Changes from v1 to v2

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Aave v2 improves many aspects over v1, opening up a vast new design space for developers to build products and services.
If you previously built on Aave v1, the following changes should be noted.
Tokenization
Keeping track of user positions
In Aave v2, all positions are tokenised. Therefore to calculate the debt that a user has, you just need to call balanceOf() for the associated debt token of that user.
See both aTokens and Debt Tokens sections for more info.
Approving ERC20/EIP20 tokens: LendingPool vs LendingPoolCore
In Aave v2, there is no more LendingPoolCore contract that holds all the protocol's assets. Asset's are held directly in the associated aToken contracts, with the LendingPool being the 'core' contract of the protocol.
WETH support
In Aave v2, to ensure consistency throughout the protocol we now use WETH (instead of ETH and a placeholder reserve value as was done in v1). Therefore to call a function using the ETH as an asset, use the WETH address.
There is also a WETH helper contract that will help with wrapping/unwrapping ETH, when used within the protocol.
Deposits
Redeem and Withdraw via LendingPool, not the aToken
In Aave v2, nearly all actions should be performed via the LendingPool contract. This is different from v1, where a redeem/withdraw of an aTokens needed to be called on the aToken contract. 
In v2, you will need to call withdraw on the LendingPool contract only.
Removal of interest redirection feature
In the initial release of Aave v2, interest redirection is not supported. However it is possible that the feature is added back later, via the governance process.
Borrowing
Native Credit Delegation
When a user has deposited collateral in the protocol, they can easily delegate credit to any address by calling approveDelegation() on the relevant debt token. 
See the Credit Delegation guide for more details.
Flash Loans
Within the protocol
In Aave v2, flash loans are possible within the protocol itself. In fact, they are used extensively within the protocol for position swapping and other 'trading' like features. 
See the Flash loans section for more details.
Batch flash loans
Flash loans can now be performed in batches, i.e. multiple flash loans with different parameters in the same call. This will allow powerful new use cases, such as paying back multiple assets and positions with one flash loan transaction.
Flash loans modes
You can now perform a mix of 'traditional' flash loans, which are paid back immediately, along with flash loans where a debt is incurred (i.e. the flash loan isn't paid back immediately). 
See the Flash loans section for more details.
Structural Changes
Addresses Provider Registry
With multiple markets, there will be multiple AddressesProviders. The AddressesProviderRegistry will maintain a registry of all the Aave market addresses providers.
Replacing LendingPoolCore
LendingPoolCore is no longer used. Only LendingPool is used, which simplifies integrations and building on Aave v2.
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