AWS SNS Unsubscribe loop is not releasing memory

Problem

When I unsubscribe to my endpoints subscriptions, instruments memory graph goes from 32MB to 300MB and app is killed Jetsam. It was my understanding that AWS BFTasks clean up after themselves… If I remove the do...while and allow it to process the first 100 subscriptions, all works fine.

Process

I have about 8000 subscriptions in my development app that uses AWS SNS to push alerts to iOS devices and email. After the user selects the subscriptions they want, I remove their existing subscriptions, then subscribe them to the new ones. The [_awsSnsClient listSubscriptions:request] returns a max of 100 subscriptions, I loop through these, performing [_awsSnsClient unsubscribe:unsubscribeRequest] as needed. Then get the next 100...

Code

- (void) awsUsubscribeAllSubscriptions {
    DLog(@"Removing existing subscriptions");

    AWSSNSListSubscriptionsInput *request = [AWSSNSListSubscriptionsInput new];

    __block Boolean sentLastUnsubscribe = NO; // YES when the last unsubscribe request was sent
    __block int pendingUnsubscribe = 0; // Number of outsianding unsubscribes (async process not yet complete)

    do {
        [[[_awsSnsClient listSubscriptions:request] continueWithSuccessBlock:^id(BFTask *task) {
            AWSSNSListSubscriptionsResponse *response = (AWSSNSListSubscriptionsResponse *)task.result;
            NSString *nextToken = response.nextToken;
            NSArray *subscriptions = response.subscriptions;
            for (AWSSNSSubscription *subscription in subscriptions) {
                if ([subscription.endpoint isEqualToString:_awsPlatformEndpoint.endpointArn]) {
                    AWSSNSUnsubscribeInput *unsubscribeRequest = [AWSSNSUnsubscribeInput new];
                    unsubscribeRequest.subscriptionArn = subscription.subscriptionArn;
                    pendingUnsubscribe++;
                    [[[_awsSnsClient unsubscribe:unsubscribeRequest] continueWithSuccessBlock:^id(BFTask *task) {
                        DLog(@"Unsubscribed from:%@",subscription.subscriptionArn);
                        return nil;
                    }] continueWithBlock:^id(BFTask *task) {
                        pendingUnsubscribe--;
                        if (task.error) {
                            // failed with error
                            ALog(@"Error unsubscribing to: %@, %@, %@", subscription, [task.error localizedDescription], [task.error localizedFailureReason]);
                        }
                        // If we have processed the last unsubscribe, then create topics and subscribe
                        if (sentLastUnsubscribe  && (pendingUnsubscribe <= 0)) {
                            [self awsCreateTopicsAndSubscriptions];
                        }
                        return nil;
                    }];
                }
            }
            request.nextToken = nextToken;
            if(!nextToken) sentLastUnsubscribe = YES; // Reached the end of the SNS subscriptions
            return nil;
        }] continueWithBlock:^id(BFTask *task) {
            if (task.error) {
                // failed with error
                ALog(@"Error listing subscriptions: %@, %@", [task.error localizedDescription], [task.error localizedFailureReason]);
            }
            return nil;
        }];
    } while (!sentLastUnsubscribe);
}

Is there a better way to remove all SNS subscriptions from an endpoint? Is there a way for force release any retained data during this loop?

Solution

Using both recursion and - waitUntilFinished . Memory load is now much lower.

-(void) awsUnsubscribeFromAllSubscriptionsWithNextToken:(NSString *)nextToken AndSubscribe:(BOOL)subscribe {
AWSSNSListSubscriptionsInput *request = [AWSSNSListSubscriptionsInput new];
request.nextToken = nextToken;
[[[_awsSnsClient listSubscriptions:request] continueWithSuccessBlock:^id(BFTask *task) {
    AWSSNSListSubscriptionsResponse *response = (AWSSNSListSubscriptionsResponse *)task.result;
    NSArray *subscriptions = response.subscriptions;
    for (AWSSNSSubscription *subscription in subscriptions) {
        if ([subscription.endpoint isEqualToString:_awsPlatformEndpoint.endpointArn]) {
            DLog(@"Unsubscribe from %@", subscription.topicArn);
            AWSSNSUnsubscribeInput *unsubscribeRequest = [AWSSNSUnsubscribeInput new];
            unsubscribeRequest.subscriptionArn = subscription.subscriptionArn;
            [[_awsSnsClient unsubscribe:unsubscribeRequest] waitUntilFinished];
        }
    }
    if(response.nextToken) {
        [self awsUnsubscribeFromAllSubscriptionsWithNextToken:response.nextToken AndSubscribe:subscribe];
    } else if (subscribe) {
        [self awsCreateTopicsAndSubscriptions];
    }
    return nil;
}] continueWithBlock:^id(BFTask *task) {
    if (task.error) {
        // failed with error
        ALog(@"Error listing subscriptions: %@, %@", [task.error localizedDescription], [task.error localizedFailureReason]);
    }
    return nil;
}];

}

---

Please note that - listSubscriptions: is an asynchronous method and returns immediately. You are calling an async method in a for loop. It means you are potentially calling - listSubscriptions: hundreds or even thousands of times in a short period.

The easiest thing you can do to fix it is to call - waitUntilFinished at the end of the async block. It makes the entire method synchronous

However, in general, you should avoid calling - waitUntilFinished as much as possible. AWSKinesisRecorderTests.m has a function to call - getRecords: recursively. You can adopt a similar pattern.

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