Harcon - Proven and reliable microservice solution for the harmonic convergence of JS entities. Designed to be insanly scalable to serve enterprise components of real-time systems.

Harcon is strong to build a scalable execution-chain or workflow-based or machine-learning solution for NodeJS applications.

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!Note: From version 8.0.0, harcon supports only Node v8 and await functions. For callback-based version please use v7 or below.

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harcon is a microservice solution for NodeJS/Browser giving superior abstraction layer for interoperability between entities in a highly structured and fragmented ecosystem. It allows you to design and implement complex workflows and microservices where context and causality of messages are important.

The library has a stunning feature list beyond basic messaging functionality.

• Channel-agnostic: harcon represents a very abstract messaging framework allowing you to use any underlaying technology your application requires: AMQP, MQTT, Amazon SQS, NATS etc... For amqp integration, please check this: harcon-amqp For sqs integration, please check this: harcon-sqs For mqtt integration, please check this: harcon-mqtt For nats.io integration, please check this: harcon-nats

• Tracking: you can monitor every message delivered (request or response) by only few lines of code

• Flow control / Reproducibility: A flow of communication / messages can be halted / continued / reinitiated anytime with no effort

• Free orchestration: your system can be orchestrated and distributed as you wish, message delivery is not limited to nodes or hosts

• Short learning curve: no need to learn hundred of pages, communication has to be simple after all

• Log-free coding: no more mixture of logging and business logic. Harcon logs all messages exchanged.

• Transparent: although harcon introduces lots of complex types and structures, your code will be kept clean and pure, everything is (un)packed in the background in a transparent way

• Smooth infiltration: your objects / functions will possess the necessary services via injection, no need to create complex structures and compounds

• Advanced routing & listening: system fragmentation, qualified names, regular expressions, wildcards, etc.

• Execution-chain: toolset to easily define execution paths between entities to relay messages and results.

• Business-transaction: small helpers to manage business executions paths as (distributed) transactions.

!Note: Harcon's concept is to introduce a clean and high abstraction layer over messaging between entities. Like in case of every abstraction tool, for webapps which are simple as 1, it can be proven as a liability.

This library starts to shine in a highly structured and distributed environment.

Installation

$ npm install harcon

Quick setup

var Harcon = require('harcon')
let harcon = new Harcon( { /* opts */ } )
let inflicter = await harcon.init()


// define a listener function listening every message related to "greet" like "greet.goodmorning" or "greet.goodday"
await inflicter.deploy( null, 'peter', 'greet.*', async function (greetings1, greetings2) {
	return 'Hi there!'
} )

// define an plain object serving as listener withing the context "greet" to messages "warm"
marie = {
	name: 'marie',
	context: 'greet',
	warm: async function (greetings1, greetings2) {
		return 'Bonjour!'
	}
}
await inflicter.deploy( marie )

// sends a communication 'greet.everyone' with parameters and waits for responses
// will receive back 2 answers: 'Hi there!' and 'Bonjour!'
await harcon.simpleRequest( 'greet.everyone', 'Whatsup?', 'How do you do?' )

Logical view - Terms of Harcon

Send messages between entities

harcon distinguishes three message flow:

• request: entity A sends a message to B and requests an answer. Normal RPC model.
• inform: entity A sends a message to B and continues right away not considering whether B even received / processed the message. Signals / notifications are meant to be realised.
• delegate: entity A sends a message to B and requests an answer to a specified address. Usual delegation model.

These flows are available as services for all entities within harcon.

Unique messages

Every communication exchanged possesses the following properties (not exclusively):

• unique ID
• reference to the parent message if exists
• uniqued ID of the workflow itself
• external ID of the workflow started by an external communication involving a reference number to consider
• timestamp

Any time you sends a message or receives an answer, such objects are bypassing through the harcon system which logs and tracks all of them.

By default, harcon uses 32 as length of the IDs which are unique over time and among computer nodes. You can override this default when initiating Harcon

harcon = new Harcon( { /* ... */ idLength: 32 } )

Entities

In harcon, the communication unit is simply called entity. One can define 2 type of entities:

• simple function: when you associate a function with an event-pattern. Recommended to be used as observer, job-like, surveillance-, or interface-related asset.

