diff --git a/internal/partitions/partitions.go b/internal/partitions/partitions.go
index 278f875b..01419808 100644
--- a/internal/partitions/partitions.go
+++ b/internal/partitions/partitions.go
@@ -119,7 +119,7 @@ const (
 func PartitionCollectionWithSize(
 	ctx context.Context,
 	uuidEntry *uuidutil.NamespaceAndUUID,
-	retryer retry.Retryer,
+	retryer *retry.Retryer,
 	srcClient *mongo.Client,
 	replicatorList []Replicator,
 	subLogger *logger.Logger,
@@ -137,7 +137,7 @@ func PartitionCollectionWithSize(
 	partitions, docCount, byteCount, err := PartitionCollectionWithParameters(
 		ctx,
 		uuidEntry,
-		&retryer,
+		retryer,
 		srcClient,
 		replicatorList,
 		defaultSampleRate,
@@ -153,7 +153,7 @@ func PartitionCollectionWithSize(
 		return PartitionCollectionWithParameters(
 			ctx,
 			uuidEntry,
-			&retryer,
+			retryer,
 			srcClient,
 			replicatorList,
 			defaultSampleRate,
diff --git a/internal/retry/retry.go b/internal/retry/retry.go
index 362d3f00..470da5b5 100644
--- a/internal/retry/retry.go
+++ b/internal/retry/retry.go
@@ -44,9 +44,9 @@ type RetryCallback = func(context.Context, *FuncInfo) error
 // This returns an error if the duration limit is reached, or if f() returns a
 // non-transient error.
 func (r *Retryer) Run(
-	ctx context.Context, logger *logger.Logger, f ...RetryCallback,
+	ctx context.Context, logger *logger.Logger, funcs ...RetryCallback,
 ) error {
-	return r.runRetryLoop(ctx, logger, f)
+	return r.runRetryLoop(ctx, logger, funcs)
 }
 
 // runRetryLoop contains the core logic for the retry loops.
@@ -74,8 +74,18 @@ func (r *Retryer) runRetryLoop(
 	sleepTime := minSleepTime
 
 	for {
+		if beforeFunc, hasBefore := r.before.Get(); hasBefore {
+			beforeFunc()
+		}
+
 		eg, egCtx := errgroup.WithContext(ctx)
 		for i, curFunc := range funcs {
+			if curFunc == nil {
+				panic("curFunc should be non-nil")
+			}
+			if funcinfos[i] == nil {
+				panic(fmt.Sprintf("funcinfos[%d] should be non-nil", i))
+			}
 
 			eg.Go(func() error {
 				err := curFunc(egCtx, funcinfos[i])
diff --git a/internal/retry/retryer.go b/internal/retry/retryer.go
index 15ba15d9..6c269113 100644
--- a/internal/retry/retryer.go
+++ b/internal/retry/retryer.go
@@ -2,24 +2,27 @@ package retry
 
 import (
 	"time"
+
+	"github.com/10gen/migration-verifier/option"
 )
 
 // Retryer handles retrying operations that fail because of network failures.
 type Retryer struct {
 	retryLimit           time.Duration
 	retryRandomly        bool
+	before               option.Option[func()]
 	additionalErrorCodes []int
 }
 
 // New returns a new retryer.
-func New(retryLimit time.Duration) Retryer {
+func New(retryLimit time.Duration) *Retryer {
 	return NewWithRandomlyRetries(retryLimit, false)
 }
 
 // NewWithRandomlyRetries returns a new retryer, but allows the option of setting the
 // retryRandomly field.
-func NewWithRandomlyRetries(retryLimit time.Duration, retryRandomly bool) Retryer {
-	return Retryer{
+func NewWithRandomlyRetries(retryLimit time.Duration, retryRandomly bool) *Retryer {
+	return &Retryer{
 		retryLimit:    retryLimit,
 		retryRandomly: retryRandomly,
 	}
@@ -29,9 +32,21 @@ func NewWithRandomlyRetries(retryLimit time.Duration, retryRandomly bool) Retrye
 // this method. This allows for a single function to customize the codes it
 // wants to retry on. Note that if the Retryer already has additional custom
 // error codes set, these are _replaced_ when this method is called.
-func (r Retryer) WithErrorCodes(codes ...int) Retryer {
-	r2 := r
+func (r *Retryer) WithErrorCodes(codes ...int) *Retryer {
+	r2 := *r
 	r2.additionalErrorCodes = codes
 
