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<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="writing-oracle-services">
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<h1>Writing oracle services<a class="headerlink" href="#writing-oracle-services" title="Permalink to this headline">¶</a></h1>
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<p>This article covers <em>oracles</em>: network services that link the ledger to the outside world by providing facts that
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affect the validity of transactions.</p>
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<p>The current prototype includes an example oracle that provides an interest rate fixing service. It is used by the
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IRS trading demo app.</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>Oracles are a key concept in the block chain/decentralised ledger space. They can be essential for many kinds of
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application, because we often wish to condition a transaction on some fact being true or false, but the ledger itself
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has a design that is essentially functional: all transactions are <em>pure</em> and <em>immutable</em>. Phrased another way, a
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smart contract cannot perform any input/output or depend on any state outside of the transaction itself. There is no
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way to download a web page or interact with the user, in a smart contract. It must be this way because everyone must
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be able to independently check a transaction and arrive at an identical conclusion for the ledger to maintan its
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integrity: if a transaction could evaluate to “valid” on one computer and then “invalid” a few minutes later on a
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different computer, the entire shared ledger concept wouldn’t work.</p>
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<p>But it is often essential that transactions do depend on data from the outside world, for example, verifying that an
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interest rate swap is paying out correctly may require data on interest rates, verifying that a loan has reached
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maturity requires knowledge about the current time, knowing which side of a bet receives the payment may require
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arbitrary facts about the real world (e.g. the bankruptcy or solvency of a company or country) ... and so on.</p>
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<p>We can solve this problem by introducing services that create digitally signed data structures which assert facts.
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These structures can then be used as an input to a transaction and distributed with the transaction data itself. Because
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the statements are themselves immutable and signed, it is impossible for an oracle to change its mind later and
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invalidate transactions that were previously found to be valid. In contrast, consider what would happen if a contract
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could do an HTTP request: it’s possible that an answer would change after being downloaded, resulting in loss of
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consensus (breaks).</p>
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</div>
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<div class="section" id="the-two-basic-approaches">
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<h2>The two basic approaches<a class="headerlink" href="#the-two-basic-approaches" title="Permalink to this headline">¶</a></h2>
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<p>The architecture provides two ways of implementing oracles with different tradeoffs:</p>
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<ol class="arabic simple">
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<li>Using commands</li>
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<li>Using attachments</li>
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</ol>
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<p>When a fact is encoded in a command, it is embedded in the transaction itself. The oracle then acts as a co-signer to
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the entire transaction. The oracle’s signature is valid only for that transaction, and thus even if a fact (like a
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stock price) does not change, every transaction that incorporates that fact must go back to the oracle for signing.</p>
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<p>When a fact is encoded as an attachment, it is a separate object to the transaction and is referred to by hash.
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Nodes download attachments from peers at the same time as they download transactions, unless of course the node has
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already seen that attachment, in which case it won’t fetch it again. Contracts have access to the contents of
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attachments when they run.</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">Currently attachments do not support digital signing, but this is a planned feature.</p>
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</div>
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<p>As you can see, both approaches share a few things: they both allow arbitrary binary data to be provided to transactions
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(and thus contracts). The primary difference is whether the data is a freely reusable, standalone object or whether it’s
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integrated with a transaction.</p>
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<p>Here’s a quick way to decide which approach makes more sense for your data source:</p>
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<ul class="simple">
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<li>Is your data <em>continuously changing</em>, like a stock price, the current time, etc? If yes, use a command.</li>
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<li>Is your data <em>commercially valuable</em>, like a feed which you are not allowed to resell unless it’s incorporated into
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a business deal? If yes, use a command, so you can charge money for signing the same fact in each unique business
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context.</li>
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<li>Is your data <em>very small</em>, like a single number? If yes, use a command.</li>
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<li>Is your data <em>large</em>, <em>static</em> and <em>commercially worthless</em>, for instance, a holiday calendar? If yes, use an
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attachment.</li>
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<li>Is your data <em>intended for human consumption</em>, like a PDF of legal prose, or an Excel spreadsheet? If yes, use an
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attachment.</li>
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</ul>
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</div>
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<div class="section" id="asserting-continuously-varying-data">
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<h2>Asserting continuously varying data<a class="headerlink" href="#asserting-continuously-varying-data" title="Permalink to this headline">¶</a></h2>
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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">A future version of the platform will include a complete tutorial on implementing this type of oracle.</p>
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</div>
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<p>Let’s look at the interest rates oracle that can be found in the <code class="docutils literal"><span class="pre">NodeInterestRates</span></code> file. This is an example of
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an oracle that uses a command because the current interest rate fix is a constantly changing fact.</p>
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<p>The obvious way to implement such a service is like this:</p>
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<ol class="arabic simple">
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<li>The creator of the transaction that depends on the interest rate sends it to the oracle.</li>
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<li>The oracle inserts a command with the rate and signs the transaction.</li>
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<li>The oracle sends it back.</li>
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</ol>
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<p>But this has a problem - it would mean that the oracle has to be the first entity to sign the transaction, which might impose
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ordering constraints we don’t want to deal with (being able to get all parties to sign in parallel is a very nice thing).
