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MLS-C01 Data Engineering Practice Question

This MLS-C01 practice question tests your understanding of data engineering. 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 is using AWS Glue to run ETL jobs that process data from an Amazon RDS for PostgreSQL database. The jobs are failing with connection timeouts. The security group for the RDS instance allows inbound traffic from the Glue job's security group. 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.

A
The security group inbound rule is incorrect
Why wrong: The rule allowing inbound from the job's security group is correct.
B
The VPC does not have an S3 VPC endpoint
Glue jobs need an S3 VPC endpoint to access the catalog.
C
RDS is in a different subnet
Why wrong: Subnet differences are handled by routing; not likely cause.
D
The IAM role does not have permission to access RDS
Why wrong: Connection timeouts indicate network, not permissions.

Full breakdown with real-world context →

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 VPC does not have an S3 VPC endpoint

The most likely cause is that the VPC does not have an S3 VPC endpoint. AWS Glue jobs running in a VPC require access to S3 for storing temporary data, scripts, and logs. Without an S3 VPC endpoint, traffic to S3 is routed through a public IP, which fails in private subnets lacking internet connectivity. This failure can cause the Glue job to hang or fail during initialization, and the attempted connection to RDS may time out as a result. The security group inbound rule is correctly configured, so ingress is not the issue. The problem is network egress for S3, not access to RDS itself.

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 security group inbound rule is incorrect
Why it's wrong here
The rule allowing inbound from the job's security group is correct.
✓
The VPC does not have an S3 VPC endpoint
Why this is correct
Glue jobs need an S3 VPC endpoint to access the catalog.
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.
✗
RDS is in a different subnet
Why it's wrong here
Subnet differences are handled by routing; not likely cause.
✗
The IAM role does not have permission to access RDS
Why it's wrong here
Connection timeouts indicate network, not permissions.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates focus on the security group inbound rule (which is correct) and overlook that Glue jobs in a VPC need outbound connectivity to S3 and other services, often requiring a VPC endpoint or NAT gateway.

Detailed technical explanation

How to think about this question

When Glue jobs run in a VPC, they use Elastic Network Interfaces (ENIs) in the specified subnets. Without an S3 VPC endpoint, all traffic to S3 must traverse the internet via a NAT gateway or internet gateway. If the subnets are private and lack a NAT gateway, the Glue job cannot download scripts, upload temporary data, or write logs to S3, leading to timeouts. The S3 VPC endpoint uses AWS PrivateLink to route S3 traffic within the AWS network, avoiding internet dependency.

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 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.

Visual reference

Quick reference

AWS S3 Storage Class Comparison

Storage Class	Min Duration	Retrieval	Use Case
S3 Standard	None	Immediate	Frequently accessed data
S3 Standard-IA	30 days	Immediate	Infrequent access, rapid retrieval
S3 One Zone-IA	30 days	Immediate	Non-critical infrequent data
S3 Intelligent-Tiering	None	Immediate–hours	Unknown or changing access patterns
S3 Glacier Instant	90 days	Milliseconds	Archive with instant retrieval
S3 Glacier Flexible	90 days	Minutes–hours	Archive, flexible retrieval
S3 Glacier Deep Archive	180 days	Hours	Long-term compliance archive

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 MLS-C01 question test?

Data Engineering — This question tests Data Engineering — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The VPC does not have an S3 VPC endpoint — The most likely cause is that the VPC does not have an S3 VPC endpoint. AWS Glue jobs running in a VPC require access to S3 for storing temporary data, scripts, and logs. Without an S3 VPC endpoint, traffic to S3 is routed through a public IP, which fails in private subnets lacking internet connectivity. This failure can cause the Glue job to hang or fail during initialization, and the attempted connection to RDS may time out as a result. The security group inbound rule is correctly configured, so ingress is not the issue. The problem is network egress for S3, not access to RDS itself.

What should I do if I get this MLS-C01 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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This MLS-C01 practice question is part of Courseiva's free Amazon Web Services 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 MLS-C01 exam.