MongoDB Sharding and Horizontal Scaling Flashcards

What is a chunk in MongoDB sharding?

Answer: A contiguous range of shard key values that is stored on a single shard

A chunk is a contiguous range of shard key values assigned to a specific shard. As data grows, MongoDB splits large chunks and the balancer migrates chunks between shards. The default chunk size is 128 MB in recent MongoDB versions.

What is zone sharding (also called tag-aware sharding) in MongoDB?

Answer: Associating ranges of shard key values with specific shards to control data placement

Zone sharding allows you to define ranges of shard key values (zones) and associate them with specific shards. This is used for data sovereignty (keeping data in a specific region), tiered storage, or workload isolation — ensuring certain documents always reside on designated shards.

What is the minimum number of config servers required for a production MongoDB sharded cluster?

Answer: 3

Production MongoDB sharded clusters require 3 config servers deployed as a replica set (CSRS — Config Server Replica Set). Three config servers ensure high availability of cluster metadata. Using a single config server is only suitable for testing.

What happens when you shard a collection in MongoDB that already has data?

Answer: MongoDB creates an initial chunk and begins distributing data during subsequent balancer runs

When you enable sharding on an existing collection, MongoDB creates an initial chunk containing all existing documents on the primary shard. The balancer then gradually migrates chunks to other shards over time. Initial sync doesn't happen instantaneously.

What is a jumbo chunk in MongoDB sharding and why is it a problem?

Answer: A chunk that cannot be split because all documents share the same shard key value, preventing rebalancing

A jumbo chunk forms when all documents within a chunk share the same shard key value, preventing further splitting. The balancer cannot move jumbo chunks, causing imbalanced data distribution. Choosing a high-cardinality shard key prevents jumbo chunk formation.

Which command enables sharding for a specific database in MongoDB?

Answer: sh.enableSharding('dbName')

sh.enableSharding('dbName') enables sharding at the database level, which is required before sharding individual collections. This is run via the mongos router. Then sh.shardCollection('dbName.collectionName', { shardKey: 1 }) shards a specific collection.