Collection Management Commands

A collection is a single logical index that uses a single Solr configuration file (solrconfig.xml) and a single index schema.

All of the examples in this section assume you are running the "techproducts" Solr example:

bin/solr -c -e techproducts

CREATE: Create a Collection

The CREATE action is used to create new collection of data.

V1 API

http://localhost:8983/solr/admin/collections?action=CREATE&name=techproducts_v2&collection.configName=techproducts&numShards=1

V2 API

With the v2 API, the create command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections -H 'Content-Type: application/json' -d '
  {
    "create": {
      "name": "techproducts_v2",
      "config": "techproducts",
      "numShards": 1
    }
  }
'

CREATE Parameters

The CREATE action allows the following parameters:

name

Required

Default: none

The name of the collection to be created.

router.name

Optional

Default: compositeId

The router name that will be used. The router defines how documents will be distributed among the shards. Possible values are implicit or compositeId.

The implicit router does not automatically route documents to different shards. Whichever shard you indicate on the indexing request (or within each document) will be used as the destination for those documents.

The compositeId router hashes the value in the uniqueKey field and looks up that hash in the collection’s clusterstate to determine which shard will receive the document, with the additional ability to manually direct the routing.

When using the implicit router, the shards parameter is required. When using the compositeId router, the numShards parameter is required.

For more information, see also the section Document Routing.

numShards

Optional

Default: none

The number of shards to be created as part of the collection. This is a required parameter when the router.name is compositeId.

shards

Optional

Default: none

A comma separated list of shard names, e.g., shard-x,shard-y,shard-z. This is a required parameter when the router.name is implicit.

replicationFactor

Optional

Default: 1

The number of replicas to be created for each shard.

This will create a NRT type of replica. If you want another type of replica, see the tlogReplicas and pullReplicas parameters below. See the section Types of Replicas for more information about replica types.

nrtReplicas

Optional

Default: none

The number of NRT (Near-Real-Time) replicas to create for this collection. This type of replica maintains a transaction log and updates its index locally. If you want all of your replicas to be of this type, you can simply use replicationFactor instead.

tlogReplicas

Optional

Default: none

The number of TLOG replicas to create for this collection. This type of replica maintains a transaction log but only updates its index via replication from a leader. See the section Types of Replicas for more information about replica types.

pullReplicas

Optional

Default: none

The number of PULL replicas to create for this collection. This type of replica does not maintain a transaction log and only updates its index via replication from a leader. This type is not eligible to become a leader and should not be the only type of replicas in the collection. See the section Types of Replicas for more information about replica types.

createNodeSet (v1), nodeSet (v2)

Optional

Default: none

Allows defining the nodes to spread the new collection across. The format is a comma-separated list of node_names, such as localhost:8983_solr,localhost:8984_solr,localhost:8985_solr.

If not provided, the CREATE operation will create shard-replicas spread across all live Solr nodes.

Alternatively, use the special value of EMPTY to initially create no shard-replica within the new collection and then later use the ADDREPLICA operation to add shard-replicas when and where required.

createNodeSet.shuffle (v1), shuffleNodes (v2)

Optional

Default: true

Controls whether or not the shard-replicas created for this collection will be assigned to the nodes specified by the createNodeSet in a sequential manner, or if the list of nodes should be shuffled prior to creating individual replicas.

A false value makes the results of a collection creation predictable and gives more exact control over the location of the individual shard-replicas, but true can be a better choice for ensuring replicas are distributed evenly across nodes.

This parameter is ignored if createNodeSet is not also specified.

collection.configName (v1), config (v2)

Optional

Default: none

Defines the name of the configuration (which must already be stored in ZooKeeper) to use for this collection.

If not provided, Solr will use the configuration of _default configset to create a new (and mutable) configset named <collectionName>.AUTOCREATED and will use it for the new collection. When such a collection is deleted, its autocreated configset will be deleted by default when it is not in use by any other collection.

router.field (v1), router (v2)

Optional

Default: none

If this parameter is specified, the router will look at the value of the field in an input document to compute the hash and identify a shard instead of looking at the uniqueKey field. If the field specified is null in the document, the document will be rejected.

Please note that RealTime Get or retrieval by document ID would also require the parameter _route_ (or shard.keys) to avoid a distributed search.

perReplicaState

Optional

Default: false

If true the states of individual replicas will be maintained as individual child of the state.json.

property.name=value

Optional

Default: none

Set core property name to value. See the section Core Discovery for details on supported properties and values.

The entries in each core.properties file are vital for Solr to function correctly. Overriding entries can result in unusable collections. Altering these entries by specifying property.name=value is an expert-level option and should only be used if you have a thorough understanding of the consequences.
waitForFinalState

Optional

Default: none

If true, the request will complete only when all affected replicas become active. The default is false, which means that the API will return the status of the single action, which may be before the new replica is online and active.

alias

Optional

Default: none

When a collection is created additionally an alias can be created that points to this collection. This parameter allows specifying the name of this alias, effectively combining this operation with CREATEALIAS.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

Collections are first created in read-write mode but can be put in readOnly mode using the MODIFYCOLLECTION action.

CREATE Response

The response will include the status of the request and the new core names. If the status is anything other than "success", an error message will explain why the request failed.

RELOAD: Reload a Collection

The RELOAD action is used when you have changed a configuration file in ZooKeeper, like uploading a new solrconfig.xml. Solr automatically reloads collections when certain files, monitored via a watch in ZooKeeper are changed, such as security.json. However, for changes to files in configsets, like uploading a new schema, you will need to manually trigger the RELOAD.

V1 API

http://localhost:8983/solr/admin/collections?action=RELOAD&name=techproducts_v2

V2 API

With the v2 API, the reload command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections/techproducts_v2 -H 'Content-Type: application/json' -d '
  {
    "reload": {}
  }
'

Additional parameters can be passed in via the reload key:

curl -X POST http://localhost:8983/api/collections/techproducts_v2 -H 'Content-Type: application/json' -d '
  {
    "reload": {
      "async": "reload1"
    }
  }
'

RELOAD Parameters

name

Optional

Default: none

The name of the collection to reload. This parameter is required by the V1 API.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

RELOAD Response

The response will include the status of the request and the cores that were reloaded. If the status is anything other than "success", an error message will explain why the request failed.

