Problem Statement
What are read concerns in MongoDB and how do they ensure data consistency?
Explanation
Read concern controls the consistency and isolation properties of data read from MongoDB. It determines what data is visible to a read operation, especially in replica sets where data may not be immediately consistent across all members.
Read concern local returns the most recent data available on the queried node, without guarantees that the data has been replicated. This is the default and provides the fastest reads but may return data that could be rolled back if the primary fails before replication.
Read concern majority returns data that has been acknowledged by the majority of replica set members. This ensures you never read data that could be rolled back during failover. It provides strong consistency guarantees but may return slightly older data if the most recent writes have not yet replicated to the majority.
Read concern linearizable provides read-your-writes consistency for single-document reads. It guarantees that read operations return data that reflects all successful writes that completed before the read started. This is useful for scenarios requiring strict consistency but has performance implications.
Read concern snapshot provides point-in-time consistency for multi-document reads within transactions. All reads in the transaction see a consistent snapshot of data as it existed at the transaction start time.
Choose read concern based on your consistency requirements. Use majority for critical reads where data accuracy is essential. Use local for better performance when eventual consistency is acceptable.
Code Solution
SolutionRead Only
// Read concern 'local' - fastest, may return rollback data
db.inventory.find({ qty: { $gt: 0 } })
.readConcern("local")
// Read concern 'majority' - durable reads, slower
db.inventory.find({ qty: { $gt: 0 } })
.readConcern("majority")
// Read concern 'linearizable' - strongest consistency
db.inventory.findOne({ _id: 1 })
.readConcern("linearizable")
// In transactions with snapshot isolation
const session = client.startSession()
session.startTransaction({
readConcern: { level: "snapshot" },
writeConcern: { w: "majority" }
})