// Qualified name - will answer to only this message
inflicter.deploy( null, 'hugh', 'allocate.ip', async function () {
	return 'Done.'
} )
// Wildcards - will answer anything within the context greet
inflicter.deploy( null, 'peter', 'greet.*', async function () {
	return 'Done.'
} )
// Regular expression - will answer anything where message name start with string 'job'
inflicter.deploy( null, 'john', /job.*/, async function ( partner ) {
	return 'Done.'
} )
...
let res = await inflicter.simpleRequest( 'job.order', { name: 'Stephen', customerID:123 } )
let res = await inflicter.simpleRequest( 'john.job', { name: 'Stephen', customerID:123 } )

• objects: plain object enclosing functions and a unique name. This is the recommended way to define entities.

var bookKeeper = {
	name: 'BookKeeper',
	...
	newOrder: async function ( customer ) {
		return 'Done.'
	},
	ordersOfToday: async function ( ) {
		return []
	}
}
...
let res = await inflicter.simpleRequest( 'BookKeeper.newOrder', {name: 'Stephen', customerID:123} )
let res = await inflicter.simpleRequest( 'BookKeeper.ordersOfToday' )

The simplest but not the only way to address is to quality entities with their names.

!Note: The following names are strictly forbidden to be used as entity names in any lower- or uppercase version: 'Barrel', 'Bender', 'Blower', 'Communication', 'Fire', 'Firestarter', 'Firestormstarter', 'FireBender', 'Flamestarter', 'FlowBuilder', 'FlowReader', 'Inflicter', 'Mortar', 'Warper'

Those are used by harcon itself.

The message flows introduced above appear as functions injected to all entities you publish. So - among many management-typed others - three functions will be injected:

• request
• inform
• delegate

One can use these functions to send messages to other entities as follows:

var bookKeeper = {
	name: 'BookKeeper',
	...
	newOrder: async function ( customer ) {
		this.request( 'Post.sendMaik', 'greetings', 'Dear Guest...' )
		return 'Done.'
	}
}

Function request will send the message "sendMaik" to the entity Post with the given parameters.

Note: keep it mind, that all service functions must be async and act accordingly!

Responses

By default, harcon returns and array of response objects returned by the entities addressed by a message sent or a single object if only one entity has been addressed by the request.

Let's have 2 simple entities:

inflicter.deploy( null, 'peter', 'greet.*', async function () {
	return 'Hi.'
} )
inflicter.deploy( null, 'camille', 'greet.*', async function () {
	return 'Hello.'
} )

let res = await inflicter.simpleRequest( 'greet.simple' )

Returns with the following:

[ 'Hi.', 'Hello.' ]

In some cases, you might find useful to know which answer comes from which entity. If you add a single parameter to the harcon:

new Harcon( { namedResponses: true } )

The returned object will look like this:

{ peter: 'Hi.', camille: 'Hello.' }

!Note: Harcon allows you to enforce the "responses are always arrays" behaviour. Please read Unfolding for details.

Error responses

Your functions might receive an error object in unwanted situations. The default transport channel of harcon will stop the message processing at the first error occurring as follows:

inflicter.deploy( null, 'peter', 'greet.*', async function () {
	throw new Error('Stay away, please.')
} )
inflicter.deploy( null, 'camille', 'greet.*', async function () {
	throw new Error('Do not bother me.')
} )

try {
	await inflicter.simpleRequest( 'greet.simple')
} catch (err) {
	console.error( err, res )
}

will result the following on your console:

[Error: Stay away, please.]

The default transport layer is designed for development purposes only. In an EE, environment, please mind the introduction of a message queue solution like: AMQP, SQS, MQTT or Nats using official plugins: harcon-amqp, harcon-sqs, harcon-mqtt and harcon-nats accordingly.

By using a real transport layer, all occurred error messages will be delegated. In such cases, harcon will retrieve an Error object encapsulating all error object received from entities.

Entity initialization

The need to pass contextual parameters to entities might rise. The options object passed to the constructure of harcon allows you to specify parameters for entities which will be passed while the init method defined in the entity is called. This function must perform well in order to be published, so the entity is not live when this function is called! The init function like all services, must return with a promise object!

harcon = new Harcon( { /* ... */ marie: {greetings: 'Hi!'} } )
var marie = {
	name: 'marie',
	context: 'test',
	init: async function (options) {
		// {greetings: 'Hi!'} will be passed
		return 'Initiated.'
	}
	// services ...
}

Keep it mind, that the function init does serve only to init the component so read files or connect to DB, should not be used to swift state or send message to other entities being deployed.

Should you want to notify entity when it has been successfully deployed, shoudl you implement a started function as follows:

harcon = new Harcon( { /* ... */ marie: {greetings: 'Hi!'} } )
var marie = {
	name: 'marie',
	context: 'test',
	init: async function (options) {
		// {greetings: 'Hi!'} will be passed
		return 'Initiated.'
	},
	started: async function () {
		return 'ok'
	}
	// services ...
}

The call of that function means that the component is up and running and ready to serve.

