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

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    • Type: Spec Change
    • Resolution: Done
    • Priority: Major - P3
    • None
    • Component/s: CRUD
    • Needed
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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.

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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.
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      Key Status/Resolution FixVersion
      CDRIVER-4224 Done 1.21.0
      CXX-2414 Works as Designed
      CSHARP-3957 Done 2.15.0
      GODRIVER-2220 Done 1.8.0
      JAVA-4396 Duplicate
      NODE-3752 Works as Designed
      PHPLIB-760 Fixed 1.10.1
      PYTHON-3012 Duplicate
      MOTOR-859 Duplicate
      RUBY-2841 Fixed 2.17.0
      RUST-1097 Duplicate
      SWIFT-1412 Duplicate
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      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)

        1. image-2021-10-29-16-41-37-013.png
          image-2021-10-29-16-41-37-013.png
          27 kB

            Assignee:
            Jeremy Mikola
            Reporter:
            Jeffrey Yemin
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              Created:
              Updated:
              Resolved: