saga模式是分布式事务中使用比较多的一种模式，主要应用在多节点长流程的应用中，对一个全局事务，如果某个节点抛出了异常，则从当前这个节点依次往前补偿事务。一阶段正向服务和二阶段补偿服务都需要由业务代码来实现。今天我们就来看看它的源码实现。
状态机定义
以一个典型的电商购物流程为例，我们定义3个服务，订单服务(OrderServer)，账户服务(AccountService)和库存服务(StorageService),这里我们把订单服务当做聚合服务，也就是TM。
当外部下单时，订单服务首先会创建一个订单，然后调用账户服务扣减金额，最后调用库存服务扣减库存。这个流程入下图：
seata的saga模式是基于状态机来实现了，状态机对状态的控制需要一个JSON文件，这个JSON文件定义如下：
代码语言：javascript复制{
"Name": "buyGoodsOnline",
"Comment": "buy a goods on line, add order, deduct account, deduct storage ",
"StartState": "SaveOrder",
"Version": "0.0.1",
"States": {
"SaveOrder": {
"Type": "ServiceTask",
"ServiceName": "orderSave",
"ServiceMethod": "saveOrder",
"CompensateState": "DeleteOrder",
"Next": "ChoiceAccountState",
"Input": [
"$.[businessKey]",
"$.[order]"
],
"Output": {
"SaveOrderResult": "$.#root"
},
"Status": {
"#root == true": "SU",
"#root == false": "FA",
"$Exception{java.lang.Throwable}": "UN"
}
},
"ChoiceAccountState":{
"Type": "Choice",
"Choices":[
{
"Expression":"[SaveOrderResult] == true",
"Next":"ReduceAccount"
}
],
"Default":"Fail"
},
"ReduceAccount": {
"Type": "ServiceTask",
"ServiceName": "accountService",
"ServiceMethod": "decrease",
"CompensateState": "CompensateReduceAccount",
"Next": "ChoiceStorageState",
"Input": [
"$.[businessKey]",
"$.[userId]",
"$.[money]",
{
"throwException" : "$.[mockReduceAccountFail]"
}
],
"Output": {
"ReduceAccountResult": "$.#root"
},
"Status": {
"#root == true": "SU",
"#root == false": "FA",
"$Exception{java.lang.Throwable}": "UN"
},
"Catch": [
{
"Exceptions": [
"java.lang.Throwable"
],
"Next": "CompensationTrigger"
}
]
},
"ChoiceStorageState":{
"Type": "Choice",
"Choices":[
{
"Expression":"[ReduceAccountResult] == true",
"Next":"ReduceStorage"
}
],
"Default":"Fail"
},
"ReduceStorage": {
"Type": "ServiceTask",
"ServiceName": "storageService",
"ServiceMethod": "decrease",
"CompensateState": "CompensateReduceStorage",
"Input": [
"$.[businessKey]",
"$.[productId]",
"$.[count]",
{
"throwException" : "$.[mockReduceStorageFail]"
}
],
"Output": {
"ReduceStorageResult": "$.#root"
},
"Status": {
"#root == true": "SU",
"#root == false": "FA",
"$Exception{java.lang.Throwable}": "UN"
},
"Catch": [
{
"Exceptions": [
"java.lang.Throwable"
],
"Next": "CompensationTrigger"
}
],
"Next": "Succeed"
},
"DeleteOrder": {
"Type": "ServiceTask",
"ServiceName": "orderSave",
"ServiceMethod": "deleteOrder",
"Input": [
"$.[businessKey]",
"$.[order]"
]
},
"CompensateReduceAccount": {
"Type": "ServiceTask",
"ServiceName": "accountService",
"ServiceMethod": "compensateDecrease",
"Input": [
"$.[businessKey]",
"$.[userId]",
"$.[money]"
]
},
"CompensateReduceStorage": {
"Type": "ServiceTask",
"ServiceName": "storageService",
"ServiceMethod": "compensateDecrease",
"Input": [
"$.[businessKey]",
"$.[productId]",
"$.[count]"
]
},
"CompensationTrigger": {
"Type": "CompensationTrigger",
"Next": "Fail"
},
"Succeed": {
"Type":"Succeed"
},
"Fail": {
"Type":"Fail",
"ErrorCode": "PURCHASE_FAILED",
"Message": "purchase failed"
}
}
}状态机是运行在TM中的，也就是我们上面定义的订单服务。订单服务创建订单时需要开启一个全局事务，这时就需要启动状态机，代码如下：
代码语言：javascript复制StateMachineEngine stateMachineEngine = (StateMachineEngine) ApplicationContextUtils.getApplicationContext().getBean("stateMachineEngine");
Map<String, Object> startParams = new HashMap<>(3);
String businessKey = String.valueOf(System.currentTimeMillis());
startParams.put("businessKey", businessKey);
startParams.put("order", order);
