Mongo.setReadPref()
- Definition
  - Parameters
- Examples

Mongo.setReadPref()

Definition

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

Note

You must call Mongo.setReadPref() on the connection objectbefore retrieving documents using that connection to use that readpreference.

[1]	To apply a read preference for a specific query or queries, you canapply `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 tomembers with the specified tag(s). `tagSet` is not availableif using read preference mode `primary`.For details, see Tag Set.

Mongo.setReadPref() does not support themaxStalenessSeconds option for readpreference.

Examples

The following operation uses the read preference mode to target theread to a secondary member.

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

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

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 secondarymembers with the datacenter: "B" tag first.

If found, MongoDB limits the eligible secondaries to those with thedatacenter: "B" tag and ignores the remaining tags.
If none are found, then, MongoDB tries to find secondary members withthe "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.