Asset Tracking

Let's use ChainScript to model a very common process in supply chain: tracking asset ownership.

The name of our process will be asset-tracker. All our links will thus have link.meta.process.name = asset-tracker.

We will create one instance of that process per asset. The ID of each instance (link.meta.mapId) will be the ID of the asset that we're tracking.

Our link.data structure will be very simple. It will be a canonical JSON object containing the name of the current asset owner:

{
  "owner": "alice"
}

Our process has only two steps:

• init creates the asset and starts the ownership trail.
• transfer transfers the asset to someone.

An asset can be transfered to only one person: we need to prevent branching. We will use the link.meta.outDegree to model that, by setting it to 1. This will ensure that a link can have only one child.

We will leverage digital signatures to ensure that the ownership changes are correct: a transfer steps needs to include a valid signature from the current owner.

Creating Assets

Let's create two assets belonging to Alice: asset-1 and asset-2. Their init links will look like:

{
  "data": "ewogICJvd25lciI6ICJhbGljZSIKfQ==",
  "meta": {
    "action": "init",
    "clientId": "github.com/stratumn/js-chainscript",
    "mapId": "asset-1",
    "outDegree": 1,
    "process": { "name": "asset-tracker", "state": "asset-created" },
    "step": "init",
    "tags": ["alice"]
  },
  "signatures": [
    {
      "payloadPath": "[version,data,meta]",
      "publicKey": "...",
      "signature": "...",
      "version": "1.0.0"
    }
  ],
  "version": "1.0.0"
}

{
  "data": "ewogICJvd25lciI6ICJhbGljZSIKfQ==",
  "meta": {
    "action": "init",
    "clientId": "github.com/stratumn/js-chainscript",
    "mapId": "asset-2",
    "outDegree": 1,
    "process": { "name": "asset-tracker", "state": "asset-created" },
    "step": "init",
    "tags": ["alice"]
  },
  "signatures": [
    {
      "payloadPath": "[version,data,meta]",
      "publicKey": "...",
      "signature": "...",
      "version": "1.0.0"
    }
  ],
  "version": "1.0.0"
}

In those links, data is the base64 encoding of the following json:

{
  "owner": "alice"
}

Transferring Assets

Now that our assets are created, Alice can transfer them to someone else. Malicious actors could try to transfer them too, but since we'll verify that the transfer link contains a signature from the current owner, cheaters will be detected and their links will be rejected automatically.

A transfer to Bob of asset-1 would look like:

{
  "data": "ewogICJvd25lciI6ICJib2IiCn0=",
  "meta": {
    "action": "transfer",
    "clientId": "github.com/stratumn/js-chainscript",
    "mapId": "asset-1",
    "outDegree": 1,
    "process": { "name": "asset-tracker", "state": "asset-transferred" },
    "step": "transfer",
    "tags": ["alice", "bob"]
  },
  "signatures": [
    {
      "payloadPath": "[version,data,meta]",
      "publicKey": "...",
      "signature": "...",
      "version": "1.0.0"
    }
  ],
  "version": "1.0.0"
}

This link's data is the base64 encoding of the following json:

{
  "owner": "bob"
}

Now Bob can keep transferring asset-1 and everyone can see a clear ownership trail for the asset by filtering by link.meta.mapId == 'asset-1'.

Sample Code

If you'd like to see some code, have a look at our Sample Apps repository. It contains all the features mentioned in this example and can be run easily from your machine.