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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-to-oracles">
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<h2>Introduction to oracles<a class="headerlink" href="#introduction-to-oracles" 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 maintain 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 class="section" id="the-two-basic-approaches">
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<h3>The two basic approaches<a class="headerlink" href="#the-two-basic-approaches" title="Permalink to this headline">¶</a></h3>
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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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<h3>Asserting continuously varying data<a class="headerlink" href="#asserting-continuously-varying-data" title="Permalink to this headline">¶</a></h3>
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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, potentially in parallel. The oracle checks that
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the command has the 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">NodeInterestRates.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">deadline</span><span class="p">:</span> <span class="n">Instant</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">ftx</span><span class="p">:</span> <span class="n">FilteredTransaction</span><span class="p">,</span> <span class="n">merkleRoot</span><span class="p">:</span> <span class="n">SecureHash</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>
|
|
<p>Because the fix contains a timestamp (the <code class="docutils literal"><span class="pre">forDay</span></code> field), that identifies the version of the data being requested,
|
|
there can be an arbitrary delay between a fix being 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>
|
|
as the Oracle can know which, potentially historical, value it is being asked to sign for. This is an important
|
|
technique for continously varying data.</p>
|
|
<p>The <code class="docutils literal"><span class="pre">query</span></code> method takes a deadline, which is a point in time the requester is willing to wait until for the necessary
|
|
data to be available. Not every oracle will need this. This can be useful where data is expected to be available on a
|
|
particular schedule and we use scheduling functionality to automatically launch the processing associated with it.
|
|
We can schedule for the expected announcement (or publish) time and give a suitable deadline at which the lack of the
|
|
information being available and the delay to processing becomes significant and may need to be escalated.</p>
|
|
</div>
|
|
<div class="section" id="hiding-transaction-data-from-the-oracle">
|
|
<h3>Hiding transaction data from the oracle<a class="headerlink" href="#hiding-transaction-data-from-the-oracle" title="Permalink to this headline">¶</a></h3>
|
|
<p>Because the transaction is sent to the oracle for signing, ordinarily the oracle would be able to see the entire contents
|
|
of that transaction including the inputs, output contract states and all the commands, not just the one (in this case)
|
|
relevant command. This is an obvious privacy leak for the other participants. We currently solve this with
|
|
<code class="docutils literal"><span class="pre">FilteredTransaction</span></code>-s and the use of Merkle Trees. These reveal only the necessary parts of the transaction to the
|
|
oracle but still allow it to sign it by providing the Merkle hashes for the remaining parts. See <a class="reference internal" href="merkle-trees.html"><span class="doc">Transaction tear-offs</span></a> for
|
|
more details.</p>
|
|
</div>
|
|
<div class="section" id="pay-per-play-oracles">
|
|
<h3>Pay-per-play oracles<a class="headerlink" href="#pay-per-play-oracles" title="Permalink to this headline">¶</a></h3>
|
|
<p>Because the signature covers the transaction, and transactions may end up being forwarded anywhere, the fact itself
|
|
is independently checkable. However, this approach can still be useful when the data itself costs money, because the act
|
|
of issuing the signature in the first place can be charged for (e.g. by requiring the submission of a fresh
|
|
<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
|
|
<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
|
|
contract could in theory include a transaction parsing and signature checking library, writing a contract in this way
|
|
would be conclusive evidence of intent to disobey the rules of the service (<em>res ipsa loquitur</em>). In an environment
|
|
where parties are legally identifiable, usage of such a contract would by itself be sufficient to trigger some sort of
|
|
punishment.</p>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="implementing-an-oracle-with-continuously-varying-data">
|
|
<h2>Implementing an oracle with continuously varying data<a class="headerlink" href="#implementing-an-oracle-with-continuously-varying-data" title="Permalink to this headline">¶</a></h2>
|
|
<div class="section" id="implement-the-core-classes">
|
|
<h3>Implement the core classes<a class="headerlink" href="#implement-the-core-classes" title="Permalink to this headline">¶</a></h3>
