[ADDED] Introduce PersistMode for configurable persistence settings in streams (#1943)

Signed-off-by: Piotr Piotrowski <piotr@synadia.com>

jetstream/stream_config.go

@@ -207,6 +207,9 @@ type (
 
 		// AllowAtomicPublish allows atomic batch publishing into the stream.
 		AllowAtomicPublish bool `json:"allow_atomic,omitempty"`
+
+		// PersistMode allows to opt-in to different persistence mode settings.
+		PersistMode PersistModeType `json:"persist_mode,omitempty"`
 	}
 
 	// StreamSourceInfo shows information about an upstream stream
@@ -428,6 +431,9 @@ type (
 
 	// StoreCompression determines how messages are compressed.
 	StoreCompression uint8
+
+	// PersistModeType determines what persistence mode the stream uses.
+	PersistModeType int
 )
 
 const (
@@ -459,6 +465,16 @@ const (
 	workQueuePolicyString = "workqueue"
 )
 
+const (
+	// DefaultPersistMode specifies the default persist mode. Writes to the stream will immediately be flushed.
+	// The publish acknowledgement will be sent after the persisting completes.
+	DefaultPersistMode = PersistModeType(iota)
+	// AsyncPersistMode specifies writes to the stream will be flushed asynchronously.
+	// The publish acknowledgement may be sent before the persisting completes.
+	// This means writes could be lost if they weren't flushed prior to a hard kill of the server.
+	AsyncPersistMode
+)
+
 func (rp RetentionPolicy) String() string {
 	switch rp {
 	case LimitsPolicy:
@@ -533,6 +549,40 @@ func (dp *DiscardPolicy) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+func (pm PersistModeType) String() string {
+	switch pm {
+	case DefaultPersistMode:
+		return "Default"
+	case AsyncPersistMode:
+		return "Async"
+	default:
+		return "Unknown Persist Mode"
+	}
+}
+
+func (pm PersistModeType) MarshalJSON() ([]byte, error) {
+	switch pm {
+	case DefaultPersistMode:
+		return json.Marshal("default")
+	case AsyncPersistMode:
+		return json.Marshal("async")
+	default:
+		return nil, fmt.Errorf("nats: can not marshal %v", pm)
+	}
+}
+
+func (pm *PersistModeType) UnmarshalJSON(data []byte) error {
+	switch strings.ToLower(string(data)) {
+	case jsonString("default"):
+		*pm = DefaultPersistMode
+	case jsonString("async"):
+		*pm = AsyncPersistMode
+	default:
+		return fmt.Errorf("nats: can not unmarshal %q", data)
+	}
+	return nil
+}
+
 const (
 	// FileStorage specifies on disk storage. It's the default.
 	FileStorage StorageType = iota