Mongos router behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The mongos acts as a query router. It directs client requests to the correct shard(s) based on the cluster's metadata, making the sharding transparent to the client.

intermediate

How does mongos know which shard to send a query to?

Mongos uses the cluster metadata stored in the config servers to determine the shard that holds the data for the query's shard key range.

beginner

True or False: mongos stores data itself in a sharded cluster.

False. Mongos does not store data. It only routes queries to the appropriate shards which hold the actual data.

advanced

What happens if a mongos instance loses connection to the config servers?

Mongos cannot route queries correctly without config server metadata. It will fail to process queries until it reconnects and refreshes metadata.

intermediate

Why might you run multiple mongos instances in a sharded cluster?

Running multiple mongos instances improves availability and load balancing for client requests, preventing a single point of failure or bottleneck.

What is the main function of a mongos router in MongoDB?

A Backup shard data

B Route queries to the correct shard

C Manage config server metadata

D Store data in the cluster

Mongos routes queries to the correct shard based on metadata; it does not store data or manage backups.

Where does mongos get the metadata to route queries?

A From the config servers

B From the shards directly

C From the client application

D From the mongod instances

Mongos retrieves cluster metadata from the config servers to know where data is located.

If mongos loses connection to config servers, what happens?

A It continues routing queries normally

B It stores data locally

C It automatically becomes a config server

D It cannot route queries correctly

Without config server metadata, mongos cannot route queries correctly.

Does mongos store any data in a sharded cluster?

A No, it only routes queries

B Yes, it stores metadata only

C Yes, it stores all data

D No, it stores backups

Mongos only routes queries; data is stored on shards.

Why run multiple mongos instances?

A To replace config servers

B To store more data

C To improve query routing availability

D To shard data

Multiple mongos instances improve availability and load balancing for query routing.

Explain how mongos routes a client query in a MongoDB sharded cluster.

Think about how mongos acts like a traffic controller.

Describe what happens if a mongos instance cannot access the config servers.

Consider the importance of config server metadata for routing.

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