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What is balancer in MongoDB?
The MongoDB balancer is a background process that monitors the number of chunks on each shard. When the number of chunks on a given shard reaches specific migration thresholds, the balancer attempts to automatically migrate chunks between shards and reach an equal number of chunks per shard.
Which of the following are the simplest NoSQL databases?
Key-value stores are the simplest NoSQL databases.
How do I shard a collection in MongoDB Atlas?
The Atlas Data Explorer only supports sharding Global Write collections inside of a global cluster. To shard non-Global-Write collections, you must use mongosh or application code. To learn more, see Deploy a Sharded Cluster.
Why do I need balancer window in MongoDB?
The balancer window must be sufficient to complete the migration of all data inserted during the day. As data insert rates can change based on activity and usage patterns, it is important to ensure that the balancing window you select will be sufficient to support the needs of your deployment.
How does shard migration work in MongoDB balancer?
To migrate multiple chunks from a shard, the balancer migrates the chunks one at a time. However, the balancer does not wait for the current migration’s delete phase to complete before starting the next chunk migration. See Chunk Migration for the chunk migration process and the delete phase.
How big is the default chunk size in MongoDB?
The default chunk size is 64 megabytes. MongoDB will not begin migrations until the imbalance of chunks in the cluster exceeds the migration threshold. This behavior helps prevent unnecessary chunk migrations, which can degrade the performance of your cluster as a whole.
How does read preference work in MongoDB cluster?
But, clients can specify a read preference to send read operations to secondaries. Read preference consists of the read preference mode and optionally, a tag set, the maxStalenessSeconds option, and the hedged read option. Hedged read option is available for MongoDB 4.4+ sharded clusters for reads that use non- primary read preference.