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<h1><a href="#example" name="example" class="anchor"><span class="anchor-link"></span></a>Example</h1>
<p>The example app is a dummy API for managing bank accounts and transferring amounts between accounts. Accounts are implemented with sharded entities, and transfers with transactions. </p>
<p>The example app can be found in the <code>example</code> module and can be run directly with <code>sbt run</code>. </p><div class="callout note "><div class="callout-title">Stress-test</div>
<p>The module also contains a multi-jvm test that demonstrates the resilience of the system, by transferring amounts between 1000 accounts while restarting nodes. </p></div>
<h2><a href="#api" name="api" class="anchor"><span class="anchor-link"></span></a>API</h2>
<p>The API has a simple CRUD for accounts and a dedicated endpoint to perform transfers between two accounts.</p>
<pre class="prettyprint"><code class="language-scala">HttpRoutes.of[IO] {
case GET -> Root / "account" / id / "balance" => server.balanceFor(AccountID(id))
case POST -> Root / "account" / id => server.open(AccountID(id))
case POST -> Root / "account" / id / "deposit" / IntVar(amount) => server.deposit(AccountID(id), amount)
case POST -> Root / "account" / id / "withdraw" / IntVar(amount) => server.withdraw(AccountID(id), amount)
case POST -> Root / "account" / origin / "transfer" / "to" / destination / IntVar(amount) => server.transfer(AccountID(origin), AccountID(destination), PosAmount(amount))
}
.orNotFound
</code></pre>
<p>The HTTP server makes uses of the <code>Accounts</code> and <code>Account</code> algebras to satisfy the requests.</p>
<h2><a href="#algebras" name="algebras" class="anchor"><span class="anchor-link"></span></a>Algebras</h2>
<h3><a href="#accounts" name="accounts" class="anchor"><span class="anchor-link"></span></a><code>Accounts</code></h3>
<p>This trait represents the repository, i.e. the ability to retrieve a handle on a specific account and orchestrate transfers between accounts. </p>
<pre class="prettyprint"><code class="language-scala">trait Accounts[F[_]]
def accountFor(name: AccountID): Account[F]
def transfer(from: AccountID, to: AccountID, amount: PosAmount): F[TransferFailure \/ Unit]
</code></pre>
<h3><a href="#account" name="account" class="anchor"><span class="anchor-link"></span></a><code>Account</code></h3>
<p>This trait represents the account entity, i.e. the ability to query and perform operations on a specific account. </p>
<pre class="prettyprint"><code class="language-scala">trait Account[F[_]]
def open: F[AlreadyExists.type \/ Unit]
def balance: F[Unknown.type \/ NonNegAmount]
def deposit(amount: PosAmount): F[Unknown.type \/ PosAmount]
def withdraw(amount: PosAmount): F[WithdrawFailure \/ NonNegAmount]
def prepareOutgoingTransfer(id: TransferID, transfer: Transfer): F[WithdrawFailure \/ Unit]
def prepareIncomingTransfer(id: TransferID, transfer: Transfer): F[IncomingTransferFailure \/ Unit]
def commitTransfer(id: TransferID): F[TransferFailure \/ Unit]
def abortTransfer(id: TransferID): F[TransferFailure \/ Unit]
</code></pre>
<p>The latter four methods are of particular interest as they are used by the transactional branch logic to carry out the various phases of the transfer. </p>
<h2><a href="#transfers" name="transfers" class="anchor"><span class="anchor-link"></span></a>Transfers</h2>
<p>Transfers are implemented using an <code>endless-transaction</code> transaction coordinator. </p>
<h3><a href="#coordinator" name="coordinator" class="anchor"><span class="anchor-link"></span></a>Coordinator</h3>
<p>A coordinator is created with <code>TransferID</code> as the transaction identifier type, <code>AccountID</code> as the branch identifier, <code>Transfer</code> as the query payload (the amount, origin, and destination), and <code>TransferFailure</code> as the error coproduct.</p>
<pre class="prettyprint"><button class="snippet-button copy-snippet" title="Copy snippet to clipboard">copy</button><a class="snippet-button go-to-source" href="https://github.com/endless4s/endless-transaction/tree/v0.4.1/example/src/main/scala/endless/transaction/example/logic/ShardedAccounts.scala#L66-L73" target="_blank" title="Go to snippet source">source</a><code class="language-scala">transactor.coordinator[TransferID, AccountID, Transfer, TransferFailure](
"transfer",
{ accountID =>
val account = sharding.entityFor(accountID)
new TransferBranch(accountID, account)
},
Some(transferParameters.timeout)
)</code></pre>