startParams.put("mockReduceAccountFail", "true");
startParams.put("userId", order.getUserId());
startParams.put("money", order.getPayAmount());
startParams.put("productId", order.getProductId());
startParams.put("count", order.getCount());
//sync test
StateMachineInstance inst = stateMachineEngine.startWithBusinessKey("buyGoodsOnline", null, businessKey, startParams);可以看到，上面代码定义的buyGoodsOnline，正是JSON文件中name的属性值。
状态机初始化
那上面创建订单代码中的stateMachineEngine这个bean是在哪里定义的呢？订单服务的demo中有一个类StateMachineConfiguration来进行定义，代码如下：
代码语言：javascript复制public class StateMachineConfiguration {
@Bean
public ThreadPoolExecutorFactoryBean threadExecutor(){
ThreadPoolExecutorFactoryBean threadExecutor = new ThreadPoolExecutorFactoryBean();
threadExecutor.setThreadNamePrefix("SAGA_ASYNC_EXE_");
threadExecutor.setCorePoolSize(1);
threadExecutor.setMaxPoolSize(20);
return threadExecutor;
}
@Bean
public DbStateMachineConfig dbStateMachineConfig(ThreadPoolExecutorFactoryBean threadExecutor, DataSource hikariDataSource) throws IOException {
DbStateMachineConfig dbStateMachineConfig = new DbStateMachineConfig();
dbStateMachineConfig.setDataSource(hikariDataSource);
dbStateMachineConfig.setThreadPoolExecutor((ThreadPoolExecutor) threadExecutor.getObject());
/**
*这里配置了json文件的路径，TM在初始化的时候，会把json文件解析成StateMachineImpl类，如果数据库没有保存这个状态机，则存入数据库seata_state_machine_def表，
*如果数据库有记录，则取最新的一条记录，并且注册到StateMachineRepositoryImpl，
*注册的Map有2个，一个是stateMachineMapByNameAndTenant，key格式是(stateMachineName "_" tenantId),
*一个是stateMachineMapById，key是stateMachine.getId()
*具体代码见StateMachineRepositoryImpl类registryStateMachine方法
*这个注册的触发方法在DefaultStateMachineConfig的初始化方法init()，这个类是DbStateMachineConfig的父类
*/
dbStateMachineConfig.setResources(new PathMatchingResourcePatternResolver().getResources("classpath*:statelang/*.json"));//json文件
dbStateMachineConfig.setEnableAsync(true);
dbStateMachineConfig.setApplicationId("order-server");
dbStateMachineConfig.setTxServiceGroup("my_test_tx_group");
return dbStateMachineConfig;
}
@Bean
public ProcessCtrlStateMachineEngine stateMachineEngine(DbStateMachineConfig dbStateMachineConfig){
ProcessCtrlStateMachineEngine stateMachineEngine = new ProcessCtrlStateMachineEngine();
stateMachineEngine.setStateMachineConfig(dbStateMachineConfig);
return stateMachineEngine;
}
@Bean
public StateMachineEngineHolder stateMachineEngineHolder(ProcessCtrlStateMachineEngine stateMachineEngine){
StateMachineEngineHolder stateMachineEngineHolder = new StateMachineEngineHolder();
stateMachineEngineHolder.setStateMachineEngine(stateMachineEngine);
return stateMachineEngineHolder;
}
}可以看到，我们在DbStateMachineConfig中配置了状态机的json文件，同时配置了applicationId和txServiceGroup。在DbStateMachineConfig初始化的时候，子类DefaultStateMachineConfig的init的方法会把json文件解析成状态机，并注册。
注册的过程中往seata_state_machine_def这张表里插入了1条记录，表里的content字段保存了我们的JOSON文件内容，其他字段值数据如下图：
附:根据前面的JSON文件，我们debug跟踪到的StateMachineImpl的内容如下：
代码语言：javascript复制id = null
tenantId = null
appName = "SEATA"
name = "buyGoodsOnline"
comment = "buy a goods on line, add order, deduct account, deduct storage "
version = "0.0.1"
startState = "SaveOrder"
status = {StateMachine$Status@9135} "AC"
recoverStrategy = null
isPersist = true
type = "STATE_LANG"
content = null
gmtCreate = null
states = {LinkedHashMap@9137} size = 11
"SaveOrder" -> {ServiceTaskStateImpl@9153}
"ChoiceAccountState" -> {ChoiceStateImpl@9155}
"ReduceAccount" -> {ServiceTaskStateImpl@9157}
"ChoiceStorageState" -> {ChoiceStateImpl@9159}
