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2016-10-11 11:30:55 +02:00
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@ -32,7 +32,7 @@
<link rel="top" title="R3 Corda latest documentation" href="index.html"/>
<link rel="next" title="Writing oracle services" href="oracles.html"/>
<link rel="prev" title="Writing a contract test" href="tutorial-test-dsl.html"/>
<link rel="prev" title="Client RPC API" href="tutorial-clientrpc-api.html"/>
<script src="_static/js/modernizr.min.js"></script>
@ -93,6 +93,7 @@
<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="persistence.html">Persistence</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>
@ -104,6 +105,7 @@
<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="tutorial-clientrpc-api.html">Client RPC API</a></li>
<li class="toctree-l1 current"><a class="current reference internal" href="#">Protocol state machines</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#introduction">Introduction</a></li>
<li class="toctree-l2"><a class="reference internal" href="#theory">Theory</a></li>
@ -120,10 +122,9 @@
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="oracles.html">Writing oracle services</a></li>
<li class="toctree-l1"><a class="reference internal" href="tutorial-attachments.html">Using attachments</a></li>
<li class="toctree-l1"><a class="reference internal" href="event-scheduling.html">Event scheduling</a></li>
<li class="toctree-l1"><a class="reference internal" href="secure-coding-guidelines.html">Secure coding guidelines</a></li>
<li class="toctree-l1"><a class="reference internal" href="secure-coding-guidelines.html#protocols">Protocols</a></li>
<li class="toctree-l1"><a class="reference internal" href="secure-coding-guidelines.html#contracts">Contracts</a></li>
</ul>
<p class="caption"><span class="caption-text">Contracts</span></p>
<ul>
@ -263,7 +264,7 @@ owner key as output, and any change cash as output. It contains a single signatu
it lacks a signature from S authorising movement of the asset.</li>
<li>S signs it and hands the now finalised <code class="docutils literal"><span class="pre">SignedTransaction</span></code> back to B.</li>
</ol>
<p>You can find the implementation of this protocol in the file <code class="docutils literal"><span class="pre">contracts/protocols/TwoPartyTradeProtocol.kt</span></code>.</p>
<p>You can find the implementation of this protocol in the file <code class="docutils literal"><span class="pre">contracts/src/main/kotlin/com/r3corda/protocols/TwoPartyTradeProtocol.kt</span></code>.</p>
<p>Assuming no malicious termination, they both end the protocol being in posession of a valid, signed transaction that
represents an atomic asset swap.</p>
<p>Note that it&#8217;s the <em>seller</em> who initiates contact with the buyer, not vice-versa as you might imagine.</p>
@ -277,7 +278,6 @@ write them and the approach is the same.</p>
</div>
<div class="codeset container">
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">object</span> <span class="nc">TwoPartyTradeProtocol</span> <span class="p">{</span>
<span class="k">val</span> <span class="py">TOPIC</span> <span class="p">=</span> <span class="s">&quot;platform.trade&quot;</span>
<span class="k">class</span> <span class="nc">UnacceptablePriceException</span><span class="p">(</span><span class="k">val</span> <span class="py">givenPrice</span><span class="p">:</span> <span class="n">Amount</span><span class="p">&lt;</span><span class="n">Currency</span><span class="p">&gt;)</span> <span class="p">:</span> <span class="n">Exception</span><span class="p">(</span><span class="s">&quot;Unacceptable price: $givenPrice&quot;</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">AssetMismatchException</span><span class="p">(</span><span class="k">val</span> <span class="py">expectedTypeName</span><span class="p">:</span> <span class="n">String</span><span class="p">,</span> <span class="k">val</span> <span class="py">typeName</span><span class="p">:</span> <span class="n">String</span><span class="p">)</span> <span class="p">:</span> <span class="n">Exception</span><span class="p">()</span> <span class="p">{</span>
@ -285,21 +285,20 @@ write them and the approach is the same.</p>
<span class="p">}</span>
<span class="c1">// This object is serialised to the network and is the first protocol message the seller sends to the buyer.</span>
<span class="k">class</span> <span class="nc">SellerTradeInfo</span><span class="p">(</span>
<span class="k">data</span> <span class="k">class</span> <span class="nc">SellerTradeInfo</span><span class="p">(</span>
