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design_pattern_desicions.md

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Design Pattern Decisions

Circuit Breaker

Zeppelin's Pausable library is extended to both the Marketplace.sol and Stores.sol contracts.

The contract owner (i.e. the address that has deployed the contract) can therefore pause any of them. When paused, all methods that cause changes in the administrators, storefronts, products or, most importantly, balances, cannot be used.

There are no functions that are only usable if the contract is paused, but because the contract is Destructible, the owner could pause the contract, withdraw the contract balance, and then manually send each store owner their storefront balances if needed. For an idea about how whenPaused functions could be used, see the Speed Bump section below.

The pausable functions have the whenNotPaused modifier.

Fail early and fail loud

Used require to trigger an error instead of if in the following modifiers and functions:

Marketplace.sol

  • onlyAdmin()
  • removeAdmin(address admin)
  • requestStoreOwnerStatus()

Stores.sol

  • onlyStoreOwner()
  • onlyStorefrontOwner(bytes32 id)
  • withdrawStorefrontBalance(bytes32 storefrontId)
  • purchaseProduct(bytes32 storefrontId, bytes32 productId, uint qty)

Restricting Access

The following modifiers restrict access to certain functions:

Marketplace.sol

  • onlyAdmin()

Stores.sol

  • onlyStoreOwner()
  • onlyStorefrontOwner(bytes32 id)

Auto Deprecation

Not used. Nothing in the application should expire/be deprecated.

Mortal

Both Marketplace.sol and Stores.sol extend Zeppelin's Destructible contract and hence are mortal.

Pull over Push Payments

The payable function in Stores.sol is purchaseProduct. This function does not transfer funds to the storeOwner directly, but increments the storefront's balance. Withdrawals can then be done via the withdrawStorefrontBalance method. Both these functions are also pausable, because they each have the whenNotPaused modifier.

State Machine

Not used, but could be interesting to use if states were added to the products (ex: purchased, shipped, etc.). For now, this is not needed.

Speed Bump

Not used. Could be interesting to use it in conjunction with the Pausable functionality. For example, if we had a SlowWithdraw function that allowed storeOwners to withdraw their storefront balances N blocks after the contract had been paused.