mirror of
https://github.com/corda/corda.git
synced 2024-12-20 05:28:21 +00:00
361 lines
19 KiB
HTML
361 lines
19 KiB
HTML
|
|
|
|
<!DOCTYPE html>
|
|
<!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]-->
|
|
<!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]-->
|
|
<head>
|
|
<meta charset="utf-8">
|
|
|
|
<meta name="viewport" content="width=device-width, initial-scale=1.0">
|
|
|
|
<title>Writing oracle services — R3 Corda latest documentation</title>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<link rel="stylesheet" href="_static/css/custom.css" type="text/css" />
|
|
|
|
|
|
|
|
|
|
|
|
<link rel="top" title="R3 Corda latest documentation" href="index.html"/>
|
|
<link rel="next" title="Event scheduling" href="event-scheduling.html"/>
|
|
<link rel="prev" title="Protocol state machines" href="protocol-state-machines.html"/>
|
|
|
|
|
|
<script src="_static/js/modernizr.min.js"></script>
|
|
|
|
</head>
|
|
|
|
<body class="wy-body-for-nav" role="document">
|
|
|
|
<div class="wy-grid-for-nav">
|
|
|
|
|
|
<nav data-toggle="wy-nav-shift" class="wy-nav-side">
|
|
<div class="wy-side-scroll">
|
|
<div class="wy-side-nav-search">
|
|
|
|
|
|
|
|
|
|
<a href="index.html" class="icon icon-home"> R3 Corda
|
|
|
|
|
|
|
|
</a>
|
|
|
|
|
|
|
|
|
|
<div class="version">
|
|
latest
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div role="search">
|
|
<form id="rtd-search-form" class="wy-form" action="search.html" method="get">
|
|
<input type="text" name="q" placeholder="Search docs" />
|
|
<input type="hidden" name="check_keywords" value="yes" />
|
|
<input type="hidden" name="area" value="default" />
|
|
</form>
|
|
</div>
|
|
|
|
|
|
<br>
|
|
<a href="api/index.html">API reference</a>
|
|
|
|
</div>
|
|
|
|
<div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
|
|
|
|
|
|
|
|
<p class="caption"><span class="caption-text">Overview</span></p>
|
|
<ul>
|
|
<li class="toctree-l1"><a class="reference internal" href="inthebox.html">What’s included?</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="getting-set-up.html">Getting set up</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="data-model.html">Data model</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="transaction-data-types.html">Data types</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="consensus.html">Consensus model</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="creating-a-cordapp.html">Creating a Cordapp</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="running-the-demos.html">Running the demos</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="node-administration.html">Node administration</a></li>
|
|
</ul>
|
|
<p class="caption"><span class="caption-text">Contracts</span></p>
|
|
<ul>
|
|
<li class="toctree-l1"><a class="reference internal" href="contract-catalogue.html">Contract catalogue</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="contract-irs.html">Interest Rate Swaps</a></li>
|
|
</ul>
|
|
<p class="caption"><span class="caption-text">Tutorials</span></p>
|
|
<ul class="current">
|
|
<li class="toctree-l1"><a class="reference internal" href="where-to-start.html">Where to start</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="tutorial-contract.html">Writing a contract</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="tutorial-contract-clauses.html">Writing a contract using clauses</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="tutorial-test-dsl.html">Writing a contract test</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
|
<li class="toctree-l1 current"><a class="current reference internal" href="#">Writing oracle services</a><ul>
|
|
<li class="toctree-l2"><a class="reference internal" href="#introduction">Introduction</a></li>
|
|
<li class="toctree-l2"><a class="reference internal" href="#the-two-basic-approaches">The two basic approaches</a></li>
|
|
<li class="toctree-l2"><a class="reference internal" href="#asserting-continuously-varying-data">Asserting continuously varying data</a></li>
|
|
<li class="toctree-l2"><a class="reference internal" href="#pay-per-play-oracles">Pay-per-play oracles</a></li>
|
|
</ul>
|
|
</li>
|
|
<li class="toctree-l1"><a class="reference internal" href="event-scheduling.html">Event scheduling</a></li>
|
|
</ul>
|
|
<p class="caption"><span class="caption-text">Appendix</span></p>
|
|
<ul>
|
|
<li class="toctree-l1"><a class="reference internal" href="release-process.html">Release process</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="release-process.html#steps-to-cut-a-release">Steps to cut a release</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="release-notes.html">Release notes</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="codestyle.html">Code style guide</a></li>
