Mongoid preferred read replica

Question

I'm building a geo redundant MongoDB replica set, but there is a problem. Every time a query is sent to the database apparently a random replica is choosen.

MOPED: x.x.x.0:p COMMAND      database=admin command={:ismaster=>1} runtime: 148.4439ms
MOPED: x.x.x.1:p COMMAND      database=admin command={:ismaster=>1} runtime: 150.5547ms
MOPED: x.x.x.2:p COMMAND      database=admin command={:ismaster=>1} runtime: 3.8707ms
MOPED: x.x.x.3:p COMMAND      database=admin command={:ismaster=>1} runtime: 289.3072ms

As you can see the third replica is the nearest, so it would be a waste of time to read from another one, but that's what happens:

MOPED: x.x.x.3:p QUERY        database=d collection=c selector={} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil runtime: 393.3878ms

So the question is, can I specify the IP address of the preferred read, or just tell MOPED to connect to the fastest one?

The mongoid.yml file now specifies all 4 replicas:

  hosts:
    - x.x.x.0:p
    - x.x.x.1:p
    - x.x.x.2:p
    - x.x.x.3:p
  options:
    read: :secondary

One option would be to just manually set only the master, and the nearest replica:

  hosts:
    - x.x.x.0:p
    - x.x.x.2:p
  options:
    read: :secondary

But I don't feel is a good solution as availability would suffer in case of failures.

Try setting the read preference to nearest instead of secondary. Secondary will pick randomly (well, pretty much), while nearest will pick based on testing ping times. — wdberkeley, Feb 09 '15 at 22:42

score 0 · Accepted Answer · answered Feb 12 '15 at 12:16

Figured out, reading this: https://github.com/mongoid/moped/blob/master/lib/moped/read_preference.rb you can see that moped supports reading from the nearest replica, so the solution is very simple, in mongoid.yml:

hosts:
  - x.x.x.0:p
  - x.x.x.2:p
options:
  read: :nearest

Mongoid preferred read replica

1 Answers1