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.

Practice

1. What is the primary role of the mongos router in a sharded MongoDB cluster?

easy

A. It stores the actual data for the database.

B. It routes queries to the correct shard based on the shard key.

C. It manages user authentication and roles.

D. It backs up the database automatically.

Solution

1. Step 1: Understand mongos role

   The mongos acts as a query router in a sharded cluster, directing queries to the right shard.

2. Step 2: Differentiate from other components

   Data storage is handled by shards, not mongos. Authentication and backups are separate concerns.

3. Final Answer:

   It routes queries to the correct shard based on the shard key. -> Option B

4. Quick Check:

   mongos routes queries [OK]

Hint: Remember: mongos routes, shards store data [OK]

Common Mistakes:

• Thinking mongos stores data
• Confusing mongos with config servers
• Assuming mongos handles backups

2. Which of the following is the correct way for an application to connect in a sharded MongoDB setup?

easy

A. Connect to the primary replica set member.

B. Connect only to the config servers.

C. Connect directly to each shard individually.

D. Connect to the mongos router.

Solution

1. Step 1: Identify connection point in sharded cluster

   Applications connect to mongos which routes queries to shards transparently.

2. Step 2: Exclude other options

   Direct connection to shards or config servers is not recommended; primary replica set member is for replica sets, not sharding.

3. Final Answer:

   Connect to the mongos router. -> Option D

4. Quick Check:

   Apps connect to mongos [OK]

Hint: Always connect apps to mongos, not shards [OK]

Common Mistakes:

• Connecting directly to shards
• Using config servers for queries
• Confusing replica sets with sharding

3. Given a sharded cluster with two shards and a mongos router, what will happen when you run this query through mongos?

db.users.find({ userId: 12345 })

Assume userId is the shard key.

medium

A. mongos returns an error because shard key is missing.

B. mongos sends the query to all shards and merges results.

C. mongos routes the query only to the shard containing userId 12345.

D. mongos sends the query to the config servers.

Solution

1. Step 1: Check query uses shard key

   The query filters on userId, which is the shard key, so mongos can route precisely.

2. Step 2: Understand mongos routing behavior

   mongos sends the query only to the shard that holds the chunk for userId 12345, not all shards.

3. Final Answer:

   mongos routes the query only to the shard containing userId 12345. -> Option C

4. Quick Check:

   Query with shard key routes to one shard [OK]

Hint: Query with shard key goes to one shard only [OK]

Common Mistakes:

• Assuming mongos queries all shards always
• Thinking config servers handle queries
• Believing shard key is optional for routing

4. You wrote a query to find documents by userId in a sharded cluster but get slow results. You realize userId is not the shard key. What is the likely cause?

medium

A. mongos is routing the query to all shards (scatter-gather).

B. mongos is routing the query to only one shard.

C. mongos is ignoring the query and returning empty results.

D. mongos is connecting directly to config servers.

Solution

1. Step 1: Identify shard key usage in query

   The query filters on userId, which is not the shard key, so mongos cannot route precisely.

2. Step 2: Understand mongos scatter-gather behavior

   mongos sends the query to all shards (scatter-gather) causing slower results.

3. Final Answer:

   mongos is routing the query to all shards (scatter-gather). -> Option A

4. Quick Check:

   Query without shard key causes scatter-gather [OK]

Hint: Queries without shard key hit all shards, slowing response [OK]

Common Mistakes:

• Assuming query routes to one shard without shard key
• Thinking mongos returns empty results on missing shard key
• Confusing config servers with query routers

5. In a sharded MongoDB cluster, you want to optimize query performance for a collection frequently queried by email. The current shard key is userId. What is the best approach involving mongos behavior?

hard

A. Change the shard key to email so mongos routes queries efficiently.

B. Add an index on email and keep the shard key as userId.

C. Connect directly to shards to bypass mongos routing.

D. Disable sharding on the collection to improve query speed.

Solution

1. Step 1: Analyze query pattern and shard key

   Queries are frequent on email, but shard key is userId, causing scatter-gather queries via mongos.

2. Step 2: Optimize shard key for routing

   Changing shard key to email lets mongos route queries directly to relevant shards, improving performance.

3. Final Answer:

   Change the shard key to email so mongos routes queries efficiently. -> Option A

4. Quick Check:

   Shard key matches query field for efficient mongos routing [OK]

Hint: Match shard key to frequent query field for fast mongos routing [OK]

Common Mistakes:

• Thinking adding index alone fixes routing
• Connecting directly to shards bypasses mongos is not recommended
• Disabling sharding reduces scalability