"ReduceStorage" -> {ServiceTaskStateImpl@9161}
"DeleteOrder" -> {ServiceTaskStateImpl@9163}
"CompensateReduceAccount" -> {ServiceTaskStateImpl@9165}
"CompensateReduceStorage" -> {ServiceTaskStateImpl@9167}
"CompensationTrigger" -> {CompensationTriggerStateImpl@9169}
"Succeed" -> {SucceedEndStateImpl@9171}
"Fail" -> {FailEndStateImpl@9173}启动状态机
在第一节创建订单的代码中，startWithBusinessKey方法进行了整个事务的启动，这个方法还有一个异步模式startWithBusinessKeyAsync，这里我们只分析同步模式，源代码如下：
代码语言：javascript复制public StateMachineInstance startWithBusinessKey(String stateMachineName, String tenantId, String businessKey,
Map<String, Object> startParams) throws EngineExecutionException {
return startInternal(stateMachineName, tenantId, businessKey, startParams, false, null);
}
private StateMachineInstance startInternal(String stateMachineName, String tenantId, String businessKey,
Map<String, Object> startParams, boolean async, AsyncCallback callback)
throws EngineExecutionException {
//省略部分源代码
//创建一个状态机实例
//默认值tenantId="000001"
StateMachineInstance instance = createMachineInstance(stateMachineName, tenantId, businessKey, startParams);
/**
* ProcessType.STATE_LANG这个枚举只有一个元素
* OPERATION_NAME_START = "start"
* callback是null
* getStateMachineConfig()返回DbStateMachineConfig
*/
ProcessContextBuilder contextBuilder = ProcessContextBuilder.create().withProcessType(ProcessType.STATE_LANG)
.withOperationName(DomainConstants.OPERATION_NAME_START).withAsyncCallback(callback).withInstruction(
new StateInstruction(stateMachineName, tenantId)).withStateMachineInstance(instance)
.withStateMachineConfig(getStateMachineConfig()).withStateMachineEngine(this);
Map<String, Object> contextVariables;
if (startParams != null) {
contextVariables = new ConcurrentHashMap<>(startParams.size());
nullSafeCopy(startParams, contextVariables);
} else {
contextVariables = new ConcurrentHashMap<>();
}
instance.setContext(contextVariables);//把启动参数赋值给状态机实例的context
//给ProcessContextImpl的variables加参数
contextBuilder.withStateMachineContextVariables(contextVariables);
contextBuilder.withIsAsyncExecution(async);
//上面定义的建造者创建一个ProcessContextImpl
ProcessContext processContext = contextBuilder.build();
//这个条件是true
if (instance.getStateMachine().isPersist() && stateMachineConfig.getStateLogStore() != null) {
//记录状态机开始状态
stateMachineConfig.getStateLogStore().recordStateMachineStarted(instance, processContext);
}
if (StringUtils.isEmpty(instance.getId())) {
instance.setId(
stateMachineConfig.getSeqGenerator().generate(DomainConstants.SEQ_ENTITY_STATE_MACHINE_INST));
}
if (async) {
stateMachineConfig.getAsyncProcessCtrlEventPublisher().publish(processContext);
} else {
//发送消息到EventBus，这里的消费者是ProcessCtrlEventConsumer，在DefaultStateMachineConfig初始化时设置
stateMachineConfig.getProcessCtrlEventPublisher().publish(processContext);
}
return instance;
}上面的代码中我们可以看出，启动状态记得时候主要做了2件事情，一个是记录状态机开始的状态，一个是发送消息到EventBus，下面我们详细看一下这2个过程。
开启全局事务
上面的代码分析中，有一个记录状态机开始状态的代码，如下：
代码语言：javascript复制stateMachineConfig.getStateLogStore().recordStateMachineStarted(instance, processContext);这里调用了类DbAndReportTcStateLogStore的recordStateMachineStarted方法，我们来看一下，代码如下：
代码语言：javascript复制public void recordStateMachineStarted(StateMachineInstance machineInstance, ProcessContext context) {
if (machineInstance != null) {
//if parentId is not null, machineInstance is a SubStateMachine, do not start a new global transaction,
//use parent transaction instead.
