- A
The service discovery namespace is of type private but the DNS records are not being created.
If DNS records are missing, service discovery fails.
- B
The task definitions are not using the awsvpc network mode.
Why wrong: Fargate tasks require awsvpc mode and it's default.
- C
The service discovery namespace is of type public and the services are in a private subnet.
Why wrong: Public namespace would not resolve within VPC.
- D
The VPC does not have an internet gateway attached.
Why wrong: Private namespace resolution does not require internet gateway.
Question 159 of 1,616
ECS Fargate Service Discovery Connectivity Issue: Troubleshooting Cloud Map Private Namespace
This DVA-C02 practice question tests your understanding of development with aws services. 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 company runs a microservices application on Amazon ECS with Fargate. Each service has its own task definition and uses service discovery via AWS Cloud Map. Recently, one service is failing to connect to another service using the service discovery endpoint. The developer verified that both services are running and the security groups allow traffic. What is the most likely cause of the connectivity issue?
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 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
✓
The service discovery namespace is of type private but the DNS records are not being created.
When using AWS Cloud Map for service discovery with Amazon ECS on Fargate, the namespace type must be 'private' (linked to a VPC) for DNS queries to resolve within the VPC. If the namespace is private but DNS records are not being created (e.g., due to missing IAM permissions for the task role to register instances, or a misconfigured service discovery service), the service will fail to resolve the endpoint even though both services are running and security groups allow traffic. This is the most likely cause because the developer has already verified the services are healthy and network-level access is open, pointing to a service discovery configuration issue.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 service discovery namespace is of type private but the DNS records are not being created.
Why this is correct
If DNS records are missing, service discovery fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The task definitions are not using the awsvpc network mode.
Why it's wrong here
Fargate tasks require awsvpc mode and it's default.
- ✗
The service discovery namespace is of type public and the services are in a private subnet.
Why it's wrong here
Public namespace would not resolve within VPC.
- ✗
The VPC does not have an internet gateway attached.
Why it's wrong here
Private namespace resolution does not require internet gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume connectivity issues are always due to security groups or network ACLs, but the question explicitly states those are verified, so the real cause is a misconfiguration in the service discovery registration process or namespace type.
Detailed technical explanation
How to think about this question
AWS Cloud Map for service discovery creates a Route 53 private hosted zone within the VPC, and each service instance registers an A or SRV record. For Fargate tasks, the task role must have the 'servicediscovery:RegisterInstance' permission to create these DNS records; if the role lacks this permission, the task runs but never registers, causing DNS queries to fail with NXDOMAIN. Additionally, the namespace must be associated with the VPC, and the 'DNS resolution' and 'DNS hostnames' settings must be enabled on the VPC for private DNS to work.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
Quick reference
Common DNS Record Types
| Record | Purpose | Example |
|---|---|---|
| A | IPv4 address mapping | example.com → 93.184.216.34 |
| AAAA | IPv6 address mapping | example.com → 2606:2800::1 |
| CNAME | Alias to another hostname | www → example.com |
| MX | Mail server for domain | example.com → mail.example.com (priority 10) |
| TXT | Text data (SPF, DKIM, verification) | v=spf1 include:_spf.example.com ~all |
| NS | Authoritative name servers | example.com NS ns1.example.com |
| PTR | Reverse DNS (IP → hostname) | 34.216.184.93.in-addr.arpa → example.com |
| SOA | Zone authority record | Primary NS, admin email, serial, TTL defaults |
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this DVA-C02 question test?
Development with AWS Services — This question tests Development with AWS Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The service discovery namespace is of type private but the DNS records are not being created. — When using AWS Cloud Map for service discovery with Amazon ECS on Fargate, the namespace type must be 'private' (linked to a VPC) for DNS queries to resolve within the VPC. If the namespace is private but DNS records are not being created (e.g., due to missing IAM permissions for the task role to register instances, or a misconfigured service discovery service), the service will fail to resolve the endpoint even though both services are running and security groups allow traffic. This is the most likely cause because the developer has already verified the services are healthy and network-level access is open, pointing to a service discovery configuration issue.
What should I do if I get this DVA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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