-	return r2
+	return &r2
+}
+
+// WithBefore sets a callback that always runs before any retryer callback.
+//
+// This is useful if there are multiple callbacks and you need to reset some
+// condition before each retryer iteration. (In the single-callback case it’s
+// largely redundant.)
+func (r *Retryer) WithBefore(todo func()) *Retryer {
+	r2 := *r
+	r2.before = option.Some(todo)
+
+	return &r2
 }
diff --git a/internal/uuidutil/get_uuid.go b/internal/uuidutil/get_uuid.go
index a8a7e576..86996933 100644
--- a/internal/uuidutil/get_uuid.go
+++ b/internal/uuidutil/get_uuid.go
@@ -27,7 +27,7 @@ type NamespaceAndUUID struct {
 	CollName string
 }
 
-func GetCollectionNamespaceAndUUID(ctx context.Context, logger *logger.Logger, retryer retry.Retryer, db *mongo.Database, collName string) (*NamespaceAndUUID, error) {
+func GetCollectionNamespaceAndUUID(ctx context.Context, logger *logger.Logger, retryer *retry.Retryer, db *mongo.Database, collName string) (*NamespaceAndUUID, error) {
 	binaryUUID, uuidErr := GetCollectionUUID(ctx, logger, retryer, db, collName)
 	if uuidErr != nil {
 		return nil, uuidErr
@@ -39,7 +39,7 @@ func GetCollectionNamespaceAndUUID(ctx context.Context, logger *logger.Logger, r
 	}, nil
 }
 
-func GetCollectionUUID(ctx context.Context, logger *logger.Logger, retryer retry.Retryer, db *mongo.Database, collName string) (*primitive.Binary, error) {
+func GetCollectionUUID(ctx context.Context, logger *logger.Logger, retryer *retry.Retryer, db *mongo.Database, collName string) (*primitive.Binary, error) {
 	filter := bson.D{{"name", collName}}
 	opts := options.ListCollections().SetNameOnly(false)
 
diff --git a/internal/verifier/compare.go b/internal/verifier/compare.go
index e619255c..802d0bae 100644
--- a/internal/verifier/compare.go
+++ b/internal/verifier/compare.go
@@ -25,7 +25,8 @@ func (verifier *Verifier) FetchAndCompareDocuments(
 	types.ByteCount,
 	error,
 ) {
-	srcChannel, dstChannel, readSrcCallback, readDstCallback := verifier.getFetcherChannelsAndCallbacks(task)
+	var srcChannel, dstChannel <-chan bson.Raw
+	var readSrcCallback, readDstCallback func(context.Context, *retry.FuncInfo) error
 
 	results := []VerificationResult{}
 	var docCount types.DocumentCount
@@ -33,23 +34,31 @@ func (verifier *Verifier) FetchAndCompareDocuments(
 
 	retryer := retry.New(retry.DefaultDurationLimit)
 
-	err := retryer.Run(
-		givenCtx,
-		verifier.logger,
-		readSrcCallback,
-		readDstCallback,
-		func(ctx context.Context, _ *retry.FuncInfo) error {
-			var err error
-			results, docCount, byteCount, err = verifier.compareDocsFromChannels(
-				ctx,
-				task,
-				srcChannel,
-				dstChannel,
-			)
+	err := retryer.
+		WithBefore(func() {
+			srcChannel, dstChannel, readSrcCallback, readDstCallback = verifier.getFetcherChannelsAndCallbacks(task)
+		}).
+		Run(
+			givenCtx,
+			verifier.logger,
+			func(ctx context.Context, fi *retry.FuncInfo) error {
+				return readSrcCallback(ctx, fi)
+			},
+			func(ctx context.Context, fi *retry.FuncInfo) error {
+				return readDstCallback(ctx, fi)
+			},
+			func(ctx context.Context, _ *retry.FuncInfo) error {
+				var err error
+				results, docCount, byteCount, err = verifier.compareDocsFromChannels(
+					ctx,
+					task,
+					srcChannel,
+					dstChannel,
+				)
 
-			return err
-		},
-	)
+				return err
+			},
+		)
 
 	return results, docCount, byteCount, err
 }
diff --git a/internal/verifier/mongos_refresh.go b/internal/verifier/mongos_refresh.go
index 5b12d6aa..ec8fa70f 100644
--- a/internal/verifier/mongos_refresh.go
+++ b/internal/verifier/mongos_refresh.go
@@ -137,7 +137,7 @@ func RefreshAllMongosInstances(
 func getAnyExistingShardConnectionStr(
 	ctx context.Context,
 	l *logger.Logger,
-	r retry.Retryer,
+	r *retry.Retryer,
 	client *mongo.Client,
 ) (string, error) {
 	res, err := runListShards(ctx, l, r, client)
@@ -169,7 +169,7 @@ func getAnyExistingShardConnectionStr(
 func runListShards(
 	ctx context.Context,
 	l *logger.Logger,
-	r retry.Retryer,
+	r *retry.Retryer,
 	client *mongo.Client,
 ) (*mongo.SingleResult, error) {
 	var res *mongo.SingleResult