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<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="event-scheduling">
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<h1>Event scheduling<a class="headerlink" href="#event-scheduling" title="Permalink to this headline">¶</a></h1>
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<p>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.</p>
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<div class="section" id="introduction">
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<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
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<p>Many financial instruments have time sensitive components to them. For example, an Interest Rate Swap has a schedule
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for when:</p>
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<ul class="simple">
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<li>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.</li>
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<li>Any payments between the parties are expected to take place.</li>
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<li>Any payments between the parties become overdue.</li>
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</ul>
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<p>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 <em>next</em> 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 <em>was</em> the next event becomes irrelevant and obsolete
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and the next time sensitive event is determined by any successor contract state.</p>
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<p>Knowing when the next time sensitive event is due to occur is useful, but typically some <em>activity</em> 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 <a class="reference internal" href="flow-state-machines.html"><span class="doc">flows</span></a>,
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so in the platform we have introduced the concept of <em>scheduled activities</em> 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.</p>
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</div>
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<div class="section" id="how-to-implement-scheduled-events">
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<h2>How to implement scheduled events<a class="headerlink" href="#how-to-implement-scheduled-events" title="Permalink to this headline">¶</a></h2>
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<p>There are two main steps to implementing scheduled events:</p>
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<ul class="simple">
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<li>Have your <code class="docutils literal"><span class="pre">ContractState</span></code> implementation also implement <code class="docutils literal"><span class="pre">SchedulableState</span></code>. This requires a method named
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<code class="docutils literal"><span class="pre">nextScheduledActivity</span></code> to be implemented which returns an optional <code class="docutils literal"><span class="pre">ScheduledActivity</span></code> instance.
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<code class="docutils literal"><span class="pre">ScheduledActivity</span></code> captures what <code class="docutils literal"><span class="pre">FlowLogic</span></code> instance each node will run, to perform the activity, and when it
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will run is described by a <code class="docutils literal"><span class="pre">java.time.Instant</span></code>. 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.</li>
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<li>If nothing suitable exists, implement a <code class="docutils literal"><span class="pre">FlowLogic</span></code> 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 <code class="docutils literal"><span class="pre">FlowLogic</span></code>, 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.</li>
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</ul>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">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 <code class="docutils literal"><span class="pre">ZonedDateTime</span></code>.</p>
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</div>
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<p>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.</p>
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<p>The production and consumption of <code class="docutils literal"><span class="pre">ContractStates</span></code> 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 <code class="docutils literal"><span class="pre">nextScheduledActivity</span></code>
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method and if they do not return <code class="docutils literal"><span class="pre">null</span></code> then that activity is scheduled based on the content of the
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<code class="docutils literal"><span class="pre">ScheduledActivity</span></code> object returned. Be aware that this <em>only</em> 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.</p>
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</div>
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<div class="section" id="an-example">
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<h2>An example<a class="headerlink" href="#an-example" title="Permalink to this headline">¶</a></h2>
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<p>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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<code class="docutils literal"><span class="pre">nextScheduledActivity</span></code> method on the <code class="docutils literal"><span class="pre">State</span></code>.</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span>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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</pre></div>
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</div>
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</div>
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<p>The first thing this does is establish if there are any remaining fixings. If there are none, then it returns <code class="docutils literal"><span class="pre">null</span></code>
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to indicate that there is no activity to schedule. Otherwise it calculates the <code class="docutils literal"><span class="pre">Instant</span></code> 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 <code class="docutils literal"><span class="pre">FlowLogic</span></code> will be handed the <code class="docutils literal"><span class="pre">StateRef</span></code> for the interest
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rate swap <code class="docutils literal"><span class="pre">State</span></code> in question, as well as a tolerance <code class="docutils literal"><span class="pre">Duration</span></code> of how long to wait after the activity is triggered
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for the interest rate before indicating an error.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">This is a way to create a reference to the FlowLogic class and its constructor parameters to
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instantiate. The reference can be checked against a per-node whitelist of approved and allowable types as
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part of our overall security sandboxing.</p>
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</div>
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<p>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 <code class="docutils literal"><span class="pre">FixingSessionInitiationHandler</span></code> which
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is responsible for starting a <code class="docutils literal"><span class="pre">FlowLogic</span></code> to perform one role in the fixing flow with the <code class="docutils literal"><span class="pre">sessionID</span></code> sent
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by the <code class="docutils literal"><span class="pre">FixingRoleDecider</span></code> 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.</p>
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