Message exchange

When your components are deployed, the need to send and receive messages arises. In a workflow, your component might initiate a message, response one or while responding one sends other ones. As introduced above, 3 different message flows are implemented in harcon: request, inform and delegate. Your components can perform these functions to send messages to each other.

var order = {
	name: 'Order',
	context: 'order',
	newVPN: async function ( customer ) {
		await this.inform( 'apn.notify', 'deviceID', 'Refresh network' )
		return this.request( 'allocate.address', '127.0.0.1' )
	}
}

The messaging functions are injected by the harcon when you publish the components.

Logging

When you create the Harcon instance, you can pass a logger object which will be respected and used to do loggings. If not set, harcon will log everything to the console. So in production, setting up a logging facility ( like winston or bunyan ) is strongly adviced.

harcon = new Harcon( { logger: logger /* ... */ } )

That logger instance will be used as logging facility all over the system, including internal services and entities. Each entity receives a function: harconlog with the signature:

function( err, message, obj, level ) { ... }

That function can be used anything within your entity object:

var Marie = {
	name: 'Marie',
	context: 'greet',
	whiny: async function (greetings) {
		this.harconlog( null, 'Some logging', { data: greetings }, 'silly' )
		return 'Pas du tout!'
	}
}

That function should be used for any logging activity you need during the flow of your app.

Important: Harcon logs all incoming and outgoing messages at the level 'igniteLevel' set in the configuration file making every-day logging activities unnecessary. You might want to change the default level of this behaviour in order to refine your logging strategy.

harcon = new Harcon( { igniteLevel: 'info' /* ... */ } )

An entity might exclude itself from the logging process by possessing the attribute "concealed":

var Marie = {
	name: 'Marie',
	concealed: true,
	...
}

That will turn off the logging of all messages sent or received by the entity 'Marie'.

Another option is to set the the list of entities unlogged.

harcon = new Harcon( { seals: [ 'Marie' ] /* ... */ } )

That will turn off the logging of all messages sent to Marie. So no answers received by other entities from Marie will be logged.

Workflows

In an enterprise-level system, one has to realize complex communication structure where lots of entities are following business logic and rules, involving subsystems and external resources, policies and other considerations, in short form: workflows. I take the liberty to define the workflow now as well defined routes and causality of messages. In a workflow, you are not dependent on the response timeframe, workflows manage distance in time and space. The recipient of a message can be on another server or city or planet. Recipient can answer right away or tomorrow or never.

In the JS world, one should mind the introduction of microservices to start with right in the beginning. Just take the advantage of better orchestration, simpler development and debugging, easier deployment, scaling and monitoring.

harcon is "just" a low-level library to leverage such concept. In a simple way, you define entities and the communications among them then publish them.

Execution-chain

In a microservice architecture, the key to design and orchestrate a working and living system is to abstract out the control in some form and write business logic fitting that abstract description.

harcon provides the Bender chapter for details, but please learn the basics of harcon before entering the rabbit hole, it is not without a reason that chapter is at the end of the documentation... ;)

Business transactions

When execution is defined by execution-chains, harcon is capable to tell when a given flow really terminated and inform all entities about the termination opening the gate to the distributed transaction management, when all entities participating to a given flow might react to its termination, performing DB operations for example.

Please check chapter Transactions for detail.

Orchestration

You have seen how to call service functions using qualified names and regular expressions (function-based entity). If a much structured system must be orchestrated, a set of finer toolset is at you disposal: contexts and divisions, representing different abstraction levels.

Context: is a named set of object-based entities and contexts. a qualified name identifying the field/purpose the entity is operating. To refine the structure of your service. For example you can have multiple entities providing service like parse but in different context like: "xml" or "json". You can structure your entities like the following:

var parser = {
	name: 'JSONParser',
	context: 'transfer.json',
	parse: async function ( document ) {
		return 'Done.'
	}
}
var observer = {
	name: 'XMLParser',
	context: 'transfer.xml',
	parse: async function ( document ) {
		return 'Done.'
	}
}

In this case, such messages can be sent:

await inflicter.simpleRequest( 'transfer.json.parse' )
or
await inflicter.simpleRequest( 'transfer.xml.parse' )
or
await inflicter.simpleRequest( 'transfer.'+document.type+'.parse' )

addressing directly to one of those entities, depending the type of the document you want to parse. Let's define the following entity:

var observer = {
	name: 'Observer',
	context: 'transfer',
	parse: async  function ( document ) {
		return 'Done.'
	}
}

Such entity will also receive those message and might do logging or measuring or perform preliminary actions. In short form: you can address multiple entities with a single message.

The matching algorithm is simple: spliting the title by '.' matching with the context you specify and fails only if the compared strings are not equals. This means, that:

The title 'transfer.xml.parse' will target the XMLParser and Observer entities.

and

The title 'transfer.parse' will target the XMLParser, JSONParser and Observer entities.

Contexts are very good way to refine your structures and entities and express overlapping functional behaviors.