MODIFYCOLLECTION: Modify Attributes of a Collection

It’s possible to edit multiple attributes at a time. Changing these values only updates the znode on ZooKeeper, they do not change the topology of the collection. For instance, increasing replicationFactor will not automatically add more replicas to the collection but will allow more ADDREPLICA commands to succeed.

An attribute can be deleted by passing an empty value. For example, yet_another_attribute_name= (with no value) will delete the yet_another_attribute_name parameter from the collection.

V1 API

http://localhost:8983/solr/admin/collections?action=MODIFYCOLLECTION&collection=techproducts_v2&<attribute-name>=<attribute-value>&<another-attribute-name>=<another-value>&<yet_another_attribute_name>=

http://localhost:8983/solr/admin/collections?action=modifycollection&collection=techproducts_v2&replicationFactor=2

V2 API

With the v2 API, the modify command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections/techproducts_v2 -H 'Content-Type: application/json' -d '
  {
    "modify": {
      "replicationFactor": 2
    }
  }
'

MODIFYCOLLECTION Parameters

collection

Required

Default: none

The name of the collection to be modified.

attribute=value

Required

Default: none

Key-value pairs of attribute names and attribute values.

At least one attribute parameter is required.

The attributes that can be modified are:

  • replicationFactor

  • collection.configName

  • readOnly

  • other custom properties that use a property. prefix

    See the CREATE action section above for details on these attributes.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

Read-Only Mode

Setting the readOnly attribute to true puts the collection in read-only mode, in which any index update requests are rejected. Other collection-level actions (e.g., adding / removing / moving replicas) are still available in this mode.

The transition from the (default) read-write to read-only mode consists of the following steps:

  • the readOnly flag is changed in collection state,

  • any new update requests are rejected with 403 FORBIDDEN error code (ongoing long-running requests are aborted, too),

  • a forced commit is performed to flush and commit any in-flight updates.

    This may potentially take a long time if there are still major segment merges running in the background.

  • a collection RELOAD action is executed.

Removing the readOnly property or setting it to false enables the processing of updates and reloads the collection.

LIST: List Collections

Fetch the names of the collections in the cluster.

V1 API

http://localhost:8983/solr/admin/collections?action=LIST

V2 API

With the v2 API, the list command is provided as part of the JSON data that contains the required parameters:

curl -X GET http://localhost:8983/api/collections

Output

{
  "responseHeader":{
    "status":0,
    "QTime":2011},
  "collections":["collection1",
    "example1",
    "example2"]}

RENAME: Rename a Collection

Renaming a collection sets up a standard alias that points to the underlying collection, so that the same (unmodified) collection can now be referred to in query, index and admin operations using the new name.

This command does NOT actually rename the underlying Solr collection - it sets up a new one-to-one alias using the new name, or renames the existing alias so that it uses the new name, while still referring to the same underlying Solr collection. However, from the user’s point of view the collection can now be accessed using the new name, and the new name can be also referred to in other aliases.

The following limitations apply:

  • the existing name must be either a SolrCloud collection or a standard alias referring to a single collection. Aliases that refer to more than 1 collection are not supported.

  • the existing name must not be a Routed Alias.

  • the target name must not be an existing alias.

V1 API

http://localhost:8983/solr/admin/collections?action=RENAME&name=techproducts_v2&target=renamedCollection

V2 API

curl -X POST http://localhost:8983/api/collections/techproducts/rename -H 'Content-Type: application/json' -d '
  {
    "to": "new_name"
  }
'

RENAME Command Parameters

name

Required

Default: none

Name of the existing SolrCloud collection or an alias that refers to exactly one collection and is not a Routed Alias.

target (v1), to (v2)

Required

Default: none

Target name of the collection. This will be the new alias that refers to the underlying SolrCloud collection. The original name (or alias) of the collection will be replaced also in the existing aliases so that they also refer to the new name. Target name must not be an existing alias.

Examples using RENAME

Assuming there are two actual SolrCloud collections named collection1 and collection2, and the following aliases already exist:

  • col1 ⇒ collection1: this resolves to collection1.

  • col2 ⇒ collection2: this resolves to collection2.

  • simpleAlias ⇒ col1: this resolves to collection1.

  • compoundAlias ⇒ col1,col2: this resolves to collection1,collection2

The RENAME of col1 to foo will change the aliases to the following:

  • foo ⇒ collection1: this resolves to collection1.

  • col2 ⇒ collection2: this resolves to collection2.

  • simpleAlias ⇒ foo: this resolves to collection1.

  • compoundAlias ⇒ foo,col2: this resolves to collection1,collection2.

If we then rename collection1 (which is an actual collection name) to collection2 (which is also an actual collection name) the following aliases will exist now:

  • foo ⇒ collection2: this resolves to collection2.

  • col2 ⇒ collection2: this resolves to collection2.

  • simpleAlias ⇒ foo: this resolves to collection2.

  • compoundAlias ⇒ foo,col2: this would resolve now to collection2,collection2 so it’s reduced to simply collection2.

  • collection1 ⇒ collection2: this newly created alias effectively hides collection1 from regular query and update commands, which are directed now to collection2.

DELETE: Delete a Collection

The DELETE action is used to delete a collection.

V1 API

http://localhost:8983/solr/admin/collections?action=DELETE&name=techproducts_v2

V2 API

curl -X DELETE http://localhost:8983/api/collections/techproducts_v2

To run a DELETE asynchronously then append the async parameter:

curl -X DELETE http://localhost:8983/api/collections/techproducts_v2?async=aaaa

DELETE Parameters

name

Required

Default: none

The name of the collection to delete.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

DELETE Response

The response will include the status of the request and the cores that were deleted. If the status is anything other than "success", an error message will explain why the request failed.

Output

<response>
  <lst name="responseHeader">
    <int name="status">0</int>
    <int name="QTime">603</int>
  </lst>
  <lst name="success">
    <lst name="10.0.1.6:8983_solr">
      <lst name="responseHeader">
        <int name="status">0</int>
        <int name="QTime">19</int>
      </lst>
    </lst>
    <lst name="10.0.1.4:8983_solr">
      <lst name="responseHeader">
        <int name="status">0</int>
        <int name="QTime">67</int>
      </lst>
    </lst>
  </lst>
</response>

COLLECTIONPROP: Collection Properties

Add, edit or delete a collection property.

