The answer is to use 'append' mode and perform a MERGE operation after the write to deduplicate based on the composite key (source_id, record_id). This is correct because 'append' mode writes all incoming data as new files without modifying existing data, and then the Delta Lake MERGE operation leverages ACID transactions to upsert rows, efficiently removing duplicates both within the new batch and against the existing table by matching on the composite key. On the DP-203 exam, this scenario tests your understanding of Delta Lake’s ability to handle late-arriving data and intra-batch duplicates without costly partition rewrites—a common trap is choosing 'overwrite' mode, which would erase historical data. Remember the memory tip: “Append then merge, don’t overwrite the purge.”
DP-203 Develop data processing Practice Question
This DP-203 practice question tests your understanding of develop data processing. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
You are designing a data processing solution using Azure Databricks with Delta Lake. The data is ingested from multiple sources and needs to be deduplicated based on a composite key (source_id, record_id). New data may have duplicates within the same batch. Which write mode and table property should you use to handle this efficiently?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use 'append' mode and perform a MERGE operation after write to deduplicate.
Option A is correct because using 'append' mode writes all incoming data as new files, and then performing a MERGE operation (upsert) based on the composite key (source_id, record_id) allows you to efficiently deduplicate both within the batch and against existing data. This approach leverages Delta Lake's ACID transactions and avoids the cost of rewriting entire partitions, making it suitable for handling duplicates from multiple sources.
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.
✓
Use 'append' mode and perform a MERGE operation after write to deduplicate.
Why this is correct
Append with a subsequent MERGE allows custom dedup logic on composite key.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use 'overwrite' mode and enable 'delta.autoOptimize.optimizeWrite' = true.
Why it's wrong here
Overwrite replaces the entire table, not suitable for incremental dedup.
✗
Use 'ignore' mode and set 'delta.autoCompact' = true.
Why it's wrong here
Ignore mode skips writing if file exists, not dedup within batch.
✗
Use 'error' mode and enable 'delta.merge.onSchemaMismatch' = true.
Why it's wrong here
Error mode fails on duplicates, not desired.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Microsoft often tests the misconception that 'overwrite' mode or table properties like 'autoOptimize' can handle deduplication, but the correct approach requires explicit deduplication logic (like MERGE) because Delta Lake does not enforce unique constraints natively.
Detailed technical explanation
How to think about this question
Under the hood, Delta Lake's MERGE operation uses a combination of read and write transactions, scanning the table for matching keys and then atomically applying inserts, updates, or deletes. This is more efficient than batch deduplication because it avoids full table scans by leveraging Delta Lake's transaction log and file skipping. In real-world scenarios with high-velocity streaming data, using 'append' + MERGE allows you to handle late-arriving data and duplicates without reprocessing historical partitions.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this DP-203 question in full detail.
Develop data processing — This question tests Develop data processing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use 'append' mode and perform a MERGE operation after write to deduplicate. — Option A is correct because using 'append' mode writes all incoming data as new files, and then performing a MERGE operation (upsert) based on the composite key (source_id, record_id) allows you to efficiently deduplicate both within the batch and against existing data. This approach leverages Delta Lake's ACID transactions and avoids the cost of rewriting entire partitions, making it suitable for handling duplicates from multiple sources.
What should I do if I get this DP-203 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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