Task as a UCAN

[andrewzhurov]

As hard as I try to squint my eyes I struggle to see the difference between the two.
Here's how it appears to me.
Task is Instruction + Meta + authorization of Executor to perform the Instruction.
UCAN's Capability is akin to Instruction, Facts is akin to Meta, aud is akin to authorization.
Perhaps Task could be expressed as a UCAN?
So the invocation spec can reuse all great effort UCAN spec's done towards authorization and could focus solely on solving the problem of invocation.

E.g., expressing Task (in v1 spec #21 ) as a UCAN

{
  "bafy...updateDns": {
    "rsc": "dns:example.com?TYPE=TXT",
    "op": "crud/update",
    "input": {
      "value": "hello world"
    },
    "nnc": "some nonce"
  },
  "bafy...updateDnsTask": {
    "run": {"/": "bafy...updateDns"},
    "meta": {"retries": 5},
    "cause": {"/": "bafy...somePriorInstructionTaskInvocationReceipt"},
    "prf": [{"/": "bafy...UCAN that authorized Executor"}]
  },
  "bafy...updateDnsTaskInvocation": {
    "task": {"/": "bafy...updateDnsTask"},
    "auth": {"/": "bafy...auth"}
  },
  "bafy...auth": {
    "scope": [{"/": "bafy...updateDnsTask"}],
    "s": {"/": { "bytes": "7aEDQIscUKVuAIB2Yj6jdX5ru9OcnQLxLutvHPjeMD3pbtHIoErFpo7OoC79Oe2ShgQMLbo2e6dvHh9scqHKEOmieA0"}}
  },
}

{
  "bafy...updateDns": {
    "dns:example.com?TYPE=TXT": {
      "crud/update": {
        "input": {
          "value": "hello world"
        }
      }
    }
  },
  "bafy...updateDnsTask": {
    "ucv": "0.2.0",
    "iss": "Executor's DID",
    "aud": "Executor Delegatee's DID",
    "nnc": "some nonce",
    "fct": {"retries": 5,
            "cause": {"/": "bafy...somePriorInstructionTaskInvocationReceipt"}},
    "cap": {"/": "bafy...updateDns"},
    "prf": [{"/": "bafy...UCAN that authorized Executor"}]
  },
  "bafy...updateDnsTaskInvocation": {
    "task": {"/": "bafy...updateDnsTask"},
    "auth": {"/": "bafy...auth"}
  },
  "bafy...auth": {
    "scope": [{"/": "bafy...updateDnsTask"}],
    "s": {"/": { "bytes": "7aEDQIscUKVuAIB2Yj6jdX5ru9OcnQLxLutvHPjeMD3pbtHIoErFpo7OoC79Oe2ShgQMLbo2e6dvHh9scqHKEOmieA0"}}
  },
}

There is one misalignment however, in that Caveats are meant to capture both restrictions on what can be called (akin to Inputs of an Instruction) and how it can be called (e.g., gas, retries, etc.), i.e., restrictions on how it can be invoked.
I.e., Caveats are used to express restrictions of what a function can be as well as restrictions of how it is to be invoked.
Whereas in Invocation spec it corresponds to Instruction's Input and Task's Meta.
I think Invocation spec got it right, it's a useful distinction. For example, it allows to reuse functions/Instructions in Await.
But this mix of the two in UCAN spec is not a problem, since UCAN provides Facts field which could be used for Meta.
We just need to agree to store Meta there.

I may be wrong, goes without saying 😄, happy to hear your thoughts!

[andrewzhurov]

Usings UCANs as Tasks allows for an interesting feature - subsequent UCANs can be issued to provide more specificity / restrictions.
I.e., becomes possible to Task -> Task -> Task

Which is an interesting property for the problem of "how to specify that a particular Receipt must be used for an await", mentioned here.
With this approach, subsequent UCANs could be used to gradually introduce "awaited" values.
So they become akin to closures, that provide missing values.
And when no missings values are left - an Invocation can be issued.
It's neat that nothing stops Executor to issue such subsequent UCANs.

E.g.

{
  "bafy...updateDns": {
    "dns:example.com?TYPE=TXT": {
      "crud/update": {
        "input": {
          "value": {"ok/await": "bafy...someInstruction"}
        }
      }
    }
  },
  "bafy...updateDnsTask": {
    "ucv": "0.2.0",
    "iss": "Invoker's DID",
    "aud": "Executor's DID",
    "nnc": "some nonce",
    "fct": {"retries": 5,
            "cause": {"/": "bafy...somePriorInstructionTaskInvocationReceipt"}},
    "cap": {"/": "bafy...updateDns"},
    "prf": [{"/": "bafyreieynwqrabzdhgl652ftsk4mlphcj3bxchkj2aw5eb6dc2wxieilau"}]
  },
  "bafy...updateDnsTask2": {
    "ucv": "0.2.0",
    "iss": "Executor's DID", // issued by the Executor
    "aud": "Executor's DID",
    "nnc": "some nonce",
    "fct": {"retries": 5,
            "cause": {"/": "bafy...somePriorInstructionTaskInvocationReceipt"},
            "awaited": [{"/": "bafy...someInstructionReceipt"}]}, // awaited Receipt
    "cap": {"/": "bafy...updateDns"},
    "prf": [{"/": "bafy...updateDnsTask"}] // previous UCAN
  }
}