领域驱动设计之银行转账：Wow框架实战

银行账户转账案例是一个经典的领域驱动设计（DDD）应用场景。接下来我们通过一个简单的银行账户转账案例，来了解如何使用 Wow 进行领域驱动设计以及服务开发。

银行转账流程

准备转账（Prepare）：用户发起转账请求，触发 Prepare 步骤。这个步骤会向源账户发送准备转账的请求。
校验余额（CheckBalance）：源账户在收到准备转账请求后，会执行校验余额的操作，确保账户有足够的余额进行转账。
锁定金额（LockAmount）：如果余额足够，源账户会锁定转账金额，防止其他操作干扰。
入账（Entry）：接着，转账流程进入到目标账户，执行入账操作。
确认转账（Confirm）：如果入账成功，确认转账；否则，执行解锁金额操作。
1. 成功路径（Success）：如果一切顺利，完成转账流程。
2. 失败路径（Fail）：如果入账失败，执行解锁金额操作，并处理失败情况。

Saga-Transfer

运行案例

运行 TransferExampleServer.java
查看 Swagger-UI : http://localhost:8080/swagger-ui.html
执行 API 测试：Transfer.http

自动生成 API 端点

运行之后，访问 Swagger-UI : http://localhost:8080/swagger-ui.html 。
该 RESTful API 端点是由 Wow 自动生成的，无需手动编写。

Wow-Transfer

模块划分

模块	说明
example-transfer-api	API 层，定义聚合命令（Command）、领域事件（Domain Event）以及查询视图模型（Query View Model），这个模块充当了各个模块之间通信的“发布语言”。
example-transfer-domain	领域层，包含聚合根和业务约束的实现。聚合根：领域模型的入口点，负责协调领域对象的操作。业务约束：包括验证规则、领域事件的处理等。
example-transfer-server	宿主服务，应用程序的启动点。负责整合其他模块，并提供应用程序的入口。涉及配置依赖项、连接数据库、启动 API 服务

领域建模

状态聚合根（AccountState）与命令聚合根（Account）分离设计保证了在执行命令过程中，不会修改状态聚合根的状态。

状态聚合根（`AccountState`）建模

public class AccountState implements Identifier {
    private final String id;
    private String name;
    /**
     * 余额
     */
    private long balanceAmount = 0L;
    /**
     * 已锁定金额
     */
    private long lockedAmount = 0L;
    /**
     * 账号已冻结标记
     */
    private boolean frozen = false;
    @JsonCreator
    public AccountState(@JsonProperty("id") String id) {
        this.id = id;
    }
    @NotNull
    @Override
    public String getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    public long getBalanceAmount() {
        return balanceAmount;
    }
    public long getLockedAmount() {
        return lockedAmount;
    }
    public boolean isFrozen() {
        return frozen;
    }
    void onSourcing(AccountCreated accountCreated) {
        this.name = accountCreated.name();
        this.balanceAmount = accountCreated.balance();
    }
    void onSourcing(AmountLocked amountLocked) {
        balanceAmount = balanceAmount - amountLocked.amount();
        lockedAmount = lockedAmount + amountLocked.amount();
    }
    void onSourcing(AmountEntered amountEntered) {
        balanceAmount = balanceAmount + amountEntered.amount();
    }
    void onSourcing(Confirmed confirmed) {
        lockedAmount = lockedAmount - confirmed.amount();
    }
    void onSourcing(AmountUnlocked amountUnlocked) {
        lockedAmount = lockedAmount - amountUnlocked.amount();
        balanceAmount = balanceAmount + amountUnlocked.amount();
    }
    void onSourcing(AccountFrozen accountFrozen) {
        this.frozen = true;
    }
}

命令聚合根（`Account`）建模


@StaticTenantId
@AggregateRoot
public class Account {
    private final AccountState state;
    public Account(AccountState state) {
        this.state = state;
    }
    AccountCreated onCommand(CreateAccount createAccount) {
        return new AccountCreated(createAccount.name(), createAccount.balance());
    }
    @OnCommand(returns = {AmountLocked.class, Prepared.class})
    List<?> onCommand(Prepare prepare) {
        checkBalance(prepare.amount());
        return List.of(new AmountLocked(prepare.amount()), new Prepared(prepare.to(), prepare.amount()));
    }
    private void checkBalance(long amount) {
        if (state.isFrozen()) {
            throw new IllegalStateException("账号已冻结无法转账.");
        }
        if (state.getBalanceAmount() < amount) {
            throw new IllegalStateException("账号余额不足.");
        }
    }
    Object onCommand(Entry entry) {
        if (state.isFrozen()) {
            return new EntryFailed(entry.sourceId(), entry.amount());
        }
        return new AmountEntered(entry.sourceId(), entry.amount());
    }
    Confirmed onCommand(Confirm confirm) {
        return new Confirmed(confirm.amount());
    }
    AmountUnlocked onCommand(UnlockAmount unlockAmount) {
        return new AmountUnlocked(unlockAmount.amount());
    }
    AccountFrozen onCommand(FreezeAccount freezeAccount) {
        return new AccountFrozen(freezeAccount.reason());
    }
}

