- Important Windows of Blocks/Time
- The Execution Lifecycle
- Sending the Execution Transaction
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.
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
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
windowStart + windowSize.
For example, if a request was scheduled with a
windowStart of block 2100
windowSize of 255 blocks, the request would be allowed to be executed
on any block such that
windowStart <= block.number <= windowStart +
As another example, if a request was scheduled with a
windowStart of block 2100
windowSize of 0 blocks, the request would only be allowed to be
executed at block 2100.
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
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.
It is worth pointing out that actual size of the execution window will
windowSize + 1 since the bounds are inclusive.
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.
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 <
It is worth pointing out that unlike the execution window the reserved execution window is not inclusive of it’s righthand bound.
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
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.
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.
During the validation phase all of the following validation checks must pass.
wasCalled attribute of the transaction request to
block.timestamp to be greater than or equal to
block.timestamp to be less than or equal to
windowStart + windowSize.
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
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
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
claimDepositis 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.
In addition to the pre-execution validation checks, the following things should be taken into considuration when sending the executing transaction for a request.
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.
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
180000gas to account for execution overhead.
- If you are proxying the execution through another contract such that during
msg.sender != tx.originthen you need to provide an additional
700 * requiredStackDepthgas 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.