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- cursor.readPref()
cursor.readPref()¶
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Definition¶
-
cursor.
readPref
(mode, tagSet)¶ Append
readPref()
to a cursor to control how the client routes the query to members of the replica set.Note
You must apply
readPref()
to the cursor before retrieving any documents from the database.
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). For details, see Tag Set. |
cursor.setReadPref()
does not support the
maxStalenessSeconds option for read
preference.
Example¶
The following operation uses the read preference mode to target the read to a secondary member.
To target secondaries with specific tags, include the tag set array:
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.
- If found, MongoDB limits the eligible secondaries to those with the
See Order of Tag Matching for details.
See also