fix: topics subscription resiliency

examples/topics.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:io';
 
 import 'package:client_sdk_dart/client_sdk_dart.dart';
@@ -13,32 +14,63 @@ void main() async {
       CredentialProvider.fromEnvironmentVariable("MOMENTO_API_KEY"),
       Mobile.latest());
 
-  var result = await topicClient.publish("cache", "topic", StringValue("hi"));
-  switch (result) {
-    case TopicPublishSuccess():
-      print("Successful publish!");
-    case TopicPublishError():
-      print("Publish error: ${result.errorCode} ${result.message}");
-  }
+  // start publishing messages in 2 seconds
+  Timer(const Duration(seconds: 2), () async {
+    // publish 10 messages spaced 1 second apart
+    for (final _ in Iterable.generate(10)) {
+      var result =
+          await topicClient.publish("cache", "topic", StringValue("hi"));
+      switch (result) {
+        case TopicPublishSuccess():
+          print("Successful publish!");
+        case TopicPublishError():
+          print("Publish error: ${result.errorCode} ${result.message}");
+      }
+      sleep(Duration(seconds: 1));
+    }
+  });
 
   var sub = topicClient.subscribe("cache", "topic");
   switch (sub) {
     case TopicSubscription():
       print("Successful subscription!");
-      await for (final msg in sub.stream) {
-        switch (msg) {
-          case TopicSubscriptionItemBinary():
-            print("Binary value: ${msg.value}");
-          case TopicSubscriptionItemText():
-            print("String value: ${msg.value}");
-          case TopicSubscriptionItemError():
-            print("Error receiving message: ${msg.errorCode}");
+
+      // cancel subscription 10 seconds from now
+      Timer(const Duration(seconds: 10), () {
+        print("Cancelling subscription!");
+        sub.unsubscribe();
+      });
+
+      try {
+        await for (final msg in sub.stream) {
+          switch (msg) {
+            case TopicSubscriptionItemBinary():
+              print("Binary value: ${msg.value}");
+            case TopicSubscriptionItemText():
+              print("String value: ${msg.value}");
+            case TopicSubscriptionItemError():
+              print("Error receiving message: ${msg.errorCode}");
+          }
         }
+      } catch (e) {
+        print("Runtime type: ${e.runtimeType}");
+        print("Error with await for loop: $e");
       }
     case TopicSubscribeError():
       print("Subscribe error: ${sub.errorCode} ${sub.message}");
   }
 
+  // unsubscribing should not affect the topic client's ability
+  // to subscribe to another topic afterwards
+  var sub2 = topicClient.subscribe("cache", "topic");
+  switch (sub2) {
+    case TopicSubscription():
+      print("Successful 2nd subscription!");
+    case TopicSubscribeError():
+      print("Subscribe error: ${sub2.errorCode} ${sub2.message}");
+  }
+
+  topicClient.close();
   print("End of Momento topics example");
   exit(0);
 }

lib/src/internal/pubsub_client.dart

@@ -1,7 +1,9 @@
 import 'package:client_sdk_dart/generated/cachepubsub.pbgrpc.dart';
 import 'package:client_sdk_dart/src/auth/credential_provider.dart';
 import 'package:client_sdk_dart/src/errors/errors.dart';
+import 'package:client_sdk_dart/src/internal/topics_grpc_manager.dart';
 import 'package:client_sdk_dart/src/messages/responses/topics/topic_subscribe.dart';
+import 'package:fixnum/fixnum.dart';
 import 'package:grpc/grpc.dart';
 
 import '../config/topic_configuration.dart';
@@ -13,22 +15,16 @@ abstract class AbstractPubsubClient {
       String cacheName, String topicName, Value value);
 
   TopicSubscribeResponse subscribe(String cacheName, String topicName);
+
+  void close();
 }
 
 class ClientPubsub implements AbstractPubsubClient {
-  late ClientChannel _channel;
-  late PubsubClient _client;
-  late TopicConfiguration _configuration;
+  final TopicConfiguration _configuration;
+  late final TopicGrpcManager _grpcManager;
 
-  ClientPubsub(
-      CredentialProvider credentialProvider, TopicConfiguration configuration) {
-    _channel = ClientChannel(credentialProvider.cacheEndpoint);
-    _client = PubsubClient(_channel,
-        options: CallOptions(metadata: {
-          'authorization': credentialProvider.apiKey,
-          'agent': 'dart:0.1.0'
-        }));
-    _configuration = configuration;
+  ClientPubsub(CredentialProvider credentialProvider, this._configuration) {
+    _grpcManager = TopicGrpcManager(credentialProvider);
   }
 
   TopicValue_ _valueToTopicValue(Value v) {
@@ -51,7 +47,7 @@ class ClientPubsub implements AbstractPubsubClient {
     request.topic = topicName;
     request.value = _valueToTopicValue(value);
     try {
-      await _client.publish(request,
+      await _grpcManager.client.publish(request,
           options: CallOptions(
               timeout: _configuration.transportStrategy.grpcConfig.deadline));
       return TopicPublishSuccess();
@@ -65,19 +61,29 @@ class ClientPubsub implements AbstractPubsubClient {
   }
 
   @override
-  TopicSubscribeResponse subscribe(String cacheName, String topicName) {
+  TopicSubscribeResponse subscribe(String cacheName, String topicName,
+      {Int64? resumeAtTopicSequenceNumber}) {
     var request = SubscriptionRequest_();
     request.cacheName = cacheName;
     request.topic = topicName;
+    request.resumeAtTopicSequenceNumber =
+        resumeAtTopicSequenceNumber ?? Int64(0);
     try {
-      var stream = _client.subscribe(request);
-      return TopicSubscription(stream);
+      var stream = _grpcManager.client.subscribe(request);
+      return TopicSubscription(stream, request.resumeAtTopicSequenceNumber,
+          this, cacheName, topicName);
     } catch (e) {
+      print("Error in pubsubclient.subscribe: $e");
       if (e is SdkException) {
         return TopicSubscribeError(e);
       }
       return TopicSubscribeError(
           UnknownException("Unexpected error: $e", null, null));
     }
   }
+
+  @override
+  void close() {
+    _grpcManager.close();
+  }
 }

lib/src/internal/topics_grpc_manager.dart

@@ -0,0 +1,30 @@
+import 'package:client_sdk_dart/client_sdk_dart.dart';
+import 'package:client_sdk_dart/generated/cachepubsub.pbgrpc.dart';
+import 'package:grpc/grpc.dart';
+import 'package:logging/logging.dart';
+
+class TopicGrpcManager {
+  late final ClientChannel _channel;
+  late final PubsubClient _client;
+  final _logger = Logger('MomentoTopicClient');
+
+  TopicGrpcManager(CredentialProvider credentialProvider) {
+    _channel = ClientChannel(credentialProvider.cacheEndpoint);
+    _client = PubsubClient(_channel,
+        options: CallOptions(metadata: {
+          'authorization': credentialProvider.apiKey,
+          'agent': 'dart:0.1.0'
+        }));
+  }
+
+  PubsubClient get client => _client;
+  ClientChannel get channel => _channel;
+
+  void close() {
+    try {
+      _channel.shutdown();
+    } catch (e) {
+      _logger.warning("Error shutting down channel: $e");
+    }
+  }
+}

lib/src/messages/responses/topics/topic_subscribe.dart

@@ -1,24 +1,64 @@
 import 'package:client_sdk_dart/generated/cachepubsub.pbgrpc.dart';
+import 'package:client_sdk_dart/src/errors/errors.dart';
 import 'package:client_sdk_dart/src/messages/responses/responses_base.dart';
 import 'package:grpc/grpc.dart';
 import 'package:logging/logging.dart';
+import 'package:fixnum/fixnum.dart';
 
