Hi Daml-Finance team
I’ve been currently exploring the new daml-finance-app
could you explain the concept of factories?
You can see it being used in the Service template when it is not using the auto “path”:
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Hi @BrunoRTL,
Factories in this example are contracts that are used to create instruments. The reason why using factories is a recommended pattern when using Daml Finance has to do with application decoupling / upgradeability of your application.
Let me try to illustrate this with an example. Suppose that you are writing Daml code to issue equity instruments, for which you want to use the template provided in Daml Finance.
Your workflow will reference Daml.Finance.Instrument.Equity 0.1.6 and at some point do a create Equity.Instrument with issuer = myParty, id = "MyCompany", .. to create the instrument.
If the equity package gets updated to Daml.Finance.Instrument.Equity 0.1.7 and a new field is added to the instrument (or a choice is changed, or a new lifecycle event is added, …) then you are forced to upgrade your Daml code in order to use the new feature and will have to deal with upgrading multiple templates on the ledger.
A safer approach is for your Daml code to only reference the Daml.Finance.Interface.Instrument.Equity package, which contains interface definitions and is updated much less frequently.
However, you now need a way to create equity instruments without referencing Daml.Finance.Instrument.Equity in your main Daml workflow. To do this, you can setup a Script to run on ledger initialisation that will create a factory contract and cast it to the corresponding interface. You can then use the factory in your main workflow code to create the instruments.
When an upgraded instrument comes along, you will need to write code to archive the old factory and create the new one, in order to issue the new instruments. However, the Daml code for your workflow can in principle stay untouched.
I hope this explanation clarifies the concept of factories. You can see this pattern being used in the Daml Finance app, as well as in our getting started tutorials.
Matteo
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This was a great insight! Thank you.