ballerina/websub Ballerina library

Overview

This module provides APIs for a WebSub Subscriber Service.

WebSub is a common mechanism for communication between publishers of any kind of Web content and their subscribers, based on HTTP webhooks. Subscription requests are relayed through hubs, which validate and verify the request. Hubs then distribute new and updated content to subscribers when it becomes available. WebSub was previously known as PubSubHubbub.

WebSub Subscriber is an implementation that discovers the hub and topic URL of a given resource URL, subscribes to updates at the hub, and accepts content distribution requests from the hub.

Basic flow with WebSub

1. The subscriber discovers (from the publisher) the topic it needs to subscribe to and the hub(s) that deliver notifications on the updates of the topic.

2. The subscriber sends a subscription request to one or more discovered hub(s) specifying the discovered topic along with the other subscription parameters such as:

   • The callback URL to which the content is expected to be delivered.
   • (Optional) The lease period (in seconds) the subscriber wants the subscription to stay active.
   • (Optional) A secret to use for the authenticated content distribution.

3. The hub sends an intent verification request to the specified callback URL. If the response indicates the verification (by echoing a challenge specified in the request) by the subscriber, the subscription is added for the topic at the hub.

4. The publisher notifies the hub of the updates to the topic and the content to deliver is identified.

5. The hub delivers the identified content to the subscribers of the topic.

Subscribing

• The WebSub Subscriber provides the mechanism to subscribe in a hub to a given topic URL.

@websub:SubscriberServiceConfig {
    target: ["<HUB_URL>", "<TOPIC_URL>"], 
    leaseSeconds: 36000
} 
service /subscriber on new websub:Listener(9090) {
    remote function onSubscriptionValidationDenied(websub:SubscriptionDeniedError msg) returns websub:Acknowledgement? {
        // implement subscription validation denied logic here
        return websub:ACKNOWLEDGEMENT;
    }

    remote function onSubscriptionVerification(websub:SubscriptionVerification msg)
                        returns websub:SubscriptionVerificationSuccess|websub:SubscriptionVerificationError {
        // implement subscription intent verification logic here
        return websub:SUBSCRIPTION_VERIFICATION_SUCCESS;
    }

    remote function onUnsubscriptionVerification(websub:UnsubscriptionVerification msg)
                        returns websub:UnsubscriptionVerificationSuccess|websub:UnsubscriptionVerificationError {
        // implement unsubscription intent verification logic here
        return websub:UNSUBSCRIPTION_VERIFICATION_SUCCESS;
    }

    remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError? {
        // implement on event notification logic here
        return websub:ACKNOWLEDGEMENT;
    }
}

Resource Discovery

• The WebSub Subscriber also provides the mechanism to discover the hub and topic URL resources dynamically via the provided resource URL and initiates the subscription.

@websub:SubscriberServiceConfig {
    target: "RESOURCE_URL", 
    leaseSeconds: 36000
} 
service /subscriber on new websub:Listener(9090) {
    remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError? {
        // implement on event notification logic here
        return websub:ACKNOWLEDGEMENT;
    }

    // other remote methods are optional to be implemented
}

Dynamic URI Generation

• The service path for a WebSub Subscriber is optional. The WebSub Subscriber service has the capability to generate the service path dynamically.

@websub:SubscriberServiceConfig {
    target: "RESOURCE_URL", 
    leaseSeconds: 36000
} 
service on new websub:Listener(9090) {
    remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError? {
        // implement on event notification logic here
        return websub:ACKNOWLEDGEMENT;
    }

    // other remote methods are optional to be implemented
}

Running Subscriber Service Locally

• ngrok is a TCP Tunneling software, which is used to expose services running locally in the public network.
• If you want to run the subscriber service in your local machine, you could use ngrok to expose it to the public network.
• First, download and install ngrok.
• Run the following command to expose the local port 9090 to the public network via HTTPS. For information, see the ngrok documentation).

ngrok http -bind-tls=true 9090

• Extract the public URL provided by ngrok and provide it as the callback URL for the subscriber service.

@websub:SubscriberServiceConfig {
    target: "RESOURCE_URL", 
    leaseSeconds: 36000,
    callback: "<NGROK_PUBLIC_URL>",
    appendServicePath: true
} 
service on new websub:Listener(9090) {
    remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError? {
        // implement on event notification logic here
        return websub:ACKNOWLEDGEMENT;
    }

    // other remote methods are optional to be implemented
}

Unsubscribing from the Hub

• The WebSub Subscriber has the capability to initiate unsubscription flow on Subscriber termination.

websub:ListenerConfiguration listenerConfigs = {
    gracefulShutdownPeriod: 15
};

@websub:SubscriberServiceConfig {
    target: ["https://sample.hub.com", "https://sample.topic1.com"], 
    leaseSeconds: 36000,
    // By default this is set to `false`, hence subscriber on default mode would not initiate unsubscription flow
    unsubscribeOnShutdown: true
}
service /subscriber on new websub:Listener(9090, listenerConfigs)  {
    isolated remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement {
        // implement logic here
        return websub:ACKNOWLEDGEMENT;
    }

    // other remote methods are optional to be implemented
}

Returning Errors from Remote Methods

• Remote functions in websub:SubscriberService can return error type.

@websub:SubscriberServiceConfig {
    target: "RESOURCE_URL", 
    leaseSeconds: 36000,
    callback: "<NGROK_PUBLIC_URL>",
    appendServicePath: true
} 
service on new websub:Listener(9090) {
    remote function onEventNotification(websub:ContentDistributionMessage event) 
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError|error? {
        boolean isValidRequest = check validateRequest(event);
        if isValidRequest {
            // implement on event notification logic here
            return websub:ACKNOWLEDGEMENT;
        }
    
    }

    // other remote methods are optional to be implemented
}

function validateRequest(websub:ContentDistributionMessage event) returns boolean|error {
    // validation logic 
}

• For each remote method error return has a different meaning. Following table depicts the meaning inferred from error returned from all available remote methods.