Flow: Track asynchronous application flow and access flow context variables

Sneak Peek

This library allows you to create custom asynchronous flow contexts and access its variables in every step of the flow execution.

Let's illustrate Flow functionality on a simple http server:

const Flow = require('liqd-flow'); // load Flow library

let requestID = 0;

const server = require('http').createServer( ( req, res ) =>
{
  // Start a new flow in request handler
  Flow.start( () =>
  {
    dispatch( req.url ).then( result => res.end( result ));
  },
  // and set its variables
  {
    start: process.hrtime(),
    requestID: ++requestID
  });
})
.listen(8080);

async function dispatch( url )
{
  Flow.set( 'url', url ); // Set a new variable in our Flow

  let data = await Model.getData( url );

  return
  {
    data,
    elapsed: process.hrtime( Flow.get( 'start' ) ), // get process.hrtime() from the current Flow (server request handler)
    requestID: Flow.get( 'requestID' )              // get requestID incremented in the current Flow (server request handler)
  };
}

Table of Contents

• Installing
• Class: Flow

Installing

npm install --save liqd-flow

Class: Flow

static start( callback[, scope = {}[, freeze = true]] )

Starts a new Flow and sets its scope object.

• callback {Function} Callback executed in a newly created Flow
• scope {Object} Scope assigned to the Flow
  • defaults to {Object} empty object {}
• freeze {Boolean} If set to true scope variables are frozen to prevent changes in the Flow
  • defaults to {Boolean} true

Flow.start( () =>
{
	// Callback is executed inside Flow scope

	let foo = Flow.get('foo'); // returns 'bar';
},
{ foo: 'bar' }); // Sets the Flow scope

static set( key, value[, freeze = true] )

Sets value for the key in the current Flow if key is not frozen.

• key {Any} Key
• value {Any} Value
• freeze {Boolean} If set to true value for key will be frozen to prevent changes in the Flow
  • defaults to {Boolean} true

Returns {Boolean}

• true value has been set for the key which was not frozen
• false key was frozen and variable have not been changed

Flow.set('foo', 'bar', false); // Sets the value inside Flow, returns true
Flow.set('foo', 'boo', true); // Rewrites the value, returns true
Flow.set('foo', 'goo'); // Does not rewrite the value, returns false

static get( key[, default_value = undefined] )

Returns value for the key in the current Flow, default_value if the key is not set.

• key {Any} Key
• default_value {Any} Value returned if the key is not set
  • defaults to undefined

Returns {Any}

let foo = Flow.get('foo');

static getPath( path[, default_value = undefined[, path_delimiter = '.']] )

Returns value for the path in the current Flow, default_value if the path is not set.

• path {Array of Strings | String} path
• default_value {Any} Value returned if the key is not set
  • defaults to undefined
• path_delimiter {String} Path delimiter if path is {String}
  • defaults to '.'

Returns {Any}

let foo = Flow.getPath('obj.foo');
let bar = Flow.getPath('obj|bar', null, '|');

static scope()

Returns the current scope of the Flow, object containing every key set in the Flow with its value and frozen state.

Returns {Object}

let scope = Flow.scope();

/* scope =
{
  [key] : { value, frozen }
}
*/

static save()

Returns the current Flow handle for later restoration.

Returns {FlowHandle}

let flow_handle = Flow.save();

static restore( FlowHandle, callback )

Restores the stored Flow and dispatches the callback. Flow can be restored by calling restore method on FlowHandle as well. Each FlowHandle can be restored only once - multiple restorations throws an Exception.

Flow.restore( flow_handle, () =>
{
	// Flow is restored
});

flow_handle.restore( () =>
{
	// Flow is restored
});

static bind( callback )

Binds the current Flow to callback function, callback can be executed outside of current flow yet the original Flow is automaticaly restored.

let callback;

Flow.start( () =>
{
	callback = Flow.bind( () =>
	{
		let foo = Flow.get('foo'); // returns 'bar'
	});
},
{ foo: 'bar' });

setTimeout( callback, 1000 ); // Executes callback outside of its Flow