Read Preference Tag Sets

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Order of Tag Matching

Tag Set and Read Preference Modes

If a replica set member or members are associated with tags, you can specify a tag set (array of tag specification documents) in the read preference to target those members.

To configure a member with tags, set members[n].tags to a document that contains the tag name and value pairs. The value of the tags must be a string.

{ "<tag1>": "<string1>", "<tag2>": "<string2>",... }

Then, you can include a tag set in the read preference to target tagged members. A tag set is an array of tag specification documents, where each tag specification document contains one or more tag/value pairs.

[ { "<tag1>": "<string1>", "<tag2>": "<string2>",... }, ... ]

To find replica set members, MongoDB tries each document in succession until a match is found. See Order of Tag Matching for details.

For example, if a secondary member has the following members[n].tags:

{ "region": "South", "datacenter": "A" }

Then, the following tags sets can direct read operations to the aforementioned secondary (or other members with the same tags):

[ { "region": "South", "datacenter": "A" }, { } ]     // Find members with both tag values. If none are found, read from any eligible member.
[ { "region": "South" }, { "datacenter": "A" }, { } ] // Find members with the specified region tag. Only if not found, then find members with the specified datacenter tag. If none are found, read from any eligible member.
[ { "datacenter": "A" }, { "region": "South" }, { } ] // Find members with the specified datacenter tag. Only if not found, then find members with the specified region tag. If none are found, read from any eligible member.
[ { "region": "South" }, { } ]                        // Find members with the specified region tag value. If none are found, read from any eligible member.
[ { "datacenter": "A" }, { } ]                        // Find members with the specified datacenter tag value. If none are found, read from any eligible member.
[ { } ]                                               // Find any eligible member.

Order of Tag Matching

If the tag set lists multiple documents, MongoDB tries each document in succession until a match is found. Once a match is found, that tag specification document is used to find all eligible matching members, and the remaining tag specification documents are ignored. If no members match any of the tag specification documents, the read operation returns with an error.

Tip

To avoid an error if no members match any of the tag specifications, you can add an empty document { } as the last element of the tag set to read from any eligible member.

For example, consider the following tag set with three tag specification documents:

[ { "region": "South", "datacenter": "A" },  { "rack": "rack-1" }, { } ]

First, MongoDB tries to find members tagged with both "region": "South" and "datacenter": "A".

{ "region": "South", "datacenter": "A" }

If a member is found, the remaining tag specification documents are not considered. Instead, MongoDB uses this tag specification document to find all eligible members.
Else, MongoDB tries to find members with the tags specified in the second document
```
{ "rack": "rack-1" }
```
- If a member is found tagged, the remaining tag specification document is not considered. Instead, MongoDB uses this tag specification document to find all eligible members.
- Else, the third document is considered.
  { }
  The empty document matches any eligible member.

Tag Set and Read Preference Modes

Tags are not compatible with mode primary, and in general, only apply when selecting a secondary member of a set for a read operation. However, the nearest read mode, when combined with a tag set, selects the matching member with the lowest network latency. This member may be a primary or secondary.

Mode	Notes
`primaryPreferred`	Specified tag set only applies if selecting eligible secondaries.
`secondary`	Specified tag set always applies.
`secondaryPreferred`	Specified tag set only applies if selecting eligible secondaries.
`nearest`	Specified tag set applies whether selecting either primary or eligible secondaries.

For information on the interaction between the modes and tag sets, refer to the specific read preference mode documentation.

For information on configuring tag sets, see the Configure Replica Set Tag Sets tutorial.

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