V1 API

http://localhost:8983/solr/admin/collections?action=COLLECTIONPROP&name=techproducts_v2&propertyName=propertyName&propertyValue=propertyValue

V2 API

curl -X POST http://localhost:8983/api/collections/techproducts_v2 -H 'Content-Type: application/json' -d '
  {
    "set-collection-property": {
      "name": "foo",
      "value": "bar"
    }
  }
'

COLLECTIONPROP Parameters

name

Optional

Default: none

The name of the collection for which the property would be set.

propertyName (v1), name (v2)

Optional

Default: none

The name of the property.

propertyValue (v1), value (v2)

Optional

Default: none

The value of the property. When not provided, the property is deleted.

COLLECTIONPROP Response

The response will include the status of the request and the properties that were updated or removed. If the status is anything other than "0", an error message will explain why the request failed.

MIGRATE: Migrate Documents to Another Collection

The MIGRATE command is used to migrate all documents having a given routing key to another collection. The source collection will continue to have the same data as-is but it will start re-routing write requests to the target collection for the number of seconds specified by the forward.timeout parameter. It is the responsibility of the user to switch to the target collection for reads and writes after the MIGRATE action completes.

V1 API

http://localhost:8983/solr/admin/collections?action=MIGRATE&collection=techproducts_v2&split.key=key1!&target.collection=postMigrationCollection&forward.timeout=60

V2 API

curl -X POST http://localhost:8983/api/collections/techproducts_v2 -H 'Content-Type: application/json' -d '
  {
    "migrate-docs": {
      "target": "postMigrationCollection",
      "splitKey": "key1!"
    }
  }
'

The routing key specified by the split.key parameter may span multiple shards on both the source and the target collections. The migration is performed shard-by-shard in a single thread. One or more temporary collections may be created by this command during the ‘migrate’ process but they are cleaned up at the end automatically.

This is a long running operation and therefore using the async parameter is highly recommended. If the async parameter is not specified then the operation is synchronous by default and keeping a large read timeout on the invocation is advised. Even with a large read timeout, the request may still timeout but that doesn’t necessarily mean that the operation has failed. Users should check logs, cluster state, source and target collections before invoking the operation again.

This command works only with collections using the compositeId router. The target collection must not receive any writes during the time the MIGRATE command is running otherwise some writes may be lost.

Please note that the MIGRATE API does not perform any de-duplication on the documents so if the target collection contains documents with the same uniqueKey as the documents being migrated then the target collection will end up with duplicate documents.

MIGRATE Parameters

collection

Required

Default: none

The name of the source collection from which documents will be split.

target.collection (v1), target (v2)

Required

Default: none

The name of the target collection to which documents will be migrated.

split.key (v1), splitKey (v2)

Required

Default: none

The routing key prefix. For example, if the uniqueKey of a document is "a!123", then you would use split.key=a!.

forward.timeout (v1), forwardTimeout (v2)

Optional

Default: 60 seconds

The timeout, in seconds, until which write requests made to the source collection for the given split.key will be forwarded to the target shard.

property.name=value

Optional

Default: none

Set core property name to value. See the section Core Discovery for details on supported properties and values.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

MIGRATE Response

The response will include the status of the request.

REINDEXCOLLECTION: Re-Index a Collection

The REINDEXCOLLECTION command reindexes a collection using existing data from the source collection.

V1 API

http://localhost:8983/solr/admin/collections?action=REINDEXCOLLECTION&name=techproducts_v2

V2 API

We do not currently have a V2 equivalent.

Reindexing is potentially a lossy operation. Some of the existing indexed data that is not available as stored fields may be lost, so users should use this command with caution, evaluating the potential impact by using different source and target collection names first, and preserving the source collection until the evaluation is complete.

The target collection must not exist (and may not be an alias). If the target collection name is the same as the source collection then first a unique sequential name will be generated for the target collection, and then after reindexing is done an alias will be created that points from the source name to the actual sequentially-named target collection.

When reindexing is started the source collection is put in read-only mode to ensure that all source documents are properly processed.

Using optional parameters a different index schema, collection shape (number of shards and replicas) or routing parameters can be requested for the target collection.

Reindexing is executed as a streaming expression daemon, which runs on one of the source collection’s replicas. It is usually a time-consuming operation so it’s recommended to execute it as an asynchronous request in order to avoid request time outs. Only one reindexing operation may execute concurrently for a given source collection. Long-running, erroneous or crashed reindexing operations may be terminated by using the abort option, which also removes partial results.

REINDEXCOLLECTION Parameters

name

Required

Default: none

Source collection name, may be an alias.

cmd

Optional

Default: start

Currently supported commands are:

  • start: starts processing if not already running.

  • abort: aborts an already running reindexing (or clears a left-over status after a crash), and deletes partial results.

  • status: returns detailed status of a running reindexing command.

target

Optional

Default: none

Target collection name. If not specified a unique name will be generated and after all documents have been copied an alias will be created that points from the source collection name to the unique sequentially-named collection. This effectively "hides" the original source collection from regular update and search operations.

q

Optional

Default: *:*

The query to select documents for reindexing.

fl

Optional

Default: *

A list of fields to reindex.

rows

Optional

Default: 100

The batch size for transferring documents. Depending on the average size of the document large batch sizes may cause memory issues.

configName
collection.configName

Optional

Default: <name of the source collection>

The name of the configset for the target collection.

removeSource

Optional

Default: false

If true then after the processing is successfully finished the source collection will be deleted.

async

Optional

Default: none

Optional request ID to track this action which will be processed asynchronously.

There are additionally a number of optional parameters that determine the target collection layout. If they are not specified in the request then their values are copied from the source collection. The following parameters are currently supported (described in detail in the CREATE collection section): numShards, replicationFactor, nrtReplicas, tlogReplicas, pullReplicas, shards, policy, createNodeSet, createNodeSet.shuffle, router.*.

When the reindexing process has completed the target collection is marked using property.rx: "finished", and the source collection state is updated to become read-write. On any errors the command will delete any temporary and target collections and also reset the state of the source collection’s read-only flag.