Division: divisions is a different angle on the plane of orchestration. A division is a closed "box" of entities, meaning that an entity can operate only within the division it is a member of. In fact, every entity belongs to a division defined explicitly or implicitly by the harcon. Divisions can be encapsulated, so a complete division-tree can be built-up in a harcon application. The reason why divisions are important, because it represents a responsibility and/or legal unit. Entities within it (in normal cases) cannot see outside and an entity published to a container division can answer to messages initiated by an entity somewhere lower in the tree. This gives you a control to define surveillance-like or control-like features and much higher complexity of communication-management.

Please find the Divisions chapter for details.

Note: contexts and divisions are not mandatory to be used. The complexity will tell you how to orchestrate your app. It might happen, that simple function-based named entities are fitting your need. Feel free to act on your own, there is no pattern to follow.

The terms object

In a workflow, a contextual object is very desired to be set for the business entities participating and sharing some environmental state or values. If you implement a financial transaction management system, currencies should be added to the terms object making the information accessible to all entities interoperating within the workflow. Or if you have a "logged in" entity, you might want to pass on the token of that given entity to be validated by other entities in the workflow in progress. It is very important to note, that the same object is used by the harcon itself to share services with the entities.

Should you define your business functions as below, the terms will be passed automatically:

module.exports = {
	name: 'Claire',
	init: async function (options) {
		return 'Initiated.'
	},
	simple: async function (greetings1, greetings2, terms) {
		return 'Pas du tout!'
	}
}

Along the parameters you need for your business logic, the terms can be added to the parameter list in the right order shown above.

Chain messages

If you work with workflows, the sequence/order of your messages will get an importance. To chain messages, define the next point in the workflow you have to add another parameter to your service function:

var order = {
	name: 'Order',
	context: 'order',
	newVPN: async function ( customer, terms ) {
		return terms.request( 'allocate.address', '127.0.0.1' )
	}
}
...
await inflicter.request( 'order.newVPN', {name: 'Stephen', customerID:123} )

That will initiate a small workflow. harcon.simpleRequest sends a message to entity Order who will send within the same workflow to the Allocator. When it answeres, then the message of the beginning will be answered. harcon will know if you initiate a message within the processing of another one and considers it as part of the ongoing workflow and tracks it. Mind the async execution to keep everything in track!

NOTE: You might have noticed, that the functions implementing the messaging flows introduced above appear in 2 forms: this.request and terms.request

var timer = {
	name: 'Timer',
	scheduling: async function ( terms ) {
		await terms.request( 'Validator.refreshData' )
		return this.request( 'Validator.validateAccounts' )
	}
}

The entity's own messaging services always initiate a new workflow. The messaging services in the terms object always continue the workflow already in progress and defines a "substep" within it. It is the matter of orchestration which one needs to be performed.

Terms can be used to share data among entities as well:

module.exports = {
	name: 'Domina',
	force: async function ( terms ) {
		var self = this
		terms.tree = 'grow'
		return terms.request( 'Claire.simple', 'It is morning!', 'Time to wake up!' )
	}
}

The 'tree' attribute set by entity 'Domina' will be seen by the entity 'Claire' when it receives the message. That is an adhoc terms definition which can be set globally in your entity as follows:

module.exports = {
	name: 'Domina',
	terms: {
		'*': { forEveryone: 'pass this' }
	},
	force: async function ( terms ) {
		var self = this
		terms.tree = 'grow'
		return terms.request( 'Claire.simple', 'It is morning!', 'Time to wake up!' )
	}
}

In harcon, an initiated workflow will be defined by the terms of the is starter entity. You can set the terms for all communication started by the entity 'Domina' by marking the objects with the key '*', or you can set the terms individually by setting the terms with the key of the externalID of the communication. In case of a rest-based service, you might want to channel the request and session information to the terms forcing you to perform the requests individually with a generated externalID and setting the "terms" of the web client with the key set to the value of the externalID. For a great example, please check harcon-radiation.

Distinguishing entity instances

There is an option for an object-based entity to enforce uniqueness:

module.exports = {
	name: 'Charlotte',
	distinguish: 'Unique',
	access: async function ( ) {
		return 'D\'accord?'
	}
}

The attribute 'distinguish' can be a boolean or a string which will be added to its name as a postfix. In case of boolean value, a random value will be generated.

The entity can be called with 'Charlotte-Unique.access' or 'Charlotte-1231441123.access' where '1231441123' is the random string generated by the harcon.

This feature is useful in a largely scaled environment, where a given entity might appear in multiple instances and distinguishing them is a must-have behavior.

The entity will be still available with its own name: 'Charlotte'. You can consider it as a synonym or alternative name.

