Python Multiprocessing: Handling Child Errors in Parent
@mrkwjc 's solution is simple, so easy to understand and implement, but there is one disadvantage of this solution. When we have few processes and we want to stop all processes if any single process has error, we need to wait until all processes are finished in order to check if p.exception. Below is the code which fixes this problem (ie when one child has error, we terminate also another child):
import multiprocessing
import traceback
from time import sleep
class Process(multiprocessing.Process):
"""
Class which returns child Exceptions to Parent.
https://stackoverflow.com/a/33599967/4992248
"""
def __init__(self, *args, **kwargs):
multiprocessing.Process.__init__(self, *args, **kwargs)
self._parent_conn, self._child_conn = multiprocessing.Pipe()
self._exception = None
def run(self):
try:
multiprocessing.Process.run(self)
self._child_conn.send(None)
except Exception as e:
tb = traceback.format_exc()
self._child_conn.send((e, tb))
# raise e # You can still rise this exception if you need to
@property
def exception(self):
if self._parent_conn.poll():
self._exception = self._parent_conn.recv()
return self._exception
class Task_1:
def do_something(self, queue):
queue.put(dict(users=2))
class Task_2:
def do_something(self, queue):
queue.put(dict(users=5))
def main():
try:
task_1 = Task_1()
task_2 = Task_2()
# Example of multiprocessing which is used:
# https://eli.thegreenplace.net/2012/01/16/python-parallelizing-cpu-bound-tasks-with-multiprocessing/
task_1_queue = multiprocessing.Queue()
task_2_queue = multiprocessing.Queue()
task_1_process = Process(
target=task_1.do_something,
kwargs=dict(queue=task_1_queue))
task_2_process = Process(
target=task_2.do_something,
kwargs=dict(queue=task_2_queue))
task_1_process.start()
task_2_process.start()
while task_1_process.is_alive() or task_2_process.is_alive():
sleep(10)
if task_1_process.exception:
error, task_1_traceback = task_1_process.exception
# Do not wait until task_2 is finished
task_2_process.terminate()
raise ChildProcessError(task_1_traceback)
if task_2_process.exception:
error, task_2_traceback = task_2_process.exception
# Do not wait until task_1 is finished
task_1_process.terminate()
raise ChildProcessError(task_2_traceback)
task_1_process.join()
task_2_process.join()
task_1_results = task_1_queue.get()
task_2_results = task_2_queue.get()
task_1_users = task_1_results['users']
task_2_users = task_2_results['users']
except Exception:
# Here usually I send email notification with error.
print('traceback:', traceback.format_exc())
if __name__ == "__main__":
main()
Thanks to kobejohn i have found a solution which is nice and stable.
I have created a subclass of multiprocessing.Process which implements some functions and overwrites the
run()method to wrap a new saferun method into a try-catch block. This Class requires a feedback_queue to initialize which is used to report info, debug, error messages back to the parent. The log methods in the class are wrappers for the globally defined log functions of the package:class EtlStepProcess(multiprocessing.Process): def __init__(self, feedback_queue): multiprocessing.Process.__init__(self) self.feedback_queue = feedback_queue def log_info(self, message): log_info(self.feedback_queue, message, self.name) def log_debug(self, message): log_debug(self.feedback_queue, message, self.name) def log_error(self, err): log_error(self.feedback_queue, err, self.name) def saferun(self): """Method to be run in sub-process; can be overridden in sub-class""" if self._target: self._target(*self._args, **self._kwargs) def run(self): try: self.saferun() except Exception as e: self.log_error(e) raise e returnI have subclassed all my other process steps from EtlStepProcess. The code to be run is implemented in the saferun() method rather than run. This ways i do not have to add a try catch block around it, since this is already done by the run() method. Example:
class MySqlWriter(EtlStepProcess): def __init__(self, mysql_host, mysql_user, mysql_passwd, mysql_schema, mysql_table, columns, commit_count, input_queue, feedback_queue): EtlStepProcess.__init__(self, feedback_queue) self.mysql_host = mysql_host self.mysql_user = mysql_user self.mysql_passwd = mysql_passwd self.mysql_schema = mysql_schema self.mysql_table = mysql_table self.columns = columns self.commit_count = commit_count self.input_queue = input_queue def saferun(self): self.log_info(self.name + " started") #create mysql connection engine = sqlalchemy.create_engine('mysql://' + self.mysql_user + ':' + self.mysql_passwd + '@' + self.mysql_host + '/' + self.mysql_schema) meta = sqlalchemy.MetaData() table = sqlalchemy.Table(self.mysql_table, meta, autoload=True, autoload_with=engine) connection = engine.connect() try: self.log_info("start MySQL insert") counter = 0 row_list = [] while True: next_row = self.input_queue.get() if isinstance(next_row, Terminator): if counter % self.commit_count != 0: connection.execute(table.insert(), row_list) # Poison pill means we should exit break row_list.append(next_row) counter += 1 if counter % self.commit_count == 0: connection.execute(table.insert(), row_list) del row_list[:] self.log_debug(self.name + ' ' + str(counter)) finally: connection.close() returnIn my main file, I submit a Process that does all the work and give it a feedback_queue. This process starts all the steps and thenreads from mongoDB and puts values to the initial queue. My main process listens to the feedback queue and prints all log messages. If it receives an error log, it print the error and terminate its child, which in return also terminates all its children before dying.
if __name__ == '__main__': feedback_q = multiprocessing.Queue() p = multiprocessing.Process(target=mongo_python_export, args=(feedback_q,)) p.start() while p.is_alive(): fb = feedback_q.get() if fb["type"] == "error": p.terminate() print "ERROR in " + fb["process"] + "\n" for child in multiprocessing.active_children(): child.terminate() else: print datetime.datetime.fromtimestamp(fb["timestamp"]).strftime('%Y-%m-%d %H:%M:%S') + " " + \ fb["process"] + ": " + fb["message"] p.join()
I think about making a module out of it and putting it up on github, but I have to do some cleaning up and commenting first.
I don't know standard practice but what I've found is that to have reliable multiprocessing I design the methods/class/etc. specifically to work with multiprocessing. Otherwise you never really know what's going on on the other side (unless I've missed some mechanism for this).
Specifically what I do is:
- Subclass
multiprocessing.Processor make functions that specifically support multiprocessing (wrapping functions that you don't have control over if necessary) - always provide a shared error
multiprocessing.Queuefrom the main process to each worker process - enclose the entire run code in a
try: ... except Exception as e. Then when something unexpected happens send an error package with:- the process id that died
- the exception with it's original context (check here). The original context is really important if you want to log useful information in the main process.
- of course handle expected issues as normal within the normal operation of the worker
- (similar to what you said already) assuming a long-running process, wrap the running code (inside the try/catch-all) with a loop
- define a stop token in the class or for functions.
- When the main process wants the worker(s) to stop, just send the stop token. to stop everyone, send enough for all the processes.
- the wrapping loop checks the input q for the token or whatever other input you want
The end result is worker processes that can survive for a long time and that can let you know what's happening when something goes wrong. They will die quietly since you can handle whatever you need to do after the catch-all exception and you will also know when you need to restart a worker.
Again, I've just come to this pattern through trial and error so I don't know how standard it is. Does that help with what you are asking for?
Why not to let the Process to take care of its own exceptions, like this:
from __future__ import print_function
import multiprocessing as mp
import traceback
class Process(mp.Process):
def __init__(self, *args, **kwargs):
mp.Process.__init__(self, *args, **kwargs)
self._pconn, self._cconn = mp.Pipe()
self._exception = None
def run(self):
try:
mp.Process.run(self)
self._cconn.send(None)
except Exception as e:
tb = traceback.format_exc()
self._cconn.send((e, tb))
# raise e # You can still rise this exception if you need to
@property
def exception(self):
if self._pconn.poll():
self._exception = self._pconn.recv()
return self._exception
Now you have, both error and traceback at your hands:
def target():
raise ValueError('Something went wrong...')
p = Process(target = target)
p.start()
p.join()
if p.exception:
error, traceback = p.exception
print(traceback)
Regards, Marek