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• Use Tag Sets in Read Preference
• Custom Multi-Datacenter Write Concerns

Use Tag Sets in Read Preference

If a replica set member or members are associated with tags, you can specify a tag set in the read preference to target those members. A tag set is an array of documents, where each document contains the tag and value pair(s). The specifications are tried in order until a match is found. Once found, that specification is used to find all eligible matching members.

For example, a replica set has the following replica set configuration (some of the fields have 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" : {
      ...
   }
}

1. Add tags to the members.

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

   copy

   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);
   

2. Verify the replica set configuration.

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

   copy

   {
      "_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" : {
         ...
      }
   }
   

3. Specify tag sets in the read preference.

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

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

     copy

     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:

     copy

     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 with tags, you can configure the replica set’s settings.getLastErrorModes setting to create a custom write concern.

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

1. a facility VA tagged dc_va
2. 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" : {
      ...
   }
}

1. Add tags to the replica set members.

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

   copy

   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);
   

2. Create a custom write concern.

   In the replica set configuration, define a custom write concern in the settings.getLastErrorModes setting. For example, the following defines the custom write concern MultipleDC that requires the write to propagate to two members with different dc_va tag values and one member with any dc_ca tag value.

   copy

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