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
Int, rather than a list or more complex type.
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
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.
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.
(fetchedContractId, fetchedContract) <- fetchByKey @ContractType contractKey
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).
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
fetchByKeyviolates the authorization rule, meaning no stakeholder authorized the
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.
optionalContractId <- lookupByKey @ContractType contractKey
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
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.
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
exerciseByKey @ContractType contractKey
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
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 ()