String parentId = machineInstance.getParentId();
if (StringUtils.hasLength(parentId)) {
if (StringUtils.isEmpty(machineInstance.getId())) {
machineInstance.setId(parentId);
}
} else {
//走这个分支，因为没有配置子状态机
/**
* 这里的beginTransaction就是开启全局事务，
* 这里是调用TC开启全局事务
*/
beginTransaction(machineInstance, context);
}
if (StringUtils.isEmpty(machineInstance.getId()) && seqGenerator != null) {
machineInstance.setId(seqGenerator.generate(DomainConstants.SEQ_ENTITY_STATE_MACHINE_INST));
}
// save to db
//dbType = "MySQL"
machineInstance.setSerializedStartParams(paramsSerializer.serialize(machineInstance.getStartParams()));
executeUpdate(stateLogStoreSqls.getRecordStateMachineStartedSql(dbType),
STATE_MACHINE_INSTANCE_TO_STATEMENT_FOR_INSERT, machineInstance);
}
}上面executeUpdate方法在子类AbstractStore,debug一下executeUpdate这个方法可以看到，这里执行的sql如下：
代码语言：javascript复制INSERT INTO seata_state_machine_inst
(id, machine_id, tenant_id, parent_id, gmt_started, business_key, start_params, is_running, status, gmt_updated)
VALUES ('192.168.59.146:8091:65853497147990016', '06a098cab53241ca7ed09433342e9f07', '000001', null, '2020-10-31 17:18:24.773',
'1604135904773', '{"@type":"java.util.HashMap","money":50.,"productId":1L,"_business_key_":"1604135904773","businessKey":"1604135904773",
"count":1,"mockReduceAccountFail":"true","userId":1L,"order":{"@type":"io.seata.sample.entity.Order","count":1,"payAmount":50,
"productId":1,"userId":1}}', 1, 'RU', '2020-10-31 17:18:24.773')可以看到，这个全局事务记录在了表seata_state_machine_inst，记录的是我们启动状态机的参数，status记录的状态是"RU"也就是RUNNING。
分支事务处理
上一节我们提到，启动状态机后，向EventBus发了一条消息，这个消息的消费者是ProcessCtrlEventConsumer，我们看一下这个类的代码：
代码语言：javascript复制public class ProcessCtrlEventConsumer implements EventConsumer<ProcessContext> {
private ProcessController processController;
@Override
public void process(ProcessContext event) throws FrameworkException {
//这里的processController是ProcessControllerImpl
processController.process(event);
}
@Override
public boolean accept(Class<ProcessContext> clazz) {
return ProcessContext.class.isAssignableFrom(clazz);
}
public void setProcessController(ProcessController processController) {
this.processController = processController;
}
}ProcessControllerImpl类的process方法有2个处理逻辑，process和route，代码如下：
代码语言：javascript复制public void process(ProcessContext context) throws FrameworkException {
try {
//这里的businessProcessor是CustomizeBusinessProcessor
businessProcessor.process(context);
businessProcessor.route(context);
} catch (FrameworkException fex) {
throw fex;
} catch (Exception ex) {
LOGGER.error("Unknown exception occurred, context = {}", context, ex);
throw new FrameworkException(ex, "Unknown exception occurred", FrameworkErrorCode.UnknownAppError);
}
}这里的处理逻辑有些复杂，先上一张UML类图，跟着这张图，可以捋清楚代码的调用逻辑：
我们先来看一下CustomizeBusinessProcessor中的process方法：
代码语言：javascript复制public void process(ProcessContext context) throws FrameworkException {
/**
*processType = {ProcessType@10310} "STATE_LANG"
*code = "STATE_LANG"
*message = "SEATA State Language"
*name = "STATE_LANG"
*ordinal = 0
*/
ProcessType processType = matchProcessType(context);
if (processType == null) {
if (LOGGER.isWarnEnabled()) {
LOGGER.warn("Process type not found, context= {}", context);
}
throw new FrameworkException(FrameworkErrorCode.ProcessTypeNotFound);
}
ProcessHandler processor = processHandlers.get(processType.getCode());
if (processor == null) {
LOGGER.error("Cannot find process handler by type {}, context= {}", processType.getCode(), context);
throw new FrameworkException(FrameworkErrorCode.ProcessHandlerNotFound);
}
//这里的是StateMachineProcessHandler
processor.process(context);
}这里的代码不好理解，我们分四步来研究。
第一步，我们看一下StateMachineProcessHandler类中process方法，这个方法代理了ServiceTaskStateHandler的process方法，代码如下：
代码语言：javascript复制public void process(ProcessContext context) throws FrameworkException {
/**
* instruction = {StateInstruction@11057}
* stateName = null
* stateMachineName = "buyGoodsOnline"
* tenantId = "000001"
* end = false
* temporaryState = null
*/
StateInstruction instruction = context.getInstruction(StateInstruction.class);
//这里的state实现类是ServiceTaskStateImpl
State state = instruction.getState(context);
String stateType = state.getType();
//这里stateHandler实现类是ServiceTaskStateHandler
StateHandler stateHandler = stateHandlers.get(stateType);
List<StateHandlerInterceptor> interceptors = null;
