A critical deployment requires a Blue/Green strategy to ensure zero downtime and instant rollback. The application is hosted on Compute Engine…

An External or Internal HTTP(S) Load Balancer is the core component for implementing a Blue/Green deployment on Google Cloud with Managed Instance Groups (MIGs). The load balancer uses URL maps and backend services to direct traffic. For a Blue/Green strategy, you configure two backend services, one pointing to the "Blue" MIG and another to the "Green" MIG. To switch traffic, you update the load balancer's URL map to point to the new "Green" backend service. This change is a configuration update within Google's network and is nearly instantaneous, fulfilling the requirements for zero downtime and instant rollback without waiting for DNS propagation.

A. Cloud Router: Cloud Router uses the Border Gateway Protocol (BGP) to manage routes between your VPC and on-premises networks; it does not manage application-level traffic between backend instance groups.

B. Cloud Storage bucket for configuration management: A Cloud Storage bucket stores static objects. While it could hold configuration files for a deployment pipeline, it is not the active networking component that directs or switches live traffic.

D. Cloud DNS: Using Cloud DNS to switch traffic involves changing a DNS record's IP address. This is not instant due to DNS propagation and caching (controlled by TTL values), which can take minutes or hours to update globally.

1. Google Cloud Documentation, "Application deployment and testing strategies": This document explicitly describes the Blue/Green deployment pattern. It states, "To switch between your blue and green environments, you can use a Google Cloud load balancer. The load balancer can be configured to direct traffic to either the blue or green environment." This directly supports using a load balancer as the switching mechanism.

2. Google Cloud Documentation, "External HTTP(S) Load Balancing overview": The section on "How External HTTP(S) Load Balancing works" details the components, including URL maps that route requests to specific backend services. Modifying this URL map is the mechanism for an instant traffic switch between Blue and Green backend services.

3. Google Cloud Documentation, "Cloud DNS overview": This documentation explains that Cloud DNS is a managed authoritative DNS service. Changes to DNS records are subject to propagation delays based on the Time-to-Live (TTL) settings, making it unsuitable for the "instant" switching requirement of a critical Blue/Green deployment.