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  1. Core Server
  2. SERVER-79330

Audit featureFlag usage in v7.0 branch in mongod binary

    • Type: Icon: Task Task
    • Resolution: Unresolved
    • Priority: Icon: Major - P3 Major - P3
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    • Cluster Scalability

      Intial sync can temporarily reset the fcv value to uninitialized and sets the new value afterwards. This can cause call sites trying to inspect the fcv value to hit this invariant. We need to audit feature flag usage and determine if the feature flag check can be run during initial sync:

      If it can never be called when initial sync is running, do nothing. Note that this can be tricky to prove as we once thought the catalog cache loader can never be run while initial sync is happening but it can.

      If it might get run during initial sync, this could be the case if the feature is run during initial sync itself, if the feature is in a background thread that runs during initial sync, or if the feature is run in a command that is allowed during initial sync, such as hello, serverStatus, etc. In this case, use one of these options:

      • Use isEnabledUseLastLTSFCVWhenUninitialized. It checks against the last LTS FCV version if the FCV version is unset, but note that this could result in the feature not being turned on even though the FCV will be set to latest once initial sync is complete.
      • Use isEnabledUseLatestFCVWhenUninitialized. This instead checks against the latest FCV version if the FCV version is unset, but note that this could result in the feature being turned on even though the FCV has not been upgraded yet and will be set to lastLTS once initial sync is complete.
      • Write your own special logic to avoid the invariant (for example, waiting for the FCV to become initialized before checking isEnabled, or uasserting instead of invariant-ing)

      See this section of the README

      List of feature flags from "git grep -ni featureflag src/ | grep "isEnabled(" | grep -v _test"

      gFeatureFlagBucketUnpackWithSort
      gFeatureFlagBulkWriteCommand
      gfeatureFlagCappedCollectionsRelaxedSize
      gFeatureFlagCatalogShard
      gFeatureFlagChangeStreamsFurtherEnrichedEvents
      gFeatureFlagColumnstoreIndexes
      gFeatureFlagCommonQueryFramework
      gFeatureFlagCompoundWildcardIndexes
      gFeatureFlagConfigurableX509ClusterAuthn
      gFeatureFlagAuditConfigClusterParameter
      gFeatureFlagFLE2CompactForProtocolV2
      gFeatureFlagFLE2Range
      gFeatureFlagGlobalIndexes
      gFeatureFlagHealthMonitoring
      gFeatureFlagInternalWritesAreReplicatedTransactionally
      gFeatureFlagServerlessChangeStreams
      gFeatureFlagTelemetry
      gFeatureFlagRequireTenantID
      gFeatureFlagResharding
      gFeatureFlagShardedTimeSeries
      gFeatureFlagUpdateDocumentShardKeyUsingTransactionApi
      gFeatureFlagUpdateOneWithoutShardKey
      gFeatureFlagUserRoles
      gGlobalIndexesShardingCatalog

            Assignee:
            backlog-server-cluster-scalability [DO NOT USE] Backlog - Cluster Scalability
            Reporter:
            randolph@mongodb.com Randolph Tan
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              Created:
              Updated: