Google ACE Planning and Configuring a Cloud Solution Practice Question
A team needs to deploy a microservice that processes events from Pub/Sub and writes the results to Firestore. The service is stateless and should not incur cost when idle. The expected load is low but can spike unpredictably. Which compute service is the most cost-effective and operationally simple?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Cloud Functions (2nd gen) triggered by Pub/Sub
Cloud Functions is event-driven, scales automatically, and charges only while code is executing. It can be triggered by Pub/Sub messages, making it ideal for this use case. It scales to zero and handles spikes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
GKE Standard with a cluster autoscaler and a Pub/Sub sidecar
Why it's wrong here
GKE Standard requires you to manage a node pool and maintain at least one node per cluster, so even with cluster autoscaler and a Pub/Sub sidecar the cluster never scales to zero, leaving idle compute and control-plane costs. The sidecar pattern also adds container and networking overhead that is unnecessary for a simple event processor. This approach gives you more control but at higher operational and financial cost than a serverless option.
- ✗
Compute Engine with a managed instance group and autoscaling based on Pub/Sub queue depth
Why it's wrong here
A managed instance group keeps a configured minimum number of VMs running even when the Pub/Sub queue is empty, so you pay for idle instances and still have to manage instance templates, health checks, and autoscaling policies. Scaling on queue depth requires exporting custom metrics to Cloud Monitoring and reacts more slowly than a serverless push-based trigger. The operational overhead and continuous baseline cost make this heavier than needed for intermittent event processing.
- ✗
Cloud Run for Anthos on-premises
Why it's wrong here
Cloud Run for Anthos is designed for on-premises or hybrid environments, requiring a GKE or Anthos cluster to run; it does not natively pull Pub/Sub events as a cloud-native service. You would need to set up and maintain the Anthos infrastructure, and your on-prem location lacks the low-latency, built-in integration with Google Cloud's Pub/Sub service. This makes it a poor fit for a cloud-native microservice that simply needs to react to Pub/Sub messages.
- ✓
Cloud Functions (2nd gen) triggered by Pub/Sub
Why this is correct
Cloud Functions (2nd gen) is a fully managed, event-driven compute service that can be triggered directly by Pub/Sub messages via Eventarc. It scales automatically from zero to thousands of concurrent invocations and bills only for the time your code runs, so there is no idle capacity or cluster infrastructure to manage. This makes it the most cost-effective and operationally simple choice for processing sporadic events.
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
Go deeper
Related to this question
Learn chapter
Google Cloud Platform Overview
Key term
Cloud Functions
Cloud Functions are serverless compute services that let you run single-purpose code in response to events without managing servers.
Key term
Pub/Sub
Pub/Sub is a messaging pattern where publishers send messages without knowing who receives them, and subscribers receive only the messages they care about.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ACE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ACE exam.