diff --git a/internal/backends/collection.go b/internal/backends/collection.go
index 97fc706cb8fd..31f679729757 100644
--- a/internal/backends/collection.go
+++ b/internal/backends/collection.go
@@ -34,7 +34,7 @@ import (
 type Collection interface {
 	Query(context.Context, *QueryParams) (*QueryResult, error)
 	InsertAll(context.Context, *InsertAllParams) (*InsertAllResult, error)
-	Update(context.Context, *UpdateParams) (*UpdateResult, error)
+	UpdateAll(context.Context, *UpdateAllParams) (*UpdateAllResult, error)
 	DeleteAll(context.Context, *DeleteAllParams) (*DeleteAllResult, error)
 	Explain(context.Context, *ExplainParams) (*ExplainResult, error)
 }
@@ -91,7 +91,7 @@ type InsertAllParams struct {
 // InsertAllResult represents the results of Collection.InsertAll method.
 type InsertAllResult struct{}
 
-// InsertAll inserts all or none documents into the collection.
+// InsertAll inserts documents into the collection.
 //
 // The operation should be atomic.
 // If some documents cannot be inserted, the operation should be rolled back,
@@ -115,25 +115,34 @@ func (cc *collectionContract) InsertAll(ctx context.Context, params *InsertAllPa
 	return res, err
 }
 
-// UpdateParams represents the parameters of Collection.Update method.
-type UpdateParams struct {
-	// that should be []*types.Document
-	// TODO https://github.com/FerretDB/FerretDB/issues/3079
-	Docs *types.Array
+// UpdateAllParams represents the parameters of Collection.Update method.
+type UpdateAllParams struct {
+	Docs []*types.Document
 }
 
-// UpdateResult represents the results of Collection.Update method.
-type UpdateResult struct {
+// UpdateAllResult represents the results of Collection.Update method.
+type UpdateAllResult struct {
 	Updated int32
 }
 
-// Update updates documents in collection.
+// UpdateAll updates documents in collection.
+//
+// The operation should be atomic.
+// If some documents cannot be updated, the operation should be rolled back,
+// and the first encountered error should be returned.
+//
+// All documents are expected to be valid and include _id fields.
+// They will be frozen.
 //
 // Database or collection may not exist; that's not an error.
-func (cc *collectionContract) Update(ctx context.Context, params *UpdateParams) (*UpdateResult, error) {
+func (cc *collectionContract) UpdateAll(ctx context.Context, params *UpdateAllParams) (*UpdateAllResult, error) {
 	defer observability.FuncCall(ctx)()
 
-	res, err := cc.c.Update(ctx, params)
+	for _, doc := range params.Docs {
+		doc.Freeze()
+	}
+
+	res, err := cc.c.UpdateAll(ctx, params)
 	checkError(err)
 
 	return res, err
diff --git a/internal/backends/doc.go b/internal/backends/doc.go
index f0e51d61f18b..8b3995fb01ed 100644
--- a/internal/backends/doc.go
+++ b/internal/backends/doc.go
@@ -27,8 +27,10 @@
 //  2. Backend objects are stateful.
 //     Database objects are almost stateless but should be Close()'d to avoid connection leaks.
 //     Collection objects are fully stateless.
-//  3. Contexts are per-operation and should not be stored.
-//  4. Errors returned by methods could be nil, [*Error], or some other opaque error type.
+//  3. The Backend maintains the list of databases and collections.
+//     It is recommended that it does so by not querying the information_schema or equivalent often.
+//  4. Contexts are per-operation and should not be stored.
+//  5. Errors returned by methods could be nil, [*Error], or some other opaque error type.
 //     *Error values can't be wrapped or be present anywhere in the error chain.
 //     Contracts enforce *Error codes; they are not documented in the code comments
 //     but are visible in the contract's code (to avoid duplication).
diff --git a/internal/backends/postgresql/collection.go b/internal/backends/postgresql/collection.go
index dcea8e09ecf0..8164d617977b 100644
--- a/internal/backends/postgresql/collection.go
+++ b/internal/backends/postgresql/collection.go
@@ -44,8 +44,8 @@ func (c *collection) InsertAll(ctx context.Context, params *backends.InsertAllPa
 	panic("not implemented")
 }
 
-// Update implements backends.Collection interface.
-func (c *collection) Update(ctx context.Context, params *backends.UpdateParams) (*backends.UpdateResult, error) {
+// UpdateAll implements backends.Collection interface.
+func (c *collection) UpdateAll(ctx context.Context, params *backends.UpdateAllParams) (*backends.UpdateAllResult, error) {
 	panic("not implemented")
 }
 
diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 4c615a1c35f8..590d8b041309 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -26,9 +26,7 @@ import (
 	"github.com/FerretDB/FerretDB/internal/backends"
 	"github.com/FerretDB/FerretDB/internal/backends/sqlite/metadata"
 	"github.com/FerretDB/FerretDB/internal/handlers/sjson"
-	"github.com/FerretDB/FerretDB/internal/types"
 	"github.com/FerretDB/FerretDB/internal/util/fsql"
-	"github.com/FerretDB/FerretDB/internal/util/iterator"
 	"github.com/FerretDB/FerretDB/internal/util/lazyerrors"
 	"github.com/FerretDB/FerretDB/internal/util/must"
 )
@@ -118,14 +116,14 @@ func (c *collection) InsertAll(ctx context.Context, params *backends.InsertAllPa
 	return new(backends.InsertAllResult), nil
 }
 
-// Update implements backends.Collection interface.
-func (c *collection) Update(ctx context.Context, params *backends.UpdateParams) (*backends.UpdateResult, error) {
+// UpdateAll implements backends.Collection interface.
+func (c *collection) UpdateAll(ctx context.Context, params *backends.UpdateAllParams) (*backends.UpdateAllResult, error) {
 	db := c.r.DatabaseGetExisting(ctx, c.dbName)
 	if db == nil {
 		return nil, lazyerrors.Errorf("no database %q", c.dbName)
 	}
 
-	var res backends.UpdateResult
+	var res backends.UpdateAllResult
 	meta := c.r.CollectionGet(ctx, c.dbName, c.name)
 	if meta == nil {
 		return &res, nil
@@ -133,40 +131,35 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 
 	q := fmt.Sprintf(`UPDATE %q SET %s = ? WHERE %s = ?`, meta.TableName, metadata.DefaultColumn, metadata.IDColumn)
 
-	iter := params.Docs.Iterator()
-	defer iter.Close()
-
-	for {
-		_, d, err := iter.Next()
-		if errors.Is(err, iterator.ErrIteratorDone) {
-			break
-		}
+	err := db.InTransaction(ctx, func(tx *fsql.Tx) error {
+		for _, doc := range params.Docs {
+			b, err := sjson.Marshal(doc)
+			if err != nil {
+				return lazyerrors.Error(err)
+			}
 
-		if err != nil {
-			return nil, lazyerrors.Error(err)
-		}
+			id, _ := doc.Get("_id")
+			must.NotBeZero(id)
 
-		doc, ok := d.(*types.Document)
-		if !ok {
-			panic(fmt.Sprintf("expected document, got %T", d))
-		}
+			arg := string(must.NotFail(sjson.MarshalSingleValue(id)))
 
-		id, _ := doc.Get("_id")
-		must.NotBeZero(id)
-		docArg := string(must.NotFail(sjson.Marshal(doc)))
-		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
+			r, err := tx.ExecContext(ctx, q, string(b), arg)
+			if err != nil {
+				return lazyerrors.Error(err)
+			}
 
-		r, err := db.ExecContext(ctx, q, docArg, idArg)
-		if err != nil {
-			return nil, lazyerrors.Error(err)
-		}
+			ra, err := r.RowsAffected()
+			if err != nil {
+				return lazyerrors.Error(err)
+			}
 
-		ra, err := r.RowsAffected()
-		if err != nil {
-			return nil, lazyerrors.Error(err)
+			res.Updated += int32(ra)
 		}
 
-		res.Updated += int32(ra)
+		return nil
+	})
+	if err != nil {
+		return nil, lazyerrors.Error(err)
 	}
 
 	return &res, nil
diff --git a/internal/backends/sqlite/database.go b/internal/backends/sqlite/database.go
index 93f913e927c4..a3430321bfa1 100644
--- a/internal/backends/sqlite/database.go
+++ b/internal/backends/sqlite/database.go
@@ -105,7 +105,7 @@ func (db *database) RenameCollection(ctx context.Context, params *backends.Renam
 
 // Stats implements backends.Database interface.
 //
-// If the database does not exist, it returns *backends.DBStatsResult filled with zeros for all the fields.
+// If the database does not exist, it returns *backends.StatsResult filled with zeros for all the fields.
 func (db *database) Stats(ctx context.Context, params *backends.StatsParams) (*backends.StatsResult, error) {
 	stats := new(backends.StatsResult)
 
diff --git a/internal/backends/sqlite/metadata/pool/pool.go b/internal/backends/sqlite/metadata/pool/pool.go
index e6ece43f3192..3296ac31cb28 100644
--- a/internal/backends/sqlite/metadata/pool/pool.go
+++ b/internal/backends/sqlite/metadata/pool/pool.go
@@ -68,7 +68,7 @@ type Pool struct {
 // so no validation is needed.
 // One exception is very long full path names for the filesystem,
 // but we don't check it.
-func openDB(name, uri string, singleConn bool, l *zap.Logger) (*fsql.DB, error) {
+func openDB(name, uri string, memory bool, l *zap.Logger) (*fsql.DB, error) {
 	db, err := sql.Open("sqlite", uri)
 	if err != nil {
 		return nil, lazyerrors.Error(err)
@@ -77,8 +77,10 @@ func openDB(name, uri string, singleConn bool, l *zap.Logger) (*fsql.DB, error)
 	db.SetConnMaxIdleTime(0)
 	db.SetConnMaxLifetime(0)
 
-	// TODO https://github.com/FerretDB/FerretDB/issues/2755
-	if singleConn {
+	// Each connection to in-memory database uses its own database.
+	// See https://www.sqlite.org/inmemorydb.html.
+	// We don't want that.
+	if memory {
 		db.SetMaxIdleConns(1)
 		db.SetMaxOpenConns(1)
 	}
@@ -123,7 +125,7 @@ func New(u string, l *zap.Logger) (*Pool, map[string]*fsql.DB, error) {
 
 		p.l.Debug("Opening existing database.", zap.String("name", name), zap.String("uri", uri))
 
-		db, err := openDB(name, uri, p.singleConn(), l)
+		db, err := openDB(name, uri, p.memory(), l)
 		if err != nil {
 			p.Close()
 			return nil, nil, lazyerrors.Error(err)
@@ -140,13 +142,6 @@ func (p *Pool) memory() bool {
 	return p.uri.Query().Get("mode") == "memory"
 }
 
-// singleConn returns true if pool size must be limited to a single connection.
-func (p *Pool) singleConn() bool {
-	// https://www.sqlite.org/inmemorydb.html
-	// TODO https://github.com/FerretDB/FerretDB/issues/2755
-	return p.memory()
-}
-
 // databaseName returns database name for given database file path.
 func (p *Pool) databaseName(databaseFile string) string {
 	return strings.TrimSuffix(filepath.Base(databaseFile), filenameExtension)
@@ -233,7 +228,7 @@ func (p *Pool) GetOrCreate(ctx context.Context, name string) (*fsql.DB, bool, er
 	}
 
 	uri := p.databaseURI(name)
-	db, err := openDB(name, uri, p.singleConn(), p.l)
+	db, err := openDB(name, uri, p.memory(), p.l)
 	if err != nil {
 		return nil, false, lazyerrors.Errorf("%s: %w", uri, err)
 	}
diff --git a/internal/backends/sqlite/metadata/pool/pool_test.go b/internal/backends/sqlite/metadata/pool/pool_test.go
index ac1520f76ef6..ec004d46e762 100644
--- a/internal/backends/sqlite/metadata/pool/pool_test.go
+++ b/internal/backends/sqlite/metadata/pool/pool_test.go
@@ -163,8 +163,9 @@ func TestDefaults(t *testing.T) {
 	}
 	require.NoError(t, rows.Err())
 	require.NoError(t, rows.Close())
-	require.Contains(t, options, "THREADSAFE=1")
-	require.Contains(t, options, "ENABLE_DBSTAT_VTAB")
+
+	require.Contains(t, options, "THREADSAFE=1")       // for it to work with database/sql
+	require.Contains(t, options, "ENABLE_DBSTAT_VTAB") // for dbStats/collStats/etc
 
 	for q, expected := range map[string]string{
 		"SELECT sqlite_version()":   "3.41.2",
diff --git a/internal/backends/sqlite/metadata/pool/uri.go b/internal/backends/sqlite/metadata/pool/uri.go
index 1754435fd9db..643a591cf76d 100644
--- a/internal/backends/sqlite/metadata/pool/uri.go
+++ b/internal/backends/sqlite/metadata/pool/uri.go
@@ -55,8 +55,7 @@ func parseURI(u string) (*url.URL, error) {
 
 	values := uri.Query()
 
-	// https://www.sqlite.org/inmemorydb.html
-	// TODO https://github.com/FerretDB/FerretDB/issues/2755
+	// it is deprecated and interacts weirdly with database/sql.Pool
 	if values.Get("cache") == "shared" {
 		return nil, fmt.Errorf(`shared cache is not supported`)
 	}
diff --git a/internal/handlers/common/delete.go b/internal/handlers/common/delete_params.go
similarity index 96%
rename from internal/handlers/common/delete.go
rename to internal/handlers/common/delete_params.go
index 9f6dc8fe9925..e62b364c70a5 100644
--- a/internal/handlers/common/delete.go
+++ b/internal/handlers/common/delete_params.go
@@ -22,6 +22,8 @@ import (
 )
 
