Objective

Scheduled transactions on the Flow blockchain enable smart contracts to autonomously execute predefined logic at specific future times without requiring continuous off-chain triggers. This functionality is currently provided by the Cadence FlowTransactionScheduler contract.

The limitation today is that Flow’s EVM environment cannot directly access this scheduler, meaning that Solidity contracts deployed in Flow EVM cannot natively leverage scheduling.

The objective of this document is to explore a design that would expose Flow’s scheduling feature to the EVM world.

EVM Scheduler Design

The implementation of the EVM Scheduler introduces new components that integrate with the existing FlowTransactionScheduler contract to enable the scheduling and execution of EVM transactions on Flow. The design ensures coordination between Cadence and Solidity contracts, enabling reliable scheduling, execution, and status tracking of cross-environment transactions.

Components

1. FlowTransactionScheduler (Existing, Unchanged)

• Description: The existing and unchanged Flow transaction scheduler contract.
• Role:
  • Provides the scheduling and triggering mechanism for transaction execution.
  • Remains unchanged and serves as the base scheduler on Flow.

2. EVM Scheduler (New, Cadence Contract)

• Description: A Cadence contract that bridges EVM transactions into the Flow scheduling framework, intended to be called by Cadence Arch. It defines a schedule function that does not track transaction details, only reserving an execution slot based on effort. During execution, control passes to the Solidity scheduler proxy, which holds the full transaction context. This separation of concerns ensures that scheduling is only possible through the EVM scheduler proxy and data is deduplicated.

• Responsibilities:

  • Registers transactions with the FlowTransactionScheduler.
  • Registers itself as the transaction handler in the FlowTransactionScheduler
  • Stores data for EVM transaction execution (gas limit, fees)
  • Withdraws EVM Flow tokens into Cadence during scheduling

• API:

  • The contract also implements the FlowTransactionScheduler.Handler interface which is used during execution by the Flow transaction scheduler contract.

  pub interface EVMScheduler {
  	// Schedules a new EVM transaction
  	// - timestamp: timestamp for scheduled transaction
  	// - priority: priority of the transaction
  	// - gasLimit: maximum gas allowed for execution
  	// - fees: Flow tokens to fund the scheduling
  	// Returns: an ID of the scheduled transaction
  	pub fun schedule(
  		timestamp: Uint64, 
  		priority: Priority, 
  		gasLimit: UInt64, 
  		fees: UFix64
  	): UInt64
  }
  

3. EVM Scheduler Proxy (New, Solidity Contract)

• Description: An EVM Solidity contract for interacting with the EVM Scheduler using Cadence Arch. The owner of the contract must be the EVM Scheduler COA.
• Motivation: Direct calls to the EVM Scheduler via Cadence Arch are possible but since they cannot pay for the operation, they will fail. The proxy enables additional features that ensure observability, fund management, and user control. It also stores all the scheduled transaction data that will be used during execution. It can add an overhead in fees for storing that data.