Rest parameters

harcon allows you to define your services with Rest parameters as follows:

comp: async function ( ...args ) {
	console.log('Arguments received: ', args)
	return 'ok'
},
shrink: async function ( terms, ...args ) {
	console.log('Arguments received: ', args)
	return 'ok'
}

The given services can receive any number of parameters and will be enclosed by the parameter 'args'. The naming is a convention which should be followed. Such service can also define the optional terms object as first parameter demonstrated above.

Divisions

When you orchastrate your system as a microservice architure over a clustered message bus for example, the individual microservicSystems can be orchastrated into divisions which is a tree structure actually. One can create divisions following the control-flow or responsibility-chain of the application. Every component you deploy will belong to a division. If not declared, then to the system division where all system-level components are put. Divisions is not just a logical grouping of components, but also an encapsulation-model. A component cannot send messages outside the own division but can send to the inner ones. This means, that system components can send to any component, but non-system components cannot reach the level of the main system or other branches of the division-tree.

Divisions give you a very easy-to-use structure to orchestrate your system. Of course, you can use the harcon without using divisions, the complexity of your system will show if you needed it or not.

Let's define components and add them to divisions:

// This will add John to the division 'workers'
harcon.deploy( 'workers', 'john', /job.*/, async function () {
	return 'Done.'
} )
// This will add Claire to the division 'entrance'
var claire = {
	name: 'claire',
	division: 'entrance',
	context: 'greet',
	simple: async function (greetings1, greetings2) {
		return 'Enchanté, mon plaisir!'
	}
}

Components in a division can be called to:

await inflicter.request( null, 'entrance', 'greet.simple', 'Hi', 'Ca vas?' )

Note: please keep in mind, that harcon.request can be and should be used only when you initiate a workflow from outside the harcon!

If you initiate a communication through the harcon instance, it means, that you wants to drop in a message "from outside" which could mean an integration with an external system or just kick off a workflow. There are 2 methods to be called:

request and simpleRequest

The different between them is the parameter list. The later does not require to specify external messageId or division, the prior one does as you can see in the example just above.

External ID is very useful, when the workflow is initiated by some external event possessing an id which must be kept for further logging or tracking or just because a communication harmonization across the complete company.

By default, harcon presumes to have one division per node following the concept of microservices. That division name can be given via its config object or will be derived from its unique name. The created division will serve as the root for every divisions and entities defined.

harcon = new Harcon( { division: 'District 8', ... } )

Subdivision is an option to represent an interoperability for distinct legal / organisational entities. Let's say, you want to build a private network, where you interconnect 3 companies working on the same business processes. they share services, data but they keep their own privacy. That means, that all companies / nodes belong to the same main division introduced above, but each of them represents a unique subdivision confining their own services and entities.

So your cross-company project is called 'antidotum' which is a theoretical medical experiment project between big market players. You involve a clinic, a research facility and a production facility. Their domain will be: 'antidotum.clinic', 'antidotum.research' and 'antidotum.production'.

Each node can scale and define own entities and can also address entities defined by other nodes as well.

Entity configuration

Entities can be configured easily. The init method of an entity if present, will be called at the end of its publishing process withing the harcon. The function can be used to set up DB connections or anything required for your entities.

module.exports = {
	name: 'Claire',
	context: 'greet',
	init: async function (options) {
		return 'Initiated.'
	}
}

The method receives the associated configuration object. That configuration object can be passed when an entity is published:

await inflicter.deploy( Claire, config )

or even before, when harcon is created.

harcon = new Harcon( { logger: logger, idLength: 32, Claire: {greetings: 'Hi!'} } )

An entity's configuration is an merged object made from the followings (in order):

• environment variables derived from process.env
• the millieu object in the configuration of harcon
• object in harcon configuration associated through the name of the entity
• the direct configuration passed to the function deploy

Should you use none of them, and your entity shall be initiated with empty object.

Timeout management

harcon has an internal handler to deal with messages not answered within a reasonable timeframe.

harcon = new Harcon( { ..., blower: { commTimeout: 2000 } } )

This configuration will tell your running harcon instance to check if the messages sent to some entities have been answered within 2000 millisecs. If not, an error will be sent to the sender with the message: 'Communication has not been received answer withing the given timeframe.'

The nature of your app might urge you to distinguish time management following some business logic. For example: operations involving third party APIs have to be performed with a wider or without time limitation. harcon supports such demand in many ways.

In the simplest case:

harcon = new Harcon( { ..., blower: { commTimeout: 2000, tolerates: [ 'Alizee.superFlegme' ] } } )

The field 'tolerates' can be defined as an array of strings turning off the global timeout management for the communication enclosing the given messages.

The array tolerate might be enlisting strings, regexp values, functions or objects evaluated as follows:

• Strings will be matched to the event of the message
• Regexp will be matched to the event of the message
• Function will be called by passing the ongoing communication object and the boolean return value will determine if timeout management should be applied.
• Object's event attribute will be matched as the string case above and the timeout will be set by the 'timeout' attribute of the object.

