Swagger Pipes

NOTE: THIS IS PRE-RELEASE SOFTWARE - SUBJECT TO CHANGE

** Quick Reference links: **

• Installation
• Pipes
  • Parallel Execution
  • Context
  • Error Handling
• Fittings
  • System Fittings
  • User Defined Fittings
  • Swagger Fittings
  • Node-Machine Fittings
• Debugging
• Change Log

What is "Swagger Pipes"?

Swagger Pipes was developed as a way to enable API flows and mashups to be created declaratively in Swagger without writing code. It works a lot like functional programming... there's no global state, data just gets passed from one function to the next down the line until we're done.

For example, to expose an API to get the latitude and longitude of an address using Google's Geocode API, one could simply define this flow:

 google_geocode:
   name: http                  # system fitting (type is optional)
   input:
     url: http://maps.googleapis.com/maps/api/geocode/json?sensor=true
     params:
       address:                # name defaults to key (ie. address)
         in: parameters

 getAddressLocation:
   - google_geocode            # call the fitting defined in this swagger
   - path: body                # system fitting: get body from output
   - parse: json               # body is a json string, parse to object
   - path: results             # get results from body
   - first                     # get first result
   - path: geometry.location   # output = { lat: n, lng: n }

But that's just a quick example, you can do much, much more... including filtering, error handling, and even parallel mashup requests to multiple services.

To better understand how a Swagger Pipe fits into the flow, note that before Swagger Pipes, an API request to a swagger-tools based API provider might be processed like this:

1. Annotated by swagger-metadata
2. Security checked by swagger-security / Volos
3. Validated by swagger-validator
4. Volos services (cache, quota, etc.) are applied
5. Routed by swagger-router
6. User writes some custom code here.
7. The result is sent back to the client

In the case of Swagger Pipes, we merely substitute a pipe where the user controller used to be. So step #6 can be viewed as broken out like this:

1. The client request is adapted onto the pipe
2. An error handler pipe may be specified
3. Pipe sections (and fittings) are invoked in turn
4. The output is adapted back onto the main response line

So each section of pipe may include both references to other pipes and references to pipe fittings and these sections are merely chained together and invoked in turn by Swagger Pipes.

Fittings

So what are these things called "fittings"? Well, simply, if a pipe is a list of steps, a fitting describes what a step actually accomplishes.

Let's take a very simple example: Say we have some data that looks like this:

[ { "name": "Scott", "city": "Los Angeles" }
  { "name": "Jeff", "city": "San Francisco" } ]

Now, we'll create a pipe that just retrieves the first name. In the definition below, we've defined a pipe called "getFirstUserName" that consists of a couple of system-provided fittings:

 getFirstUserName:
   - first
   - path: name

The "first" fitting selects the first element of an array passed in. The "path" fitting selects the "user" attribute from the object passed on by the first fitting. Thus, the result from our example is "Scott".

Or, say we want to get all the names as an array. We could simply do it like this:

 getUserNames:
   - pick: name

Obivously, these are trivial examples, but you can create pipes as long and as complex as you wish. In fact, you can even write your own fittings... but we're getting ahead of ourselves.

Fitting Definition

When you want to use a fitting, you have 2 options:

1. A system or user fitting with zero or a single input can be defined in-line, as we have shown above.
2. A fitting with configuration or more complex inputs may need to be defined before use.

Let's look at the 2nd type. Here's an example of a fitting that calls out to an API with a URL that looks like something like this: http://maps.googleapis.com/maps/api/geocode/json?sensor=true?address=Los%20Angeles. And, of course, we'll want to make the address dynamic. This requires a a little bit of configuration: We need to tell the "http" fitting the URL, the operation, and what parameters to use (and how to get them):

 geocode:
   type: system
   name: http
   input:
     operation: get
     url: http://maps.googleapis.com/maps/api/geocode/json
     params:
       sensor: true
       address:
         name: address
         in: output

As you can see above, we've give our fitting a name ("geocode") and specified which type of fitting we're creating (a "system" fitting called "http"). This fitting requires several inputs including the HTTP operation, the URL, and parameters to pass. Each of these is just a static string in this case except for the "address" parameter. The address is merely retrieved by picking the "address" property from the "output" object of whatever fitting came before it in the pipe. (Note: There are several options for input sources that will be defined later.)

By default, the output of this operation will be placed on the pipe in the "output" variable for the next fitting to use - or to be returned to the client if it's the last fitting to execute.

Reference

Installation

Swagger Pipes provides connect middleware that depends on swagger-tools metadata to be in place. If you're using apigee-127, this is already happening for you. So just npm install swagger-pipes --save in your project and add a little code to your app.js.

