Mongo replica sets load management

Question

Currently I have two node applications which use a mongo replica set (1 primary and 6 secondary). Currently the read queries of one application cause a load on mongo and affect the performance of the other application. So, I want to divide the secondary nodes such that 1 application uses the primary and 4 secondary members and the other application uses the primary and the other 2 secondary members. I don't want the load on one application to affect the other. How do i achieve the same?

Answer 1

You can set different tags per application and read from SECONDARY members per defined tag , for example:

app1--> db.collection.find({}).readPref( "secondary", [ { "app": "app1" } ] )
app2--> db.collection.find({}).readPref( "secondary", [ { "app": "app2" } ] )

And have you replicaSet as follow:

{
"_id" : "myrs",
"version" : 2,
"members" : [
         {
                 "_id" : 0,
                 "host" : "host1:27017",
                 "tags" : {
                         "app": "app1"
                 }
         }, {
                 "_id" : 1,
                 "host" : "host2:27017",
                 "tags" : {
                         "app": "app1"
                 }
         }, {
                 "_id" : 2,
                 "host" : "host3:27017",
                 "tags" : {
                         "app": "app1"
                 }
         },  {
                 "_id" : 3,
                 "host" : "host4:27017",
                 "tags" : {
                         "app": "app2"

                 }
         },  {
                 "_id" : 4,
                 "host" : "host4:27017",
                 "tags" : {
                         "app": "app2"

                 }
         }           
]
}

How to add the tags:

conf = rs.conf();
conf.members[0].tags = { "app": "app1" };
conf.members[1].tags = { "app": "app1" };
conf.members[2].tags = { "app": "app2" };
conf.members[3].tags = { "app": "app2" };
conf.members[4].tags = { "app": "app2" };
rs.reconfig(conf);