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One of the main modules of the Polygon SDK are Blockchain and State.

Blockchain is the powerhouse that deals with block reorganizations. This means that it deals with all the logic that happens when a new block is included into the blockchain.

State represents the state transition object. It deals with how the state changes when a new block is included.
Among other things, State handles:

  • Executing transactions
  • Executing the EVM
  • Changing the Merkle tries
  • Much more, which is covered in the corresponding State section ๐Ÿ™‚

The key takeaway is that these 2 parts are very connected, and they work closely together in order for the client to function.
For example, when the Blockchain layer receives a new block (and no reorganization occurred), it calls the State to perform a state transition.

Blockchain also has to deal with some parts relating to consensus (ex. is this ethHash correct?, is this PoW correct?).
In one sentence, it is the main core of logic through which all blocks are included.


One of the most important parts relating to the Blockchain layer is the WriteBlocks method:

// WriteBlocks writes a batch of blocks
func (b *Blockchain) WriteBlocks(blocks []*types.Block) error {
if len(blocks) == 0 {
return fmt.Errorf("no headers found to insert")
parent, ok := b.readHeader(blocks[0].ParentHash())
if !ok {
return fmt.Errorf("parent of %s (%d) not found: %s", blocks[0].Hash().String(), blocks[0].Number(), blocks[0].ParentHash())
// validate chain
for i := 0; i < len(blocks); i++ {
block := blocks[i]
if blocks[i].Number()-1 != parent.Number {
return fmt.Errorf("number sequence not correct at %d, %d and %d", i, blocks[i].Number(), parent.Number)
if blocks[i].ParentHash() != parent.Hash {
return fmt.Errorf("parent hash not correct")
if err := b.consensus.VerifyHeader(parent, blocks[i].Header, false, true); err != nil {
return fmt.Errorf("failed to verify the header: %v", err)
// verify body data
if hash := buildroot.CalculateUncleRoot(block.Uncles); hash != blocks[i].Header.Sha3Uncles {
return fmt.Errorf("uncle root hash mismatch: have %s, want %s", hash, blocks[i].Header.Sha3Uncles)
if hash := buildroot.CalculateTransactionsRoot(block.Transactions); hash != blocks[i].Header.TxRoot {
return fmt.Errorf("transaction root hash mismatch: have %s, want %s", hash, blocks[i].Header.TxRoot)
parent = blocks[i].Header
// Write chain
for indx, block := range blocks {
header := block.Header
body := block.Body()
if err := b.db.WriteBody(block.Header.Hash, block.Body()); err != nil {
return err
b.bodiesCache.Add(block.Header.Hash, body)
// Verify uncles. It requires to have the bodies on memory
if err := b.VerifyUncles(block); err != nil {
return err
// Process and validate the block
if err := b.processBlock(blocks[indx]); err != nil {
return err
// Write the header to the chain
evnt := &Event{}
if err := b.writeHeaderImpl(evnt, header); err != nil {
return err
// Update the average gas price
return nil

The WriteBlocks method is the entry point to write blocks into the blockchain. As a parameter, it takes in a range of blocks.
Firstly, the blocks are validated. After that, they are written to the chain.

The actual state transition is performed by calling the processBlock method within WriteBlocks.

It is worth mentioning that, because it is the entry point for writing blocks to the blockchain, other modules (such as the Sealer) utilize this method.

Blockchain Subscriptions#

There needs to be a way to monitor blockchain related changes.
This is where Subscriptions come in.

Subscriptions are a way to tap into blockchain event streams, and instantly receive meaningful data.

type Subscription interface {
// Returns a Blockchain Event channel
GetEventCh() chan *Event
// Returns the latest event (blocking)
GetEvent() *Event
// Closes the subscription

The Blockchain Events contain information regarding any changes made to the actual chain. This includes reorganizations, as well as new blocks:

type Event struct {
// Old chain removed if there was a reorg
OldChain []*types.Header
// New part of the chain (or a fork)
NewChain []*types.Header
// Difficulty is the new difficulty created with this event
Difficulty *big.Int
// Type is the type of event
Type EventType
// Source is the source that generated the blocks for the event
// right now it can be either the Sealer or the Syncer. TODO
Source string

Do you remember when we mentioned the monitor command in the CLI Commands?

The Blockchain Events are the original events that happen in Polygon SDK, and they're later mapped to a Protocol Buffers message format for easy transfer.