Settler exo

[turadg]

What is the Problem Being Solved?

Fast USDC needs a component to settle advances.

Description of the Design

A Settler exo that performs the advancement operations.

It looks them up from Status Manager and settles per product spec.

Security Considerations

Scaling Considerations

Test Plan

Exo test

Contract test is separate: https://github.com/Agoric/agoric-sdk/issues/10388

Upgrade Considerations

[dckc]

collecting details on the scope of this issue...

design sketch says:

An exo within FUC that handles settling transactions.

• Taps the SettlementAccount to be notified of deposits
• Extracts the EUD from the address parameters of the virtual address receiving the deposit
• Looks up associated transaction using an Advancer method
• Processes according to the sequence diagram
  • When in doubt, hold funds
• Informs advancer of settlement

re "the SettlementAccount" - that presumes there is one. Does this exo make it?

[dckc]

re "settles per product spec" - this bit of the diagram seems particularly relevant:

sequenceDiagram
    rect rgb(255, 245, 230)
        Note over FUC,CFA: Settlement Process
        Note over FUC,SA: Tap on account reads MintAmount,<br/>parses EUD from virtual address recipient<br/>and looks up AdvanceAmount,PoolFee,ContractFee.<br/>Matches against an unsettled transaction (by EUD and approx amount).
        %% Treat starting the advance as an atomic action. Assume it will complete once started.
        alt Advance was started:
            FUC->>SA: Initiate transfers<br>out of settlement
            SA->>P: Deposit the AdvanceAmount + PoolFee<br>(= MintAmount - ContractFee)
            SA->>CFA: Deposit ContractFee
        else Advance for matched transaction that has not yet started:
            P->>NC: PFM transfer<br>(MintAmount of Agoric USDC denom) to EUD
            NC->>EUD: deliver MintAmount<br>as final USDC denom
        else Settlement for unknown transaction:
            %% Have not received notification of this Amount,EUD from the watcher
            Note over SA: Leave funds in SettlementAccount.
            Note over SA: Wait for observation from watcher
        end
    end