PDE Designing Data Processing Systems Practice Question
Your company uses Pub/Sub to ingest clickstream data. Messages must be processed in order for the same user_id. How should you configure the Pub/Sub subscription to guarantee ordering?
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
✓
Use a pull subscription with enable_message_ordering=true
Pub/Sub ordering keys allow messages with the same key to be delivered in order to subscribers. The subscription must be created with enable_message_ordering set to true.
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 pull subscription with enable_message_ordering=true
Why this is correct
Ordering keys with enable_message_ordering ensures messages with the same key are delivered in order.
- ✗
Use a pull subscription with exactly-once delivery enabled
Why it's wrong here
Exactly-once delivery prevents duplicates but does not guarantee ordering.
- ✗
Use a push subscription with acknowledgement deadline set to 600 seconds
Why it's wrong here
Acknowledgement deadline does not guarantee ordering.
- ✗
Use a push subscription with a dead letter topic
Why it's wrong here
A dead letter topic handles messages that a push subscription fails to deliver, but it does not impose any ordering guarantee on message delivery. The scenario requires strict per-user_id sequencing, which only an ordered Pub/Sub subscription with message ordering enabled can provide. This option is tempting because dead letter topics are a standard mechanism for managing undeliverable messages in asynchronous workflows, and they would be correct if the requirement were simply to isolate failed messages for later analysis.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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