Optimizing Gas Fees with Coin Merging
In the IOTA blockchain, every transaction incurs a gas fee that must be paid for successful execution.
Coin merging allows you to pay this gas fee using multiple coins combined into one,
rather than relying on a single coin.
This method is particularly useful when you have several coins with small denominations or wish
to reduce the number of individual IOTA coins in your account.
Coin merging enhances coin management, especially when used with the GasCoin argument in programmable transaction blocks (PTBs).
How Coin Merging Works
When you include multiple coins to cover the gas fee in a transaction, IOTA automatically merges them into a single coin during execution. This merging occurs regardless of the individual coin amounts or the specified gas budget, as long as it falls within the allowed gas limits. The gas fee is then deducted from this combined coin, even if the transaction fails to execute (for example, due to an execution error). This means that after the transaction, the coins you provided for gas remain merged.
Coin merging is an efficient strategy for managing your coins by consolidating smaller amounts into a single coin.
You can use this merged coin not only to pay for gas fees but also for other operations within the same transaction.
For instance, you can merge multiple coins to cover the gas fee for a PTB,
and then have that PTB withdraw from the GasCoin.
PTBs offer a special argument that references the coin used for gas payment after the gas fee has been deducted,
allowing you to transfer the remaining IOTA to another address.
Since coin merging happens automatically when you supply multiple gas coins,
you can combine coin management with other transactions seamlessly.
It's important to note that IOTA sets a maximum limit of 256 coins that can be merged in a single PTB. If you attempt to merge more than this number, the transaction will not be processed. Additionally, during the merging process, all coins except the first one are deleted. This deletion often results in a storage rebate due to the removal of coin objects. While you cannot use this rebate to pay for the current transaction's gas fee, as it isn't credited until after execution, it may result in a refund after the transaction completes. The refund and any remaining balance after deducting the gas fee will reside in the first coin you provided in the transaction.
Unexpected Refunds When Running Out of Gas
Because coins are merged regardless of whether a transaction succeeds,
storage rebates can lead to scenarios where a transaction runs out of gas but still results in a net refund.
For example, suppose you have a transaction T with a gas budget of 5000,
and you provide coins C1, C2, C3, C4, and C5 with values 1000, 2000, 3000,
4000, and 5000 respectively to pay for gas.
If each deleted coin object provides a storage rebate of 2000,
and the actual gas fee for executing T exceeds the 5000 gas budget (causing T to run out of gas), the execution status will be OutOfGas.
After execution, the balance of coin C1 would be calculated as:
- Initial total coin value:
1000 + 2000 + 3000 + 4000 + 5000 = 15,000 - Gas fee deducted:
-5000 - Storage rebate from deleted coins:
2000 * 4 = 8000(since four coins were deleted) - Final balance:
15,000 - 5000 + 8000 = 18,000
In this case, even though the transaction failed due to insufficient gas, you receive a net refund of 3000 (since you end up with 18,000 in C1).