2.9 KiB
Transaction tear-offs
Suppose we want to construct a transaction that includes commands
containing interest rate fix data as in oracles. Before sending the transaction to the oracle
to obtain its signature, we need to filter out every part of the
transaction except for the Fix commands.
To do so, we need to create a filtering function that specifies which fields of the transaction should be included. Each field will only be included if the filtering function returns true when the field is passed in as input.
../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/tearoffs/TutorialTearOffs.kt
We can now use our filtering function to construct a
FilteredTransaction:
../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/tearoffs/TutorialTearOffs.kt
In the Oracle example this step takes place in
RatesFixFlow by overriding the filtering
function. See filtering_ref.
Both WireTransaction and
FilteredTransaction inherit from
TraversableTransaction, so access to the transaction
components is exactly the same. Note that unlike
WireTransaction, FilteredTransaction only
holds data that we wanted to reveal (after filtering).
../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/tearoffs/TutorialTearOffs.kt
The following code snippet is taken from
NodeInterestRates.kt and implements a signing part of an
Oracle.
../../samples/irs-demo/cordapp/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt
Note
The way the FilteredTransaction is constructed ensures
that after signing of the root hash it's impossible to add or remove
components (leaves). However, it can happen that having transaction with
multiple commands one party reveals only subset of them to the Oracle.
As signing is done now over the Merkle root hash, the service signs all
commands of given type, even though it didn't see all of them. In the
case however where all of the commands should be visible to an Oracle,
one can type ftx.checkAllComponentsVisible(COMMANDS_GROUP)
before invoking ftx.verify.
checkAllComponentsVisible is using a sophisticated
underlying partial Merkle tree check to guarantee that all of the
components of a particular group that existed in the original
WireTransaction are included in the received
FilteredTransaction.