Contract keys

Contract keys are an optional addition to templates. They let you specify a way of uniquely identifying contracts, 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 functions 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 that 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 execution 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 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 Participant 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.

Limiting key usage to stakeholders also means that keys cannot be used to access a divulged contract, i.e. there can be cases where fetch succeeds and fetchByKey does not. See the example at the end of this section for details.

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.

visibleByKey

boolean <- visibleByKey @ContractType contractKey

Use visibleByKey to check whether you can see an active contract for the given key with the current authorizations. If the contract exists and you have permission to see it, returns True, otherwise returns False.

To clarify, ignoring contention:

1. visibleByKey will return True if all of these are true: there exists a contract for the given key, the submitter is a stakeholder on that contract, and at the point of call we have the authorization of all of the maintainers of the key.
2. visibleByKey will return False if all of those are true: there is no contract for the given key, and at the point of call we have authorization from all the maintainers of the key.
3. visibleByKey will abort the transaction at interpretation time if, at the point of call, we are missing the authorization from any one maintainer of the key.
4. visibleByKey will fail at validation time (after returning False at interpretation time) if all of these are true: at the point of call, we have the authorization of all the maintainers, and a valid contract exists for the given key, but the submitter is not a stakeholder on that contract.

While it may at first seem too restrictive to require all maintainers to authorize the call, this is actually required in order to validate negative lookups. In the positive case, when you can see the contract, it’s easy for the transaction to mention which contract it found, and therefore for validators to check that this contract does indeed exist, and is active as of the time of executing the transaction.

For the negative case, however, the transaction submitted for execution cannot say which contract it has not found (as, by definition, it has not found it, and it may not even exist). Still, validators have to be able to reproduce the result of not finding the contract, and therefore they need to be able to look for it, which means having the authorization to ask the maintainers about it.

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 is conceptually equivalent to

lookupByKey : forall c k. (HasFetchByKey c k) => k -> Update (Optional (ContractId c))
lookupByKey k = do
  visible <- visibleByKey @c k
  if visible then do
    (contractId, _ignoredContract) <- fetchByKey @c k
    return $ Some contractId
  else
    return None

Therefore, lookupByKey needs all the same authorizations as visibleByKey, for the same reasons, and fails in the same cases.

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 do 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) 2021 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 UnkeyedFetch
      : Keyed
      with
        cid : ContractId Keyed
        delegee : Party
      controller delegee
      do
        fetch cid

    nonconsuming choice VisibleKeyed
      : Bool
      with
        key : Party
        delegee : Party
      controller delegee
      do
        visibleByKey @Keyed key

    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 see
  submit sig do
    b <- visibleByKey @Keyed sig
    assert b
  -- 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 see without authorization
  submitMustFail obs do visibleByKey @Keyed sig
  -- Stakeholder can see with authorization
  submit obs do
    b <- exercise sigDelegationCid VisibleKeyed with
      delegee = obs
      key = sig
    assert b
  -- 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_ fetch the contract directly
  submit divulgee do
    exercise obsDelegationCid UnkeyedFetch with
        delegee = divulgee
        cid = keyedCid
  -- Divulgee can't fetch through the key
  submitMustFail divulgee do fetchByKey @Keyed sig
  -- Divulgee can't see
  submitMustFail divulgee do visibleByKey @Keyed sig
  -- Divulgee can't see with stakeholder authority
  submitMustFail divulgee do
    exercise obsDelegationCid VisibleKeyed with
        delegee = divulgee
        key = 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.
  submitMustFail divulgee do
    b <- exercise sigDelegationCid VisibleKeyed with
      delegee = divulgee
      key = sig
    assert $ not b
  -- 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 see
  submitMustFail blind do visibleByKey @Keyed sig
  -- Blind party can't see with stakeholder authority
  submitMustFail blind do
    exercise obsDelegationCid VisibleKeyed with
      delegee = blind
      key = sig
  -- Blind party can't see with maintainer authority
  submitMustFail blind do
    b <- exercise sigDelegationCid VisibleKeyed with
      delegee = blind
      key = sig
    assert $ not b
  -- 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 party can see nonexistence of a contract
  submit blind do
    b <- exercise obsDelegationCid VisibleKeyed with
      delegee = blind
      key = obs
    assert $ not b
  -- 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 ()