The answer is that the batch job is not idempotent and is re-processing previously successful transactions. This is the most likely root cause because the unique constraint violation on the REF_NUM column in the ACCT_TRANS table directly signals a duplicate key attempt: the job is trying to insert a row with a REF_NUM that already exists, meaning it failed to recognize or skip records it had already processed. On the Certified Information Systems Auditor CISA exam, this scenario tests your understanding of application controls and batch processing integrity, specifically how idempotency prevents data corruption during reruns. A common trap is to blame a data entry error or a missing index, but the core issue is that the job lacks logic to check for prior success before inserting. Remember the mnemonic: “If it inserts twice, idempotency isn’t nice.”
CISA Practice Question: Information Systems Acquisition, Development and Implementation
This CISA practice question tests your understanding of information systems acquisition, development and implementation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Exhibit
Refer to the exhibit.
```
Error: ORA-00001: unique constraint (FIN.UK_ACCT_TRANS_REF) violated
INSERT INTO ACCT_TRANS (TRANS_ID, REF_NUM, AMOUNT, DATE) VALUES (?, ?, ?, ?)
Call stack:
- com.finance.service.TransactionService.processPayment(TransactionService.java:145)
- com.finance.batch.BatchJob.run(BatchJob.java:88)
```
During a nightly batch job, the above error appears in the application logs. The transaction table ACCT_TRANS has a unique constraint on the REF_NUM column. Which of the following is the MOST likely root 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 batch job is not idempotent and is re-processing previously successful transactions
The unique constraint violation on REF_NUM indicates that the batch job is attempting to insert a row with a REF_NUM value that already exists in the ACCT_TRANS table. This occurs when the job is not idempotent—meaning it does not check for or handle previously processed transactions—and re-processes the same data, leading to duplicate key errors.
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 batch job lacks sufficient privileges to insert into the ACCT_TRANS table
Why it's wrong here
Would result in an insufficient privileges error.
✗
There is a mismatch between the number of columns in the INSERT statement and the table definition
Why it's wrong here
Would cause a different error (column count mismatch).
✗
The batch job is missing an index on the REF_NUM column
Why it's wrong here
Missing index does not cause constraint violation.
✓
The batch job is not idempotent and is re-processing previously successful transactions
Why this is correct
Duplicate REF_NUM suggests reprocessing of already inserted records.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse a unique constraint violation with a permissions or schema mismatch error, but the specific error message (unique constraint on REF_NUM) directly points to duplicate data from non-idempotent processing, not structural or privilege issues.
Detailed technical explanation
How to think about this question
A unique constraint in SQL (e.g., `UNIQUE (REF_NUM)`) creates a unique index behind the scenes in most RDBMS (like Oracle, SQL Server, PostgreSQL) to enforce uniqueness. When a batch job re-processes transactions without idempotency checks (e.g., using `INSERT` instead of `MERGE` or `INSERT ... ON CONFLICT DO NOTHING` in PostgreSQL), the second attempt to insert the same REF_NUM violates the constraint, raising error code ORA-00001 (Oracle) or SQLSTATE 23505 (PostgreSQL). In real-world ETL pipelines, this is a common issue when jobs are not designed to handle retries or reprocessing of already-loaded data.
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 junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
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.
Information Systems Acquisition, Development and Implementation — This question tests Information Systems Acquisition, Development and Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The batch job is not idempotent and is re-processing previously successful transactions — The unique constraint violation on REF_NUM indicates that the batch job is attempting to insert a row with a REF_NUM value that already exists in the ACCT_TRANS table. This occurs when the job is not idempotent—meaning it does not check for or handle previously processed transactions—and re-processes the same data, leading to duplicate key errors.
What should I do if I get this CISA 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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