'Zero downtime deployment' is about changing / upgrading microservice without interruption from service's clients point of view, it should be transparent and completely online. Basically, before microservice shutting down, all of requests should be handled and results provided to the invokers (within defined timeout).
After this phase the process is finished.
NOTE that zore downtime deployment on JLupin Platform is performed on a single node, when only one instances of a microservice is run.
In case of SERVLET microservice the zero downtime deployment process is very similar, additionally Edge Balancer is involved in this process.
In that way, logged in users will not notice that restart has happened.
That's what we call TRUE ZERO DOWNTIME DEPLOYMENT :) Enjoy.