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| 1 | +--- |
| 2 | +comments: true |
| 3 | +--- |
| 4 | + |
| 5 | +Polygon zkEVM deploys smart contracts to manage transaction processing and related data transfers between itself and other networks, such as the Ethereum L1 or other L2s connected to the AggLayer. |
| 6 | + |
| 7 | +There are four key contract types built into the system design: |
| 8 | + |
| 9 | +- [Consensus contracts](#consensus-contracts). |
| 10 | +- [Rollup manager](#rollup-manager). |
| 11 | +- [Bridge contract](#bridge). |
| 12 | +- [Exit root management](#global-exit-roots). |
| 13 | + |
| 14 | +### Consensus |
| 15 | + |
| 16 | +Consensus contracts live in the L1 and expose specific functions for controlling the sequencing and verification mechanisms triggered by the sequencer and aggregator. |
| 17 | + |
| 18 | +Polygon zkEVM can be implemented as, |
| 19 | + |
| 20 | +- Either a zk-rollup, deploying the [`PolygonZkEVMEtrog.sol`](https://github.com/0xPolygonHermez/zkevm-contracts/blob/a5eacc6e51d7456c12efcabdfc1c37457f2219b2/contracts/v2/consensus/zkEVM/PolygonZkEVMEtrog.sol) consensus contract, where all transaction data is posted to Ethereum. |
| 21 | + |
| 22 | +- Or, a validium using a data-availability committee (DAC) for managing availability of transaction data, and deploying the [`PolygonValidiumEtrog.sol`](https://github.com/0xPolygonHermez/zkevm-contracts/blob/a5eacc6e51d7456c12efcabdfc1c37457f2219b2/contracts/v2/consensus/validium/PolygonValidiumEtrog.sol) consensus contract. |
| 23 | + |
| 24 | +These contracts therefore define the type of a CDK chain, either a validium or zk-rollup. |
| 25 | + |
| 26 | +### Rollup manager |
| 27 | + |
| 28 | +The [PolygonRollupManager.sol](https://github.com/0xPolygonHermez/zkevm-contracts/blob/main/contracts/v2/PolygonRollupManager.sol) contract is useful for managing CDK chains. |
| 29 | +It is responsible for creating, updating, and verifying CDK rollup and validium chains. |
| 30 | + |
| 31 | +### Bridge |
| 32 | + |
| 33 | +The unified bridge contract [PolygonZkEVMBridgeV2.sol](https://github.com/0xPolygonHermez/zkevm-contracts/blob/main/contracts/v2/PolygonZkEVMBridgeV2.sol) is responsible for bridging and claiming assets or messages across L1 and L2 chains. |
| 34 | + |
| 35 | +### Global exit roots |
| 36 | + |
| 37 | +Each bridging of an asset or a message from the zkEVM is recorded on a leaf of the zkEVM's _local exit tree_. |
| 38 | + |
| 39 | +Every update of the local exit tree at the leaf-level triggers an update of the local exit root, which in turn is appended to a leaf of the global exit tree. |
| 40 | + |
| 41 | +Similarly, each bridging of an asset or a message from the L1 to the zkEVM is recorded on a leaf of the _L1 Info tree_. |
| 42 | + |
| 43 | +The [PolygonZkEVMGlobalExitRootV2.sol](https://github.com/0xPolygonHermez/zkevm-contracts/blob/main/contracts/v2/PolygonZkEVMGlobalExitRootV2.sol) contract manages the exit roots across multiple networks at the Ethereum L1 level. |
| 44 | + |
| 45 | +The [PolygonZkEVMGlobalExitRootL2.sol](https://github.com/0xPolygon/cdk-validium-contracts/blob/main/contracts/PolygonZkEVMGlobalExitRootL2.sol) contract manages the zkEVM state by keeping track of the current local exit root. |
| 46 | + |
| 47 | +### Validium stacks |
| 48 | + |
| 49 | +CDK validium stacks use the [cdk-validium-contracts](https://github.com/0xPolygon/cdk-validium-contracts/tree/main) which has slightly adjusted behavior to take account of data-availability components and custom CDK requirements. |
| 50 | + |
| 51 | +The CDK repo is a fork of the zkEVM main contracts repo and all contracts, therefore, extend from common interfaces. |
| 52 | + |
| 53 | +!!! important |
| 54 | + - A CDK validium stack starts life as a rollup stack. |
| 55 | + - It may interchangeably be referred to as such when discussing aspects shared by the two options. |
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