mirror of
https://github.com/corda/corda.git
synced 2024-12-24 23:26:48 +00:00
fc38e4160a
* Updates the states API page. * Minor fixes. * Further fixes. * Adds ToC * Addresses review feedback.
154 lines
5.9 KiB
ReStructuredText
154 lines
5.9 KiB
ReStructuredText
API: States
|
|
===========
|
|
|
|
.. note:: Before reading this page, you should be familiar with the key concepts of :doc:`key-concepts-states`.
|
|
|
|
.. contents::
|
|
|
|
ContractState
|
|
-------------
|
|
In Corda, states are instances of classes that implement ``ContractState``. The ``ContractState`` interface is defined
|
|
as follows:
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/ContractState.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 1
|
|
:end-before: DOCEND 1
|
|
|
|
``ContractState`` has a single field, ``participants``. ``participants`` is a ``List`` of the ``AbstractParty`` that
|
|
are considered to have a stake in the state. Among other things, the ``participants`` will:
|
|
|
|
* Usually store the state in their vault (see below)
|
|
|
|
* Need to sign any notary-change and contract-upgrade transactions involving this state
|
|
|
|
* Receive any finalised transactions involving this state as part of ``FinalityFlow``
|
|
|
|
ContractState sub-interfaces
|
|
----------------------------
|
|
The behaviour of the state can be further customised by implementing sub-interfaces of ``ContractState``. The two most
|
|
common sub-interfaces are:
|
|
|
|
* ``LinearState``
|
|
|
|
* ``OwnableState``
|
|
|
|
``LinearState`` models shared facts for which there is only one current version at any point in time. ``LinearState``
|
|
states evolve in a straight line by superseding themselves. On the other hand, ``OwnableState`` is meant to represent
|
|
assets that can be freely split and merged over time. Cash is a good example of an ``OwnableState`` - two existing $5
|
|
cash states can be combined into a single $10 cash state, or split into five $1 cash states. With ``OwnableState``, its
|
|
the total amount held that is important, rather than the actual units held.
|
|
|
|
We can picture the hierarchy as follows:
|
|
|
|
.. image:: resources/state-hierarchy.png
|
|
|
|
LinearState
|
|
^^^^^^^^^^^
|
|
The ``LinearState`` interface is defined as follows:
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 2
|
|
:end-before: DOCEND 2
|
|
|
|
Remember that in Corda, states are immutable and can't be updated directly. Instead, we represent an evolving fact as a
|
|
sequence of ``LinearState`` states that share the same ``linearId`` and represent an audit trail for the lifecycle of
|
|
the fact over time.
|
|
|
|
When we want to extend a ``LinearState`` chain (i.e. a sequence of states sharing a ``linearId``), we:
|
|
|
|
* Use the ``linearId`` to extract the latest state in the chain from the vault
|
|
|
|
* Create a new state that has the same ``linearId``
|
|
|
|
* Create a transaction with:
|
|
|
|
* The current latest state in the chain as an input
|
|
|
|
* The newly-created state as an output
|
|
|
|
The new state will now become the latest state in the chain, representing the new current state of the agreement.
|
|
|
|
``linearId`` is of type ``UniqueIdentifier``, which is a combination of:
|
|
|
|
* A Java ``UUID`` representing a globally unique 128 bit random number
|
|
* An optional external-reference string for referencing the state in external systems
|
|
|
|
OwnableState
|
|
^^^^^^^^^^^^
|
|
The ``OwnableState`` interface is defined as follows:
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 3
|
|
:end-before: DOCEND 3
|
|
|
|
Where:
|
|
|
|
* ``owner`` is the ``PublicKey`` of the asset's owner
|
|
|
|
* ``withNewOwner(newOwner: AbstractParty)`` creates an copy of the state with a new owner
|
|
|
|
Because ``OwnableState`` models fungible assets that can be merged and split over time, ``OwnableState`` instances do
|
|
not have a ``linearId``. $5 of cash created by one transaction is considered to be identical to $5 of cash produced by
|
|
another transaction.
|
|
|
|
Other interfaces
|
|
^^^^^^^^^^^^^^^^
|
|
You can also customize your state by implementing the following interfaces:
|
|
|
|
* ``QueryableState``, which allows the state to be queried in the node's database using custom attributes (see
|
|
:doc:`api-persistence`)
|
|
|
|
* ``SchedulableState``, which allows us to schedule future actions for the state (e.g. a coupon payment on a bond) (see
|
|
:doc:`event-scheduling`)
|
|
|
|
User-defined fields
|
|
-------------------
|
|
Beyond implementing ``ContractState`` or a sub-interface, a state is allowed to have any number of additional fields
|
|
and methods. For example, here is the relatively complex definition for a state representing cash:
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../finance/src/main/kotlin/net/corda/finance/contracts/asset/Cash.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 1
|
|
:end-before: DOCEND 1
|
|
|
|
The vault
|
|
---------
|
|
Whenever a node records a new transaction, it also decides whether it should store each of the transaction's output
|
|
states in its vault. The default vault implementation makes the decision based on the following rules:
|
|
|
|
* If the state is an ``OwnableState``, the vault will store the state if the node is the state's ``owner``
|
|
* Otherwise, the vault will store the state if it is one of the ``participants``
|
|
|
|
States that are not considered relevant are not stored in the node's vault. However, the node will still store the
|
|
transactions that created the states in its transaction storage.
|
|
|
|
TransactionState
|
|
----------------
|
|
When a ``ContractState`` is added to a ``TransactionBuilder``, it is wrapped in a ``TransactionState``:
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/TransactionState.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 1
|
|
:end-before: DOCEND 1
|
|
|
|
Where:
|
|
|
|
* ``data`` is the state to be stored on-ledger
|
|
* ``contract`` is the contract governing evolutions of this state
|
|
* ``notary`` is the notary service for this state
|
|
* ``encumbrance`` points to another state that must also appear as an input to any transaction consuming this
|
|
state
|
|
* ``constraint`` is a constraint on which contract-code attachments can be used with this state |