What upgrade approach should be used to ensure no downtime during a web server update?

The correct choice is blue-green deployment, which is specifically designed to minimize downtime during updates. This approach involves running two identical environments, referred to as blue and green. During an upgrade, the new version of the application is deployed into the inactive environment (if blue is active, then the update happens in green). After the deployment and necessary testing are completed in the new environment, traffic is switched from the old environment to the updated one. This seamless transition ensures that users experience virtually no downtime, making it an effective strategy for web server updates.

While rolling updates can reduce downtime to some extent, they entail updating instances gradually, which might involve temporary unavailability of some parts of the application as instance updates occur. This method can lead to complications if not managed properly, as some servers might still run the old version while others are on the new one.

Hotfix implementation is used for urgent fixes and does not encompass a complete upgrade strategy for a web server, potentially leading to service interruptions.

Snapshot management relates to taking snapshots of the server states and is typically used for backup and recovery rather than facilitating upgrades without downtime. This strategy does not inherently include the seamless transition necessary to achieve zero downtime during an upgrade.