|
|
<li class="toctree-l1"><a class="reference internal" href="building-the-docs.html">Building the documentation</a></li>
|
|
</ul>
|
|
|
|
|
|
|
|
</div>
|
|
</div>
|
|
</nav>
|
|
|
|
<section data-toggle="wy-nav-shift" class="wy-nav-content-wrap">
|
|
|
|
|
|
<nav class="wy-nav-top" role="navigation" aria-label="top navigation">
|
|
<i data-toggle="wy-nav-top" class="fa fa-bars"></i>
|
|
<a href="index.html">R3 Corda</a>
|
|
</nav>
|
|
|
|
|
|
|
|
<div class="wy-nav-content">
|
|
<div class="rst-content">
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<div role="navigation" aria-label="breadcrumbs navigation">
|
|
<ul class="wy-breadcrumbs">
|
|
<li><a href="index.html">Docs</a> »</li>
|
|
|
|
<li>Writing oracle services</li>
|
|
<li class="wy-breadcrumbs-aside">
|
|
|
|
|
|
<a href="_sources/oracles.txt" rel="nofollow"> View page source</a>
|
|
|
|
|
|
</li>
|
|
</ul>
|
|
<hr/>
|
|
</div>
|
|
<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
|
|
<div itemprop="articleBody">
|
|
|
|
<script type="text/javascript" src="_static/jquery.js"></script>
|
|
<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="writing-oracle-services">
|
|
<h1>Writing oracle services<a class="headerlink" href="#writing-oracle-services" title="Permalink to this headline">¶</a></h1>
|
|
<p>This article covers <em>oracles</em>: network services that link the ledger to the outside world by providing facts that
|
|
affect the validity of transactions.</p>
|
|
<p>The current prototype includes an example oracle that provides an interest rate fixing service. It is used by the
|
|
IRS trading demo app.</p>
|
|
<div class="section" id="introduction">
|
|
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
|
|
<p>Oracles are a key concept in the block chain/decentralised ledger space. They can be essential for many kinds of
|
|
application, because we often wish to condition a transaction on some fact being true or false, but the ledger itself
|
|
has a design that is essentially functional: all transactions are <em>pure</em> and <em>immutable</em>. Phrased another way, a
|
|
smart contract cannot perform any input/output or depend on any state outside of the transaction itself. There is no
|
|
way to download a web page or interact with the user, in a smart contract. It must be this way because everyone must
|
|
be able to independently check a transaction and arrive at an identical conclusion for the ledger to maintan its
|
|
integrity: if a transaction could evaluate to “valid” on one computer and then “invalid” a few minutes later on a
|
|
different computer, the entire shared ledger concept wouldn’t work.</p>
|
|
<p>But it is often essential that transactions do depend on data from the outside world, for example, verifying that an
|
|
interest rate swap is paying out correctly may require data on interest rates, verifying that a loan has reached
|
|
maturity requires knowledge about the current time, knowing which side of a bet receives the payment may require
|
|
arbitrary facts about the real world (e.g. the bankruptcy or solvency of a company or country) ... and so on.</p>
|
|
<p>We can solve this problem by introducing services that create digitally signed data structures which assert facts.
|
|
These structures can then be used as an input to a transaction and distributed with the transaction data itself. Because
|
|
the statements are themselves immutable and signed, it is impossible for an oracle to change its mind later and
|
|
invalidate transactions that were previously found to be valid. In contrast, consider what would happen if a contract
|
|
could do an HTTP request: it’s possible that an answer would change after being downloaded, resulting in loss of
|
|
consensus (breaks).</p>
|
|
</div>
|
|
<div class="section" id="the-two-basic-approaches">
|
|
<h2>The two basic approaches<a class="headerlink" href="#the-two-basic-approaches" title="Permalink to this headline">¶</a></h2>
|
|
<p>The architecture provides two ways of implementing oracles with different tradeoffs:</p>
|
|
<ol class="arabic simple">
|
|
<li>Using commands</li>
|
|
<li>Using attachments</li>
|
|
</ol>
|
|
<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
|
|
the entire transaction. The oracle’s signature is valid only for that transaction, and thus even if a fact (like a
|
|
stock price) does not change, every transaction that incorporates that fact must go back to the oracle for signing.</p>
|
|
<p>When a fact is encoded as an attachment, it is a separate object to the transaction and is referred to by hash.