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So the way we actually implement it is like this:</p>
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<ol class="arabic simple">
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<li>The creator of the transaction that depends on the interest rate asks for the current rate. They can abort at this point
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if they want to.</li>
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<li>They insert a command with that rate and the time it was obtained into the transaction.</li>
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<li>They then send it to the oracle for signing, along with everyone else in parallel. The oracle checks that the command
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has correct data for the asserted time, and signs if so.</li>
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</ol>
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<p>This same technique can be adapted to other types of oracle.</p>
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<p>The oracle consists of a core class that implements the query/sign operations (for easy unit testing), and then a separate
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class that binds it to the network layer.</p>
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<p>Here is an extract from the <code class="docutils literal"><span class="pre">NodeService.Oracle</span></code> class and supporting types:</p>
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="cm">/** A [FixOf] identifies the question side of a fix: what day, tenor and type of fix ("LIBOR", "EURIBOR" etc) */</span>
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<span class="k">data</span> <span class="k">class</span> <span class="nc">FixOf</span><span class="p">(</span><span class="k">val</span> <span class="py">name</span><span class="p">:</span> <span class="n">String</span><span class="p">,</span> <span class="k">val</span> <span class="py">forDay</span><span class="p">:</span> <span class="n">LocalDate</span><span class="p">,</span> <span class="k">val</span> <span class="py">ofTenor</span><span class="p">:</span> <span class="n">Duration</span><span class="p">)</span>
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<span class="cm">/** A [Fix] represents a named interest rate, on a given day, for a given duration. It can be embedded in a tx. */</span>
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<span class="k">data</span> <span class="k">class</span> <span class="nc">Fix</span><span class="p">(</span><span class="k">val</span> <span class="py">of</span><span class="p">:</span> <span class="n">FixOf</span><span class="p">,</span> <span class="k">val</span> <span class="py">value</span><span class="p">:</span> <span class="n">BigDecimal</span><span class="p">)</span> <span class="p">:</span> <span class="n">CommandData</span>
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<span class="k">class</span> <span class="nc">Oracle</span> <span class="p">{</span>
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<span class="k">fun</span> <span class="nf">query</span><span class="p">(</span><span class="n">queries</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">FixOf</span><span class="p">>):</span> <span class="n">List</span><span class="p"><</span><span class="n">Fix</span><span class="p">></span>
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<span class="k">fun</span> <span class="nf">sign</span><span class="p">(</span><span class="n">wtx</span><span class="p">:</span> <span class="n">WireTransaction</span><span class="p">):</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>Because the fix contains a timestamp (the <code class="docutils literal"><span class="pre">forDay</span></code> field), there can be an arbitrary delay between a fix being
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requested via <code class="docutils literal"><span class="pre">query</span></code> and the signature being requested via <code class="docutils literal"><span class="pre">sign</span></code>.</p>
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</div>
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<div class="section" id="pay-per-play-oracles">
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<h2>Pay-per-play oracles<a class="headerlink" href="#pay-per-play-oracles" title="Permalink to this headline">¶</a></h2>
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<p>Because the signature covers the transaction, and transactions may end up being forwarded anywhere, the fact itself
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is independently checkable. However, this approach can still be useful when the data itself costs money, because the act
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of issuing the signature in the first place can be charged for (e.g. by requiring the submission of a fresh
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<code class="docutils literal"><span class="pre">Cash.State</span></code> that has been re-assigned to a key owned by the oracle service). Because the signature covers the
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<em>transaction</em> and not only the <em>fact</em>, this allows for a kind of weak pseudo-DRM over data feeds. Whilst a smart
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contract could in theory include a transaction parsing and signature checking library, writing a contract in this way
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would be conclusive evidence of intent to disobey the rules of the service (<em>res ipsa loquitur</em>). In an environment
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where parties are legally identifiable, usage of such a contract would by itself be sufficient to trigger some sort of
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punishment.</p>
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