The answer is that the Redshift cluster is in a read-only state due to a failover or maintenance. This is the most likely cause because the error "cannot execute INSERT in a read-only transaction" occurs when AWS Glue attempts to write to a Redshift table that has been temporarily locked into read-only mode, typically triggered by an automatic failover or a scheduled maintenance event that promotes a standby node. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this question tests your understanding of how Redshift’s high-availability architecture interacts with Glue ETL jobs, often appearing as a distractor where you must distinguish between connectivity issues and transaction state errors. A common trap is confusing this with a JDBC timeout or IAM permission error, but the specific phrase "read-only transaction" is the key clue. Memory tip: think "failover freezes writes" — if Redshift flips to read-only, your Glue INSERT will flip out.
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 runs a nightly AWS Glue ETL job that processes data from an Amazon Redshift cluster and writes to Amazon S3. The job fails intermittently with 'ERROR: cannot execute INSERT in a read-only transaction'. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Redshift cluster is in a read-only state due to a failover or maintenance
This error occurs when the Glue job tries to write to a Redshift table that is in a read-only transaction, often because the Redshift cluster is in a read-only state due to a failover or maintenance. Option A (JDBC timeout) gives a different error. Option B (connection pool) would give a different error. Option D (IAM permissions) would give an access denied error.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 IAM role used by Glue does not have permissions to insert into Redshift
Why it's wrong here
Incorrect IAM permissions would result in an access denied error, not a read-only transaction error.
✗
The JDBC driver version is incompatible with the Redshift cluster
Why it's wrong here
Incompatible driver would cause connection or syntax errors, not a read-only transaction error.
✓
The Redshift cluster is in a read-only state due to a failover or maintenance
Why this is correct
During failover, the secondary cluster may be read-only, causing this error.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The Glue job's connection pool is exhausted
Why it's wrong here
Connection pool exhaustion would cause a timeout or 'too many connections' error.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related MLS-C01 NAT questions on configuration and troubleshooting.
Data Engineering — This question tests Data Engineering — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The Redshift cluster is in a read-only state due to a failover or maintenance — This error occurs when the Glue job tries to write to a Redshift table that is in a read-only transaction, often because the Redshift cluster is in a read-only state due to a failover or maintenance. Option A (JDBC timeout) gives a different error. Option B (connection pool) would give a different error. Option D (IAM permissions) would give an access denied error.
What should I do if I get this MLS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related MLS-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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