The correct answer is that a DependsOn clause is missing between the DBInstance and the DBSubnetGroup. CloudFormation creates resources in parallel by default, so without an explicit DependsOn, the DBInstance may attempt to launch before the DBSubnetGroup has finished provisioning, resulting in the “DBSubnetGroup not found” error. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of CloudFormation resource dependencies and the default parallel creation behavior—a common trap is assuming resources are created in the order they appear in the template. Remember that while intrinsic functions like Ref create implicit dependencies for parameter values, they do not guarantee creation order for resources referenced only by name, as with DBSubnetGroupName. A simple memory tip: “DependsOn ensures the group is done before the instance runs.”
SOA-C02 Deployment, Provisioning, and Automation Practice Question
This SOA-C02 practice question tests your understanding of deployment, provisioning, and automation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A SysOps administrator is automating the creation of Amazon RDS instances using AWS CloudFormation. The template includes a DBInstance resource with a DBSubnetGroupName property referencing a subnet group created in the same template. The stack creation fails with the error 'DBSubnetGroup not found'. What is the MOST likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A DependsOn clause is missing between the DBInstance and the DBSubnetGroup.
Option C is correct because CloudFormation creates resources in parallel by default, and the DBSubnetGroup may not be created before the DBInstance if there is no DependsOn clause. Option A is wrong because subnet groups can be created in the same template. Option B is wrong because the VPC exists. Option D is wrong because the database subnet group is not required to be public.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The VPC ID is incorrect or does not exist.
Why it's wrong here
This would cause a different error.
✗
The DBSubnetGroup is not associated with a public subnet.
Why it's wrong here
DBSubnetGroups can use private subnets; public subnets are not required.
✓
A DependsOn clause is missing between the DBInstance and the DBSubnetGroup.
Why this is correct
Without DependsOn, CloudFormation may create resources in parallel, causing a dependency error.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The DBSubnetGroup is defined in a different CloudFormation stack.
Why it's wrong here
The question states it is in the same template.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Deployment, Provisioning, and Automation — This question tests Deployment, Provisioning, and Automation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: A DependsOn clause is missing between the DBInstance and the DBSubnetGroup. — Option C is correct because CloudFormation creates resources in parallel by default, and the DBSubnetGroup may not be created before the DBInstance if there is no DependsOn clause. Option A is wrong because subnet groups can be created in the same template. Option B is wrong because the VPC exists. Option D is wrong because the database subnet group is not required to be public.
What should I do if I get this SOA-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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