Idempotent Rest Publishing

Source/ARTBaseMessage+Private.h

@@ -15,6 +15,8 @@ ART_ASSUME_NONNULL_BEGIN
 
 @interface ARTBaseMessage ()
 
+@property (nonatomic, assign, readonly) BOOL isIdEmpty;
+
 - (id __art_nonnull)decodeWithEncoder:(ARTDataEncoder*)encoder error:(NSError *__art_nullable*__art_nullable)error;
 - (id __art_nonnull)encodeWithEncoder:(ARTDataEncoder*)encoder error:(NSError *__art_nullable*__art_nullable)error;
 

Source/ARTBaseMessage.m

@@ -93,4 +93,8 @@ - (NSInteger)messageSize {
     return finalResult;
 }
 
+- (BOOL)isIdEmpty {
+    return self.id == nil || [self.id isEqualToString:@""];
+}
+
 @end

Source/ARTClientOptions+Private.h

@@ -11,6 +11,7 @@
 @interface ARTClientOptions ()
 
 + (void)setDefaultEnvironment:(NSString *_Nullable)environment;
++ (BOOL)getDefaultIdempotentRestPublishingForVersion:(NSString *_Nonnull)version;
 - (NSURLComponents *_Nonnull)restUrlComponents;
 
 @end

Source/ARTClientOptions.h

@@ -109,6 +109,15 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (readwrite, strong, nonatomic) dispatch_queue_t internalDispatchQueue;
 
+/**
+ True when idempotent publishing is enabled for all messages published via REST.
+
+ When this feature is enabled, the client library will add a unique ID to every published message (without an ID) ensuring any failed published attempts (due to failures such as HTTP requests failing mid-flight) that are automatically retried will not result in duplicate messages being published to the Ably platform.
+
+ Note: This is a beta unsupported feature!
+ */
+@property (readwrite, assign, nonatomic) BOOL idempotentRestPublishing;
+
 - (BOOL)isBasicAuth;
 - (NSURL *)restUrl;
 - (NSURL *)realtimeUrl;

Source/ARTClientOptions.m

@@ -49,6 +49,7 @@ - (instancetype)initDefaults {
     _dispatchQueue = dispatch_get_main_queue();
     _internalDispatchQueue = dispatch_queue_create("io.ably.main", DISPATCH_QUEUE_SERIAL);
     _pushFullWait = false;
+    _idempotentRestPublishing = [ARTClientOptions getDefaultIdempotentRestPublishingForVersion:[ARTDefault version]];
     return self;
 }
 
@@ -127,6 +128,7 @@ - (id)copyWithZone:(NSZone *)zone {
     options.dispatchQueue = self.dispatchQueue;
     options.internalDispatchQueue = self.internalDispatchQueue;
     options.pushFullWait = self.pushFullWait;
+    options.idempotentRestPublishing = self.idempotentRestPublishing;
 
     return options;
 }
@@ -179,4 +181,13 @@ - (void)setDefaultTokenParams:(ARTTokenParams *)value {
     _defaultTokenParams = [[ARTTokenParams alloc] initWithTokenParams:value];
 }
 
++ (BOOL)getDefaultIdempotentRestPublishingForVersion:(NSString *)version {
+    if ([@"1.2" compare:version options:NSNumericSearch] == NSOrderedDescending) {
+        return false;
+    }
+    else {
+        return true;
+    }
+}
+
 @end

Source/ARTJsonLikeEncoder.m

@@ -373,7 +373,10 @@ - (NSArray *)presenceMessagesFromArray:(NSArray *)input {
 
 - (NSDictionary *)messageToDictionary:(ARTMessage *)message {
     NSMutableDictionary *output = [NSMutableDictionary dictionary];
-
+    if (message.id) {
+        [output setObject:message.id forKey:@"id"];
+    }
+
     if (message.timestamp) {
         [output setObject:[message.timestamp artToNumberMs] forKey:@"timestamp"];
     }

Source/ARTRestChannel.m

@@ -21,6 +21,9 @@
 #import "ARTTokenDetails.h"
 #import "ARTNSArray+ARTFunctional.h"
 #import "ARTPushChannel.h"
+#import "ARTCrypto+Private.h"
+
+static const NSUInteger kIdempotentLibraryGeneratedIdLength = 9; //bytes
 
 @implementation ARTRestChannel {
 @private
@@ -165,27 +168,53 @@ - (void)internalPostMessages:(id)data callback:(void (^)(ARTErrorInfo *__art_nul
 
     if ([data isKindOfClass:[ARTMessage class]]) {
         ARTMessage *message = (ARTMessage *)data;
+
+        NSString *baseId = nil;
+        if (self.rest.options.idempotentRestPublishing && message.isIdEmpty) {
+            NSData *baseIdData = [ARTCrypto generateSecureRandomData:kIdempotentLibraryGeneratedIdLength];
+            baseId = [baseIdData base64EncodedStringWithOptions:0];
+            message.id = [NSString stringWithFormat:@"%@:0", baseId];
+        }
+
         if (message.clientId && self.rest.auth.clientId_nosync && ![message.clientId isEqualToString:self.rest.auth.clientId_nosync]) {
             callback([ARTErrorInfo createWithCode:ARTStateMismatchedClientId message:@"attempted to publish message with an invalid clientId"]);
             return;
         }
         else {
             message.clientId = self.rest.auth.clientId_nosync;
         }
+
         NSError *encodeError = nil;
         encodedMessage = [self.rest.defaultEncoder encodeMessage:message error:&encodeError];
         if (encodeError) {
             callback([ARTErrorInfo createFromNSError:encodeError]);
             return;
         }
-    } else if ([data isKindOfClass:[NSArray class]]) {
-        __GENERIC(NSArray, ARTMessage *) *messages = (NSArray *)data;
+    }
+    else if ([data isKindOfClass:[NSArray class]]) {
+        NSArray<ARTMessage *> *messages = (NSArray *)data;
+
+        NSString *baseId = nil;
+        if (self.rest.options.idempotentRestPublishing) {
+            BOOL messagesHaveEmptyId = [messages artFilter:^BOOL(ARTMessage *m) { return !m.isIdEmpty; }].count <= 0;
+            if (messagesHaveEmptyId) {
+                NSData *baseIdData = [ARTCrypto generateSecureRandomData:kIdempotentLibraryGeneratedIdLength];
+                baseId = [baseIdData base64EncodedStringWithOptions:0];
+            }
+        }
+
+        NSInteger serial = 0;
         for (ARTMessage *message in messages) {
             if (message.clientId && self.rest.auth.clientId_nosync && ![message.clientId isEqualToString:self.rest.auth.clientId_nosync]) {
                 callback([ARTErrorInfo createWithCode:ARTStateMismatchedClientId message:@"attempted to publish message with an invalid clientId"]);
                 return;
             }
+            if (baseId) {
+                message.id = [NSString stringWithFormat:@"%@:%ld", baseId, (long)serial];
+            }
+            serial += 1;
         }
+
         NSError *encodeError = nil;
         encodedMessage = [self.rest.defaultEncoder encodeMessages:data error:&encodeError];
         if (encodeError) {

Spec/RestClient.swift

@@ -1255,7 +1255,7 @@ class RestClient: QuickSpec {
                 }
 
                 let transport = realtime.transport as! TestProxyTransport
-                let object = AblyTests.msgpackToJSON(transport.rawDataSent.last! as NSData)
+                let object = AblyTests.msgpackToJSON(transport.rawDataSent.last!)
                 expect(object["messages"][0]["data"].string).to(equal("message"))
             }