corda/docs/source/api-transactions.rst

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API: Transactions
=================
.. note:: Before reading this page, you should be familiar with the key concepts of :doc:`key-concepts-transactions`.
Transaction types
-----------------
There are two types of transaction in Corda:
* ``TransactionType.NotaryChange``, used to change the notary for a set of states
* ``TransactionType.General``, for transactions other than notary-change transactions
Notary-change transactions
^^^^^^^^^^^^^^^^^^^^^^^^^^
A single Corda network will usually have multiple notary services. To commit a transaction, we require a signature
from the notary service associated with each input state. If we tried to commit a transaction where the input
states were associated with different notary services, the transaction would require a signature from multiple notary
services, creating a complicated multi-phase commit scenario. To prevent this, every input state in a transaction
must be associated the same notary.
However, we will often need to create a transaction involving input states associated with different notaries. Before
we can create this transaction, we will need to change the notary service associated with each state by:
* Deciding which notary service we want to notarise the transaction
* For each set of inputs states that point to the same notary service that isn't the desired notary service, creating a
``TransactionType.NotaryChange`` transaction that:
* Consumes the input states pointing to the old notary
* Outputs the same states, but that now point to the new notary
* Using the outputs of the notary-change transactions as inputs to a standard ``TransactionType.General`` transaction
In practice, this process is handled automatically by a built-in flow called ``NotaryChangeFlow``. See
:doc:`api-flows` for more details.
Transaction workflow
--------------------
There are four states the transaction can occupy:
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* ``TransactionBuilder``, a builder for a transaction in construction
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* ``WireTransaction``, an immutable transaction
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* ``SignedTransaction``, an immutable transaction with 1+ associated signatures
* ``LedgerTransaction``, a transaction that can be checked for validity
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Here are the possible transitions between transaction states:
.. image:: resources/transaction-flow.png
TransactionBuilder
------------------
Creating a builder
^^^^^^^^^^^^^^^^^^
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The first step when creating a transaction is to instantiate a ``TransactionBuilder``. We can create a builder for each
transaction type as follows:
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Transaction components
^^^^^^^^^^^^^^^^^^^^^^
Once we have a ``TransactionBuilder``, we need to gather together the various transaction components the transaction
will include.
Input states
~~~~~~~~~~~~
Input states are added to a transaction as ``StateAndRef`` instances. A ``StateAndRef`` combines:
* A ``ContractState`` representing the input state itself
* A ``StateRef`` pointing to the input among the outputs of the transaction that created it
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A ``StateRef`` uniquely identifies an input state, allowing the notary to mark it as historic. It is made up of:
* The hash of the transaction that generated the state
* The state's index in the outputs of that transaction
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The ``StateRef`` create a chain of pointers from the input states back to the transactions that created them. This
allows a node to work backwards and verify the entirety of the transaction chain.
Output states
~~~~~~~~~~~~~
Since a transaction's output states do not exist until the transaction is committed, they cannot be referenced as the
outputs of previous transactions. Instead, we create the desired output states as ``ContractState`` instances, and
add them to the transaction:
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In many cases (e.g. when we have a transaction that updates an existing state), we may want to create an output by
copying from the input state:
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Commands
~~~~~~~~
Commands are added to the transaction as ``Command`` instances. ``Command`` combines:
* A ``CommandData`` instance representing the type of the command
* A list of the command's required signers
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Attachments
~~~~~~~~~~~
Attachments are identified by their hash. The attachment with the corresponding hash must have been uploaded ahead of
time via the node's RPC interface:
.. container:: codeset
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Time-windows
~~~~~~~~~~~~
Time windows represent the period of time during which the transaction must be notarised. They can have a start and an
end time, or be open at either end:
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:language: kotlin
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We can also define a time window as an ``Instant`` +/- a time tolerance (e.g. 30 seconds):
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:language: kotlin
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Or as a start-time plus a duration:
.. container:: codeset
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:language: kotlin
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:end-before: DOCEND 43
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Adding items
^^^^^^^^^^^^
The transaction builder is mutable. We add items to it using the ``TransactionBuilder.withItems`` method:
.. container:: codeset
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``withItems`` takes a ``vararg`` of objects and adds them to the builder based on their type:
* ``StateAndRef`` objects are added as input states
* ``TransactionState`` and ``ContractState`` objects are added as output states
* ``Command`` objects are added as commands
Passing in objects of any other type will cause an ``IllegalArgumentException`` to be thrown.
