根据capability matrix和an early post on Quora
，看来Google Dataflow在某种程度上支持状态管道。但是，尽管我有一个状态管道可以在DirectRunner上正常运行，但在DataflowRunner上却遇到此错误：
...
File "pipelines/StatefulPipeline.py", line 197, in process
states = list(location_state.read())
File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/bundle_processor.py", line 318, in __iter__
for elem in self.first:
File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/bundle_processor.py", line 187, in __iter__
data, continuation_token = self._state_handler.blocking_get(self._state_key)
File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 489, in blocking_get
continuation_token=continuation_token)))
File "/usr/local/lib/python2.7/dist-packages/apache_beam/runners/worker/sdk_worker.py", line 518, in _blocking_request
raise RuntimeError(response.error)
RuntimeError: java.lang.IllegalStateException: Tried to access state for stateless step: NameContext{stageName=s02, originalName=s12, systemName=s12, userName=AlertEngine}
at org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkState(Preconditions.java:444)
at org.apache.beam.runners.dataflow.worker.StreamingModeExecutionContext$StepContext.stateInternals(StreamingModeExecutionContext.java:685)
at org.apache.beam.runners.dataflow.worker.StreamingModeExecutionContext$UserStepContext.stateInternals(StreamingModeExecutionContext.java:737)
at org.apache.beam.runners.dataflow.worker.fn.control.RegisterAndProcessBundleOperation.lambda$handleBagUserState$5(RegisterAndProcessBundleOperation.java:509)
at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
at org.apache.beam.runners.dataflow.worker.fn.control.RegisterAndProcessBundleOperation.handleBagUserState(RegisterAndProcessBundleOperation.java:505)
at org.apache.beam.runners.dataflow.worker.fn.control.RegisterAndProcessBundleOperation.delegateByStateKeyType(RegisterAndProcessBundleOperation.java:384)
at org.apache.beam.runners.fnexecution.state.GrpcStateService$Inbound.onNext(GrpcStateService.java:130)
at org.apache.beam.runners.fnexecution.state.GrpcStateService$Inbound.onNext(GrpcStateService.java:118)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.stub.ServerCalls$StreamingServerCallHandler$StreamingServerCallListener.onMessage(ServerCalls.java:248)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.ForwardingServerCallListener.onMessage(ForwardingServerCallListener.java:33)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.Contexts$ContextualizedServerCallListener.onMessage(Contexts.java:76)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.messagesAvailable(ServerCallImpl.java:263)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1MessagesAvailable.runInContext(ServerImpl.java:683)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
at org.apache.beam.vendor.grpc.v1p13p1.io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:123)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
[while running 'generatedPtransform-327']
...
我的代码看起来像：
class AlertEngine(DoFn):
LOCATION_STATE = BagStateSpec('location', StrUtf8Coder())
def process(self, msg, location_state=DoFn.StateParam(LOCATION_STATE)):
location = msg.get('location')
states = list(location_state.read())
previous_location = states[0] if states else None
if previous_location != location:
yield location
location_state.clear()
location_state.add(location)
因此，我开始不确定Dataflow是否仅支持Python SDK的有状态管道。我在Python SDK发行说明中发现的信息很少。