Flow-wing

A simple library to easily build complex flows through composed/piped tasks flows

Flow-wing is a flow-control library with support for composable, pipeable tasks flows with shared runtime context.

It is built around two components: Flow and Task.

A Flow is the representation of a list/object of one or more Task or Flow instances that will be executed in series, waterfall or parallel.

A Task could be a normal function, a task created with Task.create([id], handler, [...args]) or even another Flow instance converted to Task.

A task can be synchronous or asynchronous using callbacks or promises.

Install

$ npm install --save flow-wing

Usage

const VError = require('verror');
const flow = require('flow-wing');
 
const delayed = number => ctx =>
  new Promise(resolve => setTimeout(() => resolve(number), number * ctx.delay));
 
const delayedWithCallback = number => (ctx, previousResult, cb) => {
  // When running in waterfall {previousResult} will be the previous task result
  // or when running in a flow that was piped into the running one as well
  const callback = cb || previousResult;
  setTimeout(() => callback(null, number), number * ctx.delay);
};
 
const context = { delay: 100 };
 
const tasks = {
  one: delayed(1),
  two: delayed(2),
  three: delayed(3),
  four: delayed(4),
  five: delayed(5)
};
 
const numbersSeriesFlow = flow(tasks, { name: 'numbers', resultsAsArray: true });
const numbersParallelFlow = flow.parallel(tasks, { name: 'numbers', resultsAsArray: true });
const numbersWaterfallFlow = flow.waterfall(tasks, { name: 'numbers' });
 
const multiplyTasks = [
  numbersParallelFlow.asTask(),
  (context, numbers) => numbers.concat([6, 7, 8, 9, 10]),
  (context, numbers) => {
    const tasks = numbers.map(number => delayed(number * 5));
    return flow.parallel(tasks)
      .run({ delay: 50 })
      .then(data => data.results);
  }
];
 
const multiplyFlow = flow.waterfall(multiplyTasks, { name: 'multiply' });
 
const errorHandler = (err) => {
  // err is a TaskError, a VError instance
  console.error(VError.fullStack(err));
  // Get err's info
  console.error(VError.info(err));
  // The error cause
  console.error(err.cause());
};
 
console.time('series run time');
numbersSeriesFlow.run(context)
  .then(data => {
    console.timeEnd('series run time');
    console.log(data);
    // series run time: 1526.753ms
    // { results: [ 1, 2, 3, 4, 5 ],
    //   errors: [],
    //   context: { delay: 100 } }
  })
  .catch(errorHandler);
 
console.time('waterfall run time');
numbersWaterfallFlow.run(context)
  .then(data => {
    console.timeEnd('waterfall run time');
    console.log(data);
    // waterfall run time: 1524.577ms
    // { results: { one: 1, two: 2, three: 3, four: 4, five: 5 },
    //   errors: [],
    //   context: { delay: 100 } }
  })
  .catch(errorHandler);
 
console.time('parallel run time');
numbersParallelFlow.run(context)
  .then(data => {
    console.timeEnd('parallel run time');
    console.log(data);
    // parallel run time: 511.154ms
    // { results: [ 1, 2, 3, 4, 5 ],
    //   errors: [],
    //   context: { delay: 100 } }
  })
  .catch(errorHandler);
 
console.time('multiply run time');
multiplyFlow.run(context)
  .then(data => {
    console.timeEnd('multiply run time');
    console.log(data);
    // multiply run time: 3022.582ms
    // { results: [ 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 ],
    //   errors: [],
    //   context: { delay: 100 } }
  })
  .catch(errorHandler);

Examples

You can see some usage examples here.

API

flow(tasks, [Options]) → Flow

An alias for flow.series(tasks, [Options])

The main exposed function to create flows

• tasks Array<Function|Task|Flow> | Object{ string: Function|Task|Flow, ... } - The array or object of normal functions, Task or Flow instances to be executed every time the flow runs flow.run().
• options Options - The Flow's options

Flow factory functions

• flow.series(tasks, [Options]) → Flow
• flow.waterfall(tasks, [Options]) → Flow
• flow.parallel(tasks, [Options]) → Flow

const flow = require('flow-wing');
const Task = flow.Task;
 
const options = {
  abortOnError: true,
  concurrency: 2
};
const taskTwoArg = 'some argument';
const tasks = [
  (context) => Promise.resolve(1),
  Task.create('taskTwo', (context, customArg) => Promise.resolve(customArg), taskTwoArg)
];
 
const seriesFlow = flow.series(tasks, options); // same as flow(tasks, options);
const waterfallFlow = flow.waterfall(tasks, options);
const parallelFlow = flow.parallel(tasks, options);

flow.Task

The Task object that exposes the static .create() method.

Options

The allowed options that determine the flow's runtime (error handling, concurrency) and how the results should be returned.

