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.