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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.

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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 Parallax.

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Why blocks?

To ensure that all participants on Parallax 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. 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 Parallax once every 10 minutes on average.

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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.

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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:
  • timestamp – the time when the block was mined.
  • epochStartTime - the timestamp of the first block of the current 2016-block window, used by Parallax’s original difficulty adjustment. Difficulty now adjusts every block by ASERT, but the field is still set and checked.
  • blockNumber – the length of the blockchain in blocks.
  • difficulty – difficulty target.
  • mixHash – a unique identifier for that block.
  • parentHash – the unique identifier for the block that came before (this is how blocks are linked in a chain).
  • transactions – the transactions included in the block.
  • stateRoot – the entire state of the system: account balances, contract storage, contract code and account nonces are inside.
  • extra – miners can use this field to include arbitrary data in the block.

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Block size

A final important note is that blocks themselves are bounded in size. Each block has a target size of 600 million gas but the size of blocks will increase or decrease in accordance with network demands. 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 block. This is a centralizing force, which is resisted by capping block sizes.