C100DBA Philosophy and Features Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Embedding events within the shipment document ensures atomic updates and fast reads for the event history.

  Why this is correct

  Embedding events in the shipment document allows the application to retrieve the entire event history with a single query, and updates to the shipment and its events can be atomic within a single document. This aligns with MongoDB's philosophy of storing data that is accessed together. However, it requires monitoring document growth to avoid hitting the 16 MB limit, especially if events are unbounded.

• ✗

  Embedding events is always preferred because it eliminates the need for multi-document transactions.

  Why it's wrong here

  While embedding can reduce the need for multi-document transactions by keeping related data in one document, it is not always preferred. If events grow without bound, embedding can lead to document size issues and performance degradation. The decision should be based on access patterns, data growth, and atomicity requirements, not an absolute rule.

• ✗

  Referencing events requires using $lookup for every read, which is as fast as an embedded document read.

  Why it's wrong here

  $lookup performs a left outer join, which is generally slower than reading an embedded document because it involves additional processing and potentially multiple shards. While $lookup is powerful, it does not match the performance of a single-document read. Therefore, referencing is not a direct substitute for embedding when read speed is critical.

• ✗

  Embedding events ensures that the shipment document remains small and efficient for writes.

  Why it's wrong here

  Embedding events actually increases the size of the shipment document as events are added. If events are frequently added, the document can grow large, leading to more expensive updates and potential fragmentation. To keep documents small, referencing events in a separate collection is a better strategy when event volume is high.

• ✓

  Referencing events in a separate collection allows unlimited growth and avoids the 16 MB document size limit.

  Why this is correct

  Referencing events in a separate collection prevents the shipment document from growing unbounded, which is crucial if events accumulate indefinitely. This approach avoids the 16 MB size limit and allows independent querying of events. However, it requires additional queries or $lookup to join data, which can impact performance. The choice depends on access patterns and growth expectations.