Problem Statement
What is a rolling update in Docker Swarm and how does it differ from a blue-green deployment strategy?
Explanation
In Swarm mode a rolling update means you update a service image and Swarm replaces tasks one by one (or per update-config) while keeping old ones running, maintaining service availability. Command: `docker service update --image new:tag <service>`. A blue-green deployment instead runs a parallel version (green) alongside current (blue), then switches traffic over, then removes the old version. Rolling updates are simpler for small services; blue-green gives strong isolation and quick rollback. Knowing both shows maturity in deployment strategy.