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Here's an example usage of ``TransactionBuilder.withItems``:
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You can also pass in objects one-by-one. This is the only way to add attachments:
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:language: kotlin
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:language: java
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To set the transaction builder's time-window, we can either set a time-window directly:
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:language: java
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Or define the time-window as a time plus a duration (e.g. 45 seconds):
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:language: kotlin
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:start-after: DOCSTART 45
:end-before: DOCEND 45
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Signing the builder
^^^^^^^^^^^^^^^^^^^
Once the builder is ready, we finalize it by signing it and converting it into a ``SignedTransaction``:
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This will sign the transaction with your legal identity key. You can also choose to use another one of your public keys:
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Either way, the outcome of this process is to create a ``SignedTransaction``, which can no longer be modified.
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SignedTransaction
-----------------
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A ``SignedTransaction`` is a combination of:
* An immutable ``WireTransaction``
* A list of signatures over that transaction
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/transactions/SignedTransaction.kt
:language: kotlin
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:end-before: DOCEND 1
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Before adding our signature to the transaction, we'll want to verify both the transaction itself and its signatures.
Verifying the transaction
^^^^^^^^^^^^^^^^^^^^^^^^^
To verify a transaction, we need to retrieve any states in the transaction chain that our node doesn't
currently have in its local storage from the proposer(s) of the transaction. This process is handled by a built-in flow
called ``ResolveTransactionsFlow``. See :doc:`api-flows` for more details.
When verifying a ``SignedTransaction``, we don't verify the ``SignedTransaction`` *per se*, but rather the
``WireTransaction`` it contains. We extract this ``WireTransaction`` as follows:
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:language: kotlin
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However, this still isn't enough. The ``WireTransaction`` holds its inputs as ``StateRef`` instances, and its
attachments as hashes. These do not provide enough information to properly validate the transaction's contents. To
resolve these into actual ``ContractState`` and ``Attachment`` instances, we need to use the ``ServiceHub`` to convert
the ``WireTransaction`` into a ``LedgerTransaction``:
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We can now *verify* the transaction to ensure that it satisfies the contracts of all the transaction's input and output
states:
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We will generally also want to conduct some additional validation of the transaction, beyond what is provided for in
the contract. Here's an example of how we might do this:
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Verifying the signatures
^^^^^^^^^^^^^^^^^^^^^^^^
We also need to verify the signatures over the transaction to prevent tampering. We do this using
``SignedTransaction.verifySignatures``:
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Optionally, we can pass ``verifySignatures`` a ``vararg`` of the public keys for which the signatures are allowed
to be missing:
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If the transaction is missing any signatures without the corresponding public keys being passed in, a
``SignaturesMissingException`` is thrown.
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We can also choose to simply verify the signatures that are present:
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However, BE VERY CAREFUL - this function provides no guarantees that the signatures are correct, or that none are
missing.
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Signing the transaction
^^^^^^^^^^^^^^^^^^^^^^^
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Once we are satisfied with the contents and existing signatures over the transaction, we can add our signature to the
``SignedTransaction`` using:
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As with the ``TransactionBuilder``, we can also choose to sign using another one of our public keys:
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We can also generate a signature over the transaction without adding it to the transaction directly by using:
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Or using another one of our public keys, as follows:
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Notarising and recording
^^^^^^^^^^^^^^^^^^^^^^^^
Notarising and recording a transaction is handled by a built-in flow called ``FinalityFlow``. See
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:doc:`api-flows` for more details.