|
|
<p>The key is to implement your oracle in a similar way to the <code class="docutils literal"><span class="pre">NodeInterestRates.Oracle</span></code> outline we gave above with
|
|
both <code class="docutils literal"><span class="pre">query</span></code> and <code class="docutils literal"><span class="pre">sign</span></code> methods. Typically you would want one class that encapsulates the parameters to the <code class="docutils literal"><span class="pre">query</span></code>
|
|
method (<code class="docutils literal"><span class="pre">FixOf</span></code> above), and a <code class="docutils literal"><span class="pre">CommandData</span></code> implementation (<code class="docutils literal"><span class="pre">Fix</span></code> above) that encapsulates both an instance of
|
|
that parameter class and an instance of whatever the result of the <code class="docutils literal"><span class="pre">query</span></code> is (<code class="docutils literal"><span class="pre">BigDecimal</span></code> above).</p>
|
|
<p>The <code class="docutils literal"><span class="pre">NodeInterestRates.Oracle</span></code> allows querying for multiple <code class="docutils literal"><span class="pre">Fix</span></code>-es but that is not necessary and is
|
|
provided for the convenience of callers who might need multiple and can do it all in one query request. Likewise
|
|
the <em>deadline</em> functionality is optional and can be avoided initially.</p>
|
|
<p>Let’s see what parameters we pass to the constructor of this oracle.</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Oracle</span><span class="p">(</span><span class="k">val</span> <span class="py">identity</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span> <span class="k">private</span> <span class="k">val</span> <span class="py">signingKey</span><span class="p">:</span> <span class="n">KeyPair</span><span class="p">,</span> <span class="k">val</span> <span class="py">clock</span><span class="p">:</span> <span class="n">Clock</span><span class="p">)</span> <span class="p">=</span> <span class="n">TODO</span><span class="p">()</span>
|
|
</pre></div>
|
|
</div>
|
|
<p>Here we see the oracle needs to have its own identity, so it can check which transaction commands it is expected to
|
|
sign for, and also needs a pair of signing keys with which it signs transactions. The clock is used for the deadline
|
|
functionality which we will not discuss further here.</p>
|
|
<p>Assuming you have a data source and can query it, it should be very easy to implement your <code class="docutils literal"><span class="pre">query</span></code> method and the
|
|
parameter and <code class="docutils literal"><span class="pre">CommandData</span></code> classes.</p>
|
|
<p>Let’s see how the <code class="docutils literal"><span class="pre">sign</span></code> method for <code class="docutils literal"><span class="pre">NodeInterestRates.Oracle</span></code> is written:</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span> <span class="k">fun</span> <span class="nf">sign</span><span class="p">(</span><span class="n">ftx</span><span class="p">:</span> <span class="n">FilteredTransaction</span><span class="p">,</span> <span class="n">merkleRoot</span><span class="p">:</span> <span class="n">SecureHash</span><span class="p">):</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span> <span class="p">{</span>
|
|
<span class="k">if</span> <span class="p">(!</span><span class="n">ftx</span><span class="p">.</span><span class="n">verify</span><span class="p">(</span><span class="n">merkleRoot</span><span class="p">))</span> <span class="p">{</span>
|
|
<span class="k">throw</span> <span class="n">MerkleTreeException</span><span class="p">(</span><span class="s">"Rate Fix Oracle: Couldn't verify partial Merkle tree."</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="c1">// Reject if we have something different than only commands.</span>
|
|
<span class="k">val</span> <span class="py">leaves</span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span>
|
|
<span class="n">require</span><span class="p">(</span><span class="n">leaves</span><span class="p">.</span><span class="n">inputs</span><span class="p">.</span><span class="n">isEmpty</span><span class="p">()</span> <span class="p">&&</span> <span class="n">leaves</span><span class="p">.</span><span class="n">outputs</span><span class="p">.</span><span class="n">isEmpty</span><span class="p">()</span> <span class="p">&&</span> <span class="n">leaves</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">isEmpty</span><span class="p">())</span>
|
|
|
|
<span class="k">val</span> <span class="py">fixes</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">Fix</span><span class="p">></span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">commands</span><span class="p">.</span>
|
|
<span class="n">filter</span> <span class="p">{</span> <span class="n">identity</span><span class="p">.</span><span class="n">owningKey</span> <span class="k">in</span> <span class="n">it</span><span class="p">.</span><span class="n">signers</span> <span class="p">&&</span> <span class="n">it</span><span class="p">.</span><span class="n">value</span> <span class="k">is</span> <span class="n">Fix</span> <span class="p">}.</span>