Examples using REINDEXCOLLECTION

Input

http://localhost:8983/solr/admin/collections?action=REINDEXCOLLECTION&name=techproducts_v2&numShards=3&configName=conf2&q=id:aa*&fl=id,string_s

This request specifies a different schema for the target collection, copies only some of the fields, selects only the documents matching a query, and also potentially re-shapes the collection by explicitly specifying 3 shards. Since the target collection hasn’t been specified in the parameters, a collection with a unique name, e.g., .rx_techproducts_v2_2, will be created and on success an alias pointing from techproducts_v2 to .rx_techproducts_v2_2 will be created, effectively replacing the source collection for the purpose of indexing and searching. The source collection is assumed to be small so a synchronous request was made.

Output

{
  "responseHeader":{
    "status":0,
    "QTime":10757},
  "reindexStatus":{
    "phase":"done",
    "inputDocs":13416,
    "processedDocs":376,
    "actualSourceCollection":".rx_techproducts_v2_1",
    "state":"finished",
    "actualTargetCollection":".rx_techproducts_v2_2",
    "checkpointCollection":".rx_ck_techproducts_v2"
  }
}

As a result a new collection .rx_techproducts_v2_2 has been created, with selected documents reindexed to 3 shards, and with an alias pointing from techproducts_v2 to this one. The status also shows that the source collection was already an alias to .rx_techproducts_v2_1, which was likely a result of a previous reindexing.

COLSTATUS: Detailed Status of a Collection’s Indexes

The COLSTATUS command provides a detailed description of the collection status, including low-level index information about segments and field data. There isn’t a good equivalent V2 API that supports all the parameters below.

V1 API

http://localhost:8983/solr/admin/collections?action=COLSTATUS&collection=techproducts_v2&coreInfo=true&segments=true&fieldInfo=true&sizeInfo=true

V2 API

The closest V2 API is this one, but doesn’t support all the features of the V1 equivalent.

curl -X GET http://localhost:8983/api/collections/techproducts_v2

This command also checks the compliance of Lucene index field types with the current Solr collection schema and indicates the names of non-compliant fields, i.e., Lucene fields with field types incompatible (or different) from the corresponding Solr field types declared in the current schema. Such incompatibilities may result from incompatible schema changes or after migration of data to a different major Solr release.

COLSTATUS Parameters

collection

Optional

Default: none

Collection name. If missing then it means all collections.

coreInfo

Optional

Default: false

If true then additional information will be provided about SolrCore of shard leaders.

segments

Optional

Default: false

If true then segment information will be provided.

fieldInfo

Optional

Default: false

If true then detailed Lucene field information will be provided and their corresponding Solr schema types.

sizeInfo

Optional

Default: false

If true then additional information about the index files size and their RAM usage will be provided.

Index Size Analysis Tool

The COLSTATUS command also provides a tool for analyzing and estimating the composition of raw index data. Please note that this tool should be used with care because it generates a significant IO load on all shard leaders of the analyzed collections. A sampling threshold and a sampling percent parameters can be adjusted to reduce this load to some degree.

Size estimates produced by this tool are only approximate and represent the aggregated size of uncompressed index data. In reality these values would never occur, because Lucene (and Solr) always stores data in a compressed format - still, these values help to understand what occupies most of the space and the relative size of each type of data and each field in the index.

In the following sections whenever "size" is mentioned it means an estimated aggregated size of uncompressed (raw) data.

The following parameters are specific to this tool:

rawSize

Optional

Default: false

If true then run the raw index data analysis tool (other boolean options below imply this option if any of them are true). Command response will include sections that show estimated breakdown of data size per field and per data type.

rawSizeSummary

Optional

Default: false

If true then include also a more detailed breakdown of data size per field and per type.

rawSizeDetails

Optional

Default: false

If true then provide exhaustive details that include statistical distribution of items per field and per type as well as top 20 largest items per field.

rawSizeSamplingPercent

Optional

Default: 5.0

When the index is larger than a certain threshold (100k documents per shard) only a part of data is actually retrieved and analyzed in order to reduce the IO load, and then the final results are extrapolated.

Values must be greater than 0 and less or equal to 100.0. Very small values (between 0.0 and 1.0) may introduce significant estimation errors. Also, values that would result in less than 10 documents being sampled are rejected with an exception.

The response for this command always contains two sections:

  • fieldsBySize: a map where field names are keys and values are estimated sizes of raw (uncompressed) data that belongs to the field. The map is sorted by size so that it’s easy to see what field occupies most space.

  • typesBySize: a map where data types are the keys and values are estimates sizes of raw (uncompressed) data of particular type. This map is also sorted by size.

Optional sections added with above parameters include:

  • summary section containing a breakdown of data sizes for each field by data type.

  • details section containing detailed statistical summary of size distribution within each field, per data type. This section also shows topN values by size from each field.

Data types shown in the response can be roughly divided into the following groups:

  • storedFields - represents the raw uncompressed data in stored fields. For example, for UTF-8 strings this represents the aggregated sum of the number of bytes in the strings' UTF-8 representation, for long numbers this is 8 bytes per value, etc.

  • terms_terms - represents the aggregated size of the term dictionary. The size of this data is affected by the number and length of unique terms, which in turn depends on the field size and the analysis chain.

  • terms_postings - represents the aggregated size of all term position and offset information, if present. This information may be absent if position-based searching, such as phrase queries, is not needed.

  • terms_payloads - represents the aggregated size of all per-term payload data, if present.

  • norms - represents the aggregated size of field norm information. This information may be omitted if a field has an omitNorms flag in the schema, which is common for fields that don’t need weighting or scoring by field length.

  • termVectors - represents the aggregated size of term vectors.

  • docValues_* - represents aggregated size of doc values, by type (e.g., docValues_numeric, docValues_binary, etc).

  • points - represents aggregated size of point values.

COLSTATUS Response

The response will include an overview of the collection status, the number of active or inactive shards and replicas, and additional index information of shard leaders.