if (stateHandler instanceof InterceptableStateHandler) {
//list上有1个元素ServiceTaskHandlerInterceptor
interceptors = ((InterceptableStateHandler)stateHandler).getInterceptors();
}
List<StateHandlerInterceptor> executedInterceptors = null;
Exception exception = null;
try {
if (interceptors != null && interceptors.size() > 0) {
executedInterceptors = new ArrayList<>(interceptors.size());
for (StateHandlerInterceptor interceptor : interceptors) {
executedInterceptors.add(interceptor);
interceptor.preProcess(context);
}
}
stateHandler.process(context);
} catch (Exception e) {
exception = e;
throw e;
} finally {
if (executedInterceptors != null && executedInterceptors.size() > 0) {
for (int i = executedInterceptors.size() - 1; i >= 0; i--) {
StateHandlerInterceptor interceptor = executedInterceptors.get(i);
interceptor.postProcess(context, exception);
}
}
}
}从这个方法我们看到，代理对stateHandler.process加入了前置和后置增强，增强类是ServiceTaskHandlerInterceptor，前置后置增强分别调用了interceptor的preProcess和postProcess。
第二步，我们来看一下增强逻辑。ServiceTaskHandlerInterceptor的preProcess和postProcess方法，代码如下：
代码语言：javascript复制public class ServiceTaskHandlerInterceptor implements StateHandlerInterceptor {
//省略部分代码
@Override
public void preProcess(ProcessContext context) throws EngineExecutionException {
StateInstruction instruction = context.getInstruction(StateInstruction.class);
StateMachineInstance stateMachineInstance = (StateMachineInstance)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_INST);
StateMachineConfig stateMachineConfig = (StateMachineConfig)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONFIG);
//如果超时,修改状态机状态为FA
if (EngineUtils.isTimeout(stateMachineInstance.getGmtUpdated(), stateMachineConfig.getTransOperationTimeout())) {
String message = "Saga Transaction [stateMachineInstanceId:" stateMachineInstance.getId()
"] has timed out, stop execution now.";
EngineUtils.failStateMachine(context, exception);
throw exception;
}
StateInstanceImpl stateInstance = new StateInstanceImpl();
Map<String, Object> contextVariables = (Map<String, Object>)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONTEXT);
ServiceTaskStateImpl state = (ServiceTaskStateImpl)instruction.getState(context);
List<Object> serviceInputParams = null;
Object isForCompensation = state.isForCompensation();
if (isForCompensation != null && (Boolean)isForCompensation) {
CompensationHolder compensationHolder = CompensationHolder.getCurrent(context, true);
StateInstance stateToBeCompensated = compensationHolder.getStatesNeedCompensation().get(state.getName());
if (stateToBeCompensated != null) {
stateToBeCompensated.setCompensationState(stateInstance);
stateInstance.setStateIdCompensatedFor(stateToBeCompensated.getId());
} else {
LOGGER.error("Compensation State[{}] has no state to compensate, maybe this is a bug.",
state.getName());
}
//加入补偿集合
CompensationHolder.getCurrent(context, true).addForCompensationState(stateInstance.getName(),
stateInstance);
}
//省略部分代码
stateInstance.setInputParams(serviceInputParams);
if (stateMachineInstance.getStateMachine().isPersist() && state.isPersist()
&& stateMachineConfig.getStateLogStore() != null) {
try {
//记录一个分支事务的状态RU到数据库
/**
*INSERT INTO seata_state_inst (id, machine_inst_id, name, type, gmt_started, service_name, service_method, service_type, is_for_update, input_params, status, business_key, state_id_compensated_for, state_id_retried_for)
*VALUES ('4fe5f602452c84ba5e88fd2ee9c13b35', '192.168.59.146:8091:65853497147990016', 'SaveOrder', 'ServiceTask', '2020-10-31 17:18:40.84', 'orderSave',
*'saveOrder', null, 1, '["1604135904773",{"@type":"io.seata.sample.entity.Order","count":1,"payAmount":50,"productId":1,"userId":1}]', 'RU', null, null, null)
*/
stateMachineConfig.getStateLogStore().recordStateStarted(stateInstance, context);
}
}
//省略部分代码
stateMachineInstance.putStateInstance(stateInstance.getId(), stateInstance);//放入StateMachineInstanceImpl的stateMap用于重试或交易补偿
((HierarchicalProcessContext)context).setVariableLocally(DomainConstants.VAR_NAME_STATE_INST, stateInstance);//记录状态后面传给TaskStateRouter判断全局事务结束
}
@Override
public void postProcess(ProcessContext context, Exception exp) throws EngineExecutionException {
StateInstruction instruction = context.getInstruction(StateInstruction.class);
