mirror of
https://github.com/corda/corda.git
synced 2024-12-22 14:22:28 +00:00
82 lines
4.9 KiB
ReStructuredText
82 lines
4.9 KiB
ReStructuredText
.. highlight:: kotlin
|
|
.. raw:: html
|
|
|
|
<script type="text/javascript" src="_static/jquery.js"></script>
|
|
<script type="text/javascript" src="_static/codesets.js"></script>
|
|
|
|
Event scheduling
|
|
================
|
|
|
|
This article explains our approach to modelling time based events in code. It explains how a contract
|
|
state can expose an upcoming event and what action to take if the scheduled time for that event is reached.
|
|
|
|
Introduction
|
|
------------
|
|
|
|
Many financial instruments have time sensitive components to them. For example, an Interest Rate Swap has a schedule
|
|
for when:
|
|
|
|
* Interest rate fixings should take place for floating legs, so that the interest rate used as the basis for payments
|
|
can be agreed.
|
|
* Any payments between the parties are expected to take place.
|
|
* Any payments between the parties become overdue.
|
|
|
|
Each of these is dependent on the current state of the financial instrument. What payments and interest rate fixings
|
|
have already happened should already be recorded in the state, for example. This means that the *next* time sensitive
|
|
event is thus a property of the current contract state. By next, we mean earliest in chronological terms, that is still
|
|
due. If a contract state is consumed in the UTXO model, then what *was* the next event becomes irrelevant and obsolete
|
|
and the next time sensitive event is determined by any successor contract state.
|
|
|
|
Knowing when the next time sensitive event is due to occur is useful, but typically some *activity* is expected to take
|
|
place when this event occurs. We already have a model for business processes in the form of :doc:`flows <flow-state-machines>`,
|
|
so in the platform we have introduced the concept of *scheduled activities* that can invoke flow state machines
|
|
at a scheduled time. A contract state can optionally described the next scheduled activity for itself. If it omits
|
|
to do so, then nothing will be scheduled.
|
|
|
|
How to implement scheduled events
|
|
---------------------------------
|
|
|
|
There are two main steps to implementing scheduled events:
|
|
|
|
* Have your ``ContractState`` implementation also implement ``SchedulableState``. This requires a method named
|
|
``nextScheduledActivity`` to be implemented which returns an optional ``ScheduledActivity`` instance.
|
|
``ScheduledActivity`` captures what ``FlowLogic`` instance each node will run, to perform the activity, and when it
|
|
will run is described by a ``java.time.Instant``. Once your state implements this interface and is tracked by the
|
|
vault, it can expect to be queried for the next activity when committed to the vault. The ``FlowLogic`` must be
|
|
annotated with ``@SchedulableFlow``.
|
|
* If nothing suitable exists, implement a ``FlowLogic`` to be executed by each node as the activity itself.
|
|
The important thing to remember is that in the current implementation, each node that is party to the transaction
|
|
will execute the same ``FlowLogic``, so it needs to establish roles in the business process based on the contract
|
|
state and the node it is running on. Each side will follow different but complementary paths through the business logic.
|
|
|
|
.. note:: The scheduler's clock always operates in the UTC time zone for uniformity, so any time zone logic must be
|
|
performed by the contract, using ``ZonedDateTime``.
|
|
|
|
The production and consumption of ``ContractStates`` is observed by the scheduler and the activities associated with
|
|
any consumed states are unscheduled. Any newly produced states are then queried via the ``nextScheduledActivity``
|
|
method and if they do not return ``null`` then that activity is scheduled based on the content of the
|
|
``ScheduledActivity`` object returned. Be aware that this *only* happens if the vault considers the state
|
|
"relevant", for instance, because the owner of the node also owns that state. States that your node happens to
|
|
encounter but which aren't related to yourself will not have any activities scheduled.
|
|
|
|
An example
|
|
----------
|
|
|
|
Let's take an example of the interest rate swap fixings for our scheduled events. The first task is to implement the
|
|
``nextScheduledActivity`` method on the ``State``.
|
|
|
|
.. container:: codeset
|
|
|
|
.. literalinclude:: ../../samples/irs-demo/cordapp/contracts-irs/src/main/kotlin/net/corda/irs/contract/IRS.kt
|
|
:language: kotlin
|
|
:start-after: DOCSTART 1
|
|
:end-before: DOCEND 1
|
|
:dedent: 8
|
|
|
|
The first thing this does is establish if there are any remaining fixings. If there are none, then it returns ``null``
|
|
to indicate that there is no activity to schedule. Otherwise it calculates the ``Instant`` at which the interest rate
|
|
should become available and schedules an activity at that time to work out what roles each node will take in the fixing
|
|
business process and to take on those roles. That ``FlowLogic`` will be handed the ``StateRef`` for the interest
|
|
rate swap ``State`` in question, as well as a tolerance ``Duration`` of how long to wait after the activity is triggered
|
|
for the interest rate before indicating an error.
|