Hi,
At work we are using lagom for last 3 months. We would like
to add flowId feature in logger i.e. (Logs will contain flowId which
will help us track from which flow the unit was invoked). This will come
in handy for debugging, like if we want to track what happened to a request and it was interacting with multiple services during its flow, using the flowId we can track the same request in other services as well. An example would be If a request in
service A with flowId “flowIdA” invokes service B then the flowId for
that request in service B will be prefixed by flowIdA (flowIdA:flowIdB)
and so on.
**A sample snippet**
public class TestOneServiceImpl implements TestOneService{
@Override
public ServiceCall<TestOneInput, Done> testCallToSecond() {
return incomingStr -> {
logger.info("incoming String in testOne is "+ incomingStr.getInput());
return twoService.testCallToThird("Hello").invoke().thenApply(returnStr -> {
return returnStr;
});
};
}
}
public class TestTwoServiceImpl implements TestTwoService{
@Override
public ServiceCall<NotUsed, Done> testCallToThird(String incomingStr) {
return notUsed -> {
logger.info("incoming String in testSecond is "+ incomingStr.getInput());
return thirdService.testMe("flowIdAppended").invoke().thenApply(returnStr -> {
return returnStr;
});
};
}
}
In this case the logs will look something like
04:14:55.387 [info] [Flow-101102010] incoming String in testOne is test
04:14:55.387 [info] [Flow-101102010:121700433] incoming String in testSecond is hello
04:14:55.387 [info] [Flow-101102010:121700433:234777649] incoming String in testThird is flowIdAppended
This can be implemented by
- Having a new logger module which will internally use slf4j but will add additional MDC attribute “FlowId” from ThreadLocal.
- FlowId has to be passed across services and this can be done using headers
- Since we write the code in asynchronous manner, The flowId needs to be passed to the futures as well. This is the part which i am not sure how to implement. On way could be we return an extended implementation of Promise which will wrap flowId in it. Then before executing the onComplete/exceptionally part we will set the flowId in threadLocal. But i guess this would require changes in Play Framework as it is creating the future object during service calls. I am not able to think of any alternatives to do the same.
- We we want flowId to pass to ReadSides as well the the flowId has to be wrapped into events of PersistentEntity which looks like a dirty hack to me as we are adding data to events which previously contained only business related data. Same is the case for services which are listening to kafka events.
I would like someone to clear my doubts which are mentioned above 
. There is also a possibility that we actually do not need a flowId feature in lagom, in that case i would like to know how users debug a complete flow of a request when we communicate across services.