Writing files asynchronously - asynchronous

Question

I've been trying to create a server-process that receives an input file path and an output path from client processes asynchronously. The server does some database-reliant transformations, but for the sake of simplicity let's assume it merely puts everything to the upper case. Here is a toy example of the server:
import asyncio
import aiofiles as aiof
import logging
import sys
ADDRESS = ("localhost", 10000)
logging.basicConfig(level=logging.DEBUG,
format="%(name)s: %(message)s",
stream=sys.stderr)
log = logging.getLogger("main")
loop = asyncio.get_event_loop()
async def server(reader, writer):
log = logging.getLogger("process at {}:{}".format(*ADDRESS))
paths = await reader.read()
in_fp, out_fp = paths.splitlines()
log.debug("connection accepted")
log.debug("processing file {!r}, writing output to {!r}".format(in_fp, out_fp))
async with aiof.open(in_fp, loop=loop) as inp, aiof.open(out_fp, "w", loop=loop) as out:
async for line in inp:
out.write(line.upper())
out.flush()
writer.write(b"done")
await writer.drain()
log.debug("closing")
writer.close()
return
factory = asyncio.start_server(server, *ADDRESS)
server = loop.run_until_complete(factory)
log.debug("starting up on {} port {}".format(*ADDRESS))
try:
loop.run_forever()
except KeyboardInterrupt:
pass
finally:
log.debug("closing server")
server.close()
loop.run_until_complete(server.wait_closed())
log.debug("closing event loop")
loop.close()
The client:
import asyncio
import logging
import sys
import random
ADDRESS = ("localhost", 10000)
MESSAGES = ["/path/to/a/big/file.txt\n",
"/output/file_{}.txt\n".format(random.randint(0, 99999))]
logging.basicConfig(level=logging.DEBUG,
format="%(name)s: %(message)s",
stream=sys.stderr)
log = logging.getLogger("main")
loop = asyncio.get_event_loop()
async def client(address, messages):
log = logging.getLogger("client")
log.debug("connecting to {} port {}".format(*address))
reader, writer = await asyncio.open_connection(*address)
writer.writelines([bytes(line, "utf8") for line in messages])
if writer.can_write_eof():
writer.write_eof()
await writer.drain()
log.debug("waiting for response")
response = await reader.read()
log.debug("received {!r}".format(response))
writer.close()
return
try:
loop.run_until_complete(client(ADDRESS, MESSAGES))
finally:
log.debug("closing event loop")
loop.close()
I activated the server and several clients at once. The server's logs:
asyncio: Using selector: KqueueSelector
main: starting up on localhost port 10000
process at localhost:10000: connection accepted
process at localhost:10000: processing file b'/path/to/a/big/file.txt', writing output to b'/output/file_79609.txt'
process at localhost:10000: connection accepted
process at localhost:10000: processing file b'/path/to/a/big/file.txt', writing output to b'/output/file_68917.txt'
process at localhost:10000: connection accepted
process at localhost:10000: processing file b'/path/to/a/big/file.txt', writing output to b'/output/file_2439.txt'
process at localhost:10000: closing
process at localhost:10000: closing
process at localhost:10000: closing
All clients print this:
asyncio: Using selector: KqueueSelector
client: connecting to localhost port 10000
client: waiting for response
client: received b'done'
main: closing event loop
The output files are created, but they remain empty. I believe they are not being flushed. Any way I can fix it?

Answer 1

You are missing an await before out.write() and out.flush():
import asyncio
from pathlib import Path
import aiofiles as aiof
FILENAME = "foo.txt"
async def bad():
async with aiof.open(FILENAME, "w") as out:
out.write("hello world")
out.flush()
print("done")
async def good():
async with aiof.open(FILENAME, "w") as out:
await out.write("hello world")
await out.flush()
print("done")
loop = asyncio.get_event_loop()
server = loop.run_until_complete(bad())
print(Path(FILENAME).stat().st_size) # prints 0
server = loop.run_until_complete(good())
print(Path(FILENAME).stat().st_size) # prints 11
However, I would strongly recommend trying to skip aiofiles and use regular, synchronized disk I/O, and keep asyncio for network activity:
with open(file, "w") as out: # regular file I/O
async for s in network_request(): # asyncio for slow network work. measure it!
out.write(s) # should be really quick, measure it!