Supported Languages

NStack is language-agnostic and allows you to write modules in multiple languages and connect them together – currently we support Python, with R and Java coming soon. More languages will be added over time – if there’s something you really need, please let us know!

Python

Basic Structure

NStack services in Python inherit from a base class, called BaseService within the nstack module:

import nstack

class Service(nstack.BaseService):
    def numChars(self, msg):
        return len(msg)

Note

Ensure you import the nstack module in your service, e.g. import nstack

Any function that you export within the API section of your nstack yaml must exist as a method on this class (you can add private methods on this class for internal use as expected in Python).

Data comes into this function via the method arguments - for nstack all the data is passed within a single argument that follows the self parameter. For instance, in the example above there is a single argument msg consisting of a single string element that may be used as-is. However if your function was defined as taking (Double, Double), you would need to unpack the tuple in Python first as follows,

def test(self, msg):
    x, y = msg
    ...

Similarly, to return data from your NStack function, simply return it as a single element within your Python method, as in the top example above.

The NStack object lasts for the life-time of the workflow, so if there is any initialisation you need to do within your service you can perform this within the object __init__ method, e.g. open a database connection, load a data-set. However remember to call the parent object __init__ method to ensure NStack is initialised correctly, i.e..

import nstack

class Service(nstack.BaseService):
    def __init__(self):
        super().__init__()
        # custom initialisation here

Notes

  • Anything your``print`` will show up in nstack log to aid debugging. (all output on stdout and stderr is sent to the NStack logs)
  • Extra libraries from pypi using pip can be installed by adding them to the requirements.txt file in the project directory - they will be installed during nstack build

Mapping NStack types to Python types

The table below show you what python types to expect and to return when dealing with types defined in the NStack IDL as defined in NStack Types:

NStack Type Python Type
Integer int
Double double.Double
Boolean bool
Text str
Tuple tuple
Struct collections.namedtuple *
Array list
[Byte] bytes
x optional None or x
Json a json-encoded string **

*You can return a normal tuple (see Structs section below)

**Allows you to specify services that either take or receive Json-encoded strings as parameters.

Structs

Given the following example definitions of types in the nstack IDL:

type URL = Text;
type Event = Click { referrer: URL, target: URL };

When you receive this data into your service method from nstack it will appear as a namedtuple from the collections module in the standard-library. eg:

ClickData = collections.namedtuple("ClickData", ["referrer", "target"])

This means you can treat the data as both a normal tuple (each field appears in the order it was defined) but also access each field as a property of the value:

>> input = nstack.Event.Click(("http://www.nstack.com/", "http://demo.nstack.com/"))
>> input.getClick().referrer
"http://www.nstack.com/"
>> input.getClick().target
"http://demo.nstack.com/"

In the example IDL, we didn’t give the struct a name: it was defined in-line inside the Click branch of the Event type. This means we can’t construct it directly if we need to return it from our method. That is not problematic, as namedtuples are just tuples so we can just return a normal tuple and nstack ensures it is correct. We can see this at work in the code example above. The Click constructor is called with a standard python tuple, but when we inspect the value, we get a namedtuple with the referrer and target properties.

R

Coming soon

Java

Coming soon