Creating the LinkFactory Class

To help us construct links we'll use the LinkFactory class. This class is responsible for creating Link objects based on two common scenarios:

1. createFromSeed - creates the first link in the chain using a seed since we won't have a prior link.
2. createFromSettled - creates a new "tip of the chain" link when someone closes / settles a Link using the last settled link.

This class takes care of the heavy lifting for creating a Link so that we can easily test our code, and the consumers of this code aren't burdened by the implementation details of creating a Link.

As we previously talked about, we'll be using digital signatures. This class has a dependency on the IMessageSigner interface. This interface provides two methods:

1. one for signing a message using your Lightning Network node
2. one for verifying a received signature

export interface IMessageSigner {
  /**
   * Signs a message using the Lightning Network node
   */
  sign(msg: string): Promise<string>;

  /**
   * Verifies a message using the Lightning Network node
   */
  verify(msg: Buffer, signature: string): Promise<VerifySignatureResult>;
}

Under the covers, we have already implemented a LndMessageSigner class that uses LND to perform signature creation and verification. This will be wired up later but feel free to explore this code in the server/src/data/lnd folder.

Exercise: Implement createFromSeed

As we previously discussed, a Link starts out in the unsettled state, which means that no one has taken ownership of it. Logically, the application starts off without any ownership and in an unsettled state. Since we don't have any prior links, we'll simply create a link from some seed value.

In order to create a link we do two things:

1. Sign the seed value using our Lightning Network node using the IMessageSigner instance.
2. Construct a new Link and supply the seed as the linkId, the signature our application server made for the seed, and the starting satoshis value required for the first owner.

Go ahead and implement the createFromSeed method.

Tip: The sign method is a asynchronous so be sure to use it with await, for example: const sig = await this.signer.sign(some_msg)

public async createFromSeed(seed: string, startSats: number): Promise<Link> {
    // Exercise
}

When you are finished you can verify you successfully implemented the method with the following command:

npm run test:server -- --grep createFromSeed

Exercise: Implement createFromSettled

Now that we know how to create a link to start the application. A person could become the leader by paying the invoice. Once that invoice is paid, the first link will become settled. We need a method to create a new link so that the next person can try to become the leader.

We will create the createFromSettled method which will create the next unsettled link from a link that has been settled.

Instead of a seed, we'll use the nextLinkId property from the Link, which we implemented in the previous section, as the link's identifier.

The createFromSettled method will need to do three things:

1. Use the IMessageSigner.sign method to sign the nextLinkId value using our Lightning Network node
2. Increment the minimum satoshis to +1 more than the settled invoice
3. Construct the new unsettled Link

Go ahead and implement the createFromSettled method.

Dev Tip: You will need to look at the settling invoice satoshi value to determine the next increment. This value is a string, so be sure to cast it to a number with Number(some_string).

public async createFromSettled(settled: Link): Promise<Link> {
    // Exercise
}

When you are finished you can verify you successfully implemented the method with the following command:

npm run test:server -- --grep createFromSettled