|
|
<span class="n">map</span> <span class="p">{</span> <span class="n">it</span><span class="p">.</span><span class="n">value</span> <span class="k">as</span> <span class="n">Fix</span> <span class="p">}</span>
|
|
|
|
<span class="c1">// Reject signing attempt if we received more commands than we should.</span>
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">fixes</span><span class="p">.</span><span class="n">size</span> <span class="p">!=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">commands</span><span class="p">.</span><span class="n">size</span><span class="p">)</span>
|
|
<span class="k">throw</span> <span class="n">IllegalArgumentException</span><span class="p">()</span>
|
|
|
|
<span class="c1">// Reject this signing attempt if there are no commands of the right kind.</span>
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">fixes</span><span class="p">.</span><span class="n">isEmpty</span><span class="p">())</span>
|
|
<span class="k">throw</span> <span class="n">IllegalArgumentException</span><span class="p">()</span>
|
|
|
|
<span class="c1">// For each fix, verify that the data is correct.</span>
|
|
<span class="k">val</span> <span class="py">knownFixes</span> <span class="p">=</span> <span class="n">knownFixes</span> <span class="c1">// Snapshot</span>
|
|
<span class="k">for</span> <span class="p">(</span><span class="n">fix</span> <span class="k">in</span> <span class="n">fixes</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">known</span> <span class="p">=</span> <span class="n">knownFixes</span><span class="p">[</span><span class="n">fix</span><span class="p">.</span><span class="n">of</span><span class="p">]</span>
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">known</span> <span class="p">==</span> <span class="k">null</span> <span class="p">||</span> <span class="n">known</span> <span class="p">!=</span> <span class="n">fix</span><span class="p">)</span>
|
|
<span class="k">throw</span> <span class="n">UnknownFix</span><span class="p">(</span><span class="n">fix</span><span class="p">.</span><span class="n">of</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="c1">// It all checks out, so we can return a signature.</span>
|
|
<span class="c1">//</span>
|
|
<span class="c1">// Note that we will happily sign an invalid transaction, as we are only being presented with a filtered</span>
|
|
<span class="c1">// version so we can't resolve or check it ourselves. However, that doesn't matter much, as if we sign</span>
|
|
<span class="c1">// an invalid transaction the signature is worthless.</span>
|
|
<span class="k">return</span> <span class="n">signingKey</span><span class="p">.</span><span class="n">signWithECDSA</span><span class="p">(</span><span class="n">merkleRoot</span><span class="p">.</span><span class="n">bytes</span><span class="p">,</span> <span class="n">identity</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
<p>Here we can see that there are several steps:</p>
|
|
<ol class="arabic simple">
|
|
<li>Ensure that the transaction we have been sent is indeed valid and passes verification, even though we cannot see all
|
|
of it.</li>
|
|
<li>Check that we only received commands as expected, and each of those commands expects us to sign for them and is of
|
|
the expected type (<code class="docutils literal"><span class="pre">Fix</span></code> here).</li>
|
|
<li>Iterate over each of the commands we identified in the last step and check that the data they represent matches
|
|
exactly our data source. The final step, assuming we have got this far, is to generate a signature for the
|
|
transaction and return it.</li>
|
|
</ol>
|
|
</div>
|
|
<div class="section" id="binding-to-the-network-via-a-cordapp-plugin">
|
|
<h3>Binding to the network via a CorDapp plugin<a class="headerlink" href="#binding-to-the-network-via-a-cordapp-plugin" title="Permalink to this headline">¶</a></h3>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">Before reading any further, we advise that you understand the concept of flows and how to write them and use
|
|
them. See <a class="reference internal" href="flow-state-machines.html"><span class="doc">Writing flows</span></a>. Likewise some understanding of Cordapps, plugins and services will be helpful.
|
|
See <a class="reference internal" href="creating-a-cordapp.html"><span class="doc">CorDapp basics</span></a>.</p>
|
|
</div>
|
|
<p>The first step is to create a service to host the oracle on the network. Let’s see how that’s implemented:</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span> class Service(val services: PluginServiceHub) : AcceptsFileUpload, SingletonSerializeAsToken() {
|
|
val oracle: Oracle by lazy {
|
|
val myNodeInfo = services.myInfo
|
|
val myIdentity = myNodeInfo.serviceIdentities(type).first()
|
|
val mySigningKey = services.keyManagementService.toKeyPair(myIdentity.owningKey.keys)
|
|
Oracle(myIdentity, mySigningKey, services.clock)
|
|
}
|
|
|
|
init {
|
|
// Note: access to the singleton oracle property is via the registered SingletonSerializeAsToken Service.
|
|
// Otherwise the Kryo serialisation of the call stack in the Quasar Fiber extends to include
|
|
// the framework Oracle and the flow will crash.
