db.collection.analyzeShardKey()

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Definition

Compatibility
Syntax
Fields
Behavior
Access Control
Output
Examples
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Definition

db.collection.analyzeShardKey(key, opts): Calculates metrics for evaluating a shard key for an unsharded or sharded collection. Metrics are based on sampled queries. You can use configureQueryAnalyzer to configure query sampling on a collection.

Compatibility

This method is available in deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Note

This command is supported in all MongoDB Atlas clusters. For information on Atlas support for all commands, see Unsupported Commands.

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

db.collection.analyzeShardKey() has this syntax:

db.collection.analyzeShardKey(
   <shardKey>,
   {
     keyCharacteristics: <bool>,
     readWriteDistribution: <bool>,
     sampleRate: <double>,
     sampleSize: <int>
   }
 )

Fields

Field	Type	Necessity	Description
`key`	document	Required	Shard key to analyze. This can be a candidate shard key for an unsharded collection or sharded collection or the current shard key for a sharded collection. There is no default value.
`opts.keyCharacteristics`	boolean	Optional	Whether or not the metrics about the characteristics of the shard key are calculated. For details, see keyCharacteristics. Defaults to `true`.
`opts.readWriteDistribution`	boolean	Optional	Whether or not the metrics about the read and write distribution are calculated. For details, see readWriteDistribution. Defaults to `true`.
`opts.sampleRate`	double	Optional	The proportion of the documents in the collection to sample when calculating the metrics about the characteristics of the shard key. If you set `sampleRate`, you cannot set `sampleSize`. Must greater than `0`, up to and including `1`. There is no default value.
`opts.sampleSize`	integer	Optional	The number of documents to sample when calculating the metrics about the characteristics of the shard key. If you set `sampleSize`, you cannot set `sampleRate`. If not specified and `sampleRate` is not specified, the sample size defaults to sample size set by `analyzeShardKeyCharacteristicsDefaultSampleSize`.

Behavior

For behavior, see analyzeShardKey Behavior.

Access Control

For details, see analyzeShardKey Access Control.

Output

For sample output, see analyzeShardKey Output.

Examples

Consider a simplified version of a social media app. The collection we are trying to shard is the post collection.

Documents in the post collection have the following schema:

{
   userId: <uuid>,
   firstName: <string>,
   lastName: <string>,
   body: <string>,  // the field that can be modified.
   date: <date>,    // the field that can be modified.
}

Background Information

The app has 1500 users.
There are 30 last names and 45 first names, some more common than others.
There are three celebrity users.
Each user follows exactly five other users and has a very high probability of following at least one celebrity user.

Sample Workload

Each user posts about two posts a day at random times. They edit each post once, right after it is posted.
Each user logs in every six hours to read their own profile and posts by the users they follow from the past 24 hours. They also reply under a random post from the past three hours.
For every user, the app removes posts that are more than three days old at midnight.

Workload Query Patterns

This workload has the following query patterns:

find command with filter { userId: , firstName: , lastName: }
find command with filter { $or: [{ userId: , firstName: , lastName:, date: { $gte: }, ] }
findAndModify command with filter { userId: , firstName: , lastName: , date: } to update the body and date field.
update command with multi: false and filter { userId: , firstName: , lastName: , date: { $gte: , $lt: } } to update the body and date field.
delete command with multi: true and filter { userId: , firstName: , lastName: , date: { $lt: } }

Below are example metrics returned by db.collection.analyzeShardKey for some candidate shard keys, with sampled queries collected from seven days of workload.

Note

Before you run the db.collection.analyzeShardKey method, read the Supporting Indexes section. If you require supporting indexes for the shard key you are analyzing, use the db.collection.createIndex() method to create the indexes.

