corda/docs/source/tutorial-tear-offs.rst

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/kotlin/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/kotlin/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/kotlin/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.