Keeping Data and Logic Separately via Eternal Storage
Eternal Storage means to define all possible data types such that no more types will be needed in the future. Lets take an example of storing unsigned integers (uint), if we can give a unique name to every uint we want to store then we can store as many uint's as we want as long as we keep giving them unique names and an efficient way to achieve this is through mapping where the key is bytes32. The bytes32 comes from taking keccak256(UNIQUE_NAME) where UNIQUE_NAME is the name we want to call our variable which stores a uint
string score = "score";
bytes32 hash = keccak256(score);
mapping(bytes32 => uint) uintStorage
Following this pattern, the eternal storage can be implemented by mappings for each data types as follows:
Taken from: https://github.com/zeppelinos/labs/blob/master/upgradeability_using_eternal_storage/contracts/EternalStorage.sol
Each of these types can be used with Logic Contract keeping a track of all variable names it uses to store the values. The eternal storage contract is completely agnostic to the actual values keys which are used, for example, if a value is no longer needed, the logic contract can stop referencing the value and instead use a new variable name for it.
This pattern can be used in the exact same way as Keeping data and logic separately, however the complexity of implementing the same approach in this is significantly higher and mistakes are much more likely.