How to implement the Inbox Pattern on the Notification service side to guarantee message processing?

The Inbox Pattern is used to guarantee message processing in distributed systems. In the Notification service, it can be implemented as follows:

1. When a message is received, the service saves it in a special "Inbox" table with a unique identifier and a "new" status.
2. Then, the service processes the message (for example, sends a notification).
3. After successful processing, it updates the record's status in the table to "processed".

This prevents reprocessing of the same message in case of failures or retries, as the presence of a record in the Inbox and its status are checked before processing.

Example of the Inbox table:

message_id	payload	status	received_at
12345	{...}	new	2024-06-01 10:00:00

Example pseudo-JavaScript code:

async function processMessage(message) {
  const exists = await inboxTable.find({ message_id: message.id });
  if (exists && exists.status === 'processed') {
    return; // Already processed
  }
  if (!exists) {
    await inboxTable.insert({ message_id: message.id, payload: message.payload, status: 'new', received_at: new Date() });
  }

  // Process the message
  await sendNotification(message.payload);

  // Update the status
  await inboxTable.update({ message_id: message.id }, { status: 'processed' });
}