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<li class="toctree-l1 current"><a class="current reference internal" href="#">Using attachments</a><ul>
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<li class="toctree-l2"><a class="reference internal" href="#attachments-demo">Attachments demo</a></li>
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<div class="section" id="using-attachments">
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<h1>Using attachments<a class="headerlink" href="#using-attachments" title="Permalink to this headline">¶</a></h1>
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<p>Attachments are (typically large) Zip/Jar files referenced within a transaction, but not included in the transaction
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itself. These files can be requested from the originating node as needed, although in many cases will be cached within
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nodes already. Examples include:</p>
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<ul class="simple">
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<li>Contract executable code</li>
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<li>Metadata about a transaction, such as PDF version of an invoice being settled</li>
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<li>Shared information to be permanently recorded on the ledger</li>
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</ul>
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<p>To add attachments the file must first be added to uploaded to the node, which returns a unique ID that can be added
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using <code class="docutils literal"><span class="pre">TransactionBuilder.addAttachment()</span></code>. Attachments can be uploaded and downloaded via RPC and HTTP. For
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instructions on HTTP upload/download please see “<a class="reference internal" href="node-administration.html"><span class="doc">Node administration</span></a>”.</p>
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<p>Normally attachments on transactions are fetched automatically via the <code class="docutils literal"><span class="pre">ResolveTransactionsFlow</span></code> when verifying
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received transactions. Attachments are needed in order to validate a transaction (they include, for example, the
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contract code), so must be fetched before the validation process can run. <code class="docutils literal"><span class="pre">ResolveTransactionsFlow</span></code> calls
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<code class="docutils literal"><span class="pre">FetchTransactionsFlow</span></code> to perform the actual retrieval.</p>
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<p>It is encouraged that where possible attachments are reusable data, so that nodes can meaningfully cache them.</p>
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<div class="section" id="attachments-demo">
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<h2>Attachments demo<a class="headerlink" href="#attachments-demo" title="Permalink to this headline">¶</a></h2>
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<p>There is a worked example of attachments, which relays a simple document from one node to another. The “two party
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trade flow” also includes an attachment, however it is a significantly more complex demo, and less well suited
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for a tutorial.</p>
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<p>The demo code is in the file <code class="docutils literal"><span class="pre">samples/attachment-demo/src/main/kotlin/net/corda/attachmentdemo/AttachmentDemo.kt</span></code>,
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with the core logic contained within the two functions <code class="docutils literal"><span class="pre">recipient()</span></code> and <code class="docutils literal"><span class="pre">sender()</span></code>. The first thing it does is set
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up an RPC connection to node B using a demo user account (this is all configured in the gradle build script for the demo
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and the nodes will be created using the <code class="docutils literal"><span class="pre">deployNodes</span></code> gradle task as normal). The <code class="docutils literal"><span class="pre">CordaRPCClient.use</span></code> method is a
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convenience helper intended for small tools that sets up an RPC connection scoped to the provided block, and brings all
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the RPCs into scope. Once connected the sender/recipient functions are run with the RPC proxy as a parameter.</p>
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<p>We’ll look at the recipient function first.</p>
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<p>The first thing it does is wait to receive a notification of a new transaction by calling the <code class="docutils literal"><span class="pre">verifiedTransactions</span></code>
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RPC, which returns both a snapshot and an observable of changes. The observable is made blocking and the next
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transaction the node verifies is retrieved. That transaction is checked to see if it has the expected attachment
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and if so, printed out.</p>
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">fun</span> <span class="nf">recipient</span><span class="p">(</span><span class="n">rpc</span><span class="p">:</span> <span class="n">CordaRPCOps</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">println</span><span class="p">(</span><span class="s">"Waiting to receive transaction ..."</span><span class="p">)</span>
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<span class="k">val</span> <span class="py">stx</span> <span class="p">=</span> <span class="n">rpc</span><span class="p">.</span><span class="n">verifiedTransactions</span><span class="p">().</span><span class="n">second</span><span class="p">.</span><span class="n">toBlocking</span><span class="p">().</span><span class="n">first</span><span class="p">()</span>
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<span class="k">val</span> <span class="py">wtx</span> <span class="p">=</span> <span class="n">stx</span><span class="p">.</span><span class="n">tx</span>
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<span class="k">if</span> <span class="p">(</span><span class="n">wtx</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">isNotEmpty</span><span class="p">())</span> <span class="p">{</span>
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<span class="n">assertEquals</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">,</span> <span class="n">wtx</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">first</span><span class="p">())</span>
