DOCSP-8771: Initial sync survives transient network errors

Author: ravindk89

source/core/replica-set-sync.txt

@@ -63,14 +63,28 @@ To perform an initial sync, see
 Fault Tolerance
 ~~~~~~~~~~~~~~~
 
-To recover from transient network or operation failures, initial sync
-has built-in retry logic.
-
-.. versionchanged:: 3.4
-
-   MongoDB 3.4 improves the initial sync retry logic to be more resilient to
-   intermittent failures on the network.
-
+If a secondary performing initial sync encounters a *non-transient*
+network error during the sync process, the secondary restarts the
+initial sync process from the beginning.
+
+Starting in MongoDB 4.4, a secondary performing initial sync can attempt
+to resume the sync process if interrupted by a *transient* network
+error. The sync source must also run MongoDB 4.4 to support resumable
+initial sync. If the sync source runs MongoDB 4.2 or earlier, the
+secondary must restart the initial sync process as if it encountered a
+non-transient network error.
+
+By default, the secondary tries to resume initial sync for 24 hours.
+MongoDB 4.4 adds the
+:parameter:`initialSyncTransientErrorRetryPeriodSeconds` server
+parameter for controlling the amount of time the secondary attempts to
+resume initial sync. If the secondary cannot successfully resume the
+initial sync process during the configured time period, it selects a new
+healthy source from the replica set and restarts the initial
+synchronization process from the beginning. 
+
+The secondary attempts to restart the initial sync up to ``10`` times
+before returning a fatal error.
 
 .. _replica-set-replication:
 

source/reference/parameters.txt

@@ -1961,6 +1961,18 @@ Replication Parameters
    The specified value must be greater than or equal to 0, with 0 to
    disable warnings. 
 
+.. parameter:: initialSyncTransientErrorRetryPeriodSeconds
+
+   .. versionadded:: 4.4
+
+   *Type*: integer
+
+   *Default*: 86400
+
+   The amount of time in seconds a secondary performing initial sync 
+   attempts to resume the process if interrupted by a transient 
+   network error. The default value is equivalent to 24 hours.
+
 .. parameter:: oplogInitialFindMaxSeconds
 
    .. versionadded:: 3.6

source/release-notes/4.4.txt

@@ -338,6 +338,28 @@ documentation, see :parameter:`tlsX509ExpirationWarningThresholdDays`.
 Replica Sets
 ------------
 
+Resumable Initial Sync
+~~~~~~~~~~~~~~~~~~~~~~
+
+Starting in MongoDB 4.4, a secondary performing initial sync can attempt
+to resume the sync process if interrupted by a *transient* network
+error. The sync source must also run MongoDB 4.4 to support resumable
+initial sync. If the sync source runs MongoDB 4.2 or earlier, the
+secondary must restart the initial sync process as if it encountered a
+non-transient network error.
+
+By default, the secondary tries to resume initial sync for 24 hours.
+MongoDB 4.4 adds the
+:parameter:`initialSyncTransientErrorRetryPeriodSeconds` server
+parameter for controlling the amount of time the secondary attempts to
+resume initial sync. If the secondary cannot successfully resume the
+initial sync process during the configured time period, it selects a new
+healthy source from the replica set and restarts the initial
+synchronization process from the beginning. 
+
+Prior to MongoDB 4.4, the secondary would restart the entire initial
+sync process if it encountered an error during the process.
+
 Rollback Directory
 ~~~~~~~~~~~~~~~~~~