// Defaults
const options = {
  resultsAsArray: true,
  abortOnError: true,
  concurrency: Infinity, // Infinity = no limit
  name: 'unnamed'
};

• resultsAsArray - To return the values as array when the passed tasks are an object
• abortOnError - Whether abort Flow's execution on error or not. When false all the occurred errors will be available on the data.errors array
• concurrency - Flow's execution concurrency, used only for parallel flows
• name - Flow's name, used only for debuggability

Context

It could be any value and will be defaulted to an empty object {} when no value was provided.

• The provided value will not be cloned, so for objects and arrays the mutations will be reflected on the original value/object
• When not provided, every time a Flow runs it will have its own clean context {}

Data

The object resolved by the promise returned by someFlow.run(context).

There are some details about the resulting data object and are as follow:

• data.results - Its value will vary depending on the flow running mode

{
  context: Context,
  results: Array<any> | Object{ string: any, ... } | any,
  errors: Array<TaskError>
}

TaskError

Whenever an error happens while running a flow's task, it will be wrapped into a TaskError which is a VError instance and the following additional information will be added to it.

// VError.info(err)
{
  taskID: 'task-id',
  flowName: 'flow-name',
  flowMode: 'series'
}

Error message example:

task "task-id" in flow{flow-name}:series has failed: the main error message goes here

The main/cause error could be obtained through the err.cause() method.

err.cause(); // returns the main error which was wrapped

Task

Task handler function

The task's handler function should have the following signature.

/**
 * Task's handler signature
 *
 * @param Context context     The flow's runtime shared context
 * @param Any [pipedValue]    The previous piped Flow or Task result value. It will be
 *                            available only for flows running in waterfall or flows|tasks
 *                            that were piped into the running flow.
 * @param Any [...args]       The handler's specific additional arguments
 * @param Function [callback] The handler's callback. When not used, a value or Promise should be returned
 */
function handler(context, pipedValue, ...args, callback) {
  // ...
  // call callback(null, result)
  // or return a Promise
}
 
// Task public interface
const Task = {
  id: string,
  run(Context, [value]) → Promise,
  pipe(handler, [...args]) → Task // Returns itself
}

Methods

Task.create([id], handler, [...args]) → Task

Static method to create Tasks.

The benefit it has to create a Task instead of just passing a function is that it allows to pass custom task arguments and also being able to pipe additional handlers that can also have its own custom arguments.

• id Optional - The task's id. When not provided it will be assigned as follow:
  1. The handler/function's name (if any)
  2. The corresponding index in the tasks array
  3. The corresponding key in the tasks object
• handler required (Function) - The task's handler. It should have the signature defined above
• ...args Optional - The task's specific additional arguments

run(Context, [value]) → Promise<any>

The method to run the task's handler(s).

This method is meant to be only called by the running flow the task belongs to.

pipe(handler, [...args]) → Task

This method adds additional handlers/functions to be executed when the task runs.

The previous handler result will be piped to the next piped handler and the last one's result will be the final task result.

Useful for debugging or transforming the task's returning data

Flow

Represents a list (Array/Object) of tasks or flows converted to task and optionally additional piped flows that should be run in some of the run modes: series | waterfall | parallel.

// Flow public interface
const Flow = {
  name: string, // used only for debuggability
  mode: 'series' | 'waterfall' | 'parallel', // used only for debuggability
  run([Context]) → Promise<Data>,
  asTask([id]) → Task,
  pipe(Flow) → Flow, // Returns itself
  unpipe([Flow]) → Flow // Returns itself
};

Methods

run([Context]) → Promise<Data>

The main method to run the flow's tasks execution.

someFlow.run(Context) → Promise<Data>

asTask([id]) → Task

Converts the Flow to a Task so that can be run into another Flow.

pipe(Flow) → Flow

It's like converting a list of flows to task and running them in waterfall

Pipes the provided Flow into the current one and returns itself.

• Multiple flows can be piped
• Once the main flow runs the piped ones will receive the previous one results
• Once a flow is piped it will remain piped unless it's un-piped

someFlow.pipe(someOtherFlow).run(Context) → Promise<Data>

unpipe([Flow]) → Flow

Un-pipes the provided Flow or all ones if not provided from the current one and returns itself.

someFlow.unpipe(someOtherFlow).run(Context) → Promise<Data>

Run modes

These are the differences between the different run modes.

All the modes when running with options.abortOnError = true will abort its execution whenever an error occurs in the current task execution and will not run the pending ones.

All the modes when running with options.abortOnError = false will continue its execution and will add the occurred errors to the data.errors array and the corresponding results array index or object key will be undefined.

All the modes when a flow contains a single task it will un-wrap such task result and that will be the resulting value of data.results unlike for multiple tasks flows that it will be an array or object depending on the provided tasks type.

series

It executes its tasks in series, so the next task will start running only until the previous one has finished.

waterfall

It behaves like series with the difference that it passes the previous task result as argument to the next one and the final data.results will be the last task's returned value.

Take a look at the pipedValue argument in the handler signature above.

parallel

It executes its tasks concurrently based on the options.concurrency option.

For complex/large flows it is your responsibility to control how many tasks are being run concurrently so that your application/system don't get blocked/unresponsive. It's best suited for I/O-bound tasks and not for CPU-bound/synchronous ones.