转账流程管理器（`TransferSaga`）

转账流程管理器（TransferSaga）负责协调处理转账的事件，并生成相应的命令。

onEvent(Prepared): 订阅转账已准备就绪事件（Prepared），并生成入账命令(Entry)。
onEvent(AmountEntered): 订阅转账已入账事件（AmountEntered），并生成确认转账命令(Confirm)。
onEvent(EntryFailed): 订阅转账入账失败事件（EntryFailed），并生成解锁金额命令(UnlockAmount)。


@StatelessSaga
public class TransferSaga {
    Entry onEvent(Prepared prepared, AggregateId aggregateId) {
        return new Entry(prepared.to(), aggregateId.getId(), prepared.amount());
    }
    Confirm onEvent(AmountEntered amountEntered) {
        return new Confirm(amountEntered.sourceId(), amountEntered.amount());
    }
    UnlockAmount onEvent(EntryFailed entryFailed) {
        return new UnlockAmount(entryFailed.sourceId(), entryFailed.amount());
    }
}

单元测试

借助 Wow 单元测试套件，可以轻松的编写聚合根和 Saga 的单元测试。从而提升代码覆盖率，保证代码质量。

example-transfer-jacoco

使用 aggregateVerifier 进行聚合根单元测试，可以有效的减少单元测试的编写工作量。

Account 聚合根单元测试

internal class AccountKTest {
    @Test
    fun createAccount() {
        aggregateVerifier<Account, AccountState>()
            .given()
            .`when`(CreateAccount("name", 100))
            .expectEventType(AccountCreated::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(100))
            }
            .verify()
    }
    @Test
    fun prepare() {
        aggregateVerifier<Account, AccountState>()
            .given(AccountCreated("name", 100))
            .`when`(Prepare("name", 100))
            .expectEventType(AmountLocked::class.java, Prepared::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(0))
            }
            .verify()
    }
    @Test
    fun entry() {
        val aggregateId = GlobalIdGenerator.generateAsString()
        aggregateVerifier<Account, AccountState>(aggregateId)
            .given(AccountCreated("name", 100))
            .`when`(Entry(aggregateId, "sourceId", 100))
            .expectEventType(AmountEntered::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(200))
            }
            .verify()
    }
    @Test
    fun entryGivenFrozen() {
        val aggregateId = GlobalIdGenerator.generateAsString()
        aggregateVerifier<Account, AccountState>(aggregateId)
            .given(AccountCreated("name", 100), AccountFrozen(""))
            .`when`(Entry(aggregateId, "sourceId", 100))
            .expectEventType(EntryFailed::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(100))
                assertThat(it.isFrozen, equalTo(true))
            }
            .verify()
    }
    @Test
    fun confirm() {
        val aggregateId = GlobalIdGenerator.generateAsString()
        aggregateVerifier<Account, AccountState>(aggregateId)
            .given(AccountCreated("name", 100), AmountLocked(100))
            .`when`(Confirm(aggregateId, 100))
            .expectEventType(Confirmed::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(0))
                assertThat(it.lockedAmount, equalTo(0))
                assertThat(it.isFrozen, equalTo(false))
            }
            .verify()
    }
    @Test
    fun unlockAmount() {
        val aggregateId = GlobalIdGenerator.generateAsString()
        aggregateVerifier<Account, AccountState>(aggregateId)
            .given(AccountCreated("name", 100), AmountLocked(100))
            .`when`(UnlockAmount(aggregateId, 100))
            .expectEventType(AmountUnlocked::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(100))
                assertThat(it.lockedAmount, equalTo(0))
                assertThat(it.isFrozen, equalTo(false))
            }
            .verify()
    }
    @Test
    fun freezeAccount() {
        val aggregateId = GlobalIdGenerator.generateAsString()
        aggregateVerifier<Account, AccountState>(aggregateId)
            .given(AccountCreated("name", 100))
            .`when`(FreezeAccount(""))
            .expectEventType(AccountFrozen::class.java)
            .expectState {
                assertThat(it.name, equalTo("name"))
                assertThat(it.balanceAmount, equalTo(100))
                assertThat(it.lockedAmount, equalTo(0))
                assertThat(it.isFrozen, equalTo(true))
            }
            .verify()
    }
}

使用 sagaVerifier 进行 Saga 单元测试，可以有效的减少单元测试的编写工作量。

TransferSaga 单元测试

internal class TransferSagaTest {
    @Test
    fun onPrepared() {
        val event = Prepared("to", 1)
        sagaVerifier<TransferSaga>()
            .`when`(event)
            .expectCommandBody<Entry> {
                assertThat(it.id, equalTo(event.to))
                assertThat(it.amount, equalTo(event.amount))
            }
            .verify()
    }
    @Test
    fun onAmountEntered() {
        val event = AmountEntered("sourceId", 1)
        sagaVerifier<TransferSaga>()
            .`when`(event)
            .expectCommandBody<Confirm> {
                assertThat(it.id, equalTo(event.sourceId))
                assertThat(it.amount, equalTo(event.amount))
            }
            .verify()
    }
    @Test
    fun onEntryFailed() {
        val event = EntryFailed("sourceId", 1)
        sagaVerifier<TransferSaga>()
            .`when`(event)
            .expectCommandBody<UnlockAmount> {
                assertThat(it.id, equalTo(event.sourceId))
                assertThat(it.amount, equalTo(event.amount))
            }
            .verify()
    }
}