Please find some examples below:

harcon = new Harcon( { ..., blower: { commTimeout: 2000, tolerates: [ 'Alizee.superFlegme' ] } } )
... or ...
harcon = new Harcon( { ..., blower: { commTimeout: 2000, tolerates: [ /Alizee\.\w+/g ] } } )
... or ...
harcon = new Harcon( { ..., blower: { commTimeout: 2000, tolerates: [ function (comm) { return comm.event === 'Alizee.flegme' } ] } } )
... or ...
harcon = new Harcon( { ..., blower: { commTimeout: 2000, tolerates: [ { event: 'Alizee.flegme', timeout: 2000 } ] } } )

You can mix the techniques as your business logic demands.

!Note: Please, keep it mind, that harcon is transport layer agnostic, so a preferred message broker can also provide some security over interaction.

Extension

harcon can be easily extended by using pure harcon components listening to system events:

var extension = {
	name: 'As you design it',
	context: harcon.name,
	castOf: async function ( name, firestarter ) {
		return 'OK'
	},
	affiliate: async function ( firestarter ) {
		return 'OK'
	},
	close: async function () {
		return 'OK'
	}
}
await inflicter.deploy( extension )

In the current version, the harcon instance you are using will send to your components events about system closing, entity publishing and revoking. For a working example, please check harcon-radiation.

All funtions must return a promise object.

Millieu

There is a "shared" environmental object added to all entities deployed within harcon:

harcon = new Harcon( { millieu: { workDir: '/temp' } } )

That object will be inserted into the initial configuration of all Entities published:

var Marie = {
	name: 'Marie',
	init: async function (options) {
		// options = { workDir: '/temp' }
		self.options = options
		return 'OK'
	}
}

In any service of entity 'Marie', the 'self.options.workDir' will be a valid object.

!Note: please keep in mind, that some transport layers might restrict the size or the content of the packets relayed. Use millieu object wisely.

State shifting

There is a reverse-directed-like feature in harcon, allowing an entity to shift "internal state" and let other entities to know about it. Let's say, entities of a system want to know when the DB component becomes "available", or when a third party connector entity becomes "connected".

To build such constellation between entities, is pretty straightforward:

module.exports = {
	name: 'Lina',
	...
	registerForShift: function () {
		await self.request('Marie.notify', 'data', 'Lina.marieChanged' )
	},
	...
	marieChanged: async function ( payload ) {
		console.log( '>>>>>>>>>>>', payload )
		return 'OK'
	}
}

In function 'registerForShift' 'Lina' tells 'Marie' to notify her if the state 'data' is changing. The function to be called is 'marieChanged'. The new value of the given state should be passed. State is a string. It does not need to be represented as attribute or whatever, just a pure string - object pair.

How Marie can "shift" state:

module.exports = {
	name: 'Marie',
	simple: async function (greetings1, greetings2) {
		this.shifted( { data: 'content' } )
		return 'Bonjour!'
	}
}

Anytime Marie calls its internal 'shifted' function, it triggers the harcon's state changing service. You have to pass an object possessing all properties considered to be "states". So an object might shift multiple states at once if needed. The values of the attributes are the payload to send to the listener entities. That this.shifted( { data: 'content' } ) line will initiate an internal communication with the addressing 'Lina.marieChanged' as Lina specified. And the function 'marieChanged' of Lina will be called with the string 'content' passed as payload.

Entity entangling

Entangling is a feature to keep state shifting in sync between distant entities. Lets consider the following entity:

module.exports = {
	name: 'Marie',
	context: 'greet',
	entangled: 'Marion',
	...

In the code above, entity Marie is entangled to Marion which makes the harcon to sync all state changes of Marie to Marion through the bus. That will "force" Marion to be the subject of the same state shifting occurred in Marie. Such feature could be useful when really distant enties must replicate state changes in a transparent and automated way.

Interoperating with other harcon instances

By default harcon blocks acts like a blackbox allowing to exchange communication within the main division / name of the system deployed. If you pass an attribute 'connectedDivisions' to the config of harcon, it will accept communication from the enlisted divisions and allow you to connect to those ones.

Unfolding

The basic concept of harcon is to relay messages between entities where m:n quantity relationship for delivery is supported. Harcon measures if the response means a single object or a list of objects and returns accordingly. That behaviour can be turned off to force harcon to return always with an array:

let harcon = new Harcon( {
	name: harconName,
	unfoldAnswer: false,
	...
} )

Bender

Bender is a high-level entity over microservices aims to introduce a simple, yet powerful execution-chain-tool for architects helping to drive message relaying between entities in a regulated environment.

How it works: you define your execution logic which is a set of rules pairing

• a message
• a list of messages induced when that message is answered.