1. Inside the a127-magic init block in app.js, after app.use(a127.middleware(config)); add:

app.use(createSwaggerPipesMiddleware(config));

2. At the end of app.js, add:

function createSwaggerPipesMiddleware(config) {

  var swaggerPipes = require('swagger-pipes');
  var magic = config['a127.magic'];
  var pipesDefs = magic.swaggerObject['x-swagger-pipes'];
  var path = require('path');
  var pipesConfig = {
    userFittingsDirs: [ path.resolve(__dirname, 'api/fittings') ],
    userViewsDirs: [ path.resolve(__dirname, 'api/fittings') ],
    userControllersDirs: [ path.resolve(__dirname, 'api/controllers' ]
  };
  return swaggerPipes.create(pipesDefs, pipesConfig).connectMiddleware();
}

Pipes

A Pipe is just defined as an Array. It can be reference by its key and can reference other pipes and fittings by their keys. Each step in a pipe may be one of the following:

1. A pipe name
2. A fitting name (with an optional value)
3. An set of key/value pairs defining pipes to be performed in parallel

If a fitting reference includes a value, that value will be emitted onto the output for the fitting to consume. Most of the system fittings are able to operate solely on the output without any additional configuration - similar to a Unix pipe.

Parallel Execution

Generally, a pipe flows from top to bottom in serial manner. However, in some cases it is desirable to execute two pipes in parallel (for example, a mashup of two external APIs).

Parallel execution of pipes can be done by using key/value pairs on the pipe in place of a single step. The output from each pipe will be assigned to the key associated with it. It's probably easiest to explain by example:

getRestaurantsAndWeather:
  - getAddressLocation
  - restaurants: getRestaurants
    weather: getWeather

This pipe will first flow through getAddressLocation step. Then, because the restaurants and weather keys are both on the same step, it will execute the getRestaurants and getWeather pipes concurrently. The final output of this pipe will be an object that looks like this: { restaurants: {...}, weather: {...} } where the values will be the output from the respective pipes.

Context

The context object that is passed through the pipe has the following properties that should be generally used by the fittings to accept input and deliver output via the pipe to other fittings or to the client:

• input: the input defined in the fitting definition (string, number, object, array)
• statusCode: status to be delivered to the client
• headers: headers to be delivered to the client
• output: output to be delivered to the next fitting or client

In addition, the context has the following properties that should not be modified - and, in general, you shouldn't need to access them at all:

• request: the http request from the client
• response: the http response to be sent to the client
• _errorHandler: the keys and values as defined in the fitting's inputs definition
• _finish: used by connect middleware to return data to the client

Finally, the context object will contain any properties that you've assigned to it via the 'output' option on your fitting definition.

Notes:

The context object is extensible as well. The names listed above as well as any name starting with '_' should be considered reserved, but you may assign other additional properties to the object should you need to for communication between fittings.

When implementing a fitting, strongly prefer to emit content to the callback - not to the response. If you write directly to the response, you are bypassing the ability of later fittings to modify the content or to potentially modify the response at all and it may cause errors.

Error Handling

By default, errors that occur in fittings will be sent to the client with a statusCode = 500 and the error message only (no stack trace). However, you may install custom error handlers in the pipe by specifying them using the system onError fitting (see onError in fittings section).

Fittings

All fittings may have the following values (all of which are optional):

• type: one of: system, user, swagger, node-machine
• name: the name of the fitting of the type specified (note: not the Swagger reference)
• config: static values passed to the fitting during construction
• input: dynamic values passed to the fitting during execution
• output: The name of the context key to which the output value is assigned

Type

If type is omitted (as it must be for in-line useage), the swagger-pipes will first check the user fittings then the system fittings for the name and use the first fitting found. Thus be aware that if you define a fitting with the same name as a system one, your fitting will override it.

Input

The input may be a hash, array, or constant. The value or subvalues of the input is defined as either:

• a constant string or number value
• a reference to a value

A reference is a value populated either from data on the request or from the output of previous fittings on the pipe. It is defined like so:

 key:
   name: the name of the variable to pick (use '*' to pick the entire object)
   in: (optional, default = output) the object to retrieve the value from
   default: (optional) value to use when the referenced value is undefined

Valid "in" object names:

parameters

Parameters declared on the Swagger path or operation

body | form | formData

Field names in the body (body must have been parsed)

header

Header values. Names are lowercased.

query

Query parameters.

output

Output from previous fitting. Used as default source.

path

The path of the URL. This grants access to the full path (name: '*' or 'path) or to the subpath of the Swagger operation that was called (name: 'subpath).

context

Any key on the context object. (See Context for more information.)

