The answer is the JDBC driver missing the 'redshift' compatibility mode setting. This is the most likely cause because the AWS Glue JDBC connector for Redshift requires the compatibility mode parameter to be explicitly set in the connection string; without it, the driver defaults to a PostgreSQL protocol that can cause unexpected connection closures, especially under load or during large data transfers. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how Glue integrates with Redshift via JDBC, often appearing as a trick where network or VPC issues seem plausible but the real culprit is a missing configuration parameter. A common trap is to blame VPC routing or SSL enforcement, but the key insight is that Redshift’s JDBC driver is not a standard PostgreSQL driver—it needs the `redshift` compatibility mode to maintain stable connections. Memory tip: think “Glue needs Redshift mode, not PG mode” to recall that the driver must explicitly declare its target dialect.
DEA-C01 Data Operations and Support Practice Question
This DEA-C01 practice question tests your understanding of data operations and support. 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 nightly AWS Glue ETL job that writes results to an Amazon Redshift table using the JDBC connector. Recently, the job has been failing with the error 'ERROR: connection to server at ... failed: server closed the connection unexpectedly'. The Redshift cluster is in a private subnet with a VPC endpoint for S3. The Glue job runs in the same VPC with enhanced VPC routing enabled. Which 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 JDBC driver is missing the 'redshift' compatibility mode setting.
Setting 'redshift' compatibility in the JDBC URL (e.g., '?compatible=redshift') ensures proper handling of Redshift-specific features and prevents unexpected disconnections.
B
SSL is not enabled on the Redshift cluster.
Why wrong: SSL is not required for connectivity; the error message does not indicate SSL issues.
C
The Glue job's security group does not allow outbound traffic to the Redshift cluster.
Why wrong: If the job is in the same VPC, security group rules should allow traffic; the error indicates a connection drop, not a timeout.
D
AWS Glue does not support Redshift as a data source.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The JDBC driver is missing the 'redshift' compatibility mode setting.
Option D is correct because Redshift compatibility mode in the JDBC driver is required for Glue to properly handle connections. Without it, the driver may close the connection unexpectedly. Option A is wrong because Redshift does not enforce SSL by default unless configured. Option B is wrong because the Glue job is in the same VPC and uses enhanced VPC routing, so the connection should work. Option C is wrong because Glue supports Redshift as a data source.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The JDBC driver is missing the 'redshift' compatibility mode setting.
Why this is correct
Setting 'redshift' compatibility in the JDBC URL (e.g., '?compatible=redshift') ensures proper handling of Redshift-specific features and prevents unexpected disconnections.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
SSL is not required for connectivity; the error message does not indicate SSL issues.
✗
The Glue job's security group does not allow outbound traffic to the Redshift cluster.
Why it's wrong here
If the job is in the same VPC, security group rules should allow traffic; the error indicates a connection drop, not a timeout.
✗
AWS Glue does not support Redshift as a data source.
Why it's wrong here
Glue fully supports Redshift via JDBC.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
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
How to think about this question
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.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
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
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DEA-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Data Operations and Support — This question tests Data Operations and Support — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The JDBC driver is missing the 'redshift' compatibility mode setting. — Option D is correct because Redshift compatibility mode in the JDBC driver is required for Glue to properly handle connections. Without it, the driver may close the connection unexpectedly. Option A is wrong because Redshift does not enforce SSL by default unless configured. Option B is wrong because the Glue job is in the same VPC and uses enhanced VPC routing, so the connection should work. Option C is wrong because Glue supports Redshift as a data source.
What should I do if I get this DEA-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DEA-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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