Let's say you want to see the following informal execution to be performed:

'Order.registerOrder' -> 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif'

The event registerOrder will trigger 3 order messages to be sent. Rolling the execution of your business workflow further. In other way, you can liberate your execution chain from low-level source code. The entity Bender is dedicated to help with you on this road.

One can activate it by using the following configuration:

let harcon = new Harcon( {
	name: harconName,
	bender: { enabled: true },
	FireBender: {
		defs: {
			// rules here
		}
	}
	...
} )

One has 2 ways to set chain definitions:

• read flow description files from a folder. For this option please check harcon-flow for details...
• manual flow definition rules

Syntax of a rule definition:

{event_name}: {
	type: {execution_type},
	primers: [ {target} ]
}

{execution_type} : 'series' || 'waterfall' || 'spread'

{target} : { division: {division_name}, event: {event_name}, skipIf: {condition} } || {event_name}

{condition} : {event_name} || function

Each rule is set of a message, a list of steps to make (called primers), and an optional validation object For example :

'Order.registerOrder': { primers: [ 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif' ], validation: { minlength: '3' } }

or

'Order.registerOrder': { primers: [ 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif' ], validation: function (payload) { return true } }

That works as follows: an entity or external event triggers the message 'registerOrder' sent to entity 'Order'. The call will be performed and the result of that call will be passed as parameter to all messages enlisted in the array 'primer'. So the entity 'DB' will receive the message 'storeOrder' automatically triggered by the message 'Order.registerOrder'. When 'DB' is finished, 'checkStore' will be triggered and so on. Default execution type is 'series' means sequential execution of the given list.

Validation object can be a JS object or a function. It is used to validate the payload of the message 'registerOrder' sent to entity 'Order'. The functions should return a boolean value to tell if the payload is valid. If the validation is an object, the vindication.js will be used for validation.

Bender is turns harcon into a strict message delivery environment, which means if an event is not recognised by Bender or in other words, there is no rule for that given event, an exception will be thrown to the caller and no message delivery will be performed.

There are several type of execution defined by the {execution_type} above.

• 'series' means to execute all steps in sequential order passing the result of 'Order.registerOrder' to each of them and returning the collected results to the caller of 'Order.registerOrder'.

'Order.registerOrder': { type: 'series', primers: [ 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif' ] }

• 'waterfall' means a waterfall-like execution. Bender passes the result of 'Order.registerOrder' to 'DB.storeOrder' and 'DB.checkStore' will receive the result of 'DB.storeOrder' and so on...

'Order.registerOrder': { type: 'waterfall', primers: [ 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif' ] }

• 'spread' means concurrent message sending to all entities enlisted in the array of 'primers'. Basically this is the bulked execution of message sending of entities.

'Order.registerOrder': { type: 'spread', primers: [ 'DB.storeOrder', 'DB.checkStore', 'Mailer.sendNotif' ] }

The attribute 'skipIf' defines the 'execution-gate' for a given step. It can be a message string or a function. If it is

• a string, so a message, the given message will be sent to the referred entity and the answer should possess the "allowed" attribute with a value coercing to true to allow the execution of the given step. Will be ignored otherwise. If error is sent back, all remaining steps will be ignored as well.

• a function, the answer coerces to true will allow to perform the step. Will be ignored otherwise. If error is thrown, all remaining steps will be ignored as well.

Attribute 'skipIf' can be used only for 'series' and 'waterfall' as logic follows.

Important: The Bender entity has a message 'completeness' telling which entities or services are still missing breaking the business flow(s) defined.

Note: harcon tries to validate the passed definitions searching for circles or crossing references. The Bender entity will fail to initialise if the validation process on the definitions falls.

Transactions

When Bender is activated and your flows are well defined through its services, your entities became capable of reacting the termination of your business flows. That serves to handle flows as transactions.

To be straightforward, when a business flow ends, your entities will be notified and can commit DB transactions to finalize their operations.

Each entity you define will possess the following functions:

flowFailed: async function ( flowID, errMessage, terms ) {
	...
}
flowSucceeded: async function ( flowID, result, terms ) {
	...
}

By default, it does not do anything. If your entity aims to react to 'flowFailed' or 'flowSucceeded' events, override those functions in the source code of your entity. The function must return a promise...

During your business logic, you business logic should access the flowID through the variable 'terms':

sign: async function ( document, terms ) {
	// terms.sourceComm.flowID hold the flowID of the current business flow
	// use it to store some records in your favorite in-memory storage to access it when 'flowTerminated' is induced.
}

If you want to use Bender without transaction management, the following configuration will turn it off :

let harcon = new Harcon( {
	bender: { enabled: true, igniteTermination: false },
	...
} )

Note: Please keep in mind, that if you are using a scaling solution, for the safety of the transactions, harcon does not distinguish the nodes, one has to facilitate the state sharing between nodes using Redis or similar tool.

