Call scheduling is the core of the Ethereum Alarm Service. Calls can be scheduled on any block at least 40 blocks (~10 minutes) in the future.

When a call is scheduled, the service deploys a new contract that represents the scheduled function call. This contract is referred to as the call contract. It holds all of the metadata associated with the call as well as the funds that will be used to pay for the call.

Lifecycle of a Call Contract

  • creation - The call contract is created as part of call scheduling.
  • pending - The call can be cancelled anytime prior to the claim stage.
  • claim - The contract can be claimed which will grant the claimer exclusive rights to execution during the first 16 blocks of the call window.
  • frozen - The contract is frozen, preventing cancellation starting 10 blocks before the call’s target block through the last block in the call window.
  • execution - The executing transaction is sent, which triggers the call contract to execute the function call.
  • payment - payments are sent to the executor and the creator of the alarm service.
  • finalization - The contract sends any remaining funds to the address which scheduled the call.

Scheduling the Call

Function calls are scheduled with the scheduleCall function on the Alarm service. This creates a new call contract that represents the function call. The primary signature for this function which accepts all allowed configuration options is as follows.

function scheduleCall(address contractAddress,
                      bytes4 abiSignature,
                      bytes callData,
                      uint16 requiredStackDepth,
                      uint8 gracePeriod,
                      uint[5] args) public returns (address);

The uint[5] args is an array of 5 uint256 values which are respectively callValue, targetBlock, requiredGas, basePayment, baseDonation.

var (callValue, targetBlock, requiredGas, basePayment, baseDonation) = args;

There are a total of 10 available configuration options for a scheduled call.

  • address contractAddress
  • bytes4 abiSignature
  • bytes callData
  • uint16 requiredStackDepth
  • uint8 gracePeriod
  • callValue
  • targetBlock
  • requiredGas
  • basePayment
  • baseDonation

Call Configuration

address contractAddress:

The address of the contract that the call should be called on.

  • Default: msg.sender of the scheduling transaction.

bytes4 abiSignature:

The 4 byte ABI signature of the function to be called.

  • Default: N/A. If omitted this value is not used.


This configuration is a convenience feature. It is perfectly fine to exclude this value and have the 4-byte function signature as part of the callData.

bytes callData:

  • Default: N/A. If omitted this value is not used.


If the abiSignature argument was specified then this should not include the 4-byte function signature. Otherwise the call will be double prefixed with the function signature which is likely not what you want.

uint callValue:

The amount in wei that will be sent as part of call execution.

  • Default: 0

uint targetBlock:

The block number the call should be executed on. This must be at least 10 blocks in the future.

  • Default: 10 blocks from the current block.

uint8 gracePeriod:

The number of blocks after targetBlock during which the call may still be executed. Cannot be less than 64 or greater than 255.

  • Default: 255

uint requiredGas:

The amount of gas required to be sent along with the executing transaction. This value cannot be less than 200,000 or more than the block gas limit minus 200,000 (block.gaslimit - 200000).

Call execution requires that at least this amount of gas be provided

  • Default: 200,000.

uint16 requiredStackDepth:

The number of call stack frames should be checked prior to execution of the function call cannot be less than 10 or greater than 1,000.

If the call is being executed by another contract, call execution will verify that the stack depth can be extended by this value.

  • Default: 10

uint basePayment:

The base amount in wei that will be used to calculate the amount paid to the executor of the call.

  • Default: The current market value of a scheduled call.

uint baseDonation:

The base amount in wei that will be used to calculate the amount donated to the creator of the service.

  • Default: 1/100th of the current market value of a scheduled call.

Alternate Call Signatures

The scheduleCall function has many alternate call signatures that are intended for simpler use in common use cases.

  • scheduleCall()

If called with no arguments then the scheduled call will execute a bare where addr is the msg.sender from when the call was scheduled.

  • scheduleCall(bytes callData)

In this case, the target of the call will be msg.sender from when the call was scheduled, but the callData will be passed into the call (

  • scheduleCall(bytes4 abiSignature)

This is very similar to scheduleCall(bytes callData) except that it can make it easy to execute a specific function that takes no arguments.

  • scheduleCall(uint256 callValue, address contractAddress)
  • scheduleCall(address contractAddress, uint256 targetBlock, uint256 callValue)

These signature can be used to easily schedule sending ether to another address.

The exhaustive list of signatures for scheduleCall can be found below.

Call Contract Address

Since each scheduled call is deployed as a standalone contract it can be useful to have the address for the call contract.

If the scheduleCall function is being used from within a contract, the address of the newly created call contract is returned. If instead, the function is being called directly in a transaction, the address of the call contract can be extracted from the transaction logs under the CallScheduled event.

