Mongos router behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

How does mongos route a query to shards?

Consider a sharded MongoDB cluster with a mongos router. When a query is sent to mongos, how does it decide which shards to query?

A It broadcasts the query to all shards regardless of shard key.

B It uses the shard key in the query to route the query only to relevant shards.

C It sends the query only to the primary shard of the database.

D It randomly selects one shard to send the query to.

💡 Hint

Think about how sharding improves query efficiency by targeting specific shards.

mongos uses the shard key in the query to determine which shards hold the relevant data. This avoids querying all shards and improves performance.

🧠 Conceptual

intermediate

What role does mongos play in a sharded cluster?

In a MongoDB sharded cluster, what is the primary role of the mongos process?

A It acts as a query router and interface between clients and shards.

B It stores the actual data for the shards.

C It manages the config servers and metadata storage.

D It performs backups of the cluster data.

💡 Hint

Think about how clients interact with a sharded cluster.

mongos acts as a router that directs client queries to the correct shards based on metadata. It does not store data or manage config servers.

📝 Syntax

advanced

Identify the correct mongos query to find documents by shard key

Given a sharded collection with shard key { userId: 1 }, which mongos query will efficiently find documents for userId 123?

MongoDB

db.orders.find({ userId: 123 })

A db.orders.find({ userId: 123 })

B db.orders.find({ _id: 123 })

C db.orders.find({ userId: { $gt: 100 } })

D db.orders.find({ customerId: 123 })

💡 Hint

Use the shard key field exactly in the query to target shards.

Querying by the shard key field userId allows mongos to route the query to the correct shard efficiently.

❓ optimization

advanced

How to optimize queries with mongos in a sharded cluster?

You notice that queries sent through mongos are slow because they query all shards. What change can optimize query performance?

A Run queries directly on config servers instead of mongos.

B Increase the number of shards to distribute load.

C Add the shard key to the query filter to target specific shards.

D Disable sharding on the collection.

💡 Hint

Think about how mongos decides which shards to query.

Including the shard key in the query filter lets mongos route queries to only relevant shards, improving performance.

🔧 Debug

expert

Why does mongos return stale data after chunk migration?

After a chunk migration in a sharded cluster, mongos sometimes returns stale data. What is the most likely cause?

A The client query is missing the shard key field.

B The shard key was changed during migration causing data loss.

C Config servers are offline and cannot update mongos.

D mongos has cached outdated metadata and has not refreshed it yet.

💡 Hint

Consider how mongos caches cluster metadata.

mongos caches metadata about chunk locations. After chunk migration, if it does not refresh this cache, it may route queries incorrectly and return stale data.