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Polygon PoS Architecture

Polygon network is a blockchain application platform that provides hybrid Proof-of-Stake and Plasma-enabled sidechains.

Architecturally, the beauty of Polygon is its elegant design, which features a generic validation layer separated from varying execution environments like full-blown EVM sidechains and other layer 2 approaches such as zero-knowledge rollups.

To enable the PoS mechanism on our platform, a set of staking management contracts are deployed on Ethereum, as well as a set of incentivized validators running Heimdall and Bor nodes. Ethereum is the first basechain Polygon supports, but Polygon intends to offer support for additional basechains, based on community suggestions and consensus, to enable an interoperable decentralized Layer 2 blockchain platform.

Polygon PoS has a three-layer architecture:

  1. Staking smart contracts on Ethereum
  2. Heimdall (Proof of Stake layer)
  3. Bor (Block producer layer)

Polygon smart contracts (on Ethereum)​

Polygon maintains a set of smart contracts on Ethereum, which handle the following:

  • Staking management for the Proof-of-Stake layer
  • Delegation management including validator shares
  • Checkpoints/snapshots of sidechain state

Heimdall (Proof-of-Stake validator layer)​

Heimdall is the PoS validator node that works in consonance with the staking contracts on Ethereum to enable the PoS mechanism on Polygon. We have implemented this by building on top of the Tendermint consensus engine with changes to the signature scheme and various data structures. It is responsible for block validation, block producer committee selection, checkpointing a representation of the sidechain blocks to Ethereum in our architecture and various other responsibilities.

Heimdall layer handles the aggregation of blocks produced by Bor into a merkle tree and publishing the merkle root periodically to the root chain. These periodic publishings are called checkpoints. For every few blocks on Bor, a validator (on the Heimdall layer):

  1. Validates all the blocks since the last checkpoint
  2. Creates a merkle tree of the block hashes
  3. Publishes the merkle root to the main chain

Checkpoints are important for two reasons:

  1. Providing finality on the Root Chain
  2. Providing Proof of Burn in withdrawal of assets

A bird’s eye view of the process can be explained as:

  • A subset of active validators from the pool are selected to act as block producers for a span. The Selection of each span will also be consented by at least 2/3 in power. These block producers are responsible for creating blocks and broadcasting it to the remaining network.
  • A checkpoint includes the root of all blocks created during any given interval. All nodes validate the same and attach their signature to it.
  • A selected proposer from the validator set is responsible for collecting all signatures for a particular checkpoint and committing the same on the main chain.
  • The responsibility of creating blocks and also proposing checkpoints is variably dependent on a validator’s stake ratio in the overall pool.

Bor (Block Producer Layer)​

Bor is Polygon block producer layer - the entity responsible for aggregating transactions into blocks.

Block producers are periodically shuffled via committee selection on Heimdall in durations termed as a span in Polygon. Blocks are produced at the Bor node and the sidechain VM is EVM-compatible. Blocks produced on Bor are also validated periodically by Heimdall nodes, and a checkpoint consisting of the Merkle tree hash of a set of blocks on Bor is committed to Ethereum periodically.

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