the case of lots of portfolios in the system. That essentially makes all pages other than the first handled asynchronously (that is, in separately issued REST calls - not waiting on a million to return if you only want to see the first 20).