Glossary of concepts

DAML

DAML is a programming language for writing smart contracts, that you can use to build an application based on a ledger. You can run DAML contracts on many different ledgers.

Contract, contract instance

A contract is an item on a ledger. They are created from blueprints called templates, and include:

Contracts are immutable: once they are created on the ledger, the information in the contract cannot be changed. The only thing that can happen to it is that the contract can be archived.

They’re sometimes referred to as a contract instance to make clear that this is an instantiated contract, as opposed to a template.

Active contract, archived contract

When a contract is created on a ledger, it becomes active. But that doesn’t mean it will stay active forever: it can be archived. This can happen:

Once the contract is archived, it is no longer valid, and choices on the contract can no longer be exercised.

Template

A template is a blueprint for creating a contract. This is the DAML code you write.

For full documentation on what can be in a template, see Reference: templates.

Choice

A choice is something that a party can exercise on a contract. You write code in the choice body that specifies what happens when the choice is exercised: for example, it could create a new contract.

Choices give you a way to transform the data in a contract: while the contract itself is immutable, you can write a choice that archives the contract and creates a new version of it with updated data.

A choice can only be exercised by its controller. Within the choice body, you have the authorization of all of the contract’s signatories.

For full documentation on choices, see Reference: choices.

Consuming choice

A consuming choice means that, when the choices is exercised, the contract it is on will be archived. The alternative is a nonconsuming choice.

Consuming choices can be preconsuming or postconsuming.

Preconsuming choice

A choice marked preconsuming will be archived at the start of that exercise.

Postconsuming choice

A choice marked postconsuming will not be archived until the end of the exercise choice body.

Nonconsuming choice

A nonconsuming choice does NOT archive the contract it is on when exercised. This means the choice can be exercised more than once on the same contract instance.

Disjunction choice, flexible controllers

A disjunction choice has more than one controller.

If a contract uses flexible controllers, this means you don’t specify the controller of the choice at creation time of the contract, but at exercise time.

Party

A party represents a person or legal entity. Parties can create contracts and exercise choices.

Signatories, observers, controllers, and maintainers all must be parties, represented by the Party data type in DAML.

Signatory

A signatory is a party on a contract instance. The signatories MUST consent to the creation of the contract by authorizing it: if they don’t, contract creation will fail.

For documentation on signatories, see Reference: templates.

Observer

An observer is a party on a contract instance. Being an observer allows them to see that instance and all the information about it. They do NOT have to consent to the creation.

For documentation on observers, see Reference: templates.

Controller

A controller is a party that is able to exercise a particular choice on a particular contract instance.

Controllers must be at least an observer, otherwise they can’t see the contract to exercise it on. But they don’t have to be a signatory. this enables the propose-accept pattern.

Stakeholder

Stakeholder is not a term used within the DAML language, but the concept refers to the signatories and observers collectively. That is, it means all of the parties that are interested in a contract instance.

Maintainer

The maintainer is a party that is part of a contract key. They must always be a signatory on the contract that they maintain the key for.

It’s not possible for keys to be globally unique, because there is no party that will necessarily know about every contract. However, by including a party as part of the key, this ensures that the maintainer will know about all of the contracts, and so can guarantee the uniqueness of the keys that they know about.

For documentation on contract keys, see Contract keys.

Authorization, signing

The DAML runtime checks that every submitted transaction is well-authorized, according to the authorization rules of the ledger model, which guarantee the integrity of the underlying ledger.

A DAML update is the composition of update actions created with one of the items in the table below. A DAML update is well-authorized when all its contained update actions are well-authorized. Each operation has an associated set of parties that need to authorize it:

Updates and required authorization
Update action Type Authorization
create (Template c) => c -> Update (ContractId c) All signatories of the created contract instance
exercise ContractId c -> e -> Update r All controllers of the choice
fetch ContractId c -> e -> Update r One of the union of signatories and observers of the fetched contract instance
fetchByKey k -> Update (ContractId c, c) Same as fetch
lookupByKey k -> Update (Optional (ContractId c)) All key maintainers

At runtime, the DAML execution engine computes the required authorizing parties from this mapping. It also computes which parties have given authorization to the update in question. A party is giving authorization to an update in one of two ways:

  • It is the signatory of the contract that contains the update action.
  • It is element of the controllers executing the choice containing the update action.

Only if all required parties have given their authorization to an update action, the update action is well-authorized and therefore executed. A missing authorization leads to the abortion of the update action and the failure of the containing transaction.

It is noteworthy, that authorizing parties are always determined only from the local context of a choice in question, that is, its controllers and the contract’s signatories. Authorization is never inherited from earlier execution contexts.

Standard library

The DAML standard library is a set of DAML functions, classes and more that make developing with DAML easier.

For documentation, see The standard library.

Agreement

An agreement is part of a contract. It is text that explains what the contract represents.

It can be used to clarify the legal intent of a contract, but this text isn’t evaulated programmatically.

See Reference: templates.

Create

A create is an update that creates a contract instance on it the ledger.

Contract creation requires authorization from all its signatories, or the create will fail. For how to get authorization, see the propose-accept and multi-party agreement patterns.

A party submits a create command.

See Reference: updates.

Exercise

An exercise is an action that exercises a choice on a contract instance on the ledger. If the choice is consuming, the exercise will archive the contract instance; if it is nonconsuming, the contract instance will stay active.

Exercising a choice requires authorization from all of the controllers of the choice.

A party submits an exercise command.