|
|
Nodes download attachments from peers at the same time as they download transactions, unless of course the node has
|
|
already seen that attachment, in which case it won’t fetch it again. Contracts have access to the contents of
|
|
attachments when they run.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">Currently attachments do not support digital signing, but this is a planned feature.</p>
|
|
</div>
|
|
<p>As you can see, both approaches share a few things: they both allow arbitrary binary data to be provided to transactions
|
|
(and thus contracts). The primary difference is whether the data is a freely reusable, standalone object or whether it’s
|
|
integrated with a transaction.</p>
|
|
<p>Here’s a quick way to decide which approach makes more sense for your data source:</p>
|
|
<ul class="simple">
|
|
<li>Is your data <em>continuously changing</em>, like a stock price, the current time, etc? If yes, use a command.</li>
|
|
<li>Is your data <em>commercially valuable</em>, like a feed which you are not allowed to resell unless it’s incorporated into
|
|
a business deal? If yes, use a command, so you can charge money for signing the same fact in each unique business
|
|
context.</li>
|
|
<li>Is your data <em>very small</em>, like a single number? If yes, use a command.</li>
|
|
<li>Is your data <em>large</em>, <em>static</em> and <em>commercially worthless</em>, for instance, a holiday calendar? If yes, use an
|
|
attachment.</li>
|
|
<li>Is your data <em>intended for human consumption</em>, like a PDF of legal prose, or an Excel spreadsheet? If yes, use an
|
|
attachment.</li>
|
|
</ul>
|
|
</div>
|
|
<div class="section" id="asserting-continuously-varying-data">
|
|
<h2>Asserting continuously varying data<a class="headerlink" href="#asserting-continuously-varying-data" title="Permalink to this headline">¶</a></h2>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">A future version of the platform will include a complete tutorial on implementing this type of oracle.</p>
|
|
</div>
|
|
<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
|
|
an oracle that uses a command because the current interest rate fix is a constantly changing fact.</p>
|
|
<p>The obvious way to implement such a service is like this:</p>
|
|
<ol class="arabic simple">
|
|
<li>The creator of the transaction that depends on the interest rate sends it to the oracle.</li>
|
|
<li>The oracle inserts a command with the rate and signs the transaction.</li>
|
|
<li>The oracle sends it back.</li>
|
|
</ol>
|
|
<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
|
|
ordering constraints we don’t want to deal with (being able to get all parties to sign in parallel is a very nice thing).
|
|
So the way we actually implement it is like this:</p>
|
|
<ol class="arabic simple">
|
|
<li>The creator of the transaction that depends on the interest rate asks for the current rate. They can abort at this point
|
|
if they want to.</li>
|
|
<li>They insert a command with that rate and the time it was obtained into the transaction.</li>
|
|
<li>They then send it to the oracle for signing, along with everyone else in parallel. The oracle checks that the command
|
|
has correct data for the asserted time, and signs if so.</li>
|
|
</ol>
|
|
<p>This same technique can be adapted to other types of oracle.</p>
|
|
<p>The oracle consists of a core class that implements the query/sign operations (for easy unit testing), and then a separate
|
|
class that binds it to the network layer.</p>
|
|
<p>Here is an extract from the <code class="docutils literal"><span class="pre">NodeService.Oracle</span></code> class and supporting types:</p>
|
|
<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>
|
|
<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>
|
|
|
|
<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>
|
|
<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>
|
|
|
|
<span class="k">class</span> <span class="nc">Oracle</span> <span class="p">{</span>
|
|
<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>
|
|
|
|
<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>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
<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
|
|
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>
|
|
</div>
|
|
<div class="section" id="pay-per-play-oracles">
|
|
<h2>Pay-per-play oracles<a class="headerlink" href="#pay-per-play-oracles" title="Permalink to this headline">¶</a></h2>
|
|
<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>
|
|
</div>
|
|
<footer>
|
|
|
|
<div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
|
|
|
|
<a href="event-scheduling.html" class="btn btn-neutral float-right" title="Event scheduling" accesskey="n">Next <span class="fa fa-arrow-circle-right"></span></a>
|
|
|
|
|
|
<a href="protocol-state-machines.html" class="btn btn-neutral" title="Protocol state machines" accesskey="p"><span class="fa fa-arrow-circle-left"></span> Previous</a>
|
|
|
|
</div>
|
|
|
|
|
|
<hr/>
|
|
|
|
<div role="contentinfo">
|
|
<p>
|
|
© Copyright 2016, Distributed Ledger Group, LLC.
|
|
|
|
</p>
|
|
</div>
|
|
Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a <a href="https://github.com/snide/sphinx_rtd_theme">theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>.
|
|
|
|
</footer>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
</section>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<script type="text/javascript">
|
|
var DOCUMENTATION_OPTIONS = {
|
|
URL_ROOT:'./',
|
|
VERSION:'latest',
|
|
COLLAPSE_INDEX:false,
|
|
FILE_SUFFIX:'.html',
|
|
HAS_SOURCE: true
|
|
};
|
|
</script>
|
|
<script type="text/javascript" src="_static/jquery.js"></script>
|
|
<script type="text/javascript" src="_static/underscore.js"></script>
|
|
<script type="text/javascript" src="_static/doctools.js"></script>
|
|
|
|
|
|
|
|
|
|
|
|
<script type="text/javascript" src="_static/js/theme.js"></script>
|
|
|
|
|
|
|
|
|
|
<script type="text/javascript">
|
|
jQuery(function () {
|
|
SphinxRtdTheme.StickyNav.enable();
|
|
});
|
|
</script>
|
|
|
|
|
|
</body>
|
|
</html> |