<p>The coordinator is used in the implementation of the <code>transfer</code> method in <code>ShardedAccounts</code>, i.e. the implementation of <code>Accounts</code>. </p>
<h3><a href="#transaction" name="transaction" class="anchor"><span class="anchor-link"></span></a>Transaction</h3>
<p>The snippet below shows the logic: the code creates a transfer with two branches, one for the origin account, and the other for the destination account.</p>
<pre class="prettyprint"><button class="snippet-button copy-snippet" title="Copy snippet to clipboard">copy</button><a class="snippet-button go-to-source" href="https://github.com/endless4s/endless-transaction/tree/v0.4.1/example/src/main/scala/endless/transaction/example/logic/ShardedAccounts.scala#L29-L50" target="_blank" title="Go to snippet source">source</a><code class="language-scala">def transfer(from: AccountID, to: AccountID, amount: PosAmount): F[TransferFailure \/ Unit] =
coordinator
.create(TransferID.random, Transfer(from, to, amount), from, to)
.asResource
.use(_.pollForFinalStatus())
.flatMap {
case Status.Committed => ().asRight[TransferFailure].pure
case Status.Aborted(reason) =>
reason match {
case AbortReason.Timeout =>
EitherT.leftT(TransferFailure.Timeout: TransferFailure).value
case AbortReason.Branches(reasons) => EitherT.leftT(reasons.head).value
case AbortReason.Client(Some(reason)) => EitherT.leftT(reason).value
case AbortReason.Client(None) =>
new Exception("Transaction aborted by client without justification")
.raiseError[F, TransferFailure \/ Unit]
}
case Status.Failed(errors) =>
Logger[F].error(show"Transaction failed: $errors") *> new Exception(
"Transaction failed due to branch error"
).raiseError[F, TransferFailure \/ Unit]
}</code></pre>
<p>The transaction entity is sharded, so it can be running on a separate node. We therefore need to regularly check for completion so that we can respond to the API call (it’s synchronous here for the sake of simplicity). </p>
<p>To implement this polling operation, we use the <code>pollForFinalStatus()</code> built-in extension method (defined for <code>Transaction</code>): this method retrieves the status of the transaction at configurable intervals and semantically sleeps in between.</p>
<h3><a href="#branch" name="branch" class="anchor"><span class="anchor-link"></span></a>Branch</h3>
<p>An account’s involvement in a transfer is described by <code>TransferBranch</code>, as exemplified below with the implementation of the <code>prepare</code> method: </p>
<pre class="prettyprint"><button class="snippet-button copy-snippet" title="Copy snippet to clipboard">copy</button><a class="snippet-button go-to-source" href="https://github.com/endless4s/endless-transaction/tree/v0.4.1/example/src/main/scala/endless/transaction/example/logic/TransferBranch.scala#L22-L67" target="_blank" title="Go to snippet source">source</a><code class="language-scala">class TransferBranch[F[_]: Logger](accountID: AccountID, account: Account[F])(implicit
retryParameters: TransferParameters.BranchRetryParameters,
temporal: Temporal[F]
) extends Branch[F, TransferID, Transfer, TransferFailure] {
import temporal.*
private implicit val onErrorRetryParameters: RetryParameters = retryParameters.onError
def prepare(transferID: TransferID, transfer: Transfer): F[Branch.Vote[TransferFailure]] = {
if (accountID === transfer.origin)
Logger[F].debug(
show"Preparing outgoing transfer $transferID: $transfer for account $accountID"
) >>
account
.prepareOutgoingTransfer(transferID, transfer)
.onErrorRetryWithBackoff(
Logger[F]
.warn(_)(show"Error preparing outgoing transfer $transferID, retrying in a bit")
)
.onLeftRetryWithBackoff { case Account.PendingOutgoingTransfer =>
Logger[F].warn(
show"Account $accountID has a pending outgoing transfer, retrying in a bit"
)
}(retryParameters.onPendingTransfer)
.flatMap {
case Left(Account.Unknown) =>
Branch.Vote.Abort(TransferFailure.AccountNotFound(accountID)).pure[F]
case Left(InsufficientFunds(missing)) =>
Branch.Vote.Abort(TransferFailure.InsufficientFunds(missing)).pure[F]
case Left(Account.PendingOutgoingTransfer) =>
Branch.Vote.Abort(TransferFailure.OtherPendingTransfer).pure[F]
case Right(_) => Branch.Vote.Commit.pure[F]
}
else
Logger[F].debug(show"Preparing incoming $transferID: $transfer for account $accountID") >>
account
.prepareIncomingTransfer(transferID, transfer)
.onErrorRetryWithBackoff(
Logger[F]
.warn(_)(show"Error preparing incoming transfer $transferID, retrying in a bit")
)
.map {
case Left(Account.Unknown) =>
Branch.Vote.Abort(TransferFailure.AccountNotFound(accountID))
case Right(_) => Branch.Vote.Commit
}
}</code></pre>
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