|
|
services.registerFlowInitiator(RatesFixFlow.FixSignFlow::class) { FixSignHandler(it, this) }
|
|
services.registerFlowInitiator(RatesFixFlow.FixQueryFlow::class) { FixQueryHandler(it, this) }
|
|
}
|
|
|
|
private class FixSignHandler(val otherParty: Party, val service: Service) : FlowLogic<Unit>() {
|
|
@Suspendable
|
|
override fun call() {
|
|
val request = receive<RatesFixFlow.SignRequest>(otherParty).unwrap { it }
|
|
send(otherParty, service.oracle.sign(request.ftx, request.rootHash))
|
|
}
|
|
}
|
|
|
|
private class FixQueryHandler(val otherParty: Party, val service: Service) : FlowLogic<Unit>() {
|
|
companion object {
|
|
object RECEIVED : ProgressTracker.Step("Received fix request")
|
|
object SENDING : ProgressTracker.Step("Sending fix response")
|
|
}
|
|
|
|
override val progressTracker = ProgressTracker(RECEIVED, SENDING)
|
|
|
|
init {
|
|
progressTracker.currentStep = RECEIVED
|
|
}
|
|
|
|
@Suspendable
|
|
override fun call(): Unit {
|
|
val request = receive<RatesFixFlow.QueryRequest>(otherParty).unwrap { it }
|
|
val answers = service.oracle.query(request.queries, request.deadline)
|
|
progressTracker.currentStep = SENDING
|
|
send(otherParty, answers)
|
|
}
|
|
}
|
|
</pre></div>
|
|
</div>
|
|
<p>This may look complicated, but really it’s made up of some relatively simple elements (in the order they appear in the code):</p>
|
|
<ol class="arabic simple">
|
|
<li>Accept a <code class="docutils literal"><span class="pre">PluginServiceHub</span></code> in the constructor. This is your interface to the Corda node.</li>
|
|
<li>Ensure you extend the abstract class <code class="docutils literal"><span class="pre">SingletonSerializeAsToken</span></code> (see <a class="reference internal" href="corda-plugins.html"><span class="doc">The Corda plugin framework</span></a>).</li>
|
|
<li>Create an instance of your core oracle class that has the <code class="docutils literal"><span class="pre">query</span></code> and <code class="docutils literal"><span class="pre">sign</span></code> methods as discussed above.</li>
|
|
<li>Register your client sub-flows (in this case both in <code class="docutils literal"><span class="pre">RatesFixFlow</span></code>. See the next section) for querying and
|
|
signing as initiating your service flows that actually do the querying and signing using your core oracle class instance.</li>
|
|
<li>Implement your service flows that call your core oracle class instance.</li>
|
|
</ol>
|
|
<p>The final step is to register your service with the node via the plugin mechanism. Do this by
|
|
implementing a plugin. Don’t forget the resources file to register it with the <code class="docutils literal"><span class="pre">ServiceLoader</span></code> framework
|
|
(see <a class="reference internal" href="corda-plugins.html"><span class="doc">The Corda plugin framework</span></a>).</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Plugin</span> <span class="p">:</span> <span class="n">CordaPluginRegistry</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="k">override</span> <span class="k">val</span> <span class="py">servicePlugins</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">Class</span><span class="p"><*>></span> <span class="p">=</span> <span class="n">listOf</span><span class="p">(</span><span class="n">Service</span><span class="o">::</span><span class="k">class</span><span class="p">.</span><span class="n">java</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="providing-client-sub-flows-for-querying-and-signing">
|
|
<h3>Providing client sub-flows for querying and signing<a class="headerlink" href="#providing-client-sub-flows-for-querying-and-signing" title="Permalink to this headline">¶</a></h3>
|
|
<p>We mentioned the client sub-flow briefly above. They are the mechanism that clients, in the form of other flows, will
|
|
interact with your oracle. Typically there will be one for querying and one for signing. Let’s take a look at
|
|
those for <code class="docutils literal"><span class="pre">NodeInterestRates.Oracle</span></code>.</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span> <span class="k">class</span> <span class="nc">FixQueryFlow</span><span class="p">(</span><span class="k">val</span> <span class="py">fixOf</span><span class="p">:</span> <span class="n">FixOf</span><span class="p">,</span> <span class="k">val</span> <span class="py">oracle</span><span class="p">:</span> <span class="n">Party</span><span class="p">)</span> <span class="p">:</span> <span class="n">FlowLogic</span><span class="p"><</span><span class="n">Fix</span><span class="p">>()</span> <span class="p">{</span>
|
|
<span class="n">@Suspendable</span>
|
|
<span class="k">override</span> <span class="k">fun</span> <span class="nf">call</span><span class="p">():</span> <span class="n">Fix</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">deadline</span> <span class="p">=</span> <span class="n">suggestInterestRateAnnouncementTimeWindow</span><span class="p">(</span><span class="n">fixOf</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">oracle</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">fixOf</span><span class="p">.</span><span class="n">forDay</span><span class="p">).</span><span class="n">end</span>
