Contract keys

Contract keys are an optional addition to templates. They let you specify a way of uniquely identifying contract instances, using the parameters to the template - similar to a primary key for a database.

You can use contract keys to stably refer to a contract, even through iterations of instances of it.

Here’s an example of setting up a contract key for a bank account, to act as a bank account ID:

type AccountKey = (Party, Text)

template Account with
    bank : Party
    number : Text
    owner : Party
    balance : Decimal
    observers : [Party]
  where
    signatory [bank, owner]
    observer observers

    key (bank, number) : AccountKey
    maintainer key._1

What can be a contract key

The key can be an arbitrary serializable expression that does not contain contract IDs. However, it must include every party that you want to use as a maintainer (see Specifying maintainers below).

It’s best to use simple types for your keys like Text or Int, rather than a list or more complex type.

Specifying maintainers

If you specify a contract key for a template, you must also specify a maintainer or maintainers, in a similar way to specifying signatories or observers. The maintainers “own” the key in the same way the signatories “own” a contract. Just like signatories of contracts prevent double spends or use of false contract data, maintainers of keys prevent double allocation or incorrect lookups. Since the key is part of the contract, the maintainers must be signatories of the contract. However, maintainers are computed from the key instead of the template arguments. In the example above, the bank is ultimately the maintainer of the key.

Uniqueness of keys is guaranteed per template. Since multiple templates may use the same key type, some key-related function must be annotated using the @ContractType as shown in the examples below.

When you are writing DAML models, the maintainers matter since they affect authorization – much like signatories and observers. You don’t need to do anything to “maintain” the keys. In the above example, it is guaranteed, there there can only be one Account with a given number at a given bank.

Checking of the keys is done automatically at execution time, by the DAML exeuction engine: if someone tries to create a new contract that duplicates an existing contract key, the execution engine will cause that creation to fail.

Contract Lookups

The primary purpose of contract keys is to provide a stable, and possibly meaningful, identifier for contracts that can be used in DAML to fetch contracts. There are two functions to perform such lookups: fetchByKey and lookupByKey. Both types of lookup are performed at interpretation time on the submitting Partipant Node, on a best-effort basis. Currently, that best-effort means lookups only return contracts if the submitting Party is a stakeholder of that contract.

In particular, the above means that if multiple commands are submitted simultaneously, all using contract lookups to find and consume a given contract, there will be contention between these commands, and at most one will succeed.

fetchByKey

(fetchedContractId, fetchedContract) <- fetchByKey @ContractType contractKey

Use fetchByKey to fetch the ID and data of the contract with the specified key. It is an alternative to fetch and behaves the same in most ways.

It returns a tuple of the ID and the contract object (containing all its data).

Like fetch, fetchByKey needs to be authorized by at least one stakeholder.

fetchByKey fails and aborts the transaction if:

• The submitting Party is not a stakeholder on a contract with the given key, or
• A contract was found, but the fetchByKey violates the authorization rule, meaning no stakeholder authorized the fetch..

This means that if it fails, it doesn’t guarantee that a contract with that key doesn’t exist, just that the submitting Party doesn’t know about it, or there are issues with authorization.

lookupByKey

optionalContractId <- lookupByKey @ContractType contractKey

Use lookupByKey to check whether a contract with the specified key exists. If it does exist, lookupByKey returns the Some contractId, where contractId is the ID of the contract; otherwise, it returns None.

lookupByKey needs to be authorized by all maintainers of the contract you are trying to lookup. The reason for that is denial of service protection. Without this restriction, a malicious Party could legitimately request another Party to validate any number of negative contract key lookups without that other Party ever having signed anything.

Unlike fetchByKey, the transaction does not fail if no contract with the given key is found. Instead, lookupByKey just returns None. However, that does not guarantee that no such contract exists. The submitting Party may simply not know about it, in which case the transaction will be rejected during validation.

lookupByKey fails and aborts the transaction if:

• Authorization from at least one maintainer is missing. This check fails at interpretation time.
• The lookup is incorrect. This can happen either due to contention, or because the submitter didn’t know of the contract. This check fails at validation time.

To get the data from the contract once you’ve confirmed it exists, you’ll still need to use fetch.

exerciseByKey

exerciseByKey @ContractType contractKey

Use exerciseByKey to exercise a choice on a contract identified by its key (compared to exercise, which lets you exercise a contract identified by its ContractId). To run exerciseByKey you need authorization from the controllers of the choice and at least one stakeholder. This is equivalent to the authorization needed to fo a fetchByKey followed by an exercise.

Example

A complete example of possible success and failure scenarios of fetchByKey and lookupByKey is shown below.

