Persistent

Enzyme Protocol v4 (Sulu) - General Spec
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Persistent
Core
Philosophy
Enzyme has upgradable vaults, structured to give both managers and investors the opportunity to opt-in (managers) or opt-out (investors) of major updates.
For more on the philosophy and challenges of upgradability within the Enzyme Protocol, refer to this article: https://medium.com/enzymefinance/fund-in-the-shell-e82c46a0a0fa​
Approach
Essential state lives in a VaultProxy , which is moved between releases by upgrading its VaultLib and permissioned accessor via a global Dispatcher contract.
The essential state for a fund is:
holdings
shares
roles
VaultProxy
The "essential state" described above lives in per-fund VaultProxy contract instances, which are upgradable contracts following the EIP-1822 proxy pattern.
The VaultProxy specifies a VaultLib as its target logic, and these logic contracts are deployed per release, and swapped out during a migration.
A VaultLibBaseCore contract defines the absolutely essential state and logic for every VaultProxy. This includes:
a standard ERC20 implementation called SharesTokenBase 
the functions required of a IProxiableVault called by the Dispatcher 
core access control roles: owner, accessor, creator, and migrator 
The owner is the fund's owner.
The migrator is an optional role to allow a non-owner to migrate the fund.
The creator is the Dispatcher contract, which is allowed to update the accessor and vaultLib values.
The accessor is the primary account that can make state-changing calls to the VaultProxy . In practice, this is the release-level contract that interacts with a vault's assets, updates shares balances, etc.
This extremely abstract interface - in which a VaultProxy needs no knowledge about a release other than which caller can write state - allows for nearly limitless possibilities for release-level architecture.
Dispatcher
 An overarching, non-upgradable Dispatcher contract is charged with:
deploying new instances of VaultProxy 
migrating a VaultProxy from an old release to the current release
maintaining global state such as the current release, the global owner (i.e., the Enzyme Council) and the default symbol value for fund shares tokens
The Dispatcher stores the currentFundDeployer (a generic reference to the latest release's contract that is responsible for deploying and migrating funds), and only a msg.sender with that value is allowed to call functions to deploy or migrate a VaultProxy . 
This release-level FundDeployer can optionally implement migration hooks provided by IMigrationHookHandler, which give the release an opportunity to run arbitrary logic as the Dispatcher invokes those hooks at every step of the migration process.
As was the case described above with the VaultLibBaseCore , this abstracted notion of a FundDeployer - in which the Dispatcher only cares about its identity for access and for optional callbacks - is totally unrestrictive to the shape of the release-level protocol.
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Release-level contracts, then, are mostly arbitrary from the standpoint of these persistent contracts, offering maximum flexibility for future iterations and changes.
Other
In addition to the core persistent architecture that endures across all releases, there are also persistent components whose lifetimes can span one or many releases.
Used in this release:
Protocol Fee Reserve
The ProtocolFeeReserveProxy serves as a repository that stores collected fees, with upgradable logic for what can be done with the collected assets.
External Position Factory
Deploys new ExternalPositionProxy instances, which are upgradable proxy contracts used to manage positions that live outside of the vault, e.g., CDPs.
GitHub repo
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External Position Factory