- A
Use a collection group query to listen to all games at once
Why wrong: Listens to many documents, increasing reads and cost.
- B
Store an aggregate score summary document per game and listen to it
Reduces write operations and read frequency; clients get updates from a single summary document.
- C
Use a separate document per game and listeners filter by game ID
Each client listens only to the relevant document, minimizing reads.
- D
Use a single document for all games with nested fields
Why wrong: Causes contention and all clients reading a large document; not scalable.
- E
Use a subcollection of periods (quarters) to spread writes
Why wrong: Increases number of documents and reads without cost benefit.
Question 294 of 1,000
Firestore Real-Time Listeners — Minimize Reads and Reduce Costs
This PCDE practice question tests your understanding of design and implement database schemas. 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.
A company uses Firestore to power a live sports score app. Scores are updated frequently, and many clients listen to real-time updates on specific games. Which two design decisions will minimize the number of reads and reduce costs? (Choose two.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"minimum / minimize"Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Quick Answer
The answer is to use a separate document per game with listeners filtering by game ID, and to store an aggregate score summary document per game. These two design decisions minimize reads and reduce costs because the first ensures each client reads only the single document relevant to their game, avoiding unnecessary document scans, while the second batches frequent score updates into one document write, so all listeners read that single, compact document instead of multiple sub-documents. On the Google Professional Cloud Database Engineer exam, this tests your understanding of Firestore’s real-time listener cost model, where every document snapshot delivered counts as a read; a common trap is choosing a collection group query, which would listen to many documents and spike reads. Remember the memory tip: “One game, one doc, one summary—keep reads low and costs no glum.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Store an aggregate score summary document per game and listen to it
Option B is correct because storing an aggregate score summary document per game and listening to it reduces reads by consolidating frequently updated fields into a single document, minimizing the number of document reads per update. Option C is correct because using a separate document per game with listeners filtered by game ID ensures each client only listens to the specific game they care about, avoiding unnecessary reads from irrelevant documents.
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 a collection group query to listen to all games at once
Why it's wrong here
Listens to many documents, increasing reads and cost.
- ✓
Store an aggregate score summary document per game and listen to it
Why this is correct
Reduces write operations and read frequency; clients get updates from a single summary document.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Use a separate document per game and listeners filter by game ID
Why this is correct
Each client listens only to the relevant document, minimizing reads.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a single document for all games with nested fields
Why it's wrong here
Causes contention and all clients reading a large document; not scalable.
- ✗
Use a subcollection of periods (quarters) to spread writes
Why it's wrong here
Increases number of documents and reads without cost benefit.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Avoid the misconception that spreading writes across many documents (e.g., subcollections) reduces costs. In Firestore, reads are charged per document read. Consolidating frequently updated scores into a single summary document per game and having clients listen only to specific game IDs minimizes document reads and reduces costs.
Detailed technical explanation
How to think about this question
Firestore charges per document read, even for real-time listeners; each listener snapshot counts as a read. By aggregating score updates into a single summary document per game, you reduce the number of documents that need to be read per update. Additionally, Firestore's real-time listener overhead includes the full document payload on each change, so minimizing document size and frequency of updates per document directly reduces egress and read costs.
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.
What to study next
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FAQ
Questions learners often ask
What does this PCDE question test?
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Store an aggregate score summary document per game and listen to it — Option B is correct because storing an aggregate score summary document per game and listening to it reduces reads by consolidating frequently updated fields into a single document, minimizing the number of document reads per update. Option C is correct because using a separate document per game with listeners filtered by game ID ensures each client only listens to the specific game they care about, avoiding unnecessary reads from irrelevant documents.
What should I do if I get this PCDE 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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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