-- Copyright (c) 2020 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
-- SPDX-License-Identifier: Apache-2.0

module Keys where

import DA.Optional

template Keyed
  with
    sig : Party
    obs : Party
  where
    signatory sig
    observer obs

    key sig : Party
    maintainer key

template Divulger
  with
    divulgee : Party
    sig : Party
  where
    signatory divulgee

    controller sig can
      nonconsuming DivulgeKeyed
        : Keyed
        with
          keyedCid : ContractId Keyed
        do
          fetch keyedCid

template Delegation
  with
    sig : Party
    delegees : [Party]
  where
    signatory sig
    observer delegees

    nonconsuming choice CreateKeyed
      : ContractId Keyed
      with
        delegee : Party
        obs : Party
      controller delegee
      do
        create Keyed with sig; obs

    nonconsuming choice ArchiveKeyed
      : ()
      with
        delegee : Party
        keyedCid : ContractId Keyed
      controller delegee
      do
        archive keyedCid

    nonconsuming choice LookupKeyed
      : Optional (ContractId Keyed)
      with
        lookupKey : Party
        delegee : Party
      controller delegee
      do
        lookupByKey @Keyed lookupKey

    nonconsuming choice FetchKeyed
      : (ContractId Keyed, Keyed)
      with
        lookupKey : Party
        delegee : Party
      controller delegee
      do
        fetchByKey @Keyed lookupKey

lookupTest = scenario do

  -- Put four parties in the four possible relationships with a `Keyed`
  sig <- getParty "s" -- Signatory
  obs <- getParty "o" -- Observer
  divulgee <- getParty "d" -- Divulgee
  blind <- getParty "b" -- Blind

  keyedCid <- submit sig do create Keyed with ..
  divulgercid <- submit divulgee do create Divulger with ..
  submit sig do exercise divulgercid DivulgeKeyed with ..

  -- Now the signatory and observer delegate their choices
  sigDelegationCid <- submit sig do 
    create Delegation with 
      sig
      delegees = [obs, divulgee, blind]
  obsDelegationCid <- submit obs do 
    create Delegation with
      sig = obs
      delegees = [divulgee, blind]

  -- TESTING LOOKUPS AND FETCHES

  -- Maintainer can fetch
  submit sig do
    (cid, keyed) <- fetchByKey @Keyed sig
    assert (keyedCid == cid)
  -- Maintainer can lookup
  submit sig do
    mcid <- lookupByKey @Keyed sig
    assert (mcid == Some keyedCid)

  -- Stakeholder can fetch
  submit obs do
    (cid, l) <- fetchByKey @Keyed sig
    assert (keyedCid == cid)
  -- Stakeholder can't lookup without authorization
  submitMustFail obs do lookupByKey @Keyed sig
  -- Stakeholder can lookup with authorization
  submit obs do 
    mcid <- exercise sigDelegationCid LookupKeyed with 
      delegee = obs
      lookupKey = sig
    assert (mcid == Some keyedCid)

  -- Divulgee can't fetch
  submitMustFail divulgee do fetchByKey @Keyed sig
  -- Divulgee can't lookup
  submitMustFail divulgee do lookupByKey @Keyed sig
  -- Divulgee can't lookup with stakeholder authority
  submitMustFail divulgee do 
    exercise obsDelegationCid LookupKeyed with
        delegee = divulgee
        lookupKey = sig
  -- Divulgee can't do positive lookup with maintainer authority. 
  -- Note that the lookup returns `None` so the assertion passes.
  -- If the assertion is changed to `isSome`, the assertion fails,
  -- which means the error message changes. The reason is that the
  -- assertion is checked at interpretation time, before the lookup
  -- is checked at validation time.
  submitMustFail divulgee do 
    mcid <- exercise sigDelegationCid LookupKeyed with
      delegee = divulgee
      lookupKey = sig
    assert (isNone mcid)
  -- Divulgee can't fetch with stakeholder authority
  submitMustFail divulgee do 
    (cid, keyed) <- exercise obsDelegationCid FetchKeyed with
      delegee = divulgee
      lookupKey = sig
    assert (keyedCid == cid)

  -- Blind party can't fetch
  submitMustFail blind do fetchByKey @Keyed sig
  -- Blind party can't lookup
  submitMustFail blind do lookupByKey @Keyed sig
  -- Blind party can't lookup with stakeholder authority
  submitMustFail blind do
    exercise obsDelegationCid LookupKeyed with
      delegee = blind
      lookupKey = sig
  -- Blind party can't lookup with maintainer authority.
  -- The lookup initially returns `None`, but is rejected at
  -- validation time
  submitMustFail blind do
    mcid <- exercise sigDelegationCid LookupKeyed with
      delegee = blind
      lookupKey = sig
    assert (isNone mcid)
  -- Blind party can't fetch with stakeholder authority as lookup is negative
  submitMustFail blind do 
    exercise obsDelegationCid FetchKeyed with
      delegee = blind
      lookupKey = sig
  -- Blind can do a negative lookup on a truly nonexistant contract
  submit blind do
    mcid <- exercise obsDelegationCid LookupKeyed with
      delegee = blind
      lookupKey = obs
    assert (isNone mcid)
  -- TESTING CREATES AND ARCHIVES

  -- Divulgee can archive
  submit divulgee do
    exercise sigDelegationCid ArchiveKeyed with 
      delegee = divulgee
      keyedCid
  -- Divulgee can create
  keyedCid2 <- submit divulgee do 
    exercise sigDelegationCid CreateKeyed with 
      delegee = divulgee
      obs

  -- Stakeholder can archive
  submit obs do 
    exercise sigDelegationCid ArchiveKeyed with 
      delegee = obs
      keyedCid = keyedCid2
  -- Stakeholder can create
  keyedCid3 <- submit obs do
    exercise sigDelegationCid CreateKeyed with
      delegee = obs
      obs

  return ()