ServiceTaskStateImpl state = (ServiceTaskStateImpl)instruction.getState(context);
StateMachineInstance stateMachineInstance = (StateMachineInstance)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_INST);
StateInstance stateInstance = (StateInstance)context.getVariable(DomainConstants.VAR_NAME_STATE_INST);
if (stateInstance == null || !stateMachineInstance.isRunning()) {
LOGGER.warn("StateMachineInstance[id:" stateMachineInstance.getId() "] is end. stop running");
return;
}
StateMachineConfig stateMachineConfig = (StateMachineConfig)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONFIG);
if (exp == null) {
exp = (Exception)context.getVariable(DomainConstants.VAR_NAME_CURRENT_EXCEPTION);
}
stateInstance.setException(exp);
//设置事务状态
decideExecutionStatus(context, stateInstance, state, exp);
//省略部分代码
Map<String, Object> contextVariables = (Map<String, Object>)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONTEXT);
//省略部分代码
context.removeVariable(DomainConstants.VAR_NAME_OUTPUT_PARAMS);
context.removeVariable(DomainConstants.VAR_NAME_INPUT_PARAMS);
stateInstance.setGmtEnd(new Date());
if (stateMachineInstance.getStateMachine().isPersist() && state.isPersist()
&& stateMachineConfig.getStateLogStore() != null) {
//更新分支事务的状态为成功
/**
* UPDATE seata_state_inst SET gmt_end = '2020-10-31 17:18:49.919', excep = null, status = 'SU',
* output_params = 'true' WHERE id = '4fe5f602452c84ba5e88fd2ee9c13b35' AND
* machine_inst_id = '192.168.59.146:8091:65853497147990016'
*/
stateMachineConfig.getStateLogStore().recordStateFinished(stateInstance, context);
}
//省略部分代码
}
}从这个代码我们能看到，分支事务执行前，封装了一个StateInstanceImpl赋值给了ProcessContext，分支事务执行后，对这个StateInstanceImpl进行了修改，这个StateInstanceImpl有3个作用：
- 传入StateMachineInstanceImpl的stateMap用于重试或交易补偿
- 记录了分支事务的执行情况，同时支持持久化到seata_state_inst表
- 传入TaskStateRouter用作判断全局事务结束
第三步，我们看一下被代理的方法stateHandler.process(context)，正常执行逻辑中stateHandler的实现类是ServiceTaskStateHandler，代码如下：
代码语言：javascript复制public void process(ProcessContext context) throws EngineExecutionException {
StateInstruction instruction = context.getInstruction(StateInstruction.class);
ServiceTaskStateImpl state = (ServiceTaskStateImpl) instruction.getState(context);
StateInstance stateInstance = (StateInstance) context.getVariable(DomainConstants.VAR_NAME_STATE_INST);
Object result;
try {
/**
* 这里的input是我们在JSON中定义的，比如orderSave这个ServiceTask，input如下：
* 0 = "1608714480316"
* 1 = {Order@11271} "Order(id=null, userId=1, productId=1, count=1, payAmount=50, status=null)"
* JSON中定义如下：
* "Input": [
* "$.[businessKey]",
* "$.[order]"
* ]
*/
List<Object> input = (List<Object>) context.getVariable(DomainConstants.VAR_NAME_INPUT_PARAMS);
//Set the current task execution status to RU (Running)
stateInstance.setStatus(ExecutionStatus.RU);//设置状态
if (state instanceof CompensateSubStateMachineState) {
//省略子状态机的研究
} else {
StateMachineConfig stateMachineConfig = (StateMachineConfig) context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONFIG);
//这里的state.getServiceType是springBean
ServiceInvoker serviceInvoker = stateMachineConfig.getServiceInvokerManager().getServiceInvoker(
state.getServiceType());
if (serviceInvoker == null) {
throw new EngineExecutionException("No such ServiceInvoker[" state.getServiceType() "]",
FrameworkErrorCode.ObjectNotExists);
}
if (serviceInvoker instanceof ApplicationContextAware) {
((ApplicationContextAware) serviceInvoker).setApplicationContext(
stateMachineConfig.getApplicationContext());
}
//这里触发了我们在JSON中定义ServiceTask中方法，比如orderSave中的saveOrder方法
result = serviceInvoker.invoke(state, input.toArray());
}
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("<<<<<<<<<<<<<<<<<<<<<< State[{}], ServiceName[{}], Method[{}] Execute finish. result: {}",
state.getName(), serviceName, methodName, result);
}
//省略部分代码
}
//省略异常处理代码
}可以看到，process这个方法是一个核心的业务处理，它用发射触发了JSON中定义ServiceTask的方法，并且根据状态触发了Next对象，即流程中的下一个ServiceTask。
第四步，我们再看一下CustomizeBusinessProcessor的route方法，代码如下：
代码语言：javascript复制public void route(ProcessContext context) throws FrameworkException {
//code = "STATE_LANG"
//message = "SEATA State Language"
//name = "STATE_LANG"
//ordinal = 0
ProcessType processType = matchProcessType(context);
RouterHandler router = routerHandlers.get(processType.getCode());
//DefaultRouterHandler的route方法
router.route(context);
