Introduction to MongoDB collection.getShardDistribution() Method

getShardDistribution() is a method in MongoDB used to query the distribution of a sharded collection. It is used to check the distribution of a sharded collection across the sharded cluster and returns statistics including the number of documents and storage size for each shard. This method can be used to monitor and optimize the data distribution of a sharded collection, thus improving query performance and scalability.

Syntax

The syntax for the getShardDistribution() method is as follows:

db.collection.getShardDistribution()

Here, collection refers to the name of the collection to be queried.

Use Cases

The getShardDistribution() method is typically used in the following scenarios:

• Monitoring and optimizing the data distribution of a sharded collection.
• Diagnosing query performance issues.
• Identifying which shard contains a specific document.

Example

The following example demonstrates how to use the getShardDistribution() method to query the distribution of the orders collection in a sharded cluster.

First, we need to create an orders collection on a sharded cluster and insert some documents into it.

use test
sh.enableSharding("test")

db.createCollection("orders")

db.orders.insertMany([
   { _id: 1, item: "apple", quantity: 5 },
   { _id: 2, item: "orange", quantity: 10 },
   { _id: 3, item: "banana", quantity: 20 },
   { _id: 4, item: "pear", quantity: 15 }
])

Next, we shard the orders collection and distribute it across two shards.

sh.shardCollection("test.orders", { _id: 1 })
sh.addShardTag("shard0000", "east")
sh.addShardTag("shard0001", "west")

sh.addTagRange(
  "test.orders",
  { _id: MinKey },
  { _id: ObjectId("111111111111111111111111") },
  "east"
)

sh.addTagRange(
  "test.orders",
  { _id: ObjectId("111111111111111111111111") },
  { _id: MaxKey },
  "west"
)

We can now use the getShardDistribution() method to query the distribution of the orders collection across the shards:

db.orders.getShardDistribution()

The output is as follows:

Shard shard0000 at localhost:27017
 data : 8KiB docs : 2 chunks : 1
 estimated data per chunk : 8KiB
 estimated docs per chunk : 2

Shard shard0001 at localhost:27018
 data : 9KiB docs : 2 chunks : 1
 estimated data per chunk : 9KiB
 estimated docs per chunk : 2

The results show that the orders collection has 2 documents on each of the shard0000 and shard0001 shards, with storage sizes of 8KB and 9KB, respectively. The data distribution on each shard is also listed. This information can help us better understand the distribution of data in the cluster and optimize query performance and shard strategy accordingly.

Conclusion

The getShardDistribution() method provides useful information to help us understand the distribution of data in the cluster and optimize and tune it. It is a useful tool when optimizing distributed data and can help us better understand the distribution of data in a sharded cluster, thus improving query performance.