{ lastName: 1 } keyCharacteristics

This db.collection.analyzeShardKey method provides metrics on the { lastName: 1 } shard key on the social.post collection:

use social
db.post.analyzeShardKey(
   { lastName: 1 },
   {
     keyCharacteristics: true,
     readWriteDistribution: false
   }
)

The output for this command is similar to the following:

{
   "keyCharacteristics": {
     "numDocsTotal" : 9039,
     "avgDocSizeBytes" : 153,
     "numDocsSampled" : 9039,
     "isUnique" : false,
     "numDistinctValues" : 30,
     "mostCommonValues" : [
         {
           "value" : {
               "lastName" : "Smith"
           },
           "frequency" : 1013
         },
         {
           "value" : {
               "lastName" : "Johnson"
           },
           "frequency" : 984
         },
         {
           "value" : {
               "lastName" : "Jones"
           },
           "frequency" : 962
         },
         {
           "value" : {
               "lastName" : "Brown"
           },
           "frequency" : 925
         },
         {
           "value" : {
               "lastName" : "Davies"
           },
           "frequency" : 852
         }
     ],
     "monotonicity" : {
       "recordIdCorrelationCoefficient" : 0.0771959161,
       "type" : "not monotonic"
   },
 }
}

{ userId: 1 } keyCharacteristics

This db.collection.analyzeShardKey method provides metrics on the { userId: 1 } shard key on the social.post collection:

use social
db.post.analyzeShardKey(
   { userId: 1 },
   {
     keyCharacteristics: true,
     readWriteDistribution: false
   }
)

The output for this method is similar to the following:

{
  "keyCharacteristics": {
    "numDocsTotal" : 9039,
    "avgDocSizeBytes" : 162,
    "numDocsSampled" : 9039,
    "isUnique" : false,
    "numDistinctValues" : 1495,
    "mostCommonValues" : [
      {
        "value" : {
          "userId" : UUID("aadc3943-9402-4072-aae6-ad551359c596")
        },
        "frequency" : 15
      },
     {
       "value" : {
         "userId" : UUID("681abd2b-7a27-490c-b712-e544346f8d07")
       },
       "frequency" : 14
     },
     {
       "value" : {
         "userId" : UUID("714cb722-aa27-420a-8d63-0d5db962390d")
       },
       "frequency" : 14
     },
     {
       "value" : {
         "userId" : UUID("019a4118-b0d3-41d5-9c0a-764338b7e9d1")
       },
       "frequency" : 14
     },
     {
       "value" : {
         "userId" : UUID("b9c9fbea-3c12-41aa-bc69-eb316047a790")
       },
       "frequency" : 14
     }
   ],
   "monotonicity" : {
     "recordIdCorrelationCoefficient" : -0.0032039729,
     "type" : "not monotonic"
   },
 }
}

{ userId: 1 } readWriteDistribution

This db.collection.analyzeShardKey command provides metrics on the { userId: 1 } shard key on the social.post collection:

use social
db.post.analyzeShardKey(
   { userId: 1 },
   {
     keyCharacteristics: false,
     readWriteDistribution: true
   }
)

The output for this method is similar to the following:

{
   "readDistribution" : {
     "sampleSize" : {
       "total" : 61363,
       "find" : 61363,
       "aggregate" : 0,
       "count" : 0,
       "distinct" : 0
     },
     "percentageOfSingleShardReads" : 50.0008148233,
     "percentageOfMultiShardReads" : 49.9991851768,
     "percentageOfScatterGatherReads" : 0,
     "numReadsByRange" : [
       688,
       775,
       737,
       776,
       652,
       671,
       1332,
       1407,
       535,
       428,
       985,
       573,
       1496,
       ...
       ],
     },
   "writeDistribution" : {
     "sampleSize" : {
       "total" : 49638,
       "update" : 30680,
       "delete" : 7500,
       "findAndModify" : 11458
     },
     "percentageOfSingleShardWrites" : 100,
     "percentageOfMultiShardWrites" : 0,
     "percentageOfScatterGatherWrites" : 0,
     "numWritesByRange" : [
       389,
       601,
       430,
       454,
       462,
       421,
       668,
       833,
       493,
       300,
       683,
       460,
       ...
      ],
      "percentageOfShardKeyUpdates" : 0,
      "percentageOfSingleWritesWithoutShardKey" : 0,
      "percentageOfMultiWritesWithoutShardKey" : 0
    }
}

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