Lockwood's blog

Lockwood's Blog

Simple Composable Microservice Messaging

IoT

Building with simple components is easier and more fun. You can quickly build things that work.

Shortly after plugging my new Sense Hat into my Pi, I wanted to try controlling it from my laptop with NATS. Here are the results:

Components
  1. NATS: a cloud-native messaging protocol.

  2. txnats: A python NATS client using Twisted, compatible with PyPy and CPython 2.7 and 3.

  3. Raspberry Pi 2

  4. Sense HAT with 8x8 LED matrix

It took me five minutes to download and run the NATS server on my laptop. Absolutely no configuration or complex options when launching. I just downloaded, unpacked, went into its directory and typed:

./gnatsd
Basic NATS distributed messaging

Before trying the SenseHat, I had already done distributed messaging and wrote about it in an earlier post Auto-run PyPy Service on Pi

That achieved distributed, two-way communication between two systems that only had to know the address of the NATS server.

Starting from those examples, I modified them to display the messages on the LED display and to take a NATS host address with a command line argument --host so I could use the locally running server. I had to look up my laptop’s IP, on which gnatsd was running, so I could have the raspberry connect to it. I’ve added it to txnats examples with instructions.

I ssh’d into the pi, updated this repo that was already cloned to it and txnats install to the python3 site packages and ran


python3 sense_led_serve.py --host 192.168.1.150

Then from my laptop, ran:


python send_sense.py --host localhost hello world

The example in the video shows an earlier version that had the messages hard coded as 0 1 2 3.

Twisted: Easier than you think.

Twisted seems to me to have a reputation of being hard to understand and use, but things have changed: They have come up with simpler, more composable ways to do network and concurrent programming with better ways to explain it. This is a boon to the Python community. I was able to make a working NATS client in an evening.


    host = "demo.nats.io"
    port = 4222
    point = TCP4ClientEndpoint(reactor, host, port)
    nats_protocol = txnats.io.NatsProtocol(verbose=False, on_connect=listen)

    # Because NatsProtocol implements the Protocol interface, Twisted's
    # connectProtocol knows how to connected to the endpoint.
    connecting = connectProtocol(point, nats_protocol)

Clouds are not solid things.

There is a reason they’re not called brick houses.

cloud

NATS is a master piece. It’s a text based TCP protocol, meaning you can easily read the commands being sent and received. It’s trivial to diagnose because you can go minimal and interact with the server using telnet.

Networking has a lot of things that can go wrong, from wifi interference by a microwave to someone tripping over a cable in a data center or a heavy load on a service. The data may not get there in time or at all, and connections will be dropped.

NATS has check the connection built in with PING/PONG, allowing each side to check the connection. If not, gnatsd will auto-prune the subscriber and the client will be able to try another gnatsd host in a cluster.

NATS has publish, subscribe and unsubscribe to limit the messages received on a subject. This makes it trivial to do a request/reply.

Services need to scale

Service workers can subscribe with a queue group to form a distributed network of service workers. Add and remove workers at will. That’s easy scaling.

Welcoming Open Source Community

The originators of NATS realized it was so useful it deserved more attention and focus by a larger community. Apcera has built the infrustructure for an active open source community, starting with superb documentation written in markdown, a google group, a slack group and especially helpful for getting started contributing, community manager @brianflannery.

It’s simplicity and speed make it very exciting to work with.

14 Jan 2016 #microservices #IoT #Internet of Things #Raspberry Pi #Cloud