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ReStructuredText
147 lines
6.0 KiB
ReStructuredText
API: States
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===========
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.. note:: Before reading this page, you should be familiar with the key concepts of :doc:`key-concepts-states`.
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ContractState
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-------------
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In Corda, states are classes that implement ``ContractState``. The ``ContractState`` interface is defined as follows:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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Where:
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* ``contract`` is the ``Contract`` class defining the constraints on transactions involving states of this type
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* ``participants`` is a ``List`` of the ``AbstractParty`` who are considered to have a stake in the state. For example,
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all the ``participants`` will:
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* Need to sign a notary-change transaction for this state
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* Receive any committed transactions involving this state as part of ``FinalityFlow``
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The vault
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---------
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Each node has a vault, where it stores the states that are "relevant" to the node's owner. Whenever the node sees a
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new transaction, it performs a relevancy check to decide whether to add each of the transaction's output states to
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its vault. The default vault implementation decides whether a state is relevant as follows:
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* The vault will store any state for which it is one of the ``participants``
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* This behavior is overridden for states that implement ``LinearState`` or ``OwnableState`` (see below)
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If a state is not considered relevant, the node will still store the transaction in its local storage, but it will
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not track the transaction's states in its vault.
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ContractState sub-interfaces
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----------------------------
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There are two common optional sub-interfaces of ``ContractState``:
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* ``LinearState``, which helps represent objects that have a constant identity over time
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* ``OwnableState``, which helps represent fungible assets
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For example, a cash is an ``OwnableState`` - you don't have a specific piece of cash you are tracking over time, but
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rather a total amount of cash that you can combine and divide at will. A contract, on the other hand, cannot be
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merged with other contracts of the same type - it has a unique separate identity over time.
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We can picture the hierarchy as follows:
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.. image:: resources/state-hierarchy.png
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LinearState
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^^^^^^^^^^^
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``LinearState`` models facts that have a constant identity over time. Remember that in Corda, states are immutable and
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can't be updated directly. Instead, we represent an evolving fact as a sequence of states where every state is a
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``LinearState`` that shares the same ``linearId``. Each sequence of linear states represents the lifecycle of a given
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fact up to the current point in time. It represents the historic audit trail of how the fact evolved over time to its
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current "state".
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The ``LinearState`` interface is defined as follows:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 2
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:end-before: DOCEND 2
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Where:
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* ``linearId`` is a ``UniqueIdentifier`` that:
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* Allows the successive versions of the fact to be linked over time
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* Provides an ``externalId`` for referencing the state in external systems
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* ``isRelevant(ourKeys: Set<PublicKey>)`` overrides the default vault implementation's relevancy check. You would
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generally override it to check whether ``ourKeys`` is relevant to the state at hand in some way.
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The vault tracks the head (i.e. the most recent version) of each ``LinearState`` chain (i.e. each sequence of
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states all sharing a ``linearId``). To create a transaction updating a ``LinearState``, we retrieve the state from the
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vault using its ``linearId``.
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OwnableState
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^^^^^^^^^^^^
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``OwnableState`` models fungible assets. Fungible assets are assets for which it's the total amount held that is
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important, rather than the actual units held. US dollars are an example of a fungible asset - we do not track the
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individual dollar bills held, but rather the total amount of dollars.
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The ``OwnableState`` interface is defined as follows:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 3
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:end-before: DOCEND 3
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Where:
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* ``owner`` is the ``PublicKey`` of the asset's owner
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* ``OwnableState`` also override the default behavior of the vault's relevancy check. The default vault
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implementation will track any ``OwnableState`` of which it is the owner.
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* ``withNewOwner(newOwner: PublicKey)`` creates an identical copy of the state, only with a new owner
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Other interfaces
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^^^^^^^^^^^^^^^^
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``ContractState`` has several more sub-interfaces that can optionally be implemented:
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* ``QueryableState``, which allows the state to be queried in the node's database using SQL (see
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:doc:`persistence`)
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* ``SchedulableState``, which allows us to schedule future actions for the state (e.g. a coupon on a bond) (see
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:doc:`event-scheduling`)
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User-defined fields
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-------------------
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Beyond implementing ``LinearState`` or ``OwnableState``, the definition of the state is up to the CorDapp developer.
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You can define any additional class fields and methods you see fit.
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For example, here is a relatively complex state definition, for a state representing cash:
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.. container:: codeset
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.. literalinclude:: ../../finance/src/main/kotlin/net/corda/contracts/asset/Cash.kt
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:language: kotlin
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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TransactionState
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----------------
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When a ``ContractState`` is added to a ``TransactionBuilder``, it is wrapped in a ``TransactionState``:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 4
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:end-before: DOCEND 4
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Where:
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* ``data`` is the state to be stored on-ledger
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* ``notary`` is the notary service for this state
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* ``encumbrance`` points to another state that must also appear as an input to any transaction consuming this
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state |