Examples using COLSTATUS

Input

http://localhost:8983/solr/admin/collections?action=COLSTATUS&collection=gettingstarted&fieldInfo=true&sizeInfo=true

Output

{
    "responseHeader": {
        "status": 0,
        "QTime": 50
    },
    "gettingstarted": {
        "znodeVersion": 16,
        "properties": {
            "nrtReplicas": "2",
            "pullReplicas": "0",
            "replicationFactor": "2",
            "router": {
                "name": "compositeId"
            },
            "tlogReplicas": "0"
        },
        "activeShards": 2,
        "inactiveShards": 0,
        "schemaNonCompliant": [
            "(NONE)"
        ],
        "shards": {
            "shard1": {
                "state": "active",
                "range": "80000000-ffffffff",
                "replicas": {
                    "total": 2,
                    "active": 2,
                    "down": 0,
                    "recovering": 0,
                    "recovery_failed": 0
                },
                "leader": {
                    "coreNode": "core_node4",
                    "core": "gettingstarted_shard1_replica_n1",
                    "base_url": "http://192.168.0.80:8983/solr",
                    "node_name": "192.168.0.80:8983_solr",
                    "state": "active",
                    "type": "NRT",
                    "force_set_state": "false",
                    "leader": "true",
                    "segInfos": {
                        "info": {
                            "minSegmentLuceneVersion": "9.0.0",
                            "commitLuceneVersion": "9.0.0",
                            "numSegments": 40,
                            "segmentsFileName": "segments_w",
                            "totalMaxDoc": 686953,
                            "userData": {
                                "commitCommandVer": "1627350608019193856",
                                "commitTimeMSec": "1551962478819"
                            }
                        },
                        "fieldInfoLegend": [
                            "I - Indexed",
                            "D - DocValues",
                            "xxx - DocValues type",
                            "V - TermVector Stored",
                            "O - Omit Norms",
                            "F - Omit Term Frequencies & Positions",
                            "P - Omit Positions",
                            "H - Store Offsets with Positions",
                            "p - field has payloads",
                            "s - field uses soft deletes",
                            ":x:x:x - point data dim : index dim : num bytes"
                        ],
                        "segments": {
                            "_i": {
                                "name": "_i",
                                "delCount": 738,
                                "softDelCount": 0,
                                "hasFieldUpdates": false,
                                "sizeInBytes": 109398213,
                                "size": 70958,
                                "age": "2019-03-07T12:34:24.761Z",
                                "source": "merge",
                                "version": "9.0.0",
                                "createdVersionMajor": 9,
                                "minVersion": "9.0.0",
                                "diagnostics": {
                                    "os": "Mac OS X",
                                    "java.vendor": "Oracle Corporation",
                                    "java.version": "1.8.0_191",
                                    "java.vm.version": "25.191-b12",
                                    "lucene.version": "9.0.0",
                                    "mergeMaxNumSegments": "-1",
                                    "os.arch": "x86_64",
                                    "java.runtime.version": "1.8.0_191-b12",
                                    "source": "merge",
                                    "mergeFactor": "10",
                                    "os.version": "10.14.3",
                                    "timestamp": "1551962064761"
                                },
                                "attributes": {
                                    "Lucene50StoredFieldsFormat.mode": "BEST_SPEED"
                                },
                                "largestFiles": {
                                    "_i.fdt": "42.5 MB",
                                    "_i_Lucene80_0.dvd": "35.3 MB",
                                    "_i_Lucene50_0.pos": "11.1 MB",
                                    "_i_Lucene50_0.doc": "10 MB",
                                    "_i_Lucene50_0.tim": "4.3 MB"
                                }}}}}}}}}

Example of using the raw index data analysis tool:

Input

http://localhost:8983/solr/admin/collections?action=COLSTATUS&collection=gettingstarted&rawSize=true&rawSizeSamplingPercent=0.1

Output

{
    "responseHeader": {
        "status": 0,
        "QTime": 26812
    },
    "gettingstarted": {
        "znodeVersion": 33,
        "properties": {
            "nrtReplicas": "2",
            "pullReplicas": "0",
            "replicationFactor": "2",
            "router": {
                "name": "compositeId"
            },
            "tlogReplicas": "0"
        },
        "activeShards": 2,
        "inactiveShards": 0,
        "schemaNonCompliant": [
            "(NONE)"
        ],
        "shards": {
            "shard1": {
                "state": "active",
                "range": "80000000-ffffffff",
                "replicas": {
                    "total": 2,
                    "active": 2,
                    "down": 0,
                    "recovering": 0,
                    "recovery_failed": 0
                },
                "leader": {
                    "coreNode": "core_node5",
                    "core": "gettingstarted_shard1_replica_n2",
                    "base_url": "http://192.168.0.80:8983/solr",
                    "node_name": "192.168.0.80:8983_solr",
                    "state": "active",
                    "type": "NRT",
                    "force_set_state": "false",
                    "leader": "true",
                    "segInfos": {
                        "info": {
                            "minSegmentLuceneVersion": "9.0.0",
                            "commitLuceneVersion": "9.0.0",
                            "numSegments": 46,
                            "segmentsFileName": "segments_4h",
                            "totalMaxDoc": 3283741,
                            "userData": {
                                "commitCommandVer": "1635676266902323200",
                                "commitTimeMSec": "1559902446318"
                            }
                        },
                        "rawSize": {
                            "fieldsBySize": {
                                "revision.text": "7.9 GB",
                                "revision.text_str": "734.7 MB",
                                "revision.comment_str": "259.1 MB",
                                "revision": "239.2 MB",
                                "revision.sha1": "211.9 MB",
                                "revision.comment": "201.3 MB",
                                "title": "114.9 MB",
                                "revision.contributor": "103.5 MB",
                                "revision.sha1_str": "96.4 MB",
                                "revision.id": "75.2 MB",
                                "ns": "75.2 MB",
                                "revision.timestamp": "75.2 MB",
                                "revision.contributor.id": "74.7 MB",
                                "revision.format": "69 MB",
                                "id": "65 MB",
                                "title_str": "26.8 MB",
                                "revision.model_str": "25.4 MB",
                                "_version_": "24.9 MB",
                                "_root_": "24.7 MB",
                                "revision.contributor.ip_str": "22 MB",
                                "revision.contributor_str": "21.8 MB",
                                "revision_str": "15.5 MB",
                                "revision.contributor.ip": "13.5 MB",
                                "restrictions_str": "428.7 KB",
                                "restrictions": "164.2 KB",
                                "name_str": "84 KB",
                                "includes_str": "8.8 KB"
                            },
                            "typesBySize": {
                                "storedFields": "7.8 GB",
                                "docValues_sortedSet": "1.2 GB",
                                "terms_postings": "788.8 MB",
                                "terms_terms": "342.2 MB",
                                "norms": "237 MB",
                                "docValues_sortedNumeric": "124.3 MB",
                                "points": "115.7 MB",
                                "docValues_numeric": "24.9 MB",
                                "docValues_sorted": "18.5 MB"
                            }
                        }
                    }
                }
            },
            "shard2": {
                "state": "active",
                "range": "0-7fffffff",
                "replicas": {
                    "total": 2,
                    "active": 2,
                    "down": 0,
                    "recovering": 0,
                    "recovery_failed": 0
                },
                "leader": {
                    "coreNode": "core_node8",
                    "core": "gettingstarted_shard2_replica_n6",
                    "base_url": "http://192.168.0.80:8983/solr",
                    "node_name": "192.168.0.80:8983_solr",
                    "state": "active",
                    "type": "NRT",
                    "force_set_state": "false",
                    "leader": "true",
                    "segInfos": {
                        "info": {
                            "minSegmentLuceneVersion": "9.0.0",
                            "commitLuceneVersion": "9.0.0",
                            "numSegments": 55,
                            "segmentsFileName": "segments_4d",
                            "totalMaxDoc": 3284863,
                            "userData": {
                                "commitCommandVer": "1635676259742646272",
                                "commitTimeMSec": "1559902445005"
                            }
                        },
                        "rawSize": {
                            "fieldsBySize": {
                                "revision.text": "8.3 GB",
                                "revision.text_str": "687.5 MB",
                                "revision": "238.9 MB",
                                "revision.sha1": "212 MB",
                                "revision.comment_str": "211.5 MB",
                                "revision.comment": "201.7 MB",
                                "title": "115.9 MB",
                                "revision.contributor": "103.4 MB",
                                "revision.sha1_str": "96.3 MB",
                                "ns": "75.2 MB",
                                "revision.id": "75.2 MB",
                                "revision.timestamp": "75.2 MB",
                                "revision.contributor.id": "74.6 MB",
                                "revision.format": "69 MB",
                                "id": "67 MB",
                                "title_str": "29.5 MB",
                                "_version_": "24.8 MB",
                                "revision.model_str": "24 MB",
                                "revision.contributor_str": "21.7 MB",
                                "revision.contributor.ip_str": "20.9 MB",
                                "revision_str": "15.5 MB",
                                "revision.contributor.ip": "13.8 MB",
                                "restrictions_str": "411.1 KB",
                                "restrictions": "132.9 KB",
                                "name_str": "42 KB",
                                "includes_str": "41 KB"
                            },
                            "typesBySize": {
                                "storedFields": "8.2 GB",
                                "docValues_sortedSet": "1.1 GB",
                                "terms_postings": "787.4 MB",
                                "terms_terms": "337.5 MB",
                                "norms": "236.6 MB",
                                "docValues_sortedNumeric": "124.1 MB",
                                "points": "115.7 MB",
                                "docValues_numeric": "24.9 MB",
                                "docValues_sorted": "20.5 MB"
                            }
                        }
                    }
                }
            }
        }
    }
}