<span class="k">val</span> <span class="py">assetForSale</span><span class="p">:</span> <span class="n">StateAndRef</span><span class="p">&lt;</span><span class="n">OwnableState</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
<span class="k">val</span> <span class="py">sellerOwnerKey</span><span class="p">:</span> <span class="n">PublicKey</span><span class="p">,</span>
<span class="k">val</span> <span class="py">sessionID</span><span class="p">:</span> <span class="n">Long</span>
<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">&lt;</span><span class="n">Currency</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">sellerOwnerKey</span><span class="p">:</span> <span class="n">PublicKey</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">SignaturesFromSeller</span><span class="p">(</span><span class="k">val</span> <span class="py">timestampAuthoritySig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">,</span> <span class="k">val</span> <span class="py">sellerSig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">)</span>
<span class="k">data</span> <span class="k">class</span> <span class="nc">SignaturesFromSeller</span><span class="p">(</span><span class="k">val</span> <span class="py">sellerSig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">,</span>
<span class="k">val</span> <span class="py">notarySig</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">open</span> <span class="k">class</span> <span class="nc">Seller</span><span class="p">(</span><span class="k">val</span> <span class="py">otherSide</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span>
<span class="k">val</span> <span class="py">notaryNode</span><span class="p">:</span> <span class="n">NodeInfo</span><span class="p">,</span>
<span class="k">val</span> <span class="py">assetToSell</span><span class="p">:</span> <span class="n">StateAndRef</span><span class="p">&lt;</span><span class="n">OwnableState</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">&lt;</span><span class="n">Currency</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">myKeyPair</span><span class="p">:</span> <span class="n">KeyPair</span><span class="p">,</span>
<span class="k">val</span> <span class="py">buyerSessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">,</span>
<span class="k">override</span> <span class="k">val</span> <span class="py">progressTracker</span><span class="p">:</span> <span class="n">ProgressTracker</span> <span class="p">=</span> <span class="n">Seller</span><span class="p">.</span><span class="n">tracker</span><span class="p">())</span> <span class="p">:</span> <span class="n">ProtocolLogic</span><span class="p">&lt;</span><span class="n">SignedTransaction</span><span class="p">&gt;()</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">SignedTransaction</span> <span class="p">{</span>
@ -310,8 +309,7 @@ write them and the approach is the same.</p>
<span class="k">open</span> <span class="k">class</span> <span class="nc">Buyer</span><span class="p">(</span><span class="k">val</span> <span class="py">otherSide</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span>
<span class="k">val</span> <span class="py">notary</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span>
<span class="k">val</span> <span class="py">acceptablePrice</span><span class="p">:</span> <span class="n">Amount</span><span class="p">&lt;</span><span class="n">Currency</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">typeToBuy</span><span class="p">:</span> <span class="n">Class</span><span class="p">&lt;</span><span class="k">out</span> <span class="n">OwnableState</span><span class="p">&gt;,</span>
<span class="k">val</span> <span class="py">sessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">)</span> <span class="p">:</span> <span class="n">ProtocolLogic</span><span class="p">&lt;</span><span class="n">SignedTransaction</span><span class="p">&gt;()</span> <span class="p">{</span>
<span class="k">val</span> <span class="py">typeToBuy</span><span class="p">:</span> <span class="n">Class</span><span class="p">&lt;</span><span class="k">out</span> <span class="n">OwnableState</span><span class="p">&gt;)</span> <span class="p">:</span> <span class="n">ProtocolLogic</span><span class="p">&lt;</span><span class="n">SignedTransaction</span><span class="p">&gt;()</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">SignedTransaction</span> <span class="p">{</span>
<span class="n">TODO</span><span class="p">()</span>
@ -321,35 +319,23 @@ write them and the approach is the same.</p>
</pre></div>
</div>
</div>
