Upgrading Daml applications¶

Note: Cross-SDK upgrades require Daml-LF 1.8 or newer. This is the default starting from SDK 1.0. For older releases add build-options: ["--target=1.8"] to your daml.yaml to select Daml-LF 1.8.

In applications backed by a centralized database controlled by a single operator, it is possible to upgrade an application in a single step that migrates all existing data to a new data model.

As a running example, let’s imagine a centralized database containing carbon offset certificates. Its operator created the database schema with

CREATE TABLE carbon_certs (
  carbon_metric_tons VARINT,
  owner VARCHAR NOT NULL
  issuer VARCHAR NOT NULL
)

The certificate has a field for the quantity of offset carbon in metric tons, an owner and an issuer.

In the next iteration of the application, the operator decides to also store and display the carbon offset method. In the centralized case, the operator can upgrade the database by executing the single SQL command

ALTER TABLE carbon_certs ADD carbon_offset_method VARCHAR DEFAULT "unknown"

This adds a new column to the carbon_certs table and inserts the value unknown for all existing entries.

While upgrading this centralized database is simple and convenient, its data entries lack any kind of signature and hence proof of authenticity. The data consumers need to trust the operator.

In contrast, Daml templates always have at least one signatory. The consequence is that the upgrade process for a Daml application needs to be different.

Daml upgrade overview¶

In a Daml application running on a distributed ledger, the signatories of a contract have agreed to one specific version of a template. Changing the definition of a template, e.g., by extending it with a new data field or choice without agreement from its signatories would completely break the authorization guarantees provided by Daml.

Therefore, Daml takes a different approach to upgrades and extensions. Rather than having a separate concept of data migration that sidesteps the fundamental guarantees provided by Daml, upgrades are expressed as Daml contracts. This means that the same guarantees and rules that apply to other Daml contracts also apply to upgrades.

In a Daml application, it thus makes sense to think of upgrades as an extension of an existing application instead of an operation that replaces existing contracts with a newer version. The existing templates stay on the ledger and can still be used. Contracts of existing templates are not automatically replaced by newer versions. However, the application is extended with new templates. Then if all signatories of a contract agree, a choice can archive the old version of a contract and create a new contract instead.

Structuring upgrade contracts¶

Upgrade contracts are specific to the templates that are being upgraded. But most of them share common patterns. Here is the implementation of the above carbon_certs schema in Daml. We have some prescience that there will be future versions of CarbonCert, and so place the definition of CarbonCert in a module named CarbonV1

module CarbonV1 where

template CarbonCert
  with
    issuer : Party
    owner : Party
    carbon_metric_tons : Int
  where
    signatory issuer, owner

A CarbonCert has an issuer and an owner. Both are signatories. Our goal is to extend this CarbonCert template with a field that adds the method used to offset the carbon. We use a different name for the new template here for clarity. This is not required as templates are identified by the triple (PackageId, ModuleName, TemplateName).

module CarbonV2 where

template CarbonCertWithMethod
  with
    issuer : Party
    owner : Party
    carbon_metric_tons : Int
    carbon_offset_method : Text
  where
    signatory issuer, owner

Next, we need to provide a way for the signatories to agree to a contract being upgraded. It would be possible to structure this such that issuer and owner have to agree to an upgrade for each individual CarbonCert contract separately. Since the template definition for all of them is the same, this is usually not necessary for most applications. Instead, we collect agreement from the signatories only once and use that to upgrade all carbon certificates.

Since there are multiple signatories involved here, we use a Propose-Accept workflow. First, we define an UpgradeCarbonCertProposal template that will be created by the issuer. This template has an Accept choice that the owner can exercise. Upon execution it will then create an UpgradeCarbonCertAgreement.

template UpgradeCarbonCertProposal
  with
    issuer : Party
    owner : Party
  where
    signatory issuer
    observer owner
    key (issuer, owner) : (Party, Party)
    maintainer key._1
    choice Accept : ContractId UpgradeCarbonCertAgreement
      controller owner
      do create UpgradeCarbonCertAgreement with ..

Now we can define the UpgradeCarbonCertAgreement template. This template has one nonconsuming choice that takes the contract ID of a CarbonCert contract, archives this CarbonCert contract and creates a CarbonCertWithMethod contract with the same issuer and owner and the carbon_offset_method set to unknown.

template UpgradeCarbonCertAgreement
  with
    issuer : Party
    owner : Party
  where
    signatory issuer, owner
    key (issuer, owner) : (Party, Party)
    maintainer key._1
    nonconsuming choice Upgrade : ContractId CarbonCertWithMethod
      with
        certId : ContractId CarbonCert
      controller issuer
      do cert <- fetch certId
         assert (cert.issuer == issuer)
         assert (cert.owner == owner)
         archive certId
         create CarbonCertWithMethod with
           issuer = cert.issuer
           owner = cert.owner
           carbon_metric_tons = cert.carbon_metric_tons
           carbon_offset_method = "unknown"

Building and deploying carbon-1.0.0¶

Let’s see everything in action by first building and deploying carbon-1.0.0. After this we’ll see how to deploy and upgrade to carbon-2.0.0 containing the CarbonCertWithMethod template.