+import '../../../internal/pubsub_client.dart';
 import 'topic_subscription_item.dart';
 
 sealed class TopicSubscribeResponse {}
 
 class TopicSubscription implements TopicSubscribeResponse {
-  final ResponseStream<SubscriptionItem_> _stream;
+  ResponseStream<SubscriptionItem_> _stream;
+  Int64 lastSequenceNumber;
+  final ClientPubsub _client;
+  String cacheName;
+  String topicName;
+  bool retry = true;
+  final logger = Logger("TopicSubscribeResponse");
 
-  TopicSubscription(this._stream);
+  TopicSubscription(this._stream, this.lastSequenceNumber, this._client,
+      this.cacheName, this.topicName);
 
-  Stream<TopicSubscriptionItemResponse> get stream =>
-      _stream.map(_processResult).where((item) => item != null).cast();
+  Stream<TopicSubscriptionItemResponse> get stream {
+    return _handleStream();
+  }
+
+  Stream<TopicSubscriptionItemResponse> _handleStream() async* {
+    while (retry) {
+      try {
+        await for (final msg in _stream) {
+          var result = _processResult(msg);
+          if (result != null) {
+            yield result;
+          }
+        }
+      } catch (e) {
+        if (e is CancelledException ||
+            (e is GrpcError && e.codeName == "CANCELLED")) {
+          logger.fine("Subscription was cancelled, not reconnecting");
+          retry = false;
+        } else {
+          logger.fine("Attempting to reconnect after receiving error: $e");
+          var result = _client.subscribe(cacheName, topicName,
+              resumeAtTopicSequenceNumber: lastSequenceNumber);
+          if (result is TopicSubscription) {
+            _stream = result._stream;
+            lastSequenceNumber = result.lastSequenceNumber;
+          } else if (result is TopicSubscribeError) {
+            logger.fine("Error reconnecting: ${result.message}");
+          }
+        }
+      }
+    }
+  }
 
   TopicSubscriptionItemResponse? _processResult(SubscriptionItem_ item) {
-    final logger = Logger("TopicSubscribeResponse");
     switch (item.whichKind()) {
       case SubscriptionItem__Kind.item:
+        lastSequenceNumber = item.item.topicSequenceNumber;
         return createTopicItemResponse(item.item);
       case SubscriptionItem__Kind.heartbeat:
         logger.fine("topic client received a heartbeat");
@@ -30,6 +70,10 @@ class TopicSubscription implements TopicSubscribeResponse {
     }
     return null;
   }
+
+  void unsubscribe() async {
+    await _stream.cancel();
+  }
 }
 
 class TopicSubscribeError extends ErrorResponseBase

lib/src/topic_client.dart

@@ -1,5 +1,5 @@
 import 'package:client_sdk_dart/src/auth/credential_provider.dart';
-import 'package:client_sdk_dart/src/config/logger.dart';
+// import 'package:client_sdk_dart/src/config/logger.dart';
 import 'package:logging/logging.dart';
 import 'config/topic_configuration.dart';
 import 'internal/pubsub_client.dart';
@@ -12,6 +12,8 @@ abstract class ITopicClient {
       String cacheName, String topicName, Value value);
 
   TopicSubscribeResponse subscribe(String cacheName, String topicName);
+
+  void close();
 }
 
 class TopicClient implements ITopicClient {
@@ -21,7 +23,7 @@ class TopicClient implements ITopicClient {
   TopicClient(
       CredentialProvider credentialProvider, TopicConfiguration configuration)
       : _pubsubClient = ClientPubsub(credentialProvider, configuration) {
-    _logger.level = determineLoggerLevel(configuration.logLevel);
+    // _logger.level = determineLoggerLevel(configuration.logLevel);
     _logger.finest("initializing topic client");
   }
 
@@ -35,4 +37,9 @@ class TopicClient implements ITopicClient {
   TopicSubscribeResponse subscribe(String cacheName, String topicName) {
     return _pubsubClient.subscribe(cacheName, topicName);
   }
+
+  @override
+  void close() {
+    _pubsubClient.close();
+  }
 }