<p>Let&#8217;s unpack what this code does:</p>
<ul class="simple">
<li>It defines a several classes nested inside the main <code class="docutils literal"><span class="pre">TwoPartyTradeProtocol</span></code> singleton. Some of the classes
are simply protocol messages or exceptions. The other two represent the buyer and seller side of the protocol.</li>
<li>It defines the &#8220;trade topic&#8221;, which is just a string that namespaces this protocol. The prefix &#8220;platform.&#8221; is reserved
by Corda, but you can define your own protocol namespaces using standard Java-style reverse DNS notation.</li>
</ul>
<p>This code defines several classes nested inside the main <code class="docutils literal"><span class="pre">TwoPartyTradeProtocol</span></code> singleton. Some of the classes are
simply protocol messages or exceptions. The other two represent the buyer and seller side of the protocol.</p>
<p>Going through the data needed to become a seller, we have:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">otherSide:</span> <span class="pre">SingleMessageRecipient</span></code> - the network address of the node with which you are trading.</li>
<li><code class="docutils literal"><span class="pre">otherSide:</span> <span class="pre">Party</span></code> - the party with which you are trading.</li>
<li><code class="docutils literal"><span class="pre">notaryNode:</span> <span class="pre">NodeInfo</span></code> - the entry in the network map for the chosen notary. See &#8220;<a class="reference internal" href="consensus.html"><span class="doc">Consensus model</span></a>&#8221; for more
information on notaries.</li>
<li><code class="docutils literal"><span class="pre">assetToSell:</span> <span class="pre">StateAndRef&lt;OwnableState&gt;</span></code> - a pointer to the ledger entry that represents the thing being sold.</li>
<li><code class="docutils literal"><span class="pre">price:</span> <span class="pre">Amount&lt;Currency&gt;</span></code> - the agreed on price that the asset is being sold for (without an issuer constraint).</li>
<li><code class="docutils literal"><span class="pre">myKeyPair:</span> <span class="pre">KeyPair</span></code> - the key pair that controls the asset being sold. It will be used to sign the transaction.</li>
<li><code class="docutils literal"><span class="pre">buyerSessionID:</span> <span class="pre">Long</span></code> - a unique number that identifies this trade to the buyer. It is expected that the buyer
knows that the trade is going to take place and has sent you such a number already.</li>
</ul>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Session IDs will be automatically handled in a future version of the framework.</p>
</div>
<p>And for the buyer:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">acceptablePrice:</span> <span class="pre">Amount&lt;Currency&gt;</span></code> - the price that was agreed upon out of band. If the seller specifies
a price less than or equal to this, then the trade will go ahead.</li>
<li><code class="docutils literal"><span class="pre">typeToBuy:</span> <span class="pre">Class&lt;out</span> <span class="pre">OwnableState&gt;</span></code> - the type of state that is being purchased. This is used to check that the
sell side of the protocol isn&#8217;t trying to sell us the wrong thing, whether by accident or on purpose.</li>
<li><code class="docutils literal"><span class="pre">sessionID:</span> <span class="pre">Long</span></code> - the session ID that was handed to the seller in order to start the protocol.</li>
</ul>
<p>Alright, so using this protocol shouldn&#8217;t be too hard: in the simplest case we can just create a Buyer or Seller
with the details of the trade, depending on who we are. We then have to start the protocol in some way. Just
@ -387,6 +373,24 @@ be requested by untrusted code (e.g. a state that you have been sent), the types
protocol are checked against a whitelist, which can be extended by apps themselves at load time.</p>
<p>The process of starting a protocol returns a <code class="docutils literal"><span class="pre">ListenableFuture</span></code> that you can use to either block waiting for
the result, or register a callback that will be invoked when the result is ready.</p>
<p>In a two party protocol only one side is to be manually started using <code class="docutils literal"><span class="pre">ServiceHub.invokeProtocolAsync</span></code>. The other side
has to be registered by its node to respond to the initiating protocol via <code class="docutils literal"><span class="pre">ServiceHubInternal.registerProtocolInitiator</span></code>.
