Why Does ECS Deployment Succeed But the Application Fail?
This DOP-C02 practice question tests your understanding of sdlc 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.
Exhibit
CodePipeline execution history:
Pipeline: MyPipeline
Date: 2024-03-01
Stage: Deploy
Action: DeployToECS
Execution ID: e-123456789
Status: Succeeded
Revision: commit SHA abc123
Deployment details:
- ECS service: my-service
- Task definition: my-task:12
- Desired count: 2
- Running count: 2
- Pending count: 0
After deployment, application health checks fail. Rolling back to previous revision (task definition my-task:11) resolves the issue.
CloudWatch Logs from ECS tasks:
[ERROR] Failed to connect to database: TimeoutError: connect ETIMEDOUT 10.0.0.5:5432
Refer to the exhibit. The deployment succeeded but the application fails. What is the MOST likely cause?
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.
Exhibit
CodePipeline execution history:
Pipeline: MyPipeline
Date: 2024-03-01
Stage: Deploy
Action: DeployToECS
Execution ID: e-123456789
Status: Succeeded
Revision: commit SHA abc123
Deployment details:
- ECS service: my-service
- Task definition: my-task:12
- Desired count: 2
- Running count: 2
- Pending count: 0
After deployment, application health checks fail. Rolling back to previous revision (task definition my-task:11) resolves the issue.
CloudWatch Logs from ECS tasks:
[ERROR] Failed to connect to database: TimeoutError: connect ETIMEDOUT 10.0.0.5:5432
A
The CodePipeline deployment action uses the wrong cluster.
Why wrong: The deployment succeeded and tasks are running, so cluster is correct.
B
The new task definition has a misconfigured database connection string or security group.
The timeout to database IP indicates network or configuration issue introduced by the new task definition.
C
The ECS service is not registered with a target group.
Why wrong: The deployment succeeded, so ALB routing is likely fine.
D
The database is not available in the same Availability Zone.
Why wrong: The error is timeout, not connectivity, and AZ mismatch would not cause timeout.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The new task definition has a misconfigured database connection string or security group.
Option B is correct because the most common cause of a deployment succeeding but the application failing is a misconfiguration in the new task definition, such as an incorrect database connection string or a security group that does not allow traffic to the database. CodePipeline can successfully deploy the new task definition to ECS, but if the application cannot connect to its backend services due to these configuration errors, the application will fail at runtime. This aligns with the scenario where the deployment pipeline reports success but the application itself is non-functional.
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 CodePipeline deployment action uses the wrong cluster.
Why it's wrong here
The deployment succeeded and tasks are running, so cluster is correct.
✓
The new task definition has a misconfigured database connection string or security group.
Why this is correct
The timeout to database IP indicates network or configuration issue introduced by the new task definition.
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 ECS service is not registered with a target group.
Why it's wrong here
The deployment succeeded, so ALB routing is likely fine.
✗
The database is not available in the same Availability Zone.
Why it's wrong here
The error is timeout, not connectivity, and AZ mismatch would not cause timeout.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a successful deployment means the application is fully functional, but AWS separates the deployment of infrastructure (task definition, service update) from the application's runtime dependencies, so a misconfigured connection string or security group can cause application failure post-deployment.
Detailed technical explanation
How to think about this question
When ECS deploys a new task definition, it pulls the container image and starts the container with the environment variables and secrets defined in the task definition. If the database connection string is hardcoded or passed via an environment variable that points to a wrong endpoint, the application will fail to connect. Security groups act as a virtual firewall; if the ECS task's security group does not allow outbound traffic to the database's security group on the correct port (e.g., 3306 for MySQL, 5432 for PostgreSQL), or if the database's security group does not allow inbound traffic from the ECS task's security group, the TCP handshake will fail, causing the application to error out even though the deployment succeeded.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
SDLC Automation — This question tests SDLC Automation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The new task definition has a misconfigured database connection string or security group. — Option B is correct because the most common cause of a deployment succeeding but the application failing is a misconfiguration in the new task definition, such as an incorrect database connection string or a security group that does not allow traffic to the database. CodePipeline can successfully deploy the new task definition to ECS, but if the application cannot connect to its backend services due to these configuration errors, the application will fail at runtime. This aligns with the scenario where the deployment pipeline reports success but the application itself is non-functional.
What should I do if I get this DOP-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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