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Blocks

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Blocks are batches of transactions with a hash of the previous block in the chain. This links blocks together (in a chain) because hashes are cryptographically derived from the block data. This prevents fraud, because one change in any block in history would invalidate all the following blocks as all subsequent hashes would change and everyone running the blockchain would notice.

Prerequisites

Blocks are a very beginner-friendly topic. But to help you better understand this page, we recommend you first read Accounts, Transactions, and our introduction to Ethereum.

Why blocks?

To ensure that all participants on the Ethereum network maintain a synchronized state and agree on the precise history of transactions, we batch transactions into blocks. This means dozens (or hundreds) of transactions are committed, agreed on, and synchronized all at once.

A diagram showing transaction in a block causing state changes Diagram adapted from Ethereum EVM illustrated

By spacing out commits, we give all network participants enough time to come to consensus: even though transaction requests occur dozens of times per second, blocks are only created and committed on Ethereum once every twelve seconds.

How blocks work

To preserve the transaction history, blocks are strictly ordered (every new block created contains a reference to its parent block), and transactions within blocks are strictly ordered as well. Except in rare cases, at any given time, all participants on the network are in agreement on the exact number and history of blocks, and are working to batch the current live transaction requests into the next block.

Once a block is put together by some validator on the network, it is propagated to the rest of the network; all nodes add this block to the end of their blockchain, and a new validator is selected to create the next block. The exact block-assembly process and commitment/consensus process is currently specified by Ethereum’s “proof-of-stake” protocol.

Proof-of-stake protocol

Proof-of-stake means the following:

  • Validating nodes have to stake 32 ETH into a deposit contract as collateral against bad behavior. This helps protect the network because provably dishonest activity leads to some or all of that stake being destroyed.
  • In every slot (spaced twelve seconds apart) a validator is randomly selected to be the block proposer. They bundle transactions together, execute them and determine a new 'state'. They wrap this information into a block and pass it around to other validators.
  • Other validators who hear about a new block re-execute the transactions to ensure they agree with the proposed change to the global state. Assuming the block is valid they add it to their own database.
  • If a validator hears about two conflicting blocks for the same slot they use their fork-choice algorithm to pick the one supported by the most staked ETH.

More on proof-of-stake

What's in a block?

There is a lot of information contained within a block. At the highest level a block contains the following fields:

FieldDescription
slotthe slot the block belongs to
proposer_indexthe ID of the validator proposing the block
parent_rootthe hash of the preceding block
state_rootthe root hash of the state object
bodyan object containing several fields, as defined below

The block body contains several fields of its own:

FieldDescription
randao_reveala value used to select the next block proposer
eth1_datainformation about the deposit contract
graffitiarbitrary data used to tag blocks
proposer_slashingslist of validators to be slashed
attester_slashingslist of validators to be slashed
attestationslist of attestations in favor of the current block
depositslist of new deposits to the deposit contract
voluntary_exitslist of validators exiting the network
sync_aggregatesubset of validators used to serve light clients
execution_payloadtransactions passed from the execution client

The attestations field contains a list of all the attestations in the block. Attestations have their own data type that contains several pieces of data. Each attestation contains:

FieldDescription
aggregation_bitsa list of which validators participated in this attestation
dataa container with multiple subfields
signatureaggregate signature of all attesting validators

The data field in the attestation contains the following:

FieldDescription
slotthe slot the attestation relates to
indexindices for attesting validators
beacon_block_rootthe root hash of the Beacon block containing this object
sourcethe last justified checkpoint
targetthe latest epoch boundary block

Executing the transactions in the execution_payload updates the global state. All clients re-execute the transactions in the execution_payload to ensure the new state matches that in the new block state_root field. This is how clients can tell that a new block is valid and safe to add to their blockchain. The execution payload itself is an object with several fields. There is also an execution_payload_header that contains important summary information about the execution data. These data structures are organized as follows:

The execution_payload_header contains the following fields:

FieldDescription
parent_hashhash of the parent block
fee_recipientaccount address for paying transaction fees to
state_rootroot hash for the global state after applying changes in this block
receipts_roothash of the transaction receipts trie
logs_bloomdata structure containing event logs
prev_randaovalue used in random validator selection
block_numberthe number of the current block
gas_limitmaximum gas allowed in this block
gas_usedthe actual amount of gas used in this block
timestampthe block time
extra_dataarbitrary additional data as raw bytes
base_fee_per_gasthe base fee value
block_hashHash of execution block
transactions_rootroot hash of the transactions in the payload

The execution_payload itself contains the following (notice this is identical to the header except that instead of the root hash of the transactions it includes the actual list of transactions) :

FieldDescription
parent_hashhash of the parent block
fee_recipientaccount address for paying transaction fees to
state_rootroot hash for the global state after applying changes in this block
receipts_roothash of the transaction receipts trie
logs_bloomdata structure containing event logs
prev_randaovalue used in random validator selection
block_numberthe number of the current block
gas_limitmaximum gas allowed in this block
gas_usedthe actual amount of gas used in this block
timestampthe block time
extra_dataarbitrary additional data as raw bytes
base_fee_per_gasthe base fee value
block_hashHash of execution block
transactionslist of transactions to be executed

Block time

Block time refers to the time separating blocks. In Ethereum, time is divided up into twelve second units called 'slots'. In each slot a single validator is selected to propose a block. Assuming all validators are online and fully functional there will be a block in every slot, meaning the block time is 12s. However, occasionally validators might be offline when called to propose a block, meaning slots can sometimes go empty. This is different to proof-of-work based systems where block times are probabilistic and tuned by the mining difficulty.

Block size

A final important note is that blocks themselves are bounded in size. Each block has a target size of 15 million gas but the size of blocks will increase or decrease in accordance with network demands, up until the block limit of 30 million gas (2x target block size). The total amount of gas expended by all transactions in the block must be less than the block gas limit. This is important because it ensures that blocks can’t be arbitrarily large. If blocks could be arbitrarily large, then less performant full nodes would gradually stop being able to keep up with the network due to space and speed requirements. The larger the block, the greater the computing power required to process them in time for the next slot. This is a centralizing force, which is resisted by capping block sizes.

Further reading

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