In our example it doesn&#8217;t matter which protocol is the initiator and which is the initiated. For example, if we are to
take the seller as the initiator then we would register the buyer as such:</p>
<div class="codeset container">
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">val</span> <span class="py">services</span><span class="p">:</span> <span class="n">ServiceHubInternal</span> <span class="p">=</span> <span class="n">TODO</span><span class="p">()</span>
<span class="n">services</span><span class="p">.</span><span class="n">registerProtocolInitiator</span><span class="p">(</span><span class="n">Seller</span><span class="o">::</span><span class="k">class</span><span class="p">)</span> <span class="p">{</span> <span class="n">otherParty</span> <span class="p">-&gt;</span>
<span class="k">val</span> <span class="py">notary</span> <span class="p">=</span> <span class="n">services</span><span class="p">.</span><span class="n">networkMapCache</span><span class="p">.</span><span class="n">notaryNodes</span><span class="p">[</span><span class="m">0</span><span class="p">]</span>
<span class="k">val</span> <span class="py">acceptablePrice</span> <span class="p">=</span> <span class="n">TODO</span><span class="p">()</span>
<span class="k">val</span> <span class="py">typeToBuy</span> <span class="p">=</span> <span class="n">TODO</span><span class="p">()</span>
<span class="n">Buyer</span><span class="p">(</span><span class="n">otherParty</span><span class="p">,</span> <span class="n">notary</span><span class="p">,</span> <span class="n">acceptablePrice</span><span class="p">,</span> <span class="n">typeToBuy</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<p>This is telling the buyer node to fire up an instance of <code class="docutils literal"><span class="pre">Buyer</span></code> (the code in the lambda) when the initiating protocol
is a seller (<code class="docutils literal"><span class="pre">Seller::class</span></code>).</p>
</div>
<div class="section" id="implementing-the-seller">
<h2>Implementing the seller<a class="headerlink" href="#implementing-the-seller" title="Permalink to this headline"></a></h2>
@ -412,12 +416,10 @@ finished transaction.</p>
<div class="codeset container">
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="n">@Suspendable</span>
<span class="k">private</span> <span class="k">fun</span> <span class="nf">receiveAndCheckProposedTransaction</span><span class="p">():</span> <span class="n">SignedTransaction</span> <span class="p">{</span>
<span class="k">val</span> <span class="py">sessionID</span> <span class="p">=</span> <span class="n">random63BitValue</span><span class="p">()</span>
<span class="c1">// Make the first message we&#39;ll send to kick off the protocol.</span>
<span class="k">val</span> <span class="py">hello</span> <span class="p">=</span> <span class="n">SellerTradeInfo</span><span class="p">(</span><span class="n">assetToSell</span><span class="p">,</span> <span class="n">price</span><span class="p">,</span> <span class="n">myKeyPair</span><span class="p">.</span><span class="k">public</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">)</span>
<span class="k">val</span> <span class="py">hello</span> <span class="p">=</span> <span class="n">SellerTradeInfo</span><span class="p">(</span><span class="n">assetToSell</span><span class="p">,</span> <span class="n">price</span><span class="p">,</span> <span class="n">myKeyPair</span><span class="p">.</span><span class="k">public</span><span class="p">)</span>
<span class="k">val</span> <span class="py">maybeSTX</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p">&lt;</span><span class="n">SignedTransaction</span><span class="p">&gt;(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">buyerSessionID</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">,</span> <span class="n">hello</span><span class="p">)</span>
<span class="k">val</span> <span class="py">maybeSTX</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p">&lt;</span><span class="n">SignedTransaction</span><span class="p">&gt;(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">hello</span><span class="p">)</span>
<span class="n">maybeSTX</span><span class="p">.</span><span class="n">unwrap</span> <span class="p">{</span>
<span class="c1">// Check that the tx proposed by the buyer is valid.</span>
@ -442,11 +444,10 @@ finished transaction.</p>
</pre></div>
</div>
</div>
<p>Let&#8217;s break this down. We generate a session ID to identify what&#8217;s happening on the seller side, fill out
the initial protocol message, and then call <code class="docutils literal"><span class="pre">sendAndReceive</span></code>. This function takes a few arguments:</p>
<p>Let&#8217;s break this down. We fill out the initial protocol message with the trade info, and then call <code class="docutils literal"><span class="pre">sendAndReceive</span></code>.
