Storage Trigger Example

Example: A Storage Access trigger on Total Supply change

Let's say we want to use SendBlocks' storage parsing to notify us when a token's total supply changes. The trigger's slot will depend on the way the token stores its total supply:

Total Supply Variable

This method is used by most tokens: the total supply is stored as a variable in the token contract, which is usually called totalSupply. In this case we will:

1. Analyze the token contract:

2. Retrieve the results of the analysis:

3. Search through the resulting storage layout to find the matching variable:

   JSON

   {
       "storage_layout": [
           ...
           {
               "name": "totalSupply",
               "first_slot": "0x1",
               "last_slot": "0x1"
           },
           {
               "name": "balanceOf",
               "first_slot": "0x2",
               "last_slot": "0x2"
           },
           ...
       ]
   }
   

4. Set storage trigger on the variable name inside the analyzed contract.

For example, the Dai Stablecoin (DAI) stores its total supply in a variable called totalSupply.

Total Supply Slot

Some tokens store the total supply in a pre-defined slot in storage, usually calculated from the hash of some string. Finding the correct slot can be tricky, and it requires going through the token contract's code.

A good example of such a contract is the stETH token. The stETH contract's total supply slot was originally calculated using the keccak hash of a number of different values, and is now fixed to a specific address for all subsequent versions of the contract.

Using the Storage Access Trigger

Once we've got the variable name or storage slot address, we can create a function to act on changes to the value.