Building a Workflow

In the previous tutorial, we built and published a Python module, Demo. This module had a single function on it, numChars, which counted the number of characters in some text. Although it has been published, it needs to be connected to a source and a sink.


Sources generate data which gets sent to your function, and sinks receive the data which your function outputs. Learn more in Concepts

Let’s refresh ourselves on what the input and output types of our function were by asking NStack:

> nstack list functions
Demo:0.0.1-SNAPSHOT.numChars : Text -> Integer

This means that our function can be connected to any source which generates Text, and can write to any sink which can take an Integer as input.

One of the sources that NStack provides is http; if you use this source, NStack sets up an HTTP endpoint which you can send JSON-encoded data to. As a sink, we are going to use the NStack log, which is a sink for seeing the output from your function. We are going to use these two integrations in our tutorial.


See a list of available sources and sinks in Supported Integrations

Creating a workflow module

To write workflows, we create a special NStack workflow module, which we create in the same way we create a Python module – by using init.

Let’s create a new directory called DemoWorkflow, cd into the directory, and create a new workflow module.

~/DemoWorkflow/ nstack init workflow
Module 'DemoWorkflow:0.0.1-SNAPSHOT' successfully initialised at /home/nstack/Demo/DemoWorkflow

init has created a single file, workflow.nml, which is where we write our workflow module using NStack’s scripting language. If we look inside the file, we see that NStack has created an example module for us.


Just like Python modules, workflow modules are versioned.

module DemoWorkflow:0.0.1-SNAPSHOT {
  import Python.Hello:0.0.1 as Hello;
  // A sample workflow
  def w = Sources.http<Text> { http_path = "/demo" } | Hello.numChars | Sinks.log<Integer>;

This currently has a single workflow on it, w, which uses a function imported from a module called Python.Hello with the version number of 0.0.1. Like the workflow we will create, this example workflow creates an HTTP endpoint which pipes data to a function, and pipes data from the function to the NStack log.

When we created our Python module, we defined the input and output types of our function in our API. On NStack, sources and sinks also have types: this workflow specifies that the HTTP source only receives and passes on Text, and the log only accepts Integers. Because our Python function takes Text, counts it, and returns Integers, that means it can fit in the middle of the workflow.

Writing our workflow

First, let’s change the import statement to import our Demo module instead of Python.Hello.

import Demo:0.0.1-SNAPSHOT as Demo;

Now, let’s change our workflow to use the numChars function on Demo, instead of the one on Python.Hello.

def w = Sources.http<Text> { http_path = "/demo" } | Demo.numChars | Sinks.log<Integer>;


The http source is configured in this example to expose an endpoint on /demo. If you are using the demo server, we would recommend changing /demo to something more unique – as someone else may have already taken that endpoint.

Let’s break these parts to see what we’re doing:

Part Description
Sources.http<Text> { http_path = "/demo" } Use http as a source, which creates an endpoint on /demo. The <Text> statement means it can only accept and pass on Text.
Demo.numChars The name of the function which we built.
Sinks.log<Integer> Use NStack’s log as a sink. The <Integer> statement means it can only accept Integers.

NStack uses the | operator to connect statements together, just like in a shell such as bash.

Building our workflow

Before we start our workflow, we need to build it in the cloud with NStack. We do this in the same way we build a Python module. We save our workflow.nml file and run:

~/DemoWorkflow/ nstack build
Building NStack Workflow module DemoWorkflow:0.0.1-SNAPSHOT. Please wait. This may take some time.
Workflow module DemoWorkflow:0.0.1-SNAPSHOT built successfully. Use `nstack list all` to see all available functions.

We can now see our workflow is live by using the list command.

~/DemoWorkflow/ nstack list workflows
  w : Workflow

This means our workflow is ready to be started.

Starting and using our workflow

To start our workflow in the cloud, we use the start command:

~/DemoWorkflow/ $ nstack start DemoWorkflow:0.0.1-SNAPSHOT.w

We now have a live HTTP endpoint on localhost:8080/demo. The HTTP endpoint is configured to accept JSON-encoded values. We defined it to use an input schema of Text, so we will be able to send it any JSON string. In our JSON, we put params as the key, and our input as the value:

We can call it using nstack send:

~/DemoWorkflow/ $ nstack send "/demo" '"Foo"'
> Message Accepted

When workflows are started, they become processes which have numerical identifiers (_ids_). We can see the id of our process by running:

~/DemoWorkflow/ $ nstack ps

And if we look at the log of our process, which we configured as the sink, we will be able to see the result. Because our process was started with an id of 1, we run the following:

> nstack log 1
Feb 17 09:59:26 nostromo nstack-server[8925]: OUTPUT: 3

Great - we can see that the output of our function (and the number of characters in “Foo”) is 3.