TcpNetLock

Network lock based on TCP sockets

• Free software: GNU General Public License v3
• Documentation: https://tcpnetlock.readthedocs.io/
• GitHub: https://github.com/hgdeoro/tcpnetlock/
• Docker: https://hub.docker.com/r/hgdeoro/tcpnetlock/

Why?

While deploying applications to Kubernetes, I needed a way to make sure that some potential concurrent, distributed actions, are not executed concurrently. For example:

• database migrations: just one Pod in the Kubernetes cluster should be able to apply the database migrations
• for batch jobs, different workers could be working on the same resource, this can be avoided with this lock mechanism

Of course, Zookeeper is a MUCH BETTER solution, but that's too much for my use cases...

How it works

Assuming the server is running on localhost, let's get a lock using telnet:

$ telnet localhost 7654
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.

To try to acquire a lock, send:

lock,name:django-migrations

Server responds with:

ok

From that point, and while the TCP connection is open, you have the lock.

If you try the same in a different terminal, you will get:

$ telnet localhost 7654
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
lock,name:django-migrations        <= you write
not-granted                        <= server response
Connection closed by foreign host. <= server closed the connection

Here the server responded with not-granted and closed the TCP connection. The lock was not granted to you.

But, in real-life scenarios, you would use the provided utility tcpnetlock_do:

$ tcpnetlock_do --lock-name django-migrations -- python manage.py migrate

To test it, you will need the server running. To get the server running with Docker, just run:

$ docker pull hgdeoro/tcpnetlock
$ docker run -ti --rm -p 7654:7654 hgdeoro/tcpnetlock

Alternatively, you can install the package in a virtualenv and launch the server:

$ virtualenv -p python3.6 venv
$ source venv/bin/activate
$ pip install tcpnetlock
$ tcpnetlock_server --info
INFO:root:Started server listening on localhost:7654

Features

• Runs on Python 3.6 / Python 3.5
• Do not require external libraries
• Ready to use Docker image (based on Alpine)
• Includes server and python client
• Includes utility to run Linux commands while holding the lock
• Simple protocol: you can get a lock even with netcat

Appendix: netcat

Since the protocol is just text over a TCP connection, you can get a lock just writing the right text overt the TCP connection and leaving that TCP connection open, and that's the default behaviour of netcat:

$ echo 'lock,name:LOCK_NAME' | nc localhost 7654

The first line uses netcat to open the TCP connection and tries to get the lock.

The biggest problem would be to READ the response to the server (will be one of 'ok' or 'not-granted') while send nc to the background. We can use a fifo for that:

$ echo 'lock,name:LOCK_NAME' | nc -v localhost 7654 | tee /tmp/.tcpnetlock &
$ result=$(head -n 1 /tmp/.tcpnetlock)

Even though this works, using one of the two existing python clients (tnl_client and tnl_do) would be much better.

Credits

This package was created with Cookiecutter and the audreyr/cookiecutter-pypackage project template.