Execution¶

Important Windows of Blocks/Time
The Execution Lifecycle
Sending the Execution Transaction
- Gas Reimbursement
- Minimum ExecutionGas

class TransactionRequest

Warning

Anyone wishing to write their own execution client should be sure they fully understand all of the intricacies related to the execution of transaction requests. The guarantees in place for those executing requests are only in place if the executing client is written appropriately. Reading this documentation is a good start.

Important Windows of Blocks/Time ¶

Freeze Window ¶

Each request may specify a freezePeriod. This defines a number of blocks or seconds prior to the windowStart during which no actions may be performed against the request. This is primarily in place to provide some level of guarantee to those executing the request. For anyone executing requests, once the request enters the freezePeriod they can know that it will not be cancelled and that they can send the executing transaction without fear of it being cancelled at the last moment before the execution window starts.

The Execution Window ¶

The execution window is the range of blocks or timestamps during which the request may be executed. This window is defined as the range of blocks or timestamps from windowStart till windowStart + windowSize.

For example, if a request was scheduled with a windowStart of block 2100 and a windowSize of 255 blocks, the request would be allowed to be executed on any block such that windowStart <= block.number <= windowStart + windowSize.

As another example, if a request was scheduled with a windowStart of block 2100 and a windowSize of 0 blocks, the request would only be allowed to be executed at block 2100.

Very short windowSize configurations likely lower the chances of your request being executed at the desired time since it is not possible to force a transaction to be included in a specific block. The party executing your request may either fail to get the transaction included in the correct block or they may choose to not try for fear that their transaction will not be mined in the correct block, thereby not receiving their reimbursment for their gas costs.

Similarly, very short ranges of time for timestamp based calls may even make it impossible to execute the call. For example, if you were to specify a windowStart at 1480000010 and a windowSize of 5 seconds then the request would only be executable on blocks whose block.timestamp satisfied the conditions 1480000010 <= block.timestamp <= 1480000015. Given that it is entirely possible that no blocks are mined within this small range of timestamps there would never be a valid block for your request to be executed.

Note

It is worth pointing out that actual size of the execution window will always be windowSize + 1 since the bounds are inclusive.

Reserved Execution Window ¶

Each request may specify a claimWindowSize which defines a number of blocks or seconds at the beginning of the execution window during which the request may only be executed by the address which has claimed the request. Once this window has passed the request may be executed by anyone.

Note

If the request has not been claimed this window is treated no differently than the remainder of the execution window.

For example, if a request specifies a windowStart of block 2100, a windowSize of 100 blocks, and a reservedWindowSize of 25 blocks then in the case that the request was claimed then the request would only be executable by the claimer for blocks satisfying the condition 2100 <= block.number < 2125.

Note

It is worth pointing out that unlike the execution window the reserved execution window is not inclusive of it’s righthand bound.

If the reservedWindowSize is set to 0, then there will be no window of blocks during which the execution rights are exclusive to the claimer. Similarly, if the reservedWindowSize is set to be equal to the full size of the execution window or windowSize + 1 then there will be not window after the reserved execution window during which execution can be triggered by anyone.

The RequestFactory will allow a reservedWindowSize of any value from 0 up to windowSize + 1, however, it is highly recommended that you pick a number around 16 blocks or 270 seconds, leaving at least the same amount of time unreserved during the second portion of the execution window. This ensures that there is sufficient motivation for your call to be claimed because the person claiming the call knows that they will have ample opportunity to execute it when the execution window comes around. Conversely, leaving at least as much time unreserved ensures that in the event that your request is claimed but the claimer fails to execute the request that someone else has plenty of of time to fulfill the execution before the execution window ends.

The Execution Lifecycle ¶

When the :method:`TransactionRequest.execute()` function is called the contract goes through three main sections of logic which are referred to as a whole as the execution lifecycle.

Validation: Handles all of the checks that must be done to ensure that all of the conditions are correct for the requested transaction to be executed.
Execution: The actual sending of the requested transaction.
Accounting: Computing and sending of all payments to the necessary parties.

Part 1: Validation ¶

During the validation phase all of the following validation checks must pass.

Check #1: Not already called ¶

Requires the wasCalled attribute of the transaction request to be false.

Check #2: Not Cancelled ¶

Requires the isCancelled attribute of the transaction request to be false.

Check #3: Not before execution window ¶

Requires block.number or block.timestamp to be greater than or equal to the windowStart attribute.

Check #4: Not after execution window ¶

Requires block.number or block.timestamp to be less than or equal to windowStart + windowSize.

Check #5 and #6: Within the execution window and authorized ¶

If the request is claimed
- If the current time is within the reserved execution window
  
  Requires that msg.sender to be the claimedBy address
- Otherwise during the remainder of the execution window
  
  Always passes.
If the request is not claimed.
- Always passes if the current time is within the execution window

Check #8: Sufficient Call Gas ¶

Requires that the current value of msg.gas be greater than the minimum call gas. See minimum-call-gas for details on how to compute this value as it includes both the callGas amount as well as some extra for the overhead involved in execution.

Part 2: Execution ¶

The execution phase is very minimalistic. It marks the request as having been called and then dispatches the requested transaction, storing the success or failure on the wasSuccessful attribute.

Part 3: Accounting ¶

The accounting phase accounts for all of the payments and reimbursements that need to be sent.

The fee payment is the mechanism through which developers can earn a return on their development efforts on the Alarm service. When a person schedules a transaction they may choose to enter a fee amount which will get sent to the developer. This value is multiplied by the gas multiplier (see gas-multiplier) and sent to the feeRecipient address.

Next the payment for the actual execution is computed. The formula for this is as follows:

totalPayment = payment * gasMultiplier + gasUsed * tx.gasprice + claimDeposit

The three components of the totalPayment are as follows.

payment * gasMultiplier: The actual payment for execution.
gasUsed * tx.gasprice: The reimbursement for the gas costs of execution. This is not going to exactly match the actual gas costs, but it will always err on the side of overpaying slightly for gas consumption.
claimDeposit: If the request is not claimed this will be 0. Otherwise, the claimDeposit is always given to the executor of the request.

After these payments have been calculated and sent, the Executed event is logged, and any remaining ether that is not allocated to be paid to any party is sent back to the address that scheduled the request.

Sending the Execution Transaction ¶

In addition to the pre-execution validation checks, the following things should be taken into considuration when sending the executing transaction for a request.

Gas Reimbursement ¶

If the gasPrice of the network has increased significantly since the request was scheduled it is possible that it no longer has sufficient ether to pay for gas costs. The following formula can be used to compute the maximum amount of gas that a request is capable of paying:

(request.balance - 2 * (payment + fee)) / tx.gasprice

If you provide a gas value above this amount for the executing transaction then you are not guaranteed to be fully reimbursed for gas costs.

Minimum ExecutionGas ¶

When sending the execution transaction, you should use the following rules to determine the minimum gas to be sent with the transaction:

Start with a baseline of the callGas attribute.
Add 180000 gas to account for execution overhead.
If you are proxying the execution through another contract such that during execution msg.sender != tx.origin then you need to provide an additional 700 * requiredStackDepth gas for the stack depth checking.

For example, if you are sending the execution transaction directly from a private key based address, and the request specified a callGas value of 120000 gas then you would need to provide 120000 + 180000 => 300000 gas.

If you were executing the same request, except the execution transaction was being proxied through a contract, and the request specified a requiredStackDepth of 10 then you would need to provide 120000 + 180000 + 700 * 10 => 307000 gas.