Guilherme Quentel Melo created TINKERPOP-2405:
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Summary: gremlinpython: traversal hangs when the connection is
established but the servers stops responding later
Key: TINKERPOP-2405
URL: https://issues.apache.org/jira/browse/TINKERPOP-2405
Project: TinkerPop
Issue Type: Bug
Components: python
Affects Versions: 3.4.6
Environment: Ubuntu 18.04, Flask 1.1.1, python 3.8.1, Amazon Neptune,
Gremlin Server
Reporter: Guilherme Quentel Melo
On a HTTP server that connects to Amazon Neptune, I've seen some situations
where a request just hangs and never returns any response. While investigating
this, I found out that it hangs right when it is going to query Neptune.
The problem is that if the connection to Gremlin/Neptune is established and
after that the server does not respond any more, the gremlin connection never
times out, making the process/thread wait forever for a response that will
never come.
h1. How to reproduce
# Start a local gremlin server on the default port 8182
# On a terminal, run {{nc}} to listen on port 8183 with {{nc -lk 8183}}
# Run the following python code to connect to the **8183** port:
{code:python}
from gremlin_python.driver.driver_remote_connection import
DriverRemoteConnection
from gremlin_python.process.anonymous_traversal import traversal
remote_connection = DriverRemoteConnection("ws://127.0.0.1:8183/gremlin", "g")
g = traversal().withRemote(remote_connection)
g.V().limit(1).toList()
{code}
# You will see the connection request on {{nc}} output. First time, don't do
anything and the it will timeout saying the connection couldn't be established.
# Now repeat the steps, but make nc respond to establish the connection. The
quickest way I found is to manually relay the message the real gremlin server:
## Copy the whole request from `nc -l` output
## On another terminal, open a connection to the gremlin server with `nc
127.0.0.1 8182`
## Paste the request you copied before to `nc 127.0.0.1 8182` terminal
## Copy the gremlin server response and paste into `nc -l` output
## The connection will be established and the `nc -l` will receive some
unprintable chars corresponding to `g.V().limit(1).toList()`
## Now, if there is no response from `nc -l` process, the python code will hang
forever.
h1. Possible solution
As I looked into it, the problem seems that the `TornadoTransport`
implementation does not pass any timeout when reading (and writing) messages.
So, passing a timeout to {{self._loop.run_sync}} can solve the issue, at least
raising an exception when the server does not respond.
If I change the example above:
{code:python}
from gremlin_python.driver.driver_remote_connection import
DriverRemoteConnection
from gremlin_python.driver.tornado.transport import TornadoTransport
from gremlin_python.process.anonymous_traversal import traversal
class CustomTornadoTransport(TornadoTransport):
def read(self):
return self._loop.run_sync(lambda: self._ws.read_message(), timeout=5)
remote_connection = DriverRemoteConnection("ws://127.0.0.1:8183/gremlin", "g",
transport_factory=CustomTornadoTransport)
g = traversal().withRemote(remote_connection)
g.V().limit(1).toList()
{code}
and repeat the same steps, {{g.V().limit(1).toList()}} times out after not
getting any response from the server for 5 seconds.
I'm not sure if there should be any timeout for writing, but it seems it should
definitely be set for read operations.
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