See Reference: updates.

Scenario

A scenario is a way of testing DAML code during development. You can run scenarios inside DAML Studio, or write them to be executed on Sandbox when it starts up.

They’re useful for:

  • expressing clearly the intended workflow of your contracts
  • for making sure that parties can create contracts, observe contracts, and exercise choices (and that they CANNOT create contracts, observe contracts, or exercise choices that they should not be able to)
  • acting as DAML unit tests to confirm that everything keeps working correctly

Scenarios emulate a real ledger. You specify a linear sequence of actions that various parties take, and these are evaluated in order, according to the same consistency, authorization, and privacy rules as they would be on a DAML ledger. DAML Studio shows you the resulting transaction graph, and (if a scenario fails) what caused it to fail.

See Testing using scenarios.

Contract key

A contract key allows you to uniquely identify a contract instance of a particular template, similarly to a primary key in a database table.

A contract key requires a maintainer: a simple key would be something like a tuple of text and maintainer, like (accountId, bank).

See Contract keys.

DAR file, DALF file

A .dar file is the result of compiling DAML using the Assistant. Its underlying format is DAML-LF.

You upload .dar files to a ledger in order to be able to create contracts from the templates in that file.

A .dar contains multiple .dalf files. A .dalf file is the output of a compiled DAML package or library.

SDK tools

Assistant

DAML Assistant is a command-line tool for many tasks related to DAML. Using it, you can create DAML projects, compile DAML projects into .dar files, launch other SDK tools, and download new SDK versions.

See DAML Assistant (daml).

Studio

DAML Studio is a plugin for Visual Studio Code, and is the IDE for writing DAML code.

See DAML Studio.

Sandbox

Sandbox is a lightweight ledger implementation. In its normal mode, you can use it for testing.

You can also run the Sandbox connected to a PostgreSQL back end, which gives you persistence and a more production-like experience.

See DAML Sandbox.

Extractor

Extractor is a tool for extracting contract data for a single party into a PostgreSQL database.

See Extractor.

Building applications

Application, ledger client, integration

Application, ledger client and integration are all terms for an application that sits on top of the ledger. These usually read from the ledger, send commands to the ledger, or both.

There’s a lot of information available about application development, starting with the Writing applications using the Ledger API page.

Ledger API

The Ledger API is an API that’s exposed by any DAML ledger. It includes the following services.

Command submission service

Use the command submission service to submit commands - either create commands or exercise commands - to the ledger. See Command submission service.

Command completion service

Use the command completion service to find out whether or not commands you have submitted have completed, and what their status was. See Command completion service.

Command service

Use the command service when you want to submit a command and wait for it to be executed. See Command service.

Transaction service

Use the transaction service to listen to changes in the ledger, reported as a stream of transactions. See Transaction service.

Active contract service

Use the active contract service to obtain a party-specific view of all contracts currently active on the ledger. See Active contracts service.

Package service

Use the package service to obtain information about DAML packages available on the ledger. See Package service.

Ledger identity service

Use the ledger identity service to get the identity string of the ledger that your application is connected to. See Ledger identity service.

Ledger configuration service

Use the ledger configuration service to subscribe to changes in ledger configuration. See Ledger configuration service.

Ledger API libraries

The following libraries wrap the ledger API for more native experience applications development.

Java bindings

An idiomatic Java library for writing ledger applications. See Java bindings.

Node.js bindings

An idiomatic JavaScript library for writing ledger applications. See Node.js bindings.

Scala bindings

An idiomatic Scala library for writing ledger applications. See Java bindings.

gRPC API

The low-level ledger API that all of the other bindings use. Written in gRPC. See The Ledger API using gRPC.

Reading from the ledger

Applications get information about the ledger by reading from it. You can’t query the ledger, but you can subscribe to the transaction stream to get the events, or the more sophisticated active contract service.

Submitting commands, writing to the ledger

Applications make changes to the ledger by submitting commands. You can’t change it directly: an application submits a command of transactions. The command gets evaluated by the runtime, and will only be accepted if it’s valid.

For example, a command might get rejected because the transactions aren’t well-authorized; because the contract isn’t active (perhaps someone else archived it); or for other reasons.

This is echoed in scenarios, where you can mock an application by having parties submit transactions/updates to the ledger. You can use submit or submitMustFail to express what should succeed and what shouldn’t.

Commands

A command is an instruction to add a transaction to the ledger.

DAML-LF

When you compile DAML source code into a .dar file, the underlying format is DAML-LF. DAML-LF is similar to DAML, but is stripped down to a core set of features. The relationship between the surface DAML syntax and DAML-LF is loosely similar to that between Java and JVM bytecode.

As a user, you don’t need to interact with DAML-LF directly. But inside the DAML SDK, it’s used for:

  • executing DAML code on the Sandbox or on another platform
  • sending and receiving values via the Ledger API (using a protocol such as gRPC)
  • generating code in other languages for interacting with DAML models (often called “codegen”)

General concepts

Ledger, DAML ledger

Ledger can refer to a lot of things, but a ledger is essentially the underlying storage mechanism for a running DAML applications: it’s where the contracts live. A DAML ledger is a ledger that you can store DAML contracts on, because it implements the ledger API.

DAML ledgers provide various guarantees about what you can expect from it, all laid out in the DA Ledger Model page.

When you’re developing, you’ll use Sandbox as your ledger.

If you want to run DAML on a storage mechanism of your choice, you can use the DAML Integration Kit - ALPHA to help you do that.