System Fittings

There are 2 basic types of system fittings: Internal fittings that just modify output in a flow and those that are callouts to other systems. These are listed below by category:

Internal Fittings

amend: input

Amend the pipe output by copying the fields from input. Overrides output. Input and output must be objects.

emit: input

Emit the fitting's input onto onto the pipe output.

eval: 'script'

Used for testing and will likely be removed, but evaluates provided javascript directly.

first

Select the first element from an array.

jspath: jspath

Selects output using json path syntax.

memo: key

Saves the current context.output value to context[key]. Can be later retrieved via:

emit:
  name: key
  in: context

omit: key | [keys]

Omit the specified key or keys from an object.

onError: pipename

In case of error, redirect the flow to the specified pipe.

parallel: [pipenames]

Run multiple pipe flows concurrently. Generally not used directly (use shorthand syntax on pipe).

parse: json

Parses a String. Currently must only be 'json'.

path: path

Selects an element from an object by dot-delemited keys.

pick: key | [keys]

Selects only the specified key or keys from an object.

render: string | @filename

Render the object using a mustache template specified as the string or loaded from the filename in the user fittings directory.

values

Select the values of the object as an array.

Callout Fittings

http

Make a call to a URL.

config keys:

• baseUrl (optional)

input keys:

• url (optional: default = context.output)
• method (optional: default = get) (get, post, put, delete, patch, etc.)
• params (optional) key/value pairs
• headers (optional) key/value pairs

output:

{ status: statusCode headers: JSON string body: JSON string }

usergrid

Make calls to Usergrid. Work in progress...

User Defined Fittings

The user fitting is a custom function you can write and place in the fittings directory. It requires the following values:

• type: 'user'
• name: the javascript module name in the 'fittings' folder

 exampleUserFitting:
   type: user
   name: customizeResponse

Javascript implementation:

A user fitting is a fitting defined in the user's fitting's directory. It exposes a creation function that accepts a fittingDefinition and the swagger-pipes configuration. This function is executed during parsing of the Swagger. Thus, it should access the fittingDef.config (if any) and create any static resources at this time.

The creation function returns an execution function that will called during pipe flows. This function accepts a context object and a standard javascript asynchronous callback. When executed, this function should perform its intended function and then call the callback function with (error, response) when complete. Here's an example that will query Yelp for businesses near a location with an input of { latitude: n, longitude: n }:

var Yelp = require('yelp');
var util = require('util');

module.exports = function create(fittingDef, config) {

  var yelp = Yelp.createClient(fittingDef.config);

  return function yelp_search(context, cb) {

    var input = context.input;

    var options = {
      term: input.term,
      ll: util.format('%s,%s', input.latitude, input.longitude)
    };

    yelp.search(options, function(error, data) {

      if (error) { return cb(error); }
      if (data.error) { return cb(data.error); }

      cb(null, data.businesses);
    });
  }
};

Swagger fittings

You can access Swagger APIs by simply loading that Swagger. A Swagger fitting expects this:

• type: 'swagger'
• url: url to the swagger definition

 exampleSwaggerFitting:
   type: swagger
   url: http://petstore.swagger.io/v2/swagger.json

Node-machine fittings

A node-machine is a self-documenting component format that we've adapted to the a127 (see http://node-machine.org). You can use a node-machine just by using 'npm install' and declaring the fitting. The fitting definition expects a minimum of:

• type: 'node-machine'
• machinepack: the name of the machinepack
• machine: the function name (or id) of the machine

 exampleNodeMachineFitting:
   type: node-machine
   machinepack: machinepack-github
   machine: list-repos

Controller fittings

Controller fittings merely provide a call to one of the controllers you've defined in your /controllers directory for use with swagger-tools router. However, given that these controllers probably interact directly with the response and aren't designed for use within the Swagger Pipes system, proceed with extreme caution.

• type: 'controller'
• controller: the name of the controller file in your controllers directory
• function: the exported function to call on the controller

 exampleControllerFitting:
   type: controller
   controller: my_module
   function: someFunction

Debugging

Currently, debugging is limited to reading log entries and the debugger. However, there is a lot of information available to you by enabling the DEBUG log. By enabling the DEBUG=pipes log, you will be able to see the entire flow of the swagger-pipes system sent to the console:

DEBUG=pipes

You can get more debug information from the fittings with:

DEBUG=pipes:fittings

You can also emit the actual output from each step by enabling pipes:content:

DEBUG=pipes:content

Finally, you can enable all the pipes debugging by using a wildcard:

DEBUG=pipes*

Change Log

• 0.1.0
  • Breaking changes
    • pipesConfig directories (userFittingsDirs, userViewsDirs, userControllersDirs) are now all arrays
    • context.headers is now an object (key-value pairs) instead of an array

Enjoy!