Contract scheduling its own call

Contracts can take care of their own call scheduling.

contract Lottery {
    address scheduler; // set by some other mechanism.

    function beginLottery() public {
        ... // Do whatever setup needs to take place.

        // Now we schedule the picking of the winner.

        // the 4-byte signature of the local function we want to be called.
        bytes4 sig = bytes4(sha3("pickWinner()"));

        // approximately 24 hours from now
        uint targetBlock = block.number + 5760;

        // the 4-byte signature of the scheduleCall function.
        bytes4 scheduleCallSig = bytes4(sha3("scheduleCall(bytes4,uint256)"));, sig, targetBlock)

    function pickWinner() public {

In this example Lottery contract, every time the beginLottery function is called, a call to the pickWinner function is scheduled for approximately 24 hours later (5760 blocks).

Upfront Payment

The service requires that you pay upfront for all costs associated with call scheduling. This value is referred to as the endowment. Without intimate knowledge of how all of these things are calculated it can be difficult to determine how much to send.

One nice part about the service is that you can just send extra and anything unused will be returned to you. This is generally a good strategy since you are at no risk of losing your ether and it prevents situations where you come in slightly under the required endowment and have your call rejected.

The following functions are available to assist in computing this ether value.

  • getMinimumEndowment() constant returns (uint)
  • getMinimumEndowment(uint basePayment) constant returns (uint)
  • getMinimumEndowment(uint basePayment, uint baseDonation) constant returns (uint)
  • getMinimumEndowment(uint basePayment, uint baseDonation, uint callValue) constant returns (uint)
  • getMinimumEndowment(uint basePayment, uint baseDonation, uint callValue, uint requiredGas) constant returns (uint)

Call Data

If a function call requires arguments then you have two options available.

  • Provide the bytes as the callData argument at the time of scheduling.
  • Register the bytes after the call has already been scheduled.

The call contract allows for call data registration via two mechanisms. The primary mechanism is through the fallback function on the contract. This will set the call data as the full call data of the transaction.

For example, if you were registering the call data for a function with the signature myFunction(uint count, bytes32 reason) you could do it with the following solidity code.

address scheduler = 0x...;
// schedule the call
address callContract =;

// Register the call data, 'abcde');

Alternatively you can use the registerData() function which will strip the first four bytes off of and use the remainder as the call data.

In solidity, this would look something like the following.

contract Example {
    function doDataRegistration() public {
        uint arg1 = 3;
        int arg2 = -1;"registerData()")), arg1, arg2);

Once data has been registered, it cannot be modified. Attempts to do so will result in an exception.


The call() function on an address in solidity does not do any ABI encoding, so in cases where a scheduled call must pass something like a bytes variable, you will need to handle the ABI encoding yourself.

Cancelling a call

A scheduled call can be cancelled by its scheduler either before the claim window begins.

  • Solidity Function Signature: cancel()

This will cause the call to be set as cancelled, which will return any funds currently being held by the contract.

A call may also be cancelled after the call window if it has not been executed.

Looking up a Call

You can lookup whether a particular address is a known scheduled call with the isKnownCall function.

  • Solidity Function Signature: isKnownCall(address callAddress) returns (bool)

Returns a boolean as to whether this address represents a known scheduled call.

Helper Functions

The following getters can be used to return the constant values that are used by the service programatically.

  • callAPIVersion() constant returns (uint)

Returns the version of the Alarm service.

  • getMinimumGracePeriod() constant returns (uint)

The smallest value allowed for the gracePeriod of a scheduled call.

  • getDefaultDonation() constant returns (uint)

The default payment value for scheduled calls.

  • getMinimumCallGas() constant returns (uint)

The minimum allowed value for requiredGas

  • getMaximumCallGas() constant returns (uint)

The maximum allowed value for requiredGas. This value is computed as block.gaslimit - getMinimumCallGas()

  • getDefaultRequiredGas() constant returns (uint)

The default value for requiredGas

  • isKnownCall(address callAddress) constant returns (bool)

Returns whether this address was a call contract that was deployed by the alarm service. This can be useful if you need to use the service to interact with priviledged functions as you can verify that the address that is calling you is in fact a legitimate call contract.

  • getFirstSchedulableBlock() constant returns (uint)

Returns the earliest block number in the future on which a call may be scheduled.

  • getMinimumStackCheck() constant returns (uint16)

The minimum allowed value for requiredStackDepth.

  • getMaximumStackCheck() constant returns (uint16)

The maximum allowed value for requiredStackDepth.

  • getDefaultStackCheck() constant returns (uint16)

The default value for the requiredStackDepth of a scheduled call.

  • getDefaultGracePeriod() constant returns (uint8)

The default value for the gracePeriod of a scheduled call.