Quick Answer
The answer is that the VPC does not have an S3 VPC endpoint. This is the most likely cause of the AWS Glue connection timeouts to RDS because Glue jobs running inside a VPC need a gateway or interface endpoint for Amazon S3 to communicate with the Glue catalog and other AWS services; without it, the job cannot resolve metadata calls, leading to timeouts even when the RDS security group is correctly configured. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of how Glue’s network architecture differs from standard EC2-to-RDS connectivity—a common trap is assuming the security group is the culprit when the real issue is missing VPC endpoints for S3. Remember the memory tip: “Glue needs a path to S3, not just a port to RDS.”
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.
The security group inbound rule is incorrect
Why wrong: The rule allowing inbound from the job's security group is correct.
The VPC does not have an S3 VPC endpoint
Glue jobs need an S3 VPC endpoint to access the catalog.
RDS is in a different subnet
Why wrong: Subnet differences are handled by routing; not likely cause.
The IAM role does not have permission to access RDS
Why wrong: Connection timeouts indicate network, not permissions.
Answer choices
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
AWS Glue jobs run in a VPC that requires an S3 VPC endpoint to access the AWS Glue catalog and other services. Without the endpoint, the job cannot connect to Glue's metadata store, causing timeouts. Option A is wrong because the security group is correctly configured. Option C is wrong because the issue is not about IAM permissions for RDS. Option D is wrong because there is no subnet issue mentioned.
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
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
CIDR notation defines the prefix length.
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
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
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.
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
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
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related MLS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Data Engineering — This question tests Data Engineering — CIDR notation defines the prefix length..
The correct answer is: The VPC does not have an S3 VPC endpoint — AWS Glue jobs run in a VPC that requires an S3 VPC endpoint to access the AWS Glue catalog and other services. Without the endpoint, the job cannot connect to Glue's metadata store, causing timeouts. Option A is wrong because the security group is correctly configured. Option C is wrong because the issue is not about IAM permissions for RDS. Option D is wrong because there is no subnet issue mentioned.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related MLS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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