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101 lines
6.3 KiB
ReStructuredText
101 lines
6.3 KiB
ReStructuredText
.. highlight:: kotlin
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.. raw:: html
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<script type="text/javascript" src="_static/jquery.js"></script>
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<script type="text/javascript" src="_static/codesets.js"></script>
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Event scheduling
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================
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This article explains our approach to modelling time based events in code. It explains how a contract
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state can expose an upcoming event and what action to take if the scheduled time for that event is reached.
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Introduction
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------------
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Many financial instruments have time sensitive components to them. For example, an Interest Rate Swap has a schedule
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for when:
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* Interest rate fixings should take place for floating legs, so that the interest rate used as the basis for payments
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can be agreed.
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* Any payments between the parties are expected to take place.
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* Any payments between the parties become overdue.
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Each of these is dependent on the current state of the financial instrument. What payments and interest rate fixings
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have already happened should already be recorded in the state, for example. This means that the *next* time sensitive
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event is thus a property of the current contract state. By next, we mean earliest in chronological terms, that is still
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due. If a contract state is consumed in the UTXO model, then what *was* the next event becomes irrelevant and obsolete
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and the next time sensitive event is determined by any successor contract state.
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Knowing when the next time sensitive event is due to occur is useful, but typically some *activity* is expected to take
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place when this event occurs. We already have a model for business processes in the form of :doc:`flows <flow-state-machines>`,
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so in the platform we have introduced the concept of *scheduled activities* that can invoke flow state machines
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at a scheduled time. A contract state can optionally described the next scheduled activity for itself. If it omits
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to do so, then nothing will be scheduled.
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How to implement scheduled events
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---------------------------------
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There are two main steps to implementing scheduled events:
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* Have your ``ContractState`` implementation also implement ``SchedulableState``. This requires a method named
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``nextScheduledActivity`` to be implemented which returns an optional ``ScheduledActivity`` instance.
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``ScheduledActivity`` captures what ``FlowLogic`` instance each node will run, to perform the activity, and when it
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will run is described by a ``java.time.Instant``. Once your state implements this interface and is tracked by the
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wallet, it can expect to be queried for the next activity when committed to the wallet. The ``FlowLogic`` must be
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annotated with ``@SchedulableFlow``.
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* If nothing suitable exists, implement a ``FlowLogic`` to be executed by each node as the activity itself.
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The important thing to remember is that in the current implementation, each node that is party to the transaction
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will execute the same ``FlowLogic``, so it needs to establish roles in the business process based on the contract
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state and the node it is running on. Each side will follow different but complementary paths through the business logic.
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.. note:: The scheduler's clock always operates in the UTC time zone for uniformity, so any time zone logic must be
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performed by the contract, using ``ZonedDateTime``.
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In the short term, until we have automatic flow session set up, you will also likely need to install a network
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handler to help with obtaining a unique and secure random session. An example is described below.
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The production and consumption of ``ContractStates`` is observed by the scheduler and the activities associated with
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any consumed states are unscheduled. Any newly produced states are then queried via the ``nextScheduledActivity``
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method and if they do not return ``null`` then that activity is scheduled based on the content of the
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``ScheduledActivity`` object returned. Be aware that this *only* happens if the wallet considers the state
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"relevant", for instance, because the owner of the node also owns that state. States that your node happens to
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encounter but which aren't related to yourself will not have any activities scheduled.
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An example
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----------
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Let's take an example of the interest rate swap fixings for our scheduled events. The first task is to implement the
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``nextScheduledActivity`` method on the ``State``.
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.. container:: codeset
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.. sourcecode:: kotlin
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override fun nextScheduledActivity(thisStateRef: StateRef,
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flowLogicRefFactory: FlowLogicRefFactory): ScheduledActivity? {
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val nextFixingOf = nextFixingOf() ?: return null
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val (instant, duration) = suggestInterestRateAnnouncementTimeWindow(index = nextFixingOf.name,
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source = floatingLeg.indexSource,
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date = nextFixingOf.forDay)
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return ScheduledActivity(flowLogicRefFactory.create(TwoPartyDealFlow.FixingRoleDecider::class.java,
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thisStateRef, duration), instant)
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}
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The first thing this does is establish if there are any remaining fixings. If there are none, then it returns ``null``
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to indicate that there is no activity to schedule. Otherwise it calculates the ``Instant`` at which the interest rate
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should become available and schedules an activity at that time to work out what roles each node will take in the fixing
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business process and to take on those roles. That ``FlowLogic`` will be handed the ``StateRef`` for the interest
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rate swap ``State`` in question, as well as a tolerance ``Duration`` of how long to wait after the activity is triggered
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for the interest rate before indicating an error.
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.. note:: This is a way to create a reference to the FlowLogic class and its constructor parameters to instantiate.
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As previously mentioned, we currently need a small network handler to assist with session setup until the work to
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automate that is complete. See the interest rate swap specific implementation ``FixingSessionInitiationHandler`` which
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is responsible for starting a ``FlowLogic`` to perform one role in the fixing flow with the ``sessionID`` sent
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by the ``FixingRoleDecider`` on the other node which then launches the other role in the fixing flow. Currently
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the handler needs to be manually installed in the node.
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