CV0-004 Troubleshooting Practice Question
Which THREE are common reasons why a cloud database instance may become unreachable?
⚠ Common exam trap
CV0-004 often tests the distinction between 'unreachable' and 'slow' or 'failing writes'; candidates may choose storage full or hypervisor maintenance because they sound like availability issues, but the question specifically asks for reasons the instance cannot be reached at all.
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
✓
Firewall rules blocking the database port
Option A is correct because a cloud database instance listens on a specific port (e.g., 3306 for MySQL, 5432 for PostgreSQL, 1433 for SQL Server), and security group or firewall rules that fail to allow inbound traffic on that port will make the instance unreachable even though it is running. Option B is correct because an incorrect connection string—wrong hostname/endpoint, port, database name, or credentials—prevents the application from establishing a TCP session to the database, which is one of the most common causes of apparent unreachability. Option D is correct because if the database service/daemon (e.g., mysqld, postgresql, sqlservr) is not started or has crashed, the instance will refuse connections on its listening port. Option C is not a typical cause of unreachability; a full storage volume usually causes write failures, errors, or degraded performance rather than making the instance completely unreachable. Option E is not a common reason either, since hypervisor maintenance typically triggers live migration or a planned reboot, and even if a VM reboots, the database would normally come back online automatically rather than remain unreachable.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Firewall rules blocking the database port
Why this is correct
Security groups and network ACLs act as stateful or stateless packet filters; if the database listener port (for example 3306 or 5432) is not permitted inbound from the client subnet, connection attempts time out even though the instance itself is healthy and running.
- ✓
Incorrect connection string in the application
Why this is correct
The connection string supplies host, port, database name and credentials. A wrong hostname, port or credential causes resolution or authentication failure, so the client never establishes a session even though the database instance is running and reachable on the network.
- ✗
Storage volume is full on the database server
Why it's wrong here
A full storage volume stops writes and can crash the engine, but the instance endpoint typically still responds or reports errors rather than becoming unreachable. It tempts because disk exhaustion is a genuine database failure mode, and would be correct if the symptom were failed writes or read-only mode.
- ✓
Database service not started
Why this is correct
If the database engine process or service is stopped or failed, nothing listens on the database port, so TCP connections are refused or time out. The instance exists but no listener accepts sessions until the service is started.
- ✗
Hypervisor maintenance causing VM reboot
Why it's wrong here
Hypervisor maintenance reboots the underlying host, which cloud providers handle transparently; the database instance itself stays reachable. It tempts because host-level events do cause outages, but it would be correct only where the database runs on a self-managed hypervisor the administrator controls.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
This CV0-004 practice question is part of Courseiva's free CompTIA 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 CV0-004 exam.