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< h1 > Using attachments< a class = "headerlink" href = "#using-attachments" title = "Permalink to this headline" > ¶< / a > < / h1 >
< p > Attachments are (typically large) Zip/Jar files referenced within a transaction, but not included in the transaction
itself. These files can be requested from the originating node as needed, although in many cases will be cached within
nodes already. Examples include:< / p >
< ul class = "simple" >
< li > Contract executable code< / li >
< li > Metadata about a transaction, such as PDF version of an invoice being settled< / li >
< li > Shared information to be permanently recorded on the ledger< / li >
< / ul >
< 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 > ,
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
uploaded and downloaded via HTTP, to enable integration with external systems. For instructions on HTTP upload/download
please see “ < a class = "reference internal" href = "node-administration.html" > < span class = "doc" > Node administration< / span > < / a > ” .< / p >
< p > Normally attachments on transactions are fetched automatically via the < code class = "docutils literal" > < span class = "pre" > ResolveTransactionsProtocol< / span > < / code > when verifying
received transactions. Attachments are needed in order to validate a transaction (they include, for example, the
contract code), so must be fetched before the validation process can run. < code class = "docutils literal" > < span class = "pre" > ResolveTransactionsProtocol< / span > < / code > calls
< code class = "docutils literal" > < span class = "pre" > FetchTransactionsProtocol< / span > < / code > to perform the actual retrieval.< / p >
< p > It is encouraged that where possible attachments are reusable data, so that nodes can meaningfully cache them.< / p >
< div class = "section" id = "attachments-demo" >
< h2 > Attachments demo< a class = "headerlink" href = "#attachments-demo" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > There is a worked example of attachments, which relays a simple document from one node to another. The “ two party
trade protocol” also includes an attachment, however it is a significantly more complex demo, and less well suited
for a tutorial.< / p >
< p > The demo code is in the file “ src/main/kotlin/net.corda.demos/attachment/AttachmentDemo.kt” , with the core logic
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;
this subscribes to notifications of new validated transactions, and if it receives a transaction containing attachments,
loads the first attachment from storage, and checks it matches the expected attachment ID. < code class = "docutils literal" > < span class = "pre" > ResolveTransactionsProtocol< / span > < / code >
has already fetched all attachments from the remote node, and as such the attachments are available from the node’ s
storage service. Once the attachment is verified, the node shuts itself down.< / p >
< 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 >
< 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 >
< span class = "c1" > // Normally we would receive the transaction from a more specific protocol, but in this case we let [FinalityProtocol]< / span >
< span class = "c1" > // handle receiving it for us.< / span >
< 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 >
< span class = "c1" > // When the transaction is received, it' s passed through [ResolveTransactionsProtocol], which first fetches any< / span >
< span class = "c1" > // attachments for us, then verifies the transaction. As such, by the time it hits the validated transaction store,< / span >
< span class = "c1" > // we have a copy of the attachment.< / span >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< span class = "n" > thread< / span > < span class = "p" > {< / span >
< span class = "n" > node< / span > < span class = "p" > .< / span > < span class = "n" > stop< / span > < span class = "p" > ()< / span >
< span class = "p" > }< / span >
< span class = "p" > }< / span >
< span class = "p" > }< / span >
< span class = "p" > }< / span >
< / pre > < / div >
< / div >
< p > The sender correspondingly builds a transaction with the attachment, then calls < code class = "docutils literal" > < span class = "pre" > FinalityProtocol< / span > < / code > to complete the
transaction and send it to the recipient node:< / p >
< 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 >
< 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 >
< span class = "c1" > // Make sure we have the file in storage< / span >
< 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 >
< 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 >
< 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 >
< 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 >
< span class = "p" > }< / span >
< span class = "p" > }< / span >
< span class = "c1" > // Create a trivial transaction that just passes across the attachment - in normal cases there would be< / span >
< span class = "c1" > // inputs, outputs and commands that refer to this attachment.< / span >
< 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 >
< 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 >
< span class = "c1" > // Despite not having any states, we have to have at least one signature on the transaction< / span >
< 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 >
< span class = "c1" > // Send the transaction to the other recipient< / span >
< 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 >
< span class = "n" > serviceHub< / span > < span class = "p" > .< / span > < span class = "n" > startProtocol< / span > < span class = "p" > (< / span > < span class = "n" > LOG_SENDER< / span > < span class = "p" > ,< / span > < span class = "n" > FinalityProtocol< / 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 >
< span class = "n" > thread< / span > < span class = "p" > {< / span >
< 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 >
< span class = "n" > node< / span > < span class = "p" > .< / span > < span class = "n" > stop< / span > < span class = "p" > ()< / span >
< span class = "p" > }< / span >
< span class = "p" > }.< / span > < span class = "n" > failure< / span > < span class = "p" > {< / span >
< span class = "n" > println< / span > < span class = "p" > (< / span > < span class = "s" > " Failed to relay message " < / span > < span class = "p" > )< / span >
< span class = "p" > }< / span >
< span class = "p" > }< / span >
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