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Subscription Management Service

This document provides an overview of the internal workings of the Subscription Management Service, including details about the web interfaces it provides for managing subscriptions.

Overview​

The Subscription Management Service handles the lifecycle of client subscriptions, providing key functionality such as:

  • Adding new subscriptions
  • Removing existing subscriptions
  • Updating subscription metadata (e.g., last seen timestamp)
  • Fetching messages with a timeout
  • Automatically managing TTL (time-to-live) to clean up expired subscriptions

Components​

The service consists of the following key components:

  1. Subscription: Represents the subscription object containing an ID and other metadata.
  2. SubscriptionStore: Interface that abstracts storage operations for subscriptions.
  3. LastSeenTracker: Manages the last-seen timestamps for each subscription.
  4. TTLManager: Manages the automatic removal of subscriptions that have expired.
  5. Message Fetching: Allows fetching new messages with a timeout.

Sequence Diagram​

Below is a sequence diagram illustrating the interaction between different components during the Add Subscription and Get Messages processes.

Internals​

The service works as a long-running process that continuously manages subscriptions based on the following flows:

1. Add Subscription​

When a client requests to add a subscription, the service performs the following steps:

  • Checks if the subscription already exists in the SubscriptionStore.
  • If the subscription does not exist, it is added to the store.
  • The LastSeenTracker is updated to track the last-seen timestamp for the new subscription.

2. Remove Subscription​

When a client requests to remove a subscription:

  • The service retrieves the subscription by ID from the SubscriptionStore.
  • If found, the subscription is removed from the store.

3. Update Subscription​

Updating a subscription typically involves refreshing the last-seen timestamp. The service:

  • Retrieves the subscription by ID from the SubscriptionStore.
  • Updates the LastSeenTracker to the current time.

4. List Subscriptions​

The service can list all subscriptions currently stored in the SubscriptionStore. This is useful for debugging and monitoring purposes.

5. Get Messages​

When a client requests new messages for a subscription, the service:

  • Fetches the subscription by ID from the SubscriptionStore.
  • Checks for new messages related to the subscription.
  • If no messages are found, it waits for a specified timeout period before returning.

6. TTL Monitoring​

The service includes a background task that continuously checks for subscriptions that have exceeded their TTL. The TTLManager handles this by:

  • Iterating through all subscriptions.
  • Checking if their last-seen timestamp exceeds the defined TTL.
  • Removing expired subscriptions from the store.

Web Interfaces​

The following web interfaces are exposed by the Subscription Management Service:

1. Add Subscription​

Endpoint: POST /api/workflow/subscription
Action: add

Request Payload:

{
  "action": "add",
  "id": "subscription_id"
}

Response:

{
  "status": "added"
}

2. Remove Subscription​

Endpoint: POST /api/workflow/subscription
Action: remove

Request Payload:

{
  "action": "remove",
  "id": "subscription_id"
}

Response:

{
  "status": "removed"
}

3. Update Subscription​

Endpoint: POST /api/workflow/subscription
Action: update

Request Payload:

{
  "action": "update",
  "id": "subscription_id"
}

Response:

{
  "status": "updated"
}

4. List Subscriptions​

Endpoint: POST /api/workflow/subscription
Action: list

Request Payload:

{
  "action": "list"
}

Response:

[
  {
    "id": "subscription_id_1"
  },
  {
    "id": "subscription_id_2"
  }
]

5. Get Messages​

Endpoint: POST /api/workflow/subscription
Action: getMessages

Request Payload:

{
  "action": "getMessages",
  "id": "subscription_id",
  "timeout": 5
}

Response:

{
  "status": "success",
  "messages": ["message1", "message2"]
}

Timeout Response:

{
  "status": "timeout",
  "messages": []
}

Conclusion​

The Subscription Management Service is designed to be a robust and extensible system for managing client subscriptions. It includes built-in mechanisms for handling TTL expiration, ensuring that stale subscriptions are removed automatically. The web interfaces provide easy integration for clients to manage their subscriptions and retrieve messages in real-time.