This function takes a few arguments:</p>
<ul class="simple">
<li>The topic string that ensures the message is routed to the right bit of code in the other side&#8217;s node.</li>
<li>The session IDs that ensure the messages don&#8217;t get mixed up with other simultaneous trades.</li>
<li>The party on the other side.</li>
<li>The thing to send. It&#8217;ll be serialised and sent automatically.</li>
<li>Finally a type argument, which is the kind of object we&#8217;re expecting to receive from the other side. If we get
back something else an exception is thrown.</li>
@ -521,7 +522,7 @@ well (but having handled the fact that some signatures are missing ourselves).</
<span class="n">notarySignature</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">SignedTransaction</span> <span class="p">{</span>
<span class="k">val</span> <span class="py">fullySigned</span> <span class="p">=</span> <span class="n">partialTX</span> <span class="p">+</span> <span class="n">ourSignature</span> <span class="p">+</span> <span class="n">notarySignature</span>
<span class="n">logger</span><span class="p">.</span><span class="n">trace</span> <span class="p">{</span> <span class="s">&quot;Built finished transaction, sending back to secondary!&quot;</span> <span class="p">}</span>
<span class="n">send</span><span class="p">(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">buyerSessionID</span><span class="p">,</span> <span class="n">SignaturesFromSeller</span><span class="p">(</span><span class="n">ourSignature</span><span class="p">,</span> <span class="n">notarySignature</span><span class="p">))</span>
<span class="n">send</span><span class="p">(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">SignaturesFromSeller</span><span class="p">(</span><span class="n">ourSignature</span><span class="p">,</span> <span class="n">notarySignature</span><span class="p">))</span>
<span class="k">return</span> <span class="n">fullySigned</span>
<span class="p">}</span>
</pre></div>
@ -554,7 +555,7 @@ override fun call(): SignedTransaction {
val (ptx, cashSigningPubKeys) = assembleSharedTX(tradeRequest)
val stx = signWithOurKeys(cashSigningPubKeys, ptx)
val signatures = swapSignaturesWithSeller(stx, tradeRequest.sessionID)
val signatures = swapSignaturesWithSeller(stx)
logger.trace { &quot;Got signatures from seller, verifying ... &quot; }
@ -567,16 +568,14 @@ override fun call(): SignedTransaction {
@Suspendable
private fun receiveAndValidateTradeRequest(): SellerTradeInfo {
// Wait for a trade request to come in on our pre-provided session ID.
val maybeTradeRequest = receive&lt;SellerTradeInfo&gt;(sessionID)
// Wait for a trade request to come in from the other side
val maybeTradeRequest = receive&lt;SellerTradeInfo&gt;(otherParty)
maybeTradeRequest.unwrap {
// What is the seller trying to sell us?
val asset = it.assetForSale.state.data
val assetTypeName = asset.javaClass.name
logger.trace { &quot;Got trade request for a $assetTypeName: ${it.assetForSale}&quot; }
// Check the start message for acceptability.
check(it.sessionID &gt; 0)
if (it.price &gt; acceptablePrice)
throw UnacceptablePriceException(it.price)
if (!typeToBuy.isInstance(asset))
@ -591,13 +590,13 @@ private fun receiveAndValidateTradeRequest(): SellerTradeInfo {
}
@Suspendable
private fun swapSignaturesWithSeller(stx: SignedTransaction, theirSessionID: Long): SignaturesFromSeller {
private fun swapSignaturesWithSeller(stx: SignedTransaction): SignaturesFromSeller {
progressTracker.currentStep = SWAPPING_SIGNATURES
logger.trace { &quot;Sending partially signed transaction to seller&quot; }
// TODO: Protect against the seller terminating here and leaving us in the lurch without the final tx.
return sendAndReceive&lt;SignaturesFromSeller&gt;(otherSide, theirSessionID, sessionID, stx).unwrap { it }
return sendAndReceive&lt;SignaturesFromSeller&gt;(otherSide, stx).unwrap { it }
}
private fun signWithOurKeys(cashSigningPubKeys: List&lt;PublicKey&gt;, ptx: TransactionBuilder): SignedTransaction {
@ -806,7 +805,6 @@ protocols for this reason (see &#8220;<a class="reference internal" href="event-
<p>The protocol framework is a key part of the platform and will be extended in major ways in future. Here are some of
the features we have planned:</p>
<ul class="simple">
<li>Automatic session ID management</li>
<li>Identity based addressing</li>
<li>Exposing progress trackers to local (inside the firewall) clients using message queues and/or WebSockets</li>
<li>Exception propagation and management, with a &#8220;protocol hospital&#8221; tool to manually provide solutions to unavoidable
@ -830,7 +828,7 @@ reporting logic, or anything else that might be required as part of a communicat
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