Fragmented in time

Some operations cannot be performed within a reasonable short timeframe. Let's say, a task is sent to another devision requesting for manual acknowledgement. This will make a gap between request and response for sure. Could be proven the measure of days as well...

The design you app should follow for such cases as follows: The entity receiving the request must send back 'Request accepted'-like message to the sender and store the externalID and flowID of the communication:

await request( 'ManCanDo.sign', document )
console.log('Accepted.')

...

let ManCanDo = {
	auditor: true,
	sign: async function ( document, terms ) {
		await database.store( terms.sourceComm, document )
	},
	_signed: async function( document ) {
		let self = this
		let record = await database.readRequest( document )
		return self.request( record.externalId, record.flowId, record.sourceDivision, record.source + '.signed', document )
	}
}

The param terms holds references about the incoming communication by the name of sourceComm. You can store the externalID if the communication has been initiated by an external party or just the flowID if continuity must be ensured.

Of course it might happen you need a manual work to save an incoming communication and save the terms object and initiate a communication later in tame as a continuation of the give flow. Please read the following code:

let ManCanDo = {
	auditor: true,
	startToWork: async function ( terms ) {
		let comm = this.burst( terms.sourceComm, terms, '', 'ManCanDo.whatever' )
		// save the comm
		return 'ok'
	},
	returnToWork: async function( document ) {
		let comm = retrieveComm( ... )
		// 'ManCanDo.whatever' will be performed as a next execution point in the flow ...
		this.blow( comm )
	}
}

Live-reload entities

There is a built-in entity in harcon: Mortar dedicated to

• collect the entities from a folder - not considering subfolders
• publish them to harcon
• live-reload the ones which changed during runtime

To activate it:

harcon = new Harcon( { ..., mortar: { enable: true, folder: '', liveReload: false } } )

Wrappers

Wrappers are a special services in harcon allowing to specialize messages before sending them out and - as a gatekeeper, - validate them before delivering them. That functionality is meant to define some kind of contract between entities or harcon systems.

For example, in order to provide an inter-service security in a highly fragmented microservice architecture, the entities must know if the message is really cominng fron the given entity and answers also have to be ensured to be not forged by any third party, independently of any geographical distribution. A Warper can ensure, that messages are creating for example signed messages which can be verified by the other entities easily and vica versa. The system has a default global warper, defining an "always allow" policy for all services and communication. To provide a custom warper, you might want to refine the 'Barrel' settings when initializing the harcon as follows:

harcon = new Harcon( { ..., barrel: { Warper: [WarperCreatorFunction] } } )

A [WarperCreatorFunction] will be called and instantiated as follows:

let warper = new [WarperCreatorFunction]( this.division, config.connectedDivisions )

The current division and all connected divisions of the current harcon will be passed.

Warpers define the following services to be implemented:

function inpose( exposed ) ... // inposes information from other harcon nodes
function expose() ... // to share information public among harcon nodes
function conform( comm ) ... // to be called before sending out a message
function referenceMatrix ( object ) ... // to be called optionally if some external data should be considered
function allow ( communication ) ... // to be called before delivering a message

All functions are synchronous.

The harcon-ecdsa-warper defines az ECDSA-based security layer for harcon, please check for details.

Timing

Each entity published possesses functions to aid timing services:

		startCron: function ( name, spec, fn ) ...
		setTimeout: function ( fn, timeout ) ...
		setInterval: function ( fn, interval ) ...

The function startCron creates a cron-like job specified in the 2nd parameter. The functions setTimeout and setInterval are the wrapped version of the original ones defined by the JS. Please note,that all functions must be async!

All scheduled functions are stopped automatically when harcon stops.

Interveners

An entity might want to proxied by another entity ot conform / transform the input parameters and / or alter the result of a given service. One can define the followings:

module.exports = {
	name: 'Alizee',
	context: 'morning.girls',
	supervener: 'Vener',
	...

That makes harcon to intervene and catch all requests sent to 'Alizee' and allow 'Vener' to step in and perform an operation it wants then performing the original service and the result can be tranformed by the 'Vener'. That might help to implement real 'proxy' funtionality in harcon.

A possible 'Vener' could be defined as follows:

module.exports = {
	name: 'Vener',
	async supervene ( event, ...params ) {
		console.log('Accepted: ', event, params )
		return params
	},
	async superform ( event, result ) {
		console.log('Formed: ', event, result )
		return result
	}
}

The names of the interceptor functions are fixed and should be respected.

License

(The MIT License)

Copyright (c) 2019 Imre Fazekas

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Bugs

See https://github.com/imrefazekas/harcon/issues.