BACKUP: Backup Collection

Backs up Solr collections and associated configurations to a shared filesystem - for example a Network File System.

V1 API

http://localhost:8983/solr/admin/collections?action=BACKUP&name=techproducts_backup&collection=techproducts&location=file:///path/to/my/shared/drive

V2 API

With the v2 API, the backup-collection command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections -H 'Content-Type: application/json' -d '
  {
    "backup-collection": {
      "name": "techproducts_backup",
      "collection": "techproducts",
      "location": "file:///path/to/my/shared/drive"
    }
  }
'

The BACKUP command will backup Solr indexes and configurations for a specified collection. The BACKUP command takes one copy from each shard for the indexes. For configurations, it backs up the configset that was associated with the collection and metadata.

Solr ships with backup support for a number of repositories. Please refer to the Backup Storage Repositories section for a complete list.

Backup data is stored in a directory in the specified repository based on the provided name and location. Each backup location can hold multiple backups for the same collection, allowing users to later restore from any of these "backup points" as desired. Within a location backups are done incrementally, so that index files uploaded previously are skipped and not duplicated in the backup repository. Multiple collections cannot be backed up to the same location.

Previous versions of Solr supported a different snapshot-based backup method without the incremental support described above. Solr can still restore from backups that use this old format, but creating new backups of this format is not recommended and snapshot-based backups are officially deprecated. See the incremental parameter below for more information.

BACKUP Parameters

collection

Required

Default: none

The name of the collection to be backed up.

name

Required

Default: none

What to name the backup that is created. This is checked to make sure it doesn’t already exist, and otherwise an error message is raised.

location

Required

Default: none

The location on a shared drive for the backup command to write to. This parameter is required, unless a default location is defined on the repository configuration, or set as a cluster property.

If the location path is on a mounted drive, the mount must be available on the node that serves as the overseer, even if the overseer node does not host a replica of the collection being backed up. Since any node can take the overseer role at any time, a best practice to avoid possible backup failures is to ensure the mount point is available on all nodes of the cluster.

Each backup location can only hold a backup for one collection, however the same location can be used for repeated backups of the same collection. Repeated backups of the same collection are done incrementally, so that files unchanged since the last backup are not duplicated in the backup repository.

If you are using the S3 Backup Repository, then please read the S3-specific warnings about the location option.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

repository

Optional

Default: none

The name of a repository to be used for the backup. If no repository is specified then the local filesystem repository will be used automatically.

maxNumBackupPoints

Optional

Default: none

The upper-bound on how many backups should be retained at the backup location. If the current number exceeds this bound, older backups will be deleted until only maxNumBackupPoints backups remain. This parameter has no effect if incremental=false is specified.

incremental

Optional

Default: true

A boolean parameter allowing users to choose whether to create an incremental (incremental=true) or a "snapshot" (incremental=false) backup. If unspecified, backups are done incrementally by default. Incremental backups are preferred in all known circumstances and snapshot backups are deprecated, so this parameter should only be used after much consideration.