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<span class="n">require</span><span class="p">(</span><span class="n">rpc</span><span class="p">.</span><span class="n">attachmentExists</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">))</span>
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<span class="n">println</span><span class="p">(</span><span class="s">"File received - we're happy!\n\nFinal transaction is:\n\n${Emoji.renderIfSupported(wtx)}"</span><span class="p">)</span>
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<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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<span class="n">println</span><span class="p">(</span><span class="s">"Error: no attachments found in ${wtx.id}"</span><span class="p">)</span>
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<span class="p">}</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>The sender correspondingly builds a transaction with the attachment, then calls <code class="docutils literal"><span class="pre">FinalityFlow</span></code> to complete the
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transaction and send it to the recipient node:</p>
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">fun</span> <span class="nf">sender</span><span class="p">(</span><span class="n">rpc</span><span class="p">:</span> <span class="n">CordaRPCOps</span><span class="p">)</span> <span class="p">{</span>
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<span class="c1">// Get the identity key of the other side (the recipient).</span>
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<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="n">rpc</span><span class="p">.</span><span class="n">partyFromName</span><span class="p">(</span><span class="s">"Bank B"</span><span class="p">)</span><span class="o">!!</span>
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<span class="c1">// Make sure we have the file in storage</span>
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<span class="c1">// TODO: We should have our own demo file, not share the trader demo file</span>
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<span class="k">if</span> <span class="p">(!</span><span class="n">rpc</span><span class="p">.</span><span class="n">attachmentExists</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">))</span> <span class="p">{</span>
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<span class="n">Thread</span><span class="p">.</span><span class="n">currentThread</span><span class="p">().</span><span class="n">contextClassLoader</span><span class="p">.</span><span class="n">getResourceAsStream</span><span class="p">(</span><span class="s">"bank-of-london-cp.jar"</span><span class="p">).</span><span class="n">use</span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">id</span> <span class="p">=</span> <span class="n">rpc</span><span class="p">.</span><span class="n">uploadAttachment</span><span class="p">(</span><span class="n">it</span><span class="p">)</span>
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<span class="n">assertEquals</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">,</span> <span class="n">id</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="c1">// Create a trivial transaction that just passes across the attachment - in normal cases there would be</span>
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<span class="c1">// inputs, outputs and commands that refer to this attachment.</span>
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<span class="k">val</span> <span class="py">ptx</span> <span class="p">=</span> <span class="n">TransactionType</span><span class="p">.</span><span class="n">General</span><span class="p">.</span><span class="n">Builder</span><span class="p">(</span><span class="n">notary</span> <span class="p">=</span> <span class="k">null</span><span class="p">)</span>
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<span class="n">require</span><span class="p">(</span><span class="n">rpc</span><span class="p">.</span><span class="n">attachmentExists</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">))</span>
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<span class="n">ptx</span><span class="p">.</span><span class="n">addAttachment</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">)</span>
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<span class="c1">// TODO: Add a dummy state and specify a notary, so that the tx hash is randomised each time and the demo can be repeated.</span>
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<span class="c1">// Despite not having any states, we have to have at least one signature on the transaction</span>
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<span class="n">ptx</span><span class="p">.</span><span class="n">signWith</span><span class="p">(</span><span class="n">ALICE_KEY</span><span class="p">)</span>
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<span class="c1">// Send the transaction to the other recipient</span>
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<span class="k">val</span> <span class="py">stx</span> <span class="p">=</span> <span class="n">ptx</span><span class="p">.</span><span class="n">toSignedTransaction</span><span class="p">()</span>
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<span class="n">println</span><span class="p">(</span><span class="s">"Sending ${stx.id}"</span><span class="p">)</span>
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<span class="k">val</span> <span class="py">protocolHandle</span> <span class="p">=</span> <span class="n">rpc</span><span class="p">.</span><span class="n">startFlow</span><span class="p">(</span><span class="o">::</span><span class="n">FinalityFlow</span><span class="p">,</span> <span class="n">stx</span><span class="p">,</span> <span class="n">setOf</span><span class="p">(</span><span class="n">otherSide</span><span class="p">))</span>
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<span class="n">protocolHandle</span><span class="p">.</span><span class="n">progress</span><span class="p">.</span><span class="n">subscribe</span><span class="p">(</span><span class="o">::</span><span class="n">println</span><span class="p">)</span>
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<span class="n">protocolHandle</span><span class="p">.</span><span class="n">returnValue</span><span class="p">.</span><span class="n">toBlocking</span><span class="p">().</span><span class="n">first</span><span class="p">()</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
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</div>
|
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<p>This side is a bit more complex. Firstly it looks up its counterparty by name in the network map. Then, if the node
|
|
doesn’t already have the attachment in its storage, we upload it from a JAR resource and check the hash was what
|
|
we expected. Then a trivial transaction is built that has the attachment and a single signature and it’s sent to
|
|
the other side using the FinalityFlow. The result of starting the flow is a stream of progress messages and a
|
|
<code class="docutils literal"><span class="pre">returnValue</span></code> observable that can be used to watch out for the flow completing successfully.</p>
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