MongoDB Replication and Sharding Flashcards

What is a scatter-gather query in MongoDB sharding?

Answer: A query without the shard key that must be sent to all shards and results merged

Scatter-gather queries are broadcast to all shards because they lack the shard key; results are gathered and merged by mongos, making them less efficient.

What read preference allows reads from secondary replica set members?

Answer: All options except 'primary' allow secondary reads

Any read preference other than 'primary' (primaryPreferred, secondary, secondaryPreferred, nearest) can route reads to secondary members.

What does the rs.status() command display in MongoDB?

Answer: The current state, health, and replication lag of all replica set members

rs.status() returns a document describing the current state of the replica set, including each member's health, state, and optime.

What is replication lag in MongoDB?

Answer: The time difference between when the primary logs an operation and a secondary applies it

Replication lag measures how far behind a secondary is in replicating the primary's oplog, which can affect read-your-writes consistency.

What happens during a replica set failover when the primary becomes unavailable?

Answer: An election is triggered and a secondary is promoted to primary

When the primary is unreachable, eligible secondaries call an election and the member with the most up-to-date oplog and highest priority typically wins.

What is zone sharding in MongoDB?

Answer: Associating specific shard key ranges with specific shards for data locality

Zone sharding maps specific shard key ranges to specific shards, enabling data locality for compliance, latency, or hardware optimization requirements.