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Mongo.setReadPref(mode, tagSet)

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]


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.


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 Set Lists.

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


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.


Specify Read Preference Tag Set

To target secondaries with specific tags, include the tag set array:

      { "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.