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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 >
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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
using < code class = "docutils literal" > < span class = "pre" > TransactionBuilder.addAttachment()< / span > < / code > . Attachments can be uploaded and downloaded via RPC and HTTP. For
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
< 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 >
< 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
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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 > ,
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
up an RPC connection to node B using a demo user account (this is all configured in the gradle build script for the demo
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
convenience helper intended for small tools that sets up an RPC connection scoped to the provided block, and brings all
the RPCs into scope. Once connected the sender/recipient functions are run with the RPC proxy as a parameter.< / p >
< p > We’ ll look at the recipient function first.< / p >
< 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 >
RPC, which returns both a snapshot and an observable of changes. The observable is made blocking and the next
transaction the node verifies is retrieved. That transaction is checked to see if it has the expected attachment
and if so, printed out.< / p >
< 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 >
< span class = "n" > println< / span > < span class = "p" > (< / span > < span class = "s" > " Waiting to receive transaction ..." < / span > < span class = "p" > )< / span >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< span class = "p" > }< / span > < span class = "k" > else< / span > < span class = "p" > {< / span >
< 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 >
< span class = "p" > }< / span >
< / pre > < / div >
< / 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 >
< span class = "c1" > // Get the identity key of the other side (the recipient).< / 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 = "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 >
< 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 >
< 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 >
< 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 >
< 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 >
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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 >
< 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 >
< 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 >
< 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 >
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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 >
< span class = "n" > println< / span > < span class = "p" > (< / span > < span class = "s" > " Sending ${stx.id}" < / span > < span class = "p" > )< / span >
< 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 >
< 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 >
< 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 >
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< span class = "p" > }< / span >
< / pre > < / div >
< / 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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