The answer is that the custom platform has a read-only root filesystem. This is the most likely cause because Elastic Beanstalk custom platforms allow you to define a custom AMI and platform configuration, and if the platform packer template or build script sets the root volume as read-only—often for security hardening—the application will lose write access to directories like /tmp, even though /tmp is typically a local, writable filesystem on standard Amazon Linux instances. On the AWS Certified Developer Associate DVA-C02 exam, this question tests your understanding of how custom platforms differ from managed platforms, where the root filesystem is writable by default. A common trap is to blame IAM instance profiles or security groups, but since /tmp is local storage, OS-level filesystem permissions are the culprit. Memory tip: if the app can’t write to /tmp on a custom platform, think “read-only root”—the platform builder locked down the filesystem.
DVA-C02 Deployment Practice Question
This DVA-C02 practice question tests your understanding of deployment. 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 uses AWS Elastic Beanstalk to deploy a PHP application. The application requires write access to the /tmp directory on the EC2 instances. The deployment uses a custom platform. After a recent deployment, the application fails to write to /tmp. The operations team verifies that the instance security group and NACLs allow all outbound traffic. 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.
The application is running as an unprivileged user without write access to /tmp.
Why wrong: By default, /tmp is world-writable; but if permissions are changed, this could happen. However, Elastic Beanstalk typically runs as webapp user; still /tmp is usually writable. Option C is more specific to custom platforms.
B
The security group is blocking outbound traffic to the S3 bucket.
Why wrong: Outbound traffic is allowed; also /tmp is local, not S3.
C
The custom platform has a read-only root filesystem.
Custom platforms can have read-only root filesystems; /tmp may be on root and thus read-only.
D
The instance profile does not have permissions to write to the S3 bucket.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The custom platform has a read-only root filesystem.
Option B is correct: Elastic Beanstalk uses instance profiles; if the profile lacks permissions to write to /tmp (though /tmp is local filesystem, but if using EFS or similar, it might be an issue; however, /tmp is typically writable by default. But the question implies a permissions issue: the instance profile might not have permissions to mount or access certain resources. Actually, /tmp is local, so permissions are OS-level. The likely cause is that the custom platform has read-only root filesystem or the application user does not have write permission. Option B (instance profile) is plausible if /tmp is mounted from an external source. But a better answer: Option C (read-only root filesystem) is a common Elastic Beanstalk custom platform issue. However, standard Elastic Beanstalk environments have writable /tmp. Given the custom platform, the most likely is that the platform configuration sets root filesystem as read-only. So Option C is correct.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 application is running as an unprivileged user without write access to /tmp.
Why it's wrong here
By default, /tmp is world-writable; but if permissions are changed, this could happen. However, Elastic Beanstalk typically runs as webapp user; still /tmp is usually writable. Option C is more specific to custom platforms.
✗
The security group is blocking outbound traffic to the S3 bucket.
Why it's wrong here
Outbound traffic is allowed; also /tmp is local, not S3.
✓
The custom platform has a read-only root filesystem.
Why this is correct
Custom platforms can have read-only root filesystems; /tmp may be on root and thus read-only.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
The instance profile does not have permissions to write to the S3 bucket.
Why it's wrong here
/tmp is a local directory, not S3.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related DVA-C02 questions on access control and AAA configuration.
Deployment — This question tests Deployment — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The custom platform has a read-only root filesystem. — Option B is correct: Elastic Beanstalk uses instance profiles; if the profile lacks permissions to write to /tmp (though /tmp is local filesystem, but if using EFS or similar, it might be an issue; however, /tmp is typically writable by default. But the question implies a permissions issue: the instance profile might not have permissions to mount or access certain resources. Actually, /tmp is local, so permissions are OS-level. The likely cause is that the custom platform has read-only root filesystem or the application user does not have write permission. Option B (instance profile) is plausible if /tmp is mounted from an external source. But a better answer: Option C (read-only root filesystem) is a common Elastic Beanstalk custom platform issue. However, standard Elastic Beanstalk environments have writable /tmp. Given the custom platform, the most likely is that the platform configuration sets root filesystem as read-only. So Option C is correct.
What should I do if I get this DVA-C02 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related DVA-C02 questions on access control and AAA configuration.
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?
Authentication checks who the user is.
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