First we’ll need a sandbox ledger to which we can deploy.

$ daml sandbox --port 6865

Now we’ll setup the project for the original version of our certificate. The project contains the Daml for just the CarbonCert template, along with a CarbonCertProposal template which will allow us to issue some coins in the example below.

Here is the project config.

name: carbon
version: 1.0.0
dependencies:
  - daml-prim
  - daml-stdlib

Now we can build and deploy carbon-1.0.0.

$ cd example/carbon-1.0.0
$ daml build
$ daml ledger upload-dar --port 6865

Create some carbon-1.0.0 certificates¶

Let’s create some certificates!

We’ll use the navigator to connect to the ledger, and create two certificates issued by Alice, and owned by Bob.

$ cd example/carbon-1.0.0
$ daml navigator server localhost 6865

We point a browser to http://localhost:4000, and follow the steps:

Login as Alice:
1. Select Templates tab.
2. Create a CarbonCertProposal with Alice as issuer and Bob as owner and an arbitrary value for the carbon_metric_tons field.
3. Create a 2nd proposal in the same way.
Login as Bob:
1. Exercise the CarbonCertProposal_Accept choice on both proposal contracts.

Building and deploying carbon-2.0.0¶

Now we setup the project for the improved certificates containing the carbon_offset_method field. This project contains only the CarbonCertWithMethod template. The upgrade templates are in a third carbon-upgrade package. While it would be possible to include the upgrade templates in the same package, this means that the package containing the new CarbonCertWithMethod template depends on the previous version. With the approach taken here of keeping the upgrade templates in a separate package, the carbon-1.0.0 package is no longer needed once we have upgraded all certificates.

It’s worth stressing here that extensions always need to go into separate packages. We cannot just add the new definitions to the original project, rebuild and re-deploy. This is because the cryptographically computed package identifier would change. Consequently, it would not match the package identifier of the original CarbonCert contracts from carbon-1.0.0 which are live on the ledger.

Here is the new project config:

name: carbon
version: 2.0.0
dependencies:
  - daml-prim
  - daml-stdlib

Now we can build and deploy carbon-2.0.0.

$ cd example/carbon-2.0.0
$ daml build
$ daml ledger upload-dar --port 6865

Building and deploying carbon-upgrade¶

Having built and deployed carbon-1.0.0 and carbon-2.0.0 we are now ready to build the upgrade package carbon-upgrade. The project config references both carbon-1.0.0 and carbon-2.0.0 via the data-dependencies field. This allows us to import modules from the respective packages. With these imported modules we can reference templates from packages that we already uploaded to the ledger.

When following this example, path/to/carbon-1.0.0.dar and path/to/carbon-2.0.0.dar should be replaced by the relative or absolute path to the DAR file created by building the respective projects. Commonly the carbon-1.0.0 and carbon-2.0.0 projects would be sibling directories in the file systems, so this path would be: ../carbon-1.0.0/.daml/dist/carbon-1.0.0.dar.

name: carbon-upgrade
version: 1.0.0
dependencies:
  - daml-prim
  - daml-stdlib
data-dependencies:
  - path/to/carbon-1.0.0.dar
  - path/to/carbon-2.0.0.dar

The Daml for the upgrade contracts imports the modules for both the new and old certificate versions.

module UpgradeFromCarbonCertV1 where
import CarbonV1
import CarbonV2

Now we can build and deploy carbon-upgrade. Note that uploading a DAR also uploads its dependencies so if carbon-1.0.0 and carbon-2.0.0 had not already been deployed before, they would be deployed as part of deploying carbon-upgrade.

$ cd example/carbon-upgrade
$ daml build
$ daml ledger upload-dar --port 6865

Upgrade existing certificates from carbon-1.0.0 to carbon-2.0.0¶

We start the navigator again.

$ cd example/carbon-upgrade
$ daml navigator server localhost 6865

Finally, we point a browser to http://localhost:4000 and can start the carbon certificates upgrades:

Login as Alice
1. Select Templates tab.
2. Create an UpgradeCarbonCertProposal with Alice as issuer and Bob as owner.
Login as Bob
1. Exercise the Accept choice of the upgrade proposal, creating an UpgradeCarbonCertAgreement.
Login again as Alice
1. Use the UpgradeCarbonCertAgreement repeatedly to upgrade any certificate for which Alice is issuer and Bob is owner.

Further Steps¶

For the upgrade of our carbon certificate model above, we performed all steps manually via Navigator. However, if Alice had issued millions of carbon certificates, performing all upgrading steps manually becomes infeasible. It thus becomes necessary to automate these steps. We will go through a potential implementation of an automated upgrade in the next section.