Configure Replica Set Tag Sets

A replica set member or members can be configured withtags:

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

For read operations, you can specify a tag set in the readpreferences to help direct readoperations to members that have specific tag(s).

For write operations, you can use the tags to create a customwrite concern.

Use Tag Sets in Read Preference

If a replica set member or members are associated withtags, you can specify a tag set in the readpreference to target those members. A tagset is an array of documents, where each document contains the tag andvalue pair(s). The specifications are tried in order until a match isfound. Once found, that specification is used to find all eligiblematching members.

Note

You cannot specify a tag set when specifying read preference modeprimary.

For example, a replica set has the following replica setconfiguration (some of the fieldshave been omitted for brevity):

{
   "_id" : "rs0",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "writeConcernMajorityJournalDefault" : true,
   "members" : [
       { "_id" : 0, "host" : "mongodb0.example.net:27017", ...,  "tags": { }, ... },
       { "_id" : 1, "host" : "mongodb1.example.net:27017", ...,  "tags": { }, ... },
       { "_id" : 2, "host" : "mongodb2.example.net:27017", ...,  "tags": { }, ... }
   ],
   "settings" : {
      ...
   }
}

• Add tags to the members.

Connect a mongo shell to the replica set and users.reconfig() to add tags to the members:

conf = rs.conf();
conf.members[0].tags = { "dc": "east", "usage": "production" };
conf.members[1].tags = { "dc": "east", "usage": "reporting" };
conf.members[2].tags = { "dc": "west", "usage": "production" };
rs.reconfig(conf);

• Verify the replica set configuration.

Run rs.conf() to verify the replica set configuration(some of the fields have been omitted for brevity). Thers.conf() returns a document similar to the following:

{
   "_id" : "rs0",
   "version" : 2,
   "protocolVersion" : NumberLong(1),
   "writeConcernMajorityJournalDefault" : true,
   "members" : [
      {
          "_id" : 0,
          "host" : "mongodb0.example.net:27017",
          ...
          "tags" : {
              "dc": "east",
              "usage": "production"
          },
          ...
      },
      {
          "_id" : 1,
          "host" : "mongodb1.example.net:27017",
          ...
          "tags" : {
              "dc": "east",
              "usage": "reporting"
          },
          ...
       },
      {
          "_id" : 2,
          "host" : "mongodb2.example.net:27017",
          ...
          "tags" : {
              "dc": "west",
              "usage": "production"
          },
          ...
      }
   ],
   "settings" : {
      ...
   }
}

• Specify tag sets in the read preference.

To direct read operations to the secondaries tagged with a particulartag(s), in the mongo shell connected to the replica set,you can use the readPref() method to specify theread preference mode and the tagset. For example,

• To direct read operations to the secondary tagged with both"dc": "east" and "usage": "production", include thefollowing tag set:

db.collection.find({}).readPref( "secondary", [ { "dc": "east", "usage": "production" } ] )

• To direct a read operation to the secondaries tagged with "dc":"east", and if not found, to secondaries tagged with"usage": "production", include the following tag set:

db.collection.find({}).readPref( "secondary", [ { "dc": "east"}, { "usage": "production" } ] )

See also

Mongo.setReadPref()

Custom Multi-Datacenter Write Concerns

If a replica set member or members are associated withtags, you can configure the replica set’ssettings.getLastErrorModes setting to create a custom writeconcern.

Given a five member replica set with members in two data centers:

• a facility VA tagged dc_va
• a facility CA tagged dc_ca

{
   "_id" : "rs0",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "writeConcernMajorityJournalDefault" : true,
   "members" : [
       { "_id" : 0, "host" : "mongodb0.example.net:27017", ...,  "tags": { }, ... },
       { "_id" : 1, "host" : "mongodb1.example.net:27017", ...,  "tags": { }, ... },
       { "_id" : 2, "host" : "mongodb2.example.net:27017", ...,  "tags": { }, ... }
       { "_id" : 3, "host" : "mongodb3.example.net:27017", ...,  "tags": { }, ... }
       { "_id" : 4, "host" : "mongodb4.example.net:27017", ...,  "tags": { }, ... }
   ],
   "settings" : {
      ...
   }
}

• Add tags to the replica set members.

Connect a mongo shell to the replica set and users.reconfig() to add tags to the members:

conf = rs.conf();
conf.members[0].tags = { "dc_va": "rack1"};
conf.members[1].tags = { "dc_va": "rack2"};
conf.members[2].tags = { "dc_ca": "rack1"};
conf.members[3].tags = { "dc_ca": "rack2"};
conf.members[4].tags = { "dc_va": "rack1"};
rs.reconfig(conf);

• Create a custom write concern.

In the replica set configuration, define a custom write concern inthe settings.getLastErrorModes setting. For example, thefollowing defines the custom write concern MultipleDC thatrequires the write to propagate to two members with differentdc_va tag values and one member with any dc_ca tag value.

conf = rs.conf();
conf.settings = { getLastErrorModes: { MultipleDC : { "dc_va": 2, "dc_ca": 1 } } };
rs.reconfig(conf);

Note

The MultipleDC write concern is not satisfied if the writepropagates to two members with the same "dc_va" tag. Forexample, if the write has only propagated to members[0] andmembers[4], "dc_va": 2 is not satisfied since they havethe same tag value "rack1".

• Use the custom write concern.

To use the custom write concern, pass in the write concern name tothe w Option in the write concern:

db.collection.insert( { id: "xyz", status: "A" }, { writeConcern: { w: "MultipleDC" } } )