Determinism

An important principle that occurs in a few places throughout the Cardano ledger is that of determinism. Perhaps one way to summarise is would be to say: “the transaction is all you need” - that is, when trying to compute the results of a transaction, you need only look at the transaction itself rather than computing with the full ledger state.

There are two important instantiations of this principle:

Transaction Determinism

This instance tells us that, given a transaction is valid, its outputs are determined fully by the transaction itself. It may be necessary to look at certain parts of the ledger state to determine whether it is valid - for example, to check that the inputs have not been spent - but assuming it is valid, the outputs created will be exactly as described in the transaction.

Script determinism

This instance tells us that, assuming a transaction passes phase 1 validation, the validity of a script is determined only by data contained in the transaction and in the transaction outputs that it spends or references.

Implications for Node Implementors

These properties allow node implementors to safely make certain assumptions which can speed up transaction and block processing.

1. When processing historical blocks, nodes need only consider (or even deserialise) transaction bodies, since the rest of the block payload can only impact the block validity, which is already known for historical blocks.
2. Many of the more expensive checks of transaction validity need only be carried out once. In particular, the cryptographic verification and script execution need only be carried out once, when the transaction first enters the mempool. Subsequently it is required only to check that the inputs still exist.

To take advantage of these properties, ledger implementers must distinguish between these checks in the ledger in such a way as to allow transactions to be (re-)processed without repeating expensive computation.