This section describes our recommended design of a full-stack DAML application.
The above image shows the recommended architecture. Of course there are many ways how you can change the architecture and technology stack to fit your needs, which we’ll mention in the corresponding sections.
To get started quickly with the recommended application architecture clone the
create-daml-app application template:
git clone https://github.com/digital-asset/create-daml-app
create-daml-app is a small, but fully functional demo application implementing the recommended
architecture, providing you with an excellent starting point for your own application. It showcases
- using DAML React libraries
- quick iteration against the DAML Ledger Sandbox.
- deploying your application in the cloud as a Docker container
The backend for your application can be any DAML ledger implementation running your DAR (DAML Archive) file.
We recommend using the DAML JSON API as an interface to your frontend. It is served by the HTTP JSON API server connected to the ledger API server. It provides simple HTTP endpoints to interact with the ledger via GET/POST requests. However, if you prefer, you can also use the gRPC API directly.
When you use the
create-daml-app template application, you can start a local sandbox together
with a JSON API server by running
in the root of the project. This is the most simple DAML ledger implementation. Once your
application matures and becomes ready for production, the
daml deploy command helps you deploy
your frontend and DAML artefacts of your project to a production ledger. See Deploying to DAML
Ledgers for an in depth manual for specific ledgers.
We recommended building your frontend with the React framework. However, you can choose virtually any language for your frontend and interact with the ledger via HTTP JSON endpoints. In addition, we provide support libraries for Java and Scala and you can also interact with the gRPC API directly.
We provide two libraries to build your React frontend for a DAML application.
|@daml/react||React hooks to query/create/exercise DAML contracts|
|@daml/ledger||DAML ledger object to connect and directly submit commands to the ledger|
You can install any of these libraries by running
yarn add <library> in the
ui directory of
your project, e.g.
yarn add @daml/react. Please explore the
create-daml-app example project
to see the usage of these libraries.
To make your life easy when interacting with the ledger, the DAML assistant can generate corresponding typescript data definitions for the data types declared in the deployed DAR.
daml codegen ts .daml/dist/<your-project-name.dar> -o daml-ts
This command will generate a typescript project in the
daml-ts folder that needs to be connected
with your frontend code in
ui. To do so, navigate to
daml-ts and run
yarn install and
yarn workspaces run build.
When you deploy your application to a production ledger, you need to authenticate the identities of your users.
DAML ledgers support a unified interface for authentication of commands. Some DAML ledgers like for
example https://projectdabl.com offer an integrated authentication service, but you can also use an
external service provider for authentication like https://auth0.com. The DAML react libraries
support interfacing with an authenticated DAML ledger. Simply initialize your
with the token obtained by an authentication service. How authentication works and the form of the
required tokens is described in the Authentication section.
The DAML SDK enables a local development environment with fast iteration cycles. If you run
daml-reload-on-change.sh of the
create-daml-app, a local DAML sandbox ledger is started that
is updated with your most recent DAML code on any change. Next, you can start your frontend in
development mode by changing to your
ui directory and run
yarn start. This will reload your
frontend whenever you make changes to it. You can add unit tests for your DAML models by writing
DAML scenarios. These will also be reevaluated on change. A
typical DAML developer workflow is to
- Make a small change to your DAML data model
- Optionally test your DAML code and with scenarios
- Edit your React components to be aligned with changes made in DAML code
- Extend the UI to make use of the newly introduced feature
- Make further changes either to your DAML and/or React code until you’re happy with what you’ve developed
Handle failures when submitting commands¶
The interaction of a DAML application with the ledger is inherently asynchronous: applications send commands to the ledger, and some time later they see the effect of that command on the ledger.
There are several things that can fail during this time window: the application can crash, the participant node can crash, messages can be lost on the network, or the ledger may be just slow to respond due to a high load.
If you want to make sure that a command is not executed twice, your application needs to robustly handle all the various failure scenarios. DAML ledgers provide a mechanism for command deduplication to help deal this problem.
For each command applications provide a command ID and an optional parameter that specifies the deduplication time. If the latter parameter is not specified in the command submission itself, the ledger will fall back to using the configured maximum deduplication time. The ledger will then guarantee that commands for the same submitting party and command ID will be ignored within the deduplication time window.
To use command deduplication, you should:
- Use generous values for the deduplication time. It should be large enough such that you can assume the command was permanently lost if the deduplication time has passed and you still don’t observe any effect of the command on the ledger (i.e. you don’t see a transaction with the command ID via the transaction service).
- Make sure you set command IDs deterministically, that is to say: the “same” command must use the same command ID. This is useful for the recovery procedure after an application crash/restart, in which the application inspects the state of the ledger (e.g. via the Active contracts service) and sends commands to the ledger. When using deterministic command IDs, any commands that had been sent before the application restart will be discarded by the ledger to avoid duplicate submissions.
- If you are not sure whether a command was submitted successfully, just resubmit it. If the new command was submitted within the deduplication time window, the duplicate submission will safely be ignored. If the deduplication time window has passed, you can assume the command was lost or rejected and a new submission is justified.
For more details on command deduplication, see the Ledger API Services documentation.