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418 lines
22 KiB
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418 lines
22 KiB
Markdown
This document attempts to explain how Erigon organises its persistent data in its database and how this organisation is
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different from go-ethereum, the project from which it is derived. We start from a very simple genesis block and then
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apply one block containing an ETH transfer. For each step, we show visualisations produced by the code available in
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Erigon and code added to a fork of go-ethereum.
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Genesis in Erigon
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====================
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For the genesis block, we generate 3 different private keys and construct Ethereum addresses from them.
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Then, we endow one of the accounts with 9 ETH, and two others with 0.2 and 0.3 ETH, respectively.
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This is how the initial state trie looks:
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![genesis_state](state_0.png)
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In this and other illustrations, the colored boxes correspond to hexadecimal digits (a.k.a. nibbles), with values 0..f.
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Here is the palette:
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![hex_palette](hex_palette.png)
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First thing to note about the illustration of the initial state trie is that the leaves correspond to our accounts with their
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ETH endowments. Account nonces, in our case all 0s, are also shown. If you count the number of coloured boxes, from top
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to bottom, up to any of the account leaves you will get 64. Since each nibble occupies half a byte, that makes each "key"
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in the state trie 32 bytes long. However, account addresses are only 20 bytes long. The reason we get 32 and not 20 is
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that all the keys (in our case account addresses) are processed by the `Keccak256` hash function (which has 32 byte
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output) before they are inserted into the trie. If we wanted to see what the corresponding account addresses were, we
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will have to look into the database. Here is what Erigon would persist after generating such a genesis block:
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![genesis_db](changes_0.png)
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The Erigon database is a key-value store organised in tables (also commonly referred to as "buckets"). This is the list
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of the __main__ tables in the database:
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- __Headers__
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- __Block Bodies__
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- __Header Numbers__
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- __Receipts__
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- __PlainState__
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- __History Of Accounts__
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- __Change Sets__
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- __HashedState__
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- __IntermediateTrieHashes__
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- __Tx Senders__
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Table "Headers"
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---------------
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This table stores information about block headers. For each block there are three types of block header records with the following (key, value) formats:
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* __FULL HEADER__: contains the complete block header with all its information parameters
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- _key_ : 8-byte big-endian block number + 32-byte block hash
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- _value_ : the RLP-encoded block header structured as defined in the [Ethereum Yellow Paper](https://github.com/ethereum/yellowpaper):
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* parent hash: 32-byte hash of the parent block
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* uncles hash: 32-byte hash of the uncle blocks
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* coinbase: 20-byte address beneficiary of mining reward
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* state root: 32-byte root hash of the state trie
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* transactions root: 32-byte root hash of the trie structure made up of the block transactions
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* receipts root: 32-byte root hash of the trie structure made up of the transaction receipts
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* logs bloom: 256-byte Bloom filter composed from indexable information in each log entry from the transaction receipts
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* difficulty: 8-byte scalar value corresponding to the diffculty level of this block
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* block number: 8-byte scalar value equal to the number of ancestor blocks in the chain (aka block heigth)
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* gas limit: 8-byte scalar value equal to the current limit of gas expenditure per block
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* gas used: 8-byte scalar value equal to the total gas used in transactions in this block
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* timestamp: 8-byte scalar value equal to the Unix epoch timestamp at the inception of this block
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* extra data: up to 32-byte array of additional relevant data for this block (at least for Eth-hash consensus engine, for Clique could be more)
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* mix_hash: 64-byte hash proving, combined with nonce, the computation amount spent in block mining
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* nonce: 8-byte value proving, combined with mix hash, the computation amount spent in block mining
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* __TOTAL DIFFICULTY HEADER__: contains the total mining difficulty (TD) of the chain ending in such specific block
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- _key_ : 8-byte big-endian block number + 32-byte block hash + `0x74` suffix (ASCII code for `t` character)
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- _value_ : variable-length RLP-encoded total difficulty value: the cumulative difficulty value from first block to this one
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* __CANONICAL HEADER__: contains the block hash
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- key: 8-byte big-endian block number + `0x6E` suffix (ASCII code for `n` character)
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- value: 32-byte block hash
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as shown in the following picture for the block 0 (from top to bottom the three types of records, key on the left and value on the right):
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![genesis_db_headers](changes_0_Headers.png)
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You can check that for the genesis block
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* block number 0 is encoded as 0x0000000000000000 (see 8-byte zeros start of each key)
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* block hash is 0x61eb...aba1
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* logs bloom is 256-byte zeros (see the 8 white rows of all 32-byte 0s)
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* total difficulty is 0x80, which is RLP encoding of value 0
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Table "Block Bodies"
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---------------------
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![genesis_db_block_bodies](changes_0_BlockBodies.png)
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The keys in this table are concatenations of 8-byte encoding of the block number and 32-byte block hash.
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The values are RLP-encoded list of 2 structures:
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1. List of transactions
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2. List of ommers (a.k.a uncles)
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In the case of the genesis block, all of these lists are empty (RLP encodings `0xC0`), and the prefix `0xC3` means
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in RLP "some number of sub-structures with the total length of 3 bytes".
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Next three tables, "Last Header", "Last Fast", and "Last Block", always contain just one record each, and their
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keys are always the same, the ASCII-encodings of the strings `LastHeader`, `LastFast`, and `LastBlock`, respectively.
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The values record the block/header hash of the last header, receipt or block chains that the node has managed to sync
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from its peers in the network. The value in "Last Fast" bucket is not really used at the moment, because Erigon
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does not support Fast Sync.
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Table "Header Numbers"
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-----------------------
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Is a mapping of 32-byte header/block hashes to the corresponding block numbers (encoded
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in 8 bytes):
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![genesis_db_header_numbers](changes_0_HeaderNumbers.png)
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Table "Receipts"
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-----------------
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Records the list of transaction receipts for each block:
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![genesis_db_receipts](changes_0_Receipts.png)
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The 8 bytes of the key (or 16 nibbles, equaling to 0s here) encode the block number, which is 0 for the Genesis block.
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The value is the CBOR-encoded list of receipts. In our case, there were
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no transactions in the Genesis block, therefore, we have CBOR encoding of an empty list, `0xf6`.
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Table "PlainState"
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-----------------
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![genesis_db_accounts](changes_0_PlainState.png)
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Store together Accounts and Storage.
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Accounts: key="account address", value="current state of each account".
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Storage: key="account address+incarnation+storage hash", value="current value of storage".
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The accounts encoded by function `account.go:EncodeForStorage()` so that the first byte of a
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field is its length and bytes afterward the field itself.
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They all start with a fieldset of `0x02`, for each bit set into the fieldSet a field is present.
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In this case `0x02` in binary is `10` meaning that only the second field is set (the balance).
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the order in the fieldset is the following:
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* 1st bit: Nonce
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* 2nd bit: Balance
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* 3rd bit: Incarnation
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* 4th bit: Code Hash
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Therefore, immediately after the fieldset we have the length in byte of the balance: `0x08` (`8` bytes).
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the following 8 bytes (aka. 16 nibbles) contains the hexadecimal value of the
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balance that in the first record would be: `0x7ce66c50e2840000`.
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````
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$ python
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Python 2.7.15
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Type "help", "copyright", "credits" or "license" for more information.
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>>> 0x7ce66c50e2840000
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9000000000000000000
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````
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Which is 9 followed by 18 zeros, which is 9 ETH (1 ETH = 10^18 wei).
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The third bit of the field set represent the Incarnation. The Incarnation is a Erigon specific attribute, which is
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used to make removal and revival of contract accounts (now possible with `CREATE2` since Constantinopole) efficient
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with Erigon's database layout. For now it will suffice to say that all non-contract accounts will have
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incarnation 0 (not set), and all contract accounts will start their existence with incarnation 1.
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Contract accounts may also contain a contract code hash and storage root. These two pieces of information would make
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the record in the "Accounts" bucket contain 5 instead of 3 fields.
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Table "History Of Accounts"
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----------------------------
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![genesis_db_history_of_accounts](changes_0_HistoryOfAccounts.png)
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key="account addresses + incarnation", value="encoded array of block numbers, where account changed/created".
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The history of accounts records how the accounts changed at each block. But, instead of recording,
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at each change, the value that the accounts had *after* the change, it records what value the accounts
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had *before* the change. That explains the empty values here -- it records the fact that these
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three accounts in questions did not exist prior to the block 0.
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Table "Change Sets"
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--------------------
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Records the history of changes in accounts and contract storage. But, unlike in "History of Accounts"
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and "History of Storage", where keys are derived from accounts' addresses
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(key="address hash + block number"), in the "Change Sets" table, keys are derived from the block numbers:
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![genesis_db_change_sets](changes_0_AccountChanges.png)
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In the cases of our Genesis block, the key is composed from the encoding of the block number (`0x20`), and the
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ASCII-code of `hAT` (meaning **h**istory of **A**counts **T**rie).
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The "Change Set" table records changes that happen to accounts and contract storage slots at every block.
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It is important to note that the values recorded in the "Changes Set" table are not the values the accounts
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(or storage slots) had AFTER the change, it records what value the accounts (or storage slots)
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had BEFORE the change. That explains the empty values here - it records the fact that these
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three accounts in question did not exist prior to the block 0.
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The encoding of the values in the records is tailored for fast access and binary search. It has 5 parts:
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1. Number of keys-value pairs, encoded as a 4-byte (32-bit) number. In this example, it is `0x00000003`, which means
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there are 3 key-value pairs
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2. Size of each key, also encoded as a 32-bit number. All keys are the same size, which makes it possible to
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access them without deserialisation. In this example, it is `0x00000020`, which 32, meaning that all keys are
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32 bytes long.
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3. Keys themselves. In our examples, these are the coloured boxes before the streak of white 0s. Keys are sorted
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lexicographically. This, together with the keys being the same size, allows binary search without deserialisation,
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as well as linear-time merge of multiple changesets.
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4. Value offsets. These offsets mark the beginning of the next, 5th part as offset 0. First value has offset 0.
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In our example, all values are empty strings, therefore we see 3 zero offsets (24 white boxes with zeros in them).
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5. Values themselves. In our example, they are empty, so this 5th part is not present.
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Tables "HashedState" and "IntermediateTrieHashes"
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--------------------------------------------------
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These tables are used to calculate Merkle Trie root hash of whole current state. For Genesis block, these tables
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are not populated, because the root hash is known in advance.
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Block with 1 Ethereum transaction in Erigon
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===============================================
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The first block contains transaction that transfers 0.001 ETH from the account holding 9 ETH to
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a new account with the address `0x0100000000000000000000000000000000000000` (this is what one gets
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specifying `common.Address{1}` in the code).
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This is how the state trie after Block 1 is inserted looks like:
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![block1_state](state_2.png)
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First thing to note about the illustration of the state trie is that the nonce of the first account is now 1 since
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a transaction from that address has been executed and the amount of ETH of that account is now
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8.999 since the transaction had a value of 0.001 ETH. Usually, the transaction fee would also
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be paid, but in our simplified example, we set gas price to 0, therefore no fees are deducted.
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A new account has been added to the state trie, the recipient of the transaction.
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In fact, it has 0.001 ETH (the value of the transaction) and its nonce is set to 0.
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What follows are excerpts from the Erigon database after block 1 has been inserted.
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These excerpts only show the records that have changed, and
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omit the records that are the same as before the insertion of the block.
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![block1_db](changes_1.png)
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Table "Receipts"
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----------------
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Table `Receipts` now has some non-empty information for the block 1, because we had 1 transaction:
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![block1_db_receipts](changes_1_Receipts.png)
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The first 8 bytes of the key (or 16 nibbles, 15 zeroes followed by `1`) encode the block number, which is 1.
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Receipt records are only kept for "canonical" blocks, and not for forked blocks, that is why receipt keys
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do not include 32-byte block hash.
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The value is the CBOR-encoded list of receipts. In our case, a transaction was sent and as a matter of fact,
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instead of having `0xC0` (that represent an empty list) we have `0xc6c501825208c0` that is the receipt for the 0.001 ETH transaction.
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First prefix, `c6` means that the following 6 bytes is a sequence of RLP-encoded sub-structures.
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In our case, there is only one sub-structure, and its
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prefix `c5` means that the following 5 bytes is a sequence of RLP-encoded sub-structures.
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There are 3 of them: `01`, `825208`, and `c0`. The first encodes
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number 1, which means the "Success" status of the transaction (0 would mean failure).
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Next, we have a byte array of length 2 (prefix `82`), `5208`, which is
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21000 in hexadecimal, the cumulative gas used by all transactions so far.
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And the last structure, `c0` is RLP encoding of an empty list. Usually, the log (events) emitted by the transaction,
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would be here. But since our transaction did not emit anything, the list is empty.
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Table "Senders"
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---------------
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![block1_db_senders](changes_1_Senders.png)
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The reason Erigon keeps the list of transaction sender addresses for each transaction has to do with the fact that
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transactions themselves do not contain this information directly. Sender's address can be "recovered" from the digital
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signature, but this recovery can be computationally intensive, therefore we "memoise" it.
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Table "History Of Accounts"
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---------------------------
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![block1_db_history_of_accounts](changes_1_HistoryOfAccounts.png)
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The first entry has a non-empty value. This is the sender of 0.001 ETH.
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As history bucket records the value of this account *before* the change, we should
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expect the same value here as this account had at the Genesis.
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The other two records contain empty values, which means these account were non-existent previously.
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These are the account of the recepient of 0.001 ETH (`0x0100000000000000000000000000000000000000`),
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and the miner account `0x0000000000000000000000000000000000000000`.
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The keys of all three records have the common suffix of `0x21` instead of `0x20`, which is
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simply encoding of the block number 1 instead of block number 0.
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**WARNING** We are planning a change, called `THIN_HISTORY`, after which the bucket
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"History of Accounts" will not store the values of the accounts,
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but only the block numbers when those accounts changed.
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Next bucket is "Headers"
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![block1_db_headers](changes_1_Headers.png)
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A new header has been generated since a new block has been mined.
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As a matter of fact, the keys for the first two records start with a block number of 1 followed by
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the block hash (or header hash, which is the same thing) that is
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different than the Genesis block. Additionally, these two records for the new block
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that respectively represent the values in the headers and the mining difficulty differs from the Genesis Block.
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Next bucket is "Block Bodies":
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![block1_db_block_bodies](changes_1_BlockBodies.png)
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The keys in this bucket are concatenations of 8-byte encoding of the block number and 32-byte block hash.
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The new record is the representation of block 1.
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![block1_db_change_sets](changes_1_AccountChanges.png)
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Bucket "Change Sets" recorded the change in account that occurred *before* the 0.001 ETH transaction. You can notice that unlike the Genesis block, this time the key has a prefix of `0x21` since now the blockchain is 1 block apart from the Genesis block.
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Like in the ChangeSet in the Genesis block, there are 3 new records. But unlike the ChangeSet in the genesis block, one of the records (the last one) has
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a non-empty value.
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The next bucket is "PlainState":
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![block1_db_accounts](changes_1_PlainState.png)
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The values are the current state of each account.
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* The first record has a field set of `0x3` (`11`) meaning that this time the accound has a nonce as well. The nonce length is `0x01`, therefore the nonce is stored in the byte immediately after the length of the field: `0x01`. The balance is now changed to `0x7ce2ded23dbd8000`.
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The value `0x7ce2ded23dbd8000` is the hexadecimal value for:
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````
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$ python
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Python 2.7.15
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Type "help", "copyright", "credits" or "license" for more information.
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>>> 0x7ce2ded23dbd8000
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8999000000000000000
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````
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Which is 9 ETH - 0.001 ETH, this means that this account is the sender.
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* in the second value the field set is `0x02` meaning that only the balance is set.
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The value `0x1bc16d674ec80000` (balance) is the hexadecimal value for:
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````
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$ python
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Python 2.7.15
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Type "help", "copyright", "credits" or "license" for more information.
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>>> 0x1bc16d674ec80000
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2000000000000000000
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````
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Which is 2 ETH that probably represent the mining reward.
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* in the third value we have prefix `0x06` (`110`), meaning that only balance and incarnation are set. the balance is now `0x038d7ea4c68000` .Start python and do this:
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````
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$ python
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Python 2.7.15
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Type "help", "copyright", "credits" or "license" for more information.
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>>> 0x038d7ea4c68000
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1000000000000000
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````
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Which is 0.001 ETH that is exactly how much has been sent to the recipient address, meaning that this account represent the recipient of the transaction.
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Because Erigon does it completely different from go-ethereum,
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we created [dedicated article](guide.md#organising-ethereum-state-into-a-merkle-tree) for it.
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Genesis in go-ethereum
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------------------------------
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Now we will create the same Genesis state and block in go-ethereum (in archive mode to make sure we compare like for like).
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Here is how the database looks like. Since go-ethereum uses LevelDB, and LevelDB does not have a concept of "buckets" (or
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"tables"), go-ethereum emulates them by adding table-specific prefixes to all the keys, with the exception of the keys that
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describe the state trie (bucket "Hashes" in our example). In the illustration, these prefixes are mostly removed for better
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comparison with Erigon. They were not removed only for the buckets "LastBlock", "LastHeader" and "LastFast", because
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othewise they key would be empty.
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![geth_genesis_db](geth_changes_0.png)
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The buckets "Headers", "Config", "Last Header", "Last Fast", "Last Block", all look identical
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to those in the Erigon database. We will walk through the ones that are different.
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In the bucket "Block Bodies", the value is slightly different:
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![geth_genesis_block_bodies](geth_changes_0_b_5.png)
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The difference is that the block body has 2 elements instead of 3 in Erigon. The missing element is the list
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of the sender addresses that go-ethereum does not store, but recomputes after loading or caches in memory.
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The buckets "Accounts", "History Of Accounts", and "Change Sets" are missing, because go-ethereum uses a very
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different mechanism for storing the state and its history:
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![geth_genesis_hashes](geth_changes_0_hashes_0.png)
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In the illustration showing the state trie, one can find 4 parts of the diagram that consist of the coloured boxes
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(that excludes the leaves that contain account balances and nonces). These parts are usually called "trie nodes",
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and in the diagram above we see 2 types of trie nodes:
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1. Branch node. This is the horizontal line of 3 coloured boxes on the top. It branches the traversal of the state
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trie from top to bottom 3-ways.
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2. Leaf node. These are 3 vertical lines of 63 coloured boxes.
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Each type of trie nodes can be serialised (using RLP encoding), to convert it to a string of bytes. What we see in
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the values of the records in the "Hashes" bucket just above are the RLP-encodings of these 4 trie nodes.
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What we see in the keys of these records are the results of `Keccak256` function applied to the values. In a way,
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this is similar to the "Preimages" bucket, with the different type of values.
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If you look closely, you may notice that the keys of the last 3 records are actually contained inside the value
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of the first record. This is because the first value correponds to that 3-way branch node, and the hashes of the
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leaf nodes are used like "pointers" to thoese nodes. Continuing the "pointer" analogy, you can say that
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"dereferencing" these pointers mean fetching the corresponding records from this "Hashes" bucket. Using such
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"derederencing" process, one can traverse the state trie from the top to any leaf at the bottom. Each step in
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such traversal requires finding the corresponding record in the "Hashes" bucket.
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Block 1 in go-ethereum
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Now we will create the same Transaction for go-ethereum.
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![geth_block1_db](geth_changes_1.png)
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The buckets "Receipts", "Headers", "Config", "Last Header", "Last Fast", "Last Block", all look identical
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to those in the Erigon database. We will walk through the ones that are different.
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In the bucket "Block Bodies", the value is slightly different:
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![geth_block1_block_bodies](geth_changes_1_b_4.png)
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The difference is that the block body is missing the sender address since the send in go-ethereum is computed and in Erigon is stored.
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For the rest in this case the same changes occurs in terms of block number and block hash.
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![geth_block1_hashes](geth_changes_1_hashes_5.png)
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First of all a new branch node has been generated for the block.
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That's the first record because it contains in its value the key of the other key records.
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The three leaf nodes contains the `Keccak256` of the change in states of the account of the recipient,
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sender and miner of the block.
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