|
|
<span class="c1">// TODO: add deadline to receive</span>
|
|
<span class="k">val</span> <span class="py">resp</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p"><</span><span class="n">ArrayList</span><span class="p"><</span><span class="n">Fix</span><span class="p">>>(</span><span class="n">oracle</span><span class="p">,</span> <span class="n">QueryRequest</span><span class="p">(</span><span class="n">listOf</span><span class="p">(</span><span class="n">fixOf</span><span class="p">),</span> <span class="n">deadline</span><span class="p">))</span>
|
|
|
|
<span class="k">return</span> <span class="n">resp</span><span class="p">.</span><span class="n">unwrap</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">fix</span> <span class="p">=</span> <span class="n">it</span><span class="p">.</span><span class="n">first</span><span class="p">()</span>
|
|
<span class="c1">// Check the returned fix is for what we asked for.</span>
|
|
<span class="n">check</span><span class="p">(</span><span class="n">fix</span><span class="p">.</span><span class="n">of</span> <span class="p">==</span> <span class="n">fixOf</span><span class="p">)</span>
|
|
<span class="n">fix</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="k">class</span> <span class="nc">FixSignFlow</span><span class="p">(</span><span class="k">val</span> <span class="py">tx</span><span class="p">:</span> <span class="n">TransactionBuilder</span><span class="p">,</span> <span class="k">val</span> <span class="py">oracle</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span> <span class="k">val</span> <span class="py">filterFuns</span><span class="p">:</span> <span class="n">FilterFuns</span><span class="p">)</span> <span class="p">:</span> <span class="n">FlowLogic</span><span class="p"><</span><span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span><span class="p">>()</span> <span class="p">{</span>
|
|
<span class="n">@Suspendable</span>
|
|
<span class="k">override</span> <span class="k">fun</span> <span class="nf">call</span><span class="p">():</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">wtx</span> <span class="p">=</span> <span class="n">tx</span><span class="p">.</span><span class="n">toWireTransaction</span><span class="p">()</span>
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<span class="k">val</span> <span class="py">partialMerkleTx</span> <span class="p">=</span> <span class="n">FilteredTransaction</span><span class="p">.</span><span class="n">buildMerkleTransaction</span><span class="p">(</span><span class="n">wtx</span><span class="p">,</span> <span class="n">filterFuns</span><span class="p">)</span>
|
|
<span class="k">val</span> <span class="py">rootHash</span> <span class="p">=</span> <span class="n">wtx</span><span class="p">.</span><span class="n">id</span>
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<span class="k">val</span> <span class="py">resp</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p"><</span><span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span><span class="p">>(</span><span class="n">oracle</span><span class="p">,</span> <span class="n">SignRequest</span><span class="p">(</span><span class="n">rootHash</span><span class="p">,</span> <span class="n">partialMerkleTx</span><span class="p">))</span>
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<span class="k">return</span> <span class="n">resp</span><span class="p">.</span><span class="n">unwrap</span> <span class="p">{</span> <span class="n">sig</span> <span class="p">-></span>
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<span class="n">check</span><span class="p">(</span><span class="n">sig</span><span class="p">.</span><span class="n">signer</span> <span class="p">==</span> <span class="n">oracle</span><span class="p">)</span>
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<span class="n">tx</span><span class="p">.</span><span class="n">checkSignature</span><span class="p">(</span><span class="n">sig</span><span class="p">)</span>
|
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<span class="n">sig</span>
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<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
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</pre></div>
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</div>
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<p>You’ll note that the <code class="docutils literal"><span class="pre">FixSignFlow</span></code> requires a <code class="docutils literal"><span class="pre">FilterFuns</span></code> instance with the appropriate filter to include only
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the <code class="docutils literal"><span class="pre">Fix</span></code> commands. You can find a further explanation of this in <a class="reference internal" href="merkle-trees.html"><span class="doc">Transaction tear-offs</span></a>.</p>
