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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 the node’s storage service using <code class="docutils literal"><span class="pre">StorageService.importAttachment()</span></code>,
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which returns a unique ID that can be added using <code class="docutils literal"><span class="pre">TransactionBuilder.addAttachment()</span></code>. Attachments can also be
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uploaded and downloaded via HTTP, to enable integration with external systems. For instructions on HTTP upload/download
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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 “src/main/kotlin/net.corda.demos/attachment/AttachmentDemo.kt”, with the core logic
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contained within the two functions <code class="docutils literal"><span class="pre">runRecipient()</span></code> and <code class="docutils literal"><span class="pre">runSender()</span></code>. We’ll look at the recipient function first;
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this subscribes to notifications of new validated transactions, and if it receives a transaction containing attachments,
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loads the first attachment from storage, and checks it matches the expected attachment ID. <code class="docutils literal"><span class="pre">ResolveTransactionsFlow</span></code>
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has already fetched all attachments from the remote node, and as such the attachments are available from the node’s
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storage service. Once the attachment is verified, the node shuts itself down.</p>
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">private</span> <span class="k">fun</span> <span class="nf">runRecipient</span><span class="p">(</span><span class="n">node</span><span class="p">:</span> <span class="n">Node</span><span class="p">)</span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">serviceHub</span> <span class="p">=</span> <span class="n">node</span><span class="p">.</span><span class="n">services</span>
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<span class="c1">// Normally we would receive the transaction from a more specific flow, but in this case we let [FinalityFlow]</span>
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<span class="c1">// handle receiving it for us.</span>
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<span class="n">serviceHub</span><span class="p">.</span><span class="n">storageService</span><span class="p">.</span><span class="n">validatedTransactions</span><span class="p">.</span><span class="n">updates</span><span class="p">.</span><span class="n">subscribe</span> <span class="p">{</span> <span class="n">event</span> <span class="p">-></span>
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<span class="c1">// When the transaction is received, it's passed through [ResolveTransactionsFlow], which first fetches any</span>
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<span class="c1">// attachments for us, then verifies the transaction. As such, by the time it hits the validated transaction store,</span>
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<span class="c1">// we have a copy of the attachment.</span>
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<span class="k">val</span> <span class="py">tx</span> <span class="p">=</span> <span class="n">event</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">tx</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="k">val</span> <span class="py">attachment</span> <span class="p">=</span> <span class="n">serviceHub</span><span class="p">.</span><span class="n">storageService</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">openAttachment</span><span class="p">(</span><span class="n">tx</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">assertEquals</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">,</span> <span class="n">attachment</span><span class="o">?.</span><span class="n">id</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(event.tx)}"</span><span class="p">)</span>
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<span class="n">thread</span> <span class="p">{</span>
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<span class="n">node</span><span class="p">.</span><span class="n">stop</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="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">private</span> <span class="k">fun</span> <span class="nf">runSender</span><span class="p">(</span><span class="n">node</span><span class="p">:</span> <span class="n">Node</span><span class="p">,</span> <span class="n">otherSide</span><span class="p">:</span> <span class="n">Party</span><span class="p">)</span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">serviceHub</span> <span class="p">=</span> <span class="n">node</span><span class="p">.</span><span class="n">services</span>
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<span class="c1">// Make sure we have the file in storage</span>
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<span class="k">if</span> <span class="p">(</span><span class="n">serviceHub</span><span class="p">.</span><span class="n">storageService</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">openAttachment</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">)</span> <span class="p">==</span> <span class="k">null</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">net</span><span class="p">.</span><span class="n">corda</span><span class="p">.</span><span class="n">demos</span><span class="p">.</span><span class="n">Role</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="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">node</span><span class="p">.</span><span class="n">storage</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">importAttachment</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">ptx</span><span class="p">.</span><span class="n">addAttachment</span><span class="p">(</span><span class="n">serviceHub</span><span class="p">.</span><span class="n">storageService</span><span class="p">.</span><span class="n">attachments</span><span class="p">.</span><span class="n">openAttachment</span><span class="p">(</span><span class="n">PROSPECTUS_HASH</span><span class="p">)</span><span class="o">!!</span><span class="p">.</span><span class="n">id</span><span class="p">)</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">tx</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">serviceHub</span><span class="p">.</span><span class="n">startFlow</span><span class="p">(</span><span class="n">LOG_SENDER</span><span class="p">,</span> <span class="n">FinalityFlow</span><span class="p">(</span><span class="n">tx</span><span class="p">,</span> <span class="n">emptySet</span><span class="p">(),</span> <span class="n">setOf</span><span class="p">(</span><span class="n">otherSide</span><span class="p">))).</span><span class="n">success</span> <span class="p">{</span>
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<span class="n">thread</span> <span class="p">{</span>
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<span class="n">Thread</span><span class="p">.</span><span class="n">sleep</span><span class="p">(</span><span class="m">1000L</span><span class="p">)</span> <span class="c1">// Give the other side time to request the attachment</span>
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<span class="n">node</span><span class="p">.</span><span class="n">stop</span><span class="p">()</span>
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<span class="p">}</span>
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<span class="p">}.</span><span class="n">failure</span> <span class="p">{</span>
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<span class="n">println</span><span class="p">(</span><span class="s">"Failed to relay message "</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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