}我们看一下DefaultRouterHandler的route方法，代码如下：
代码语言：javascript复制public void route(ProcessContext context) throws FrameworkException {
try {
ProcessType processType = matchProcessType(context);
//这里的processRouter是StateMachineProcessRouter
ProcessRouter processRouter = processRouters.get(processType.getCode());
Instruction instruction = processRouter.route(context);
if (instruction == null) {
LOGGER.info("route instruction is null, process end");
} else {
context.setInstruction(instruction);
eventPublisher.publish(context);
}
} catch (FrameworkException e) {
throw e;
} catch (Exception ex) {
throw new FrameworkException(ex, ex.getMessage(), FrameworkErrorCode.UnknownAppError);
}
}看一下StateMachineProcessRouter的route方法，这里也是用了代理模式，代码如下：
代码语言：javascript复制public Instruction route(ProcessContext context) throws FrameworkException {
StateInstruction stateInstruction = context.getInstruction(StateInstruction.class);
State state;
if (stateInstruction.getTemporaryState() != null) {
state = stateInstruction.getTemporaryState();
stateInstruction.setTemporaryState(null);
} else {
//走这个分支
StateMachineConfig stateMachineConfig = (StateMachineConfig)context.getVariable(
DomainConstants.VAR_NAME_STATEMACHINE_CONFIG);
StateMachine stateMachine = stateMachineConfig.getStateMachineRepository().getStateMachine(
stateInstruction.getStateMachineName(), stateInstruction.getTenantId());
state = stateMachine.getStates().get(stateInstruction.getStateName());
}
String stateType = state.getType();
StateRouter router = stateRouters.get(stateType);
Instruction instruction = null;
List<StateRouterInterceptor> interceptors = null;
if (router instanceof InterceptableStateRouter) {
//这里只有EndStateRouter
interceptors = ((InterceptableStateRouter)router).getInterceptors();//EndStateRouterInterceptor
}
List<StateRouterInterceptor> executedInterceptors = null;
Exception exception = null;
try {
//前置增量实现方法是空，这里省略代码
instruction = router.route(context, state);
} catch (Exception e) {
exception = e;
throw e;
} finally {
if (executedInterceptors != null && executedInterceptors.size() > 0) {
for (int i = executedInterceptors.size() - 1; i >= 0; i--) {
StateRouterInterceptor interceptor = executedInterceptors.get(i);
interceptor.postRoute(context, state, instruction, exception);//结束状态机
}
}
//if 'Succeed' or 'Fail' State did not configured, we must end the state machine
if (instruction == null && !stateInstruction.isEnd()) {
EngineUtils.endStateMachine(context);
}
}
return instruction;
}这里的代理只实现了一个后置增强，做的事情就是结束状态机。
下面我们来看一下StateRouter，UML类图如下：
从UML类图我们看到，除了EndStateRouter，只有一个TaskStateRouter了。而EndStateRouter并没有做什么事情，因为关闭状态机的逻辑已经由代理做了。这里我们看一下TaskStateRouter，代码如下：
代码语言：javascript复制public Instruction route(ProcessContext context, State state) throws EngineExecutionException {
StateInstruction stateInstruction = context.getInstruction(StateInstruction.class);
if (stateInstruction.isEnd()) {
//如果已经结束，直接返回
//省略代码
}
//The current CompensationTriggerState can mark the compensation process is started and perform compensation
// route processing.
State compensationTriggerState = (State)context.getVariable(
DomainConstants.VAR_NAME_CURRENT_COMPEN_TRIGGER_STATE);
if (compensationTriggerState != null) {
//加入补偿集合进行补偿并返回
return compensateRoute(context, compensationTriggerState);
}
//There is an exception route, indicating that an exception is thrown, and the exception route is prioritized.
String next = (String)context.getVariable(DomainConstants.VAR_NAME_CURRENT_EXCEPTION_ROUTE);
if (StringUtils.hasLength(next)) {
context.removeVariable(DomainConstants.VAR_NAME_CURRENT_EXCEPTION_ROUTE);
} else {
next = state.getNext();
}
//If next is empty, the state selected by the Choice state was taken.
if (!StringUtils.hasLength(next) && context.hasVariable(DomainConstants.VAR_NAME_CURRENT_CHOICE)) {
next = (String)context.getVariable(DomainConstants.VAR_NAME_CURRENT_CHOICE);
context.removeVariable(DomainConstants.VAR_NAME_CURRENT_CHOICE);
}
//从当前context中取不出下一个节点了，直接返回
if (!StringUtils.hasLength(next)) {
return null;
}
StateMachine stateMachine = state.getStateMachine();
State nextState = stateMachine.getState(next);
if (nextState == null) {
throw new EngineExecutionException("Next state[" next "] is not exits",
FrameworkErrorCode.ObjectNotExists);
}
//获取到下一个要流转的状态并且赋值给stateInstruction
