mirror of
https://github.com/corda/corda.git
synced 2024-12-21 13:57:54 +00:00
76 lines
3.4 KiB
ReStructuredText
76 lines
3.4 KiB
ReStructuredText
Consensus
|
|
=========
|
|
|
|
.. topic:: Summary
|
|
|
|
* *To be committed, transactions must achieve both validity and uniqueness consensus*
|
|
* *Validity consensus requires contractual validity of the transaction and all its dependencies*
|
|
* *Uniqueness consensus prevents double-spends*
|
|
|
|
.. only:: htmlmode
|
|
|
|
Video
|
|
-----
|
|
.. raw:: html
|
|
|
|
<iframe src="https://player.vimeo.com/video/214138438" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
|
|
<p></p>
|
|
|
|
|
|
Two types of consensus
|
|
----------------------
|
|
Determining whether a proposed transaction is a valid ledger update involves reaching two types of consensus:
|
|
|
|
* *Validity consensus* - this is checked by each required signer before they sign the transaction
|
|
* *Uniqueness consensus* - this is only checked by a notary service
|
|
|
|
Validity consensus
|
|
------------------
|
|
Validity consensus is the process of checking that the following conditions hold both for the proposed transaction,
|
|
and for every transaction in the transaction chain that generated the inputs to the proposed transaction:
|
|
|
|
* The transaction is accepted by the contracts of every input and output state
|
|
* The transaction has all the required signatures
|
|
|
|
It is not enough to verify the proposed transaction itself. We must also verify every transaction in the chain of
|
|
transactions that led up to the creation of the inputs to the proposed transaction.
|
|
|
|
This is known as *walking the chain*. Suppose, for example, that a party on the network proposes a transaction
|
|
transferring us a treasury bond. We can only be sure that the bond transfer is valid if:
|
|
|
|
* The treasury bond was issued by the central bank in a valid issuance transaction
|
|
* Every subsequent transaction in which the bond changed hands was also valid
|
|
|
|
The only way to be sure of both conditions is to walk the transaction's chain. We can visualize this process as follows:
|
|
|
|
.. image:: resources/validation-consensus.png
|
|
:scale: 25%
|
|
:align: center
|
|
|
|
When verifying a proposed transaction, a given party may not have every transaction in the transaction chain that they
|
|
need to verify. In this case, they can request the missing transactions from the transaction proposer(s). The
|
|
transaction proposer(s) will always have the full transaction chain, since they would have requested it when
|
|
verifying the transaction that created the proposed transaction's input states.
|
|
|
|
Uniqueness consensus
|
|
--------------------
|
|
Imagine that Bob holds a valid central-bank-issued cash state of $1,000,000. Bob can now create two transaction
|
|
proposals:
|
|
|
|
* A transaction transferring the $1,000,000 to Charlie in exchange for £800,000
|
|
* A transaction transferring the $1,000,000 to Dan in exchange for €900,000
|
|
|
|
This is a problem because, although both transactions will achieve validity consensus, Bob has managed to
|
|
"double-spend" his USD to get double the amount of GBP and EUR. We can visualize this as follows:
|
|
|
|
.. image:: resources/uniqueness-consensus.png
|
|
:scale: 25%
|
|
:align: center
|
|
|
|
To prevent this, a valid transaction proposal must also achieve uniqueness consensus. Uniqueness consensus is the
|
|
requirement that none of the inputs to a proposed transaction have already been consumed in another transaction.
|
|
|
|
If one or more of the inputs have already been consumed in another transaction, this is known as a *double spend*,
|
|
and the transaction proposal is considered invalid.
|
|
|
|
Uniqueness consensus is provided by notaries. See :doc:`key-concepts-notaries` for more details. |