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</div>
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</div>
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<div class="section" id="using-an-oracle">
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<h2>Using an oracle<a class="headerlink" href="#using-an-oracle" title="Permalink to this headline">¶</a></h2>
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<p>The oracle is invoked through sub-flows to query for values, add them to the transaction as commands and then get
|
|
the transaction signed by the oracle. Following on from the above examples, this is all encapsulated in a sub-flow
|
|
called <code class="docutils literal"><span class="pre">RatesFixFlow</span></code>. Here’s the <code class="docutils literal"><span class="pre">call</span></code> method of that flow.</p>
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span> <span class="n">@Suspendable</span>
|
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<span class="k">override</span> <span class="k">fun</span> <span class="nf">call</span><span class="p">()</span> <span class="p">{</span>
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|
<span class="n">progressTracker</span><span class="p">.</span><span class="n">currentStep</span> <span class="p">=</span> <span class="n">progressTracker</span><span class="p">.</span><span class="n">steps</span><span class="p">[</span><span class="m">1</span><span class="p">]</span>
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|
<span class="k">val</span> <span class="py">fix</span> <span class="p">=</span> <span class="n">subFlow</span><span class="p">(</span><span class="n">FixQueryFlow</span><span class="p">(</span><span class="n">fixOf</span><span class="p">,</span> <span class="n">oracle</span><span class="p">))</span>
|
|
<span class="n">progressTracker</span><span class="p">.</span><span class="n">currentStep</span> <span class="p">=</span> <span class="n">WORKING</span>
|
|
<span class="n">checkFixIsNearExpected</span><span class="p">(</span><span class="n">fix</span><span class="p">)</span>
|
|
<span class="n">tx</span><span class="p">.</span><span class="n">addCommand</span><span class="p">(</span><span class="n">fix</span><span class="p">,</span> <span class="n">oracle</span><span class="p">.</span><span class="n">owningKey</span><span class="p">)</span>
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<span class="n">beforeSigning</span><span class="p">(</span><span class="n">fix</span><span class="p">)</span>
|
|
<span class="n">progressTracker</span><span class="p">.</span><span class="n">currentStep</span> <span class="p">=</span> <span class="n">SIGNING</span>
|
|
<span class="k">val</span> <span class="py">signature</span> <span class="p">=</span> <span class="n">subFlow</span><span class="p">(</span><span class="n">FixSignFlow</span><span class="p">(</span><span class="n">tx</span><span class="p">,</span> <span class="n">oracle</span><span class="p">,</span> <span class="n">filterFuns</span><span class="p">))</span>
|
|
<span class="n">tx</span><span class="p">.</span><span class="n">addSignatureUnchecked</span><span class="p">(</span><span class="n">signature</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
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</pre></div>
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</div>
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<p>As you can see, this:</p>
|
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<ol class="arabic simple">
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|
<li>Queries the oracle for the fact using the client sub-flow for querying from above.</li>
|
|
<li>Does some quick validation.</li>
|
|
<li>Adds the command to the transaction containing the fact to be signed for by the oracle.</li>
|
|
<li>Calls an extension point that allows clients to generate output states based on the fact from the oracle.</li>
|
|
<li>Requests the signature from the oracle using the client sub-flow for signing from above.</li>
|
|
<li>Adds the signature returned from the oracle.</li>
|
|
</ol>
|
|
<p>Here’s an example of it in action from <code class="docutils literal"><span class="pre">FixingFlow.Fixer</span></code>.</p>
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span> fun filterCommands(c: Command) = oracleParty.owningKey in c.signers && c.value is Fix
|
|
|
|
val filterFuns = FilterFuns(filterCommands = ::filterCommands)
|
|
val addFixing = object : RatesFixFlow(ptx, filterFuns, oracleParty, fixOf, BigDecimal.ZERO, BigDecimal.ONE) {
|
|
@Suspendable
|
|
override fun beforeSigning(fix: Fix) {
|
|
newDeal.generateFix(ptx, StateAndRef(txState, handshake.payload.ref), fix)
|
|
|
|
// And add a request for timestamping: it may be that none of the contracts need this! But it can't hurt
|
|
// to have one.
|
|
ptx.setTime(serviceHub.clock.instant(), 30.seconds)
|
|
}
|
|
}
|
|
subFlow(addFixing)
|
|
</pre></div>
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</div>
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