stateInstruction.setStateName(next);
return stateInstruction;
}可以看到，route的作用是帮状态机确定下一个流程节点，然后放入到当前的context中的stateInstruction。
到这里，我们就分析完成了状态机的原理，ProcessControllerImpl类中。
需要注意的是，这里获取到下一个节点后，并没有直接处理，而是使用观察者模式，先发送到EventBus，等待观察者来处理，循环往复，直到EndStateRouter结束状态机。
这里观察者模式的Event是ProcessContext，里面包含了Instruction，而Instruction里面包含了State，这个State里面就决定了下一个处理的节点直到结束。UML类图如下：
总结
seata中间件中的saga模式使用比较广泛，但是代码还是比较复杂的。我从下面几个方面进行了梳理：
- 我们定义的json文件加载到了类StateMachineImpl中。
- 启动状态机，我们也就启动了全局事务，这个普通模式启动全局事务是一样的，都会向TC发送消息。
- 处理状态机状态和控制状态流转的入口类在ProcessControllerImpl，从process方法可以跟代码。
- ProcessControllerImpl调用CustomizeBusinessProcessor的process处理当前状态，然后调用route方法获取到下一个节点并发送到EventBus。
- saga模式额外引入了3张表，我们也可以根据跟全局事务和分支事务相关的2张表来跟踪代码，我之前给出的demo，如果事务成功，这2张表的写sql按照状态机执行顺序给出一个成功sql，代码如下：
INSERT INTO seata_state_machine_inst
(id, machine_id, tenant_id, parent_id, gmt_started, business_key, start_params, is_running, status, gmt_updated)
VALUES ('192.168.59.146:8091:65853497147990016', '06a098cab53241ca7ed09433342e9f07', '000001', null, '2020-10-31 17:18:24.773', '1604135904773', '{"@type":"java.util.HashMap","money":50.,"productId":1L,"_business_key_":"1604135904773","businessKey":"1604135904773","count":1,"mockReduceAccountFail":"true","userId":1L,"order":{"@type":"io.seata.sample.entity.Order","count":1,"payAmount":50,"productId":1,"userId":1}}', 1, 'RU', '2020-10-31 17:18:24.773')
INSERT INTO seata_state_inst (id, machine_inst_id, name, type, gmt_started, service_name, service_method, service_type, is_for_update, input_params, status, business_key, state_id_compensated_for, state_id_retried_for)
VALUES ('4fe5f602452c84ba5e88fd2ee9c13b35', '192.168.59.146:8091:65853497147990016', 'SaveOrder', 'ServiceTask', '2020-10-31 17:18:40.84', 'orderSave', 'saveOrder', null, 1, '["1604135904773",{"@type":"io.seata.sample.entity.Order","count":1,"payAmount":50,"productId":1,"userId":1}]', 'RU', null, null, null)
UPDATE seata_state_inst SET gmt_end = '2020-10-31 17:18:49.919', excep = null, status = 'SU', output_params = 'true' WHERE id = '4fe5f602452c84ba5e88fd2ee9c13b35' AND machine_inst_id = '192.168.59.146:8091:65853497147990016'
INSERT INTO seata_state_inst (id, machine_inst_id, name, type, gmt_started, service_name, service_method, service_type, is_for_update, input_params, status, business_key, state_id_compensated_for, state_id_retried_for)
VALUES ('8371235cb2c66c8626e148f66123d3b4', '192.168.59.146:8091:65853497147990016', 'ReduceAccount', 'ServiceTask', '2020-10-31 17:19:00.441', 'accountService', 'decrease', null, 1, '["1604135904773",1L,50.,{"@type":"java.util.LinkedHashMap","throwException":"true"}]', 'RU', null, null, null)
UPDATE seata_state_inst SET gmt_end = '2020-10-31 17:19:09.593', excep = null, status = 'SU', output_params = 'true' WHERE id = '8371235cb2c66c8626e148f66123d3b4' AND machine_inst_id = '192.168.59.146:8091:65853497147990016'
INSERT INTO seata_state_inst (id, machine_inst_id, name, type, gmt_started, service_name, service_method, service_type, is_for_update, input_params, status, business_key, state_id_compensated_for, state_id_retried_for)
VALUES ('e70a49f1eac72f929085f4e82c2b4de2', '192.168.59.146:8091:65853497147990016', 'ReduceStorage', 'ServiceTask', '2020-10-31 17:19:18.494', 'storageService', 'decrease', null, 1, '["1604135904773",1L,1,{"@type":"java.util.LinkedHashMap"}]', 'RU', null, null, null)
UPDATE seata_state_inst SET gmt_end = '2020-10-31 17:19:26.613', excep = null, status = 'SU', output_params = 'true' WHERE id = 'e70a49f1eac72f929085f4e82c2b4de2' AND machine_inst_id = '192.168.59.146:8091:65853497147990016'
UPDATE seata_state_machine_inst SET gmt_end = '2020-10-31 17:19:33.581', excep = null, end_params = '{"@type":"java.util.HashMap","productId":1L,"count":1,"ReduceAccountResult":true,"mockReduceAccountFail":"true","userId":1L,"money":50.,"SaveOrderResult":true,"_business_key_":"1604135904773","businessKey":"1604135904773","ReduceStorageResult":true,"order":{"@type":"io.seata.sample.entity.Order","count":1,"id":60,"payAmount":50,"productId":1,"userId":1}}',status = 'SU', compensation_status = null, is_running = 0, gmt_updated = '2020-10-31 17:19:33.582' WHERE id = '192.168.59.146:8091:65853497147990016' and gmt_updated = '2020-10-31 17:18:24.773'这篇文章我主要从一个正常的流程研究了saga模式的源代码，还有好多细节没有做分析，比如全局事务失败后的回滚或补偿逻辑，以后有机会再交流。