Incremental Backup Response

{
    "response": {
        "collection": "techproducts",
        "numShards": 2,
        "backupId": 0,
        "indexVersion": "9.0.0",
        "startTime": "2022-02-11T17:20:44.157305500Z",
        "indexFileCount": 22,
        "uploadedIndexFileCount": 22,
        "indexSizeMB": 0.007,
        "uploadedIndexFileMB": 0.007,
        "shardBackupIds": [
            "md_shard2_0",
            "md_shard1_0"
        ],
        "endTime": "2022-02-11T17:20:45.245534400Z"
    }
}

LISTBACKUP: List Backups

Lists information about each backup stored at the specified repository location. Basic metadata is returned about each backup including: the timestamp the backup was created, the Lucene version used to create the index, and the size of the backup both in number of files and total filesize.

Previous versions of Solr supported a different snapshot-based backup file structure that did not support the storage of multiple backups at the same location. Solr can still restore backups stored in this old format, but it is deprecated and will be removed in subsequent versions of Solr. The LISTBACKUP API does not support the deprecated format and attempts to use this API on a location holding an older backup will result in an error message.

The file structure used by Solr internally to represent backups changed in 8.9.0. While backups created prior to this format change can still be restored, the LISTBACKUP and DELETEBACKUP API commands are only valid on this newer format. Attempting to use them on a location holding an older backup will result in an error message.

LISTBACKUP Parameters

name

Required

Default: none

The name of the backups to list. The backup name usually corresponds to the collection-name, but isn’t required to.

location

Required

Default: none

The repository location to list backups from. This parameter is required, unless a default location is defined on the repository configuration, or set as a cluster property.

If the location path is on a mounted drive, the mount must be available on the node that serves as the overseer, even if the overseer node does not host a replica of the collection being backed up. Since any node can take the overseer role at any time, a best practice to avoid possible backup failures is to ensure the mount point is available on all nodes of the cluster.

This must be the same value as was given as the location option when creating the backup.

repository

Optional

Default: none

The name of a repository to be used for accessing backup information. If no repository is specified then the local filesystem repository will be used automatically.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

LISTBACKUP Example

Input

V1 API

http://localhost:8983/solr/admin/collections?action=LISTBACKUP&name=myBackupName&location=/path/to/my/shared/drive

V2 API

curl -X POST http://localhost:8983/v2/collections/backups -H 'Content-Type: application/json' -d '
  {
    "list-backups" : {
      "name": "myBackupName",
      "location": "/path/to/my/shared/drive"
    }
  }
'

Output

{
  "responseHeader":{
    "status":0,
    "QTime":4},
  "collection":"books",
  "backups":[{
      "indexFileCount":0,
      "indexSizeMB":0.0,
      "shardBackupIds":{
        "shard2":"md_shard2_0.json",
        "shard1":"md_shard1_0.json"},
      "collection.configName":"books",
      "backupId":0,
      "collectionAlias":"books",
      "startTime":"2021-02-09T03:19:52.085653Z",
      "indexVersion":"9.0.0"},
    {
      "indexFileCount":0,
      "indexSizeMB":0.0,
      "shardBackupIds":{
        "shard2":"md_shard2_1.json",
        "shard1":"md_shard1_1.json"},
      "collection.configName":"books",
      "backupId":1,
      "collectionAlias":"books",
      "startTime":"2021-02-09T03:19:52.268804Z",
      "indexVersion":"9.0.0"}]}

RESTORE: Restore Collection

Restores Solr indexes and associated configurations to a specified collection.

V1 API

http://localhost:8983/solr/admin/collections?action=RESTORE&name=techproducts_backup&location=file:///path/to/my/shared/drive&collection=techproducts_v3

V2 API

With the v2 API, the restore-collection command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections -H 'Content-Type: application/json' -d '
  {
    "restore-collection": {
      "name": "techproducts_backup",
      "collection": "techproducts_v3",
      "location": "file:///path/to/my/shared/drive"
    }
  }
'

The RESTORE operation will replace the content of a collection with files from the specified backup.

If the provided collection value matches an existing collection, Solr will use it for restoration, assuming it is compatible (same number of shards, etc.) with the stored backup files. If the provided collection value doesn’t exist, a new collection with that name is created in a way compatible with the stored backup files. The collection created will be have the same number of shards and replicas as the original collection, preserving routing information, etc. Optionally, you can override some parameters documented below.

While restoring, if a configset with the same name exists in ZooKeeper then Solr will reuse that, or else it will upload the backed up configset in ZooKeeper and use that.

You can use the collection CREATEALIAS command to make sure clients don’t need to change the endpoint to query or index against the newly restored collection.

RESTORE Parameters

collection

Required

Default: none

The collection where the indexes will be restored into. This parameter is required.

name

Required

Default: none

The name of the existing backup that you want to restore.

location

Optional

Default: none

The location on a shared drive for the RESTORE command to read from. Alternately it can be set as a cluster property.+

This must be the same value as was given as the location option when creating the backup.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

repository

Optional

Default: none

The name of a repository to be used for the backup. If no repository is specified then the local filesystem repository will be used automatically.

backupId

Optional

Default: none

The ID of a specific backup point to restore from.

Backup locations can hold multiple backups of the same collection. This parameter allows users to choose which of those backups should be used to restore from. If not specified the most recent backup point is used.

There are also optional parameters that determine the target collection layout. The following parameters are currently supported (described in detail in the CREATE collection section): createNodeSet, createNodeSet.shuffle.

Note: for createNodeSet the special value of EMPTY is not allowed with this command.

Overridable Parameters

Additionally, there are several parameters that may have been set on the original collection that can be overridden when restoring the backup (described in detail in the CREATE collection section): collection.configName, replicationFactor, nrtReplicas, tlogReplicas, pullReplicas, property.name=value.

DELETEBACKUP: Delete backup files from the remote repository

Deletes backup files stored at the specified repository location.

Previous versions of Solr supported a different snapshot-based backup file structure that did not support the storage of multiple backups at the same location. Solr can still restore backups stored in this old format, but it is deprecated and will be removed in subsequent versions of Solr. The DELETEBACKUP API does not support the deprecated format and attempts to use this API on a location holding an older backup will result in an error message.

Solr allows storing multiple backups for the same collection at any given logical "location". These backup points are each given an identifier (backupId) which can be used to delete them specifically with this API. Alternatively Solr can be told to keep the last maxNumBackupPoints backups, deleting everything else at the given location. Deleting backup points in these ways can orphan index files that are no longer referenced by any backup points. These orphaned files can be detected and deleted using the purgeUnused option. See the parameter descriptions below for more information.

