Mongo.setReadPref()¶

On this page

Definition
Examples

Definition¶

Mongo.setReadPref(mode, tagSet, hedgeOptions)¶: Call the setReadPref() method on a Mongo connection object to control how the client will route all queries to members of the replica set. [1]

Note

You must call Mongo.setReadPref() on the connection object before retrieving documents using that connection to use that read preference.

[1]	To apply a read preference for a specific query or queries, you can apply `cursor.readPref()` to a cursor before iteration. See `cursor.readPref()` for details.

Parameters¶

Parameter Type Description

mode

string

One of the following read preference modes: primary, primaryPreferred, secondary, secondaryPreferred, or nearest

tagSet

array of documents

Optional. A tag set used to target reads to members with the specified tag(s). tagSet is not available if using read preference mode primary.

For details, see Read Preference Tag Sets.

hedgeOptions

document

Optional. A document that specifies whether to enable the use of hedged reads:

{ enabled: <boolean> }

The enabled field defaults to true; i.e. specifying an empty document { } is equivalent to specifying { enabled: true }.

Hedged reads are available starting in MongoDB 4.4 for sharded clusters. To use hedged reads, the mongos must have enabled support for hedged reads (the default) and the non-primary read preferences must enable the use of hedged reads.

Read preference nearest enables the use of hedged reads on sharded clusters by default; i.e. by default, has { enabled: true }.

New in version 4.4.

Mongo.setReadPref() does not support the Read Preference maxStalenessSeconds option for read preference.

Examples¶

Specify Read Preference Mode¶

The following operation sets the read preference mode to target the read to a secondary member. This implicitly allows reads from secondaries.

copy

db.getMongo().setReadPref('secondary')

Specify Read Preference Tag Set¶

To target secondaries with specific tags, include both the mode and the tagSet array:

copy

db.getMongo().setReadPref(
   "secondary",
   [
      { "datacenter": "B" },    // First, try matching by the datacenter tag
      { "region": "West"},      // If not found, then try matching by the region tag
      { }                       // If not found, then use the empty document to match all eligible members
   ]
)

During the secondary selection process, MongoDB tries to find secondary members with the datacenter: "B" tag first.

If found, MongoDB limits the eligible secondaries to those with the datacenter: "B" tag and ignores the remaining tags.
If none are found, then, MongoDB tries to find secondary members with the "region": "West" tag.
- If found, MongoDB limits the eligible secondaries to those with the "region": "West" tag.
- If none are found, MongoDB uses any eligible secondaries.

See Order of Tag Matching for details.

Specify Hedged Read¶

Starting in MongoDB 4.4 for sharded clusters, you can enable hedged reads for non-primary read preferences. To use hedged reads, the mongos must have enabled support for hedged reads (the default) and the non-primary read preferences must enable the use of hedged reads.

To target secondaries on 4.4+ sharded cluster using hedged reads, include both the mode and the hedgeOptions, as in the following examples:

Without a tag set

copy

db.getMongo().setReadPref(
   "secondary",            // mode
   null,                   // tag set
   { enabled: true }       // hedge options
)

With a tag set

copy

db.getMongo().setReadPref(
   "secondary",                      // mode
   [ { "datacenter": "B" },  { } ],  // tag set
   { enabled: true }                 // hedge options
)

← Mongo.setCausalConsistency() Mongo.startSession() →