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  1. Drivers
  2. DRIVERS-1969

Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

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Details

    • Spec Change
    • Status: Closed
    • Major - P3
    • Resolution: Done
    • None
    • CRUD
    • Needed
    • Hide

      In mongodb/specifications@747b748, the rules were applying a read preference for aggregations using $out and $merge was changed. Previously, drivers were instructed to attempt using the read preference and only fall back to a primary if a pre-5.0 secondary was selected.

      This was changed to require drivers to always disregard the read preference if there is any evidence of a pre-5.0 server. If there are either no available servers or all servers are 5.0+ (or load balanced, where we assume 5.0+), drivers can utilize the read preference.

      Spec tests have not been changed, as this clarification really only changes behavior for mixed version clusters or a replica set consisting of a single pre-5.0 primary, neither of which are tested.

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      In mongodb/specifications@747b748 , the rules were applying a read preference for aggregations using $out and $merge was changed. Previously, drivers were instructed to attempt using the read preference and only fall back to a primary if a pre-5.0 secondary was selected. This was changed to require drivers to always disregard the read preference if there is any evidence of a pre-5.0 server. If there are either no available servers or all servers are 5.0+ (or load balanced, where we assume 5.0+), drivers can utilize the read preference. Spec tests have not been changed, as this clarification really only changes behavior for mixed version clusters or a replica set consisting of a single pre-5.0 primary, neither of which are tested.

    Description

      Currently the specification says:

      - If an explicit (i.e. per-operation) read preference is specified for an
      		 
        aggregation with a write stage, drivers MUST attempt to use it. If that would
      		 
        result in a pre-5.0, secondary server being selected, drivers MUST instead
      		 
        select a server using a primary read preference.
      		 
      - If no explicit read preference is specified but a default read preference is
      		 
        available to inherit (e.g. from the Collection), drivers MUST attempt to use
      		 
        it. If that would result in a pre-5.0, secondary server being selected,
      		 
        drivers MUST instead select a server using a primary read preference.

      Defining it precisely in terms of available and eligible servers (the language of the server selection specification) would be more prescriptive and reduce differences in driver implementations. Something like this:

      If an explicit (i.e. per-operation) or default (i.e., specified on a collection object) 
      read preference is specified for an aggregation with a write stage, 
      drivers use this logic to determine the list of eligible servers to consider:
      		 
       
      		 
      If there are any *available* pre-5.0 servers, drivers MUST fall back to using primary 
      read preference to determine the list of *eligible* servers.  
      Otherwise, drivers MUST apply the read preference to determine the list of *eligible* servers.

      Examples (all with secondary read preference):

      1. a 5.0 primary and no secondaries -> apply read preference (no eligible servers)
      2. a 5.0 primary and a 5.0 secondary -> apply read preference (one eligible server)
      3. no available servers -> apply read preference (no eligible servers)
      4. a 4.4 primary and a 5.0 secondary -> fallback to primary (one eligible server)

      Attachments

        Issue Links

          is depended on by

          Spec Change - PYTHON-2554 Support $merge and $out executing on secondaries

          • Major - P3 - Major loss of function.
          • Closed
          is related to

          Epic - Created by Jira Software - do not edit or delete. Issue type for a big user story that needs to be broken down. DRIVERS-823 Support $merge and $out executing on secondaries

          • Major - P3 - Major loss of function.
          • Implementing

          Improvement - An improvement or enhancement to an existing feature or task. JAVA-4380 Fix server selection logic for $out/$merge on secondary

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. DRIVERS-1955 CRUD spec for $out/$merge on secondaries should consider load balancers

          • Unknown - Priority not determined yet.
          • Closed
          split to

          Bug - A problem which impairs or prevents the functions of the product. PHPLIB-760 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. CDRIVER-4224 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. CSHARP-3957 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. CXX-2414 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. GODRIVER-2220 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. JAVA-4396 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. MOTOR-859 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. NODE-3752 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. PYTHON-3012 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. RUBY-2841 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. RUST-1097 Ignore read preference for $out/$merge on secondaries if any servers are pre-5.0

          • Major - P3 - Major loss of function.
          • Closed

          Activity

            People

              jmikola@mongodb.com Jeremy Mikola
              jeff.yemin@mongodb.com Jeffrey Yemin
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                Created:
                Updated:
                Resolved: