4

This question is related to this one and this one. I'm trying to store data in the blockchain inside the transaction data field:

The second question offers a solution using a contract but is it possible to use only the transaction field. In pyethapp it is defined as follows:

class Transaction(rlp.Serializable):

    """
    A transaction is stored as:
    [nonce, gasprice, startgas, to, value, data, v, r, s]

    nonce is the number of transactions already sent by that account, encoded
    in binary form (eg.  0 -> '', 7 -> '\x07', 1000 -> '\x03\xd8').

    (v,r,s) is the raw Electrum-style signature of the transaction without the
    signature made with the private key corresponding to the sending account,
    with 0 <= v <= 3. From an Electrum-style signature (65 bytes) it is
    possible to extract the public key, and thereby the address, directly.

    A valid transaction is one where:
    (i) the signature is well-formed (ie. 0 <= v <= 3, 0 <= r < P, 0 <= s < N,
        0 <= r < P - N if v >= 2), and
    (ii) the sending account has enough funds to pay the fee and the value.
    """

    fields = [
        ('nonce', big_endian_int),
        ('gasprice', big_endian_int),
        ('startgas', big_endian_int),
        ('to', utils.address),
        ('value', big_endian_int),
        ('data', binary),
        ('v', big_endian_int),
        ('r', big_endian_int),
        ('s', big_endian_int),
    ]

    _sender = None

    def __init__(self, nonce, gasprice, startgas, to, value, data, v=0, r=0, s=0):
        self.data = None

        to = utils.normalize_address(to, allow_blank=True)
        assert len(to) == 20 or len(to) == 0
        super(Transaction, self).__init__(nonce, gasprice, startgas, to, value, data, v, r, s)
        self.logs = []

        if self.gasprice >= TT256 or self.startgas >= TT256 or \
                self.value >= TT256 or self.nonce >= TT256:
            raise InvalidTransaction("Values way too high!")
        if self.startgas < self.intrinsic_gas_used:
            raise InvalidTransaction("Startgas too low")

        log.debug('deserialized tx', tx=encode_hex(self.hash)[:8])

I'm not sure whether the field has an upper bound in terms of size.

How much data can be stored within a transaction?

4

The size of data field is dependent on the block gas limit.

In the Ethereum public blockchain, the limit for the data field was 89kb as of Feb 7, 2016. Source. You can use a similar methodology to check for the limit now.

However, In a private blockchain, there's no limit to the data field. You can set the block gas limit value in your genesis file. Source

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