DELETEBACKUP Example

Input

The following API command deletes the first backup (backupId=0) at the specified repository location.

V1 API

http://localhost:8983/solr/admin/collections?action=DELETEBACKUP&name=myBackupName&location=/path/to/my/shared/drive&backupId=0

V2 API

curl -X POST http://localhost:8983/v2/collections/backups -H 'Content-Type: application/json' -d '
  {
    "delete-backups" : {
      "name": "myBackupName",
      "location": "/path/to/my/shared/drive",
      "backupId": 0
    }
  }
'

Output

{
  "responseHeader":{
    "status":0,
    "QTime":940},
  "deleted":[[
      "startTime","2021-02-09T03:19:52.085653Z",
      "backupId",0,
      "size",28381,
      "numFiles",53]],
  "collection":"books"}

DELETEBACKUP Parameters

name

Required

Default: none

The backup name to delete backup files from.

location

Required

Default: none

The repository location to delete backups from. This parameter is required, unless a default location is defined on the repository configuration, or set as a cluster property.

If the location path is on a mounted drive, the mount must be available on the node that serves as the overseer, even if the overseer node does not host a replica of the collection being backed up. Since any node can take the overseer role at any time, a best practice to avoid possible backup failures is to ensure the mount point is available on all nodes of the cluster.

This must be the same value as was given as the location option when creating the backup.

repository

Optional

Default: none

The name of a repository to be used for deleting backup files. If no repository is specified then the local filesystem repository will be used automatically.

backupId

Optional

Default: none

Explicitly specify a single backup-ID to delete. Only one of backupId, maxNumBackupPoints, and purgeUnused may be specified per DELETEBACKUP request.

maxNumBackupPoints

Optional

Default: none

Specify how many backups should be retained, deleting all others. Only one of backupId, maxNumBackupPoints, and purgeUnused may be specified per DELETEBACKUP request.

purgeUnused

Optional

Default: none

Solr’s incremental backup support can orphan files if the backups referencing them are deleted. The purgeUnused flag parameter triggers a scan to detect these orphaned files and delete them. Administrators doing repeated backups at the same location should plan on using this parameter sporadically to reclaim disk space. Only one of backupId, maxNumBackupPoints, and purgeUnused may be specified per DELETEBACKUP request.

async

Optional

Default: none

Request ID to track this action which will be processed asynchronously.

REBALANCELEADERS: Rebalance Leaders

Reassigns leaders in a collection according to the preferredLeader property across active nodes.

V1 API

http://localhost:8983/solr/admin/collections?action=REBALANCELEADERS&collection=techproducts

V2 API

With the v2 API, the rebalance-leaders command is provided as part of the JSON data that contains the required parameters:

curl -X POST http://localhost:8983/api/collections/techproducts -H 'Content-Type: application/json' -d '
  {
    "rebalance-leaders": {
      "maxAtOnce": 3
    }
  }
'

Leaders are assigned in a collection according to the preferredLeader property on active nodes. This command should be run after the preferredLeader property has been assigned via the BALANCESHARDUNIQUE or ADDREPLICAPROP commands.

It is not required that all shards in a collection have a preferredLeader property. Rebalancing will only attempt to reassign leadership to those replicas that have the preferredLeader property set to true and are not currently the shard leader and are currently active.

REBALANCELEADERS Parameters

collection

Required

Default: none

The name of the collection to rebalance preferredLeaders on.

maxAtOnce

The maximum number of reassignments to have queue up at once. Values <=0 are use the default value Integer.MAX_VALUE.

When this number is reached, the process waits for one or more leaders to be successfully assigned before adding more to the queue.

maxWaitSeconds

Optional

Default: 60 seconds

The timeout when waiting for leaders to be reassigned. If maxAtOnce is less than the number of reassignments that will take place, this is the maximum interval that any single wait for at least one reassignment.

For example, if 10 reassignments are to take place and maxAtOnce is 1 and maxWaitSeconds is 60, the upper bound on the time that the command may wait is 10 minutes.

REBALANCELEADERS Response

The response will include the status of the request. A status of "0" indicates the request was processed, not that all assignments were successful. Examine the "Summary" section for that information.

Examples using REBALANCELEADERS

Input

Either of these commands would cause all the active replicas that had the preferredLeader property set and were not already the preferred leader to become leaders.

http://localhost:8983/solr/admin/collections?action=REBALANCELEADERS&collection=collection1&wt=json

http://localhost:8983/solr/admin/collections?action=REBALANCELEADERS&collection=collection1&maxAtOnce=5&maxWaitSeconds=30&wt=json

Output

In this example:

  • In the "alreadyLeaders" section, core_node5 was already the leader, so there were no changes in leadership for shard1.

  • In the "inactivePreferreds" section, core_node57 had the preferredLeader property set, but the node was not active, the leader for shard7 was not changed. This is considered successful.

  • In the "successes" section, core_node23 was not the leader for shard3, so leadership was assigned to that replica.

The "Summary" section with the "Success" tag indicates that the command rebalanced all active replicas with the preferredLeader property set as requried. If a replica cannot be made leader due to not being healthy (for example, it is on a Solr instance that is not running), it’s also considered success.

{
  "responseHeader":{
    "status":0,
    "QTime":3054},
  "Summary":{
    "Success":"All active replicas with the preferredLeader property set are leaders"},
  "alreadyLeaders":{
    "core_node5":{
      "status":"skipped",
      "msg":"Replica core_node5 is already the leader for shard shard1. No change necessary"}},
  "inactivePreferreds":{
    "core_node57":{
      "status":"skipped",
      "msg":"Replica core_node57 is a referredLeader for shard shard7, but is inactive. No change necessary"}},
  "successes":{
    "shard3":{
      "status":"success",
      "msg":"Successfully changed leader of slice shard3 to core_node23"}}}

Examining the clusterstate after issuing this call should show that every active replica that has the preferredLeader property should also have the "leader" property set to true.

The added work done by an NRT leader is quite small and only present when indexing. The primary use-case is to redistribute the leader role if there are a large number of leaders concentrated on a small number of nodes. Rebalancing will likely not improve performance unless the imbalance of leadership roles is measured in multiples of 10.
The BALANCESHARDUNIQUE command that distributes the preferredLeader property does not guarantee perfect distribution and in some collection topologies it is impossible to make that guarantee.