 // DeleteParams represents parameters for the delete command.
+//
+//nolint:vet // for readability
 type DeleteParams struct {
 	DB         string `ferretdb:"$db"`
 	Collection string `ferretdb:"collection"`
@@ -37,6 +39,8 @@ type DeleteParams struct {
 }
 
 // Delete represents single delete operation parameters.
+//
+//nolint:vet // for readability
 type Delete struct {
 	Filter  *types.Document `ferretdb:"q"`
 	Limited bool            `ferretdb:"limit,zeroOrOneAsBool"`
diff --git a/internal/handlers/common/update_params.go b/internal/handlers/common/update_params.go
index b320c5a5f681..1d21cd489761 100644
--- a/internal/handlers/common/update_params.go
+++ b/internal/handlers/common/update_params.go
@@ -21,11 +21,13 @@ import (
 	"github.com/FerretDB/FerretDB/internal/types"
 )
 
-// UpdatesParams represents parameters for the update command.
-type UpdatesParams struct {
-	DB         string         `ferretdb:"$db"`
-	Collection string         `ferretdb:"collection"`
-	Updates    []UpdateParams `ferretdb:"updates"`
+// UpdateParams represents parameters for the update command.
+//
+//nolint:vet // for readability
+type UpdateParams struct {
+	DB         string   `ferretdb:"$db"`
+	Collection string   `ferretdb:"collection"`
+	Updates    []Update `ferretdb:"updates"`
 
 	Comment string `ferretdb:"comment,opt"`
 
@@ -37,8 +39,10 @@ type UpdatesParams struct {
 	LSID                     any             `ferretdb:"lsid,ignored"`
 }
 
-// UpdateParams represents a single update operation parameters.
-type UpdateParams struct {
+// Update represents a single update operation parameters.
+//
+//nolint:vet // for readability
+type Update struct {
 	// TODO https://github.com/FerretDB/FerretDB/issues/2627
 	// get comment from query, e.g. db.collection.UpdateOne({"_id":"string", "$comment: "test"},{$set:{"v":"foo""}})
 	Filter *types.Document `ferretdb:"q,opt"`
@@ -54,8 +58,8 @@ type UpdateParams struct {
 }
 
 // GetUpdateParams returns parameters for update command.
-func GetUpdateParams(document *types.Document, l *zap.Logger) (*UpdatesParams, error) {
-	var params UpdatesParams
+func GetUpdateParams(document *types.Document, l *zap.Logger) (*UpdateParams, error) {
+	var params UpdateParams
 
 	err := commonparams.ExtractParams(document, "update", &params, l)
 	if err != nil {
diff --git a/internal/handlers/sqlite/msg_update.go b/internal/handlers/sqlite/msg_update.go
index 3b7228c81f59..76bd9422f5e2 100644
--- a/internal/handlers/sqlite/msg_update.go
+++ b/internal/handlers/sqlite/msg_update.go
@@ -41,6 +41,9 @@ func (h *Handler) MsgUpdate(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 		return nil, lazyerrors.Error(err)
 	}
 
+	// TODO https://github.com/FerretDB/FerretDB/issues/2612
+	_ = params.Ordered
+
 	matched, modified, upserted, err := h.updateDocument(ctx, params)
 	if err != nil {
 		return nil, lazyerrors.Error(err)
@@ -66,7 +69,7 @@ func (h *Handler) MsgUpdate(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 }
 
 // updateDocument iterate through all documents in collection and update them.
-func (h *Handler) updateDocument(ctx context.Context, params *common.UpdatesParams) (int32, int32, *types.Array, error) {
+func (h *Handler) updateDocument(ctx context.Context, params *common.UpdateParams) (int32, int32, *types.Array, error) {
 	var matched, modified int32
 	var upserted types.Array
 
@@ -214,7 +217,7 @@ func (h *Handler) updateDocument(ctx context.Context, params *common.UpdatesPara
 				continue
 			}
 
-			updateRes, err := c.Update(ctx, &backends.UpdateParams{Docs: must.NotFail(types.NewArray(doc))})
+			updateRes, err := c.UpdateAll(ctx, &backends.UpdateAllParams{Docs: []*types.Document{doc}})
 			if err != nil {
 				return 0, 0, nil, lazyerrors.Error(err)
 			}