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  1. Node.js Driver
  2. NODE-4557

MongoDB reads are not balance with 2 members selected

    • Type: Icon: Bug Bug
    • Resolution: Fixed
    • Priority: Icon: Major - P3 Major - P3
    • 4.10.0
    • Affects Version/s: 4.6.0, 4.7.0, 4.8.0, 4.8.1
    • Component/s: None
    • Labels:
    • 2
    • Not Needed

      What problem are you facing?

      When using Node.js driver versions >4.6.x in a configuration with readPreference: nearest and readPreferenceTags resulting in 2 eligible members to read from, the reads are unbalanced between the two members.

      For example if there are 1000 queries/second in total, in drivers version <4.6.x, the first members gets ~500 queries/second and second members gets ~500 queries/second. In versions >4.6.x, the second member always is preferred, getting ~750 queries/second where the first member only gets the ~250 queries/second.

      The issue seems to fix when there are 3 or more members eligible, the queries and load on all members becomes balanced.

      What driver and relevant dependency versions are you using?

      Node.js driver >4.6.x

      Steps to reproduce?

      Create a Mongo cluster setup with tags. From Node.js driver select only 2 members using readPreferenceTags and readPreference: nearest.

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        4. three-secondaries-selected-qps.png
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        5. two-secondaries-selected-qps.png
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            neal.beeken@mongodb.com Neal Beeken
            cezar.manea29@gmail.com Cezar Manea
            Neal Beeken
            3 Vote for this issue
            5 Start watching this issue