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  1. Documentation
  2. DOCS-14251

Investigate changes in SERVER-54278: Add feature flag to disable Opportunistic Reads by default



    • Task
    • Status: Closed
    • Major - P3
    • Resolution: Fixed
    • None
    • 4.2.13
    • manual, Server
    • None
    • 2
    • ServerDocs2020: Feb23 - Mar2
    • true



      Downstream Change Summary

      This ticket disables opportunistic read targeting by default, and introduces a new server parameter, "opportunisticSecondaryTargeting", to enable this feature.

      Description of Linked Ticket

      Opportunistic reads against replica-sets was originally introduced in v4.2 as part of PM-856. This feature allows mongos to direct secondary reads to any secondary with an active connection, instead of waiting for a new connection to a specific secondary.

      Under certain workloads, as described in SERVER-53853, opportunistic reads may trigger construction of unnecessary connections to mongod processes, which would regress the overall performance.

      This ticket should introduce a new feature flag to disable opportunistic reads in v4.2 by default. In particular, the following tickets from PM-856 introduce new functionality that we might want to guard with the feature flag: SERVER-41132, SERVER-41133, and SERVER-41134.

      We may wan to approach this ticket from this for loop, introduced by SERVER-41133 (see excerpt below).

      // Attempt to get a connection to every target host
      for (size_t idx = 0; idx < request.target.size() && !cmdState->finishLine.isReady(); ++idx) {
          auto connFuture = _pool->get(request.target[idx], request.sslMode, request.timeout);

      Scope of changes

      Impact to Other Docs

      MVP (Work and Date)

      Resources (Scope or Design Docs, Invision, etc.)


        Issue Links



              andrew.feierabend@mongodb.com Andrew Feierabend (Inactive)
              backlog-server-pm Backlog - Core Eng Program Management Team
              Githook User Githook User
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                1 year, 8 weeks, 3 days ago