npm package: @safefood360/cqrs-domain
Description: Node-cqrs-domain is a node.js module based on node-eventstore. It can be very useful as domain component if you work with (d)ddd, cqrs, eventdenormalizer, host, etc.
Author: safefood360
Latest version: 2.3.12


# Introduction

[![travis](https://img.shields.io/travis/adrai/node-cqrs-domain.svg)](https://travis-ci.org/adrai/node-cqrs-domain) [![npm](https://img.shields.io/npm/v/cqrs-domain.svg)](https://npmjs.org/package/cqrs-domain)

Node-cqrs-domain is a node.js module based on [node-eventstore](http://adrai.github.com/node-eventstore/).
It can be very useful as domain component if you work with (d)ddd, cqrs, eventdenormalizer, host, etc.

<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
**Table of Contents**

- [Workflow](#workflow)
- [Installation](#installation)
- [Usage](#usage)
  - [Using factory methods for event store or / and aggregate lock in domain definition](#using-factory-methods-for-event-store-or--and-aggregate-lock-in-domain-definition)
  - [Exposed errors](#exposed-errors)
  - [Catch connect ad disconnect events](#catch-connect-ad-disconnect-events)
  - [Define the command structure](#define-the-command-structure)
  - [Define the event structure](#define-the-event-structure)
  - [Define the id generator function [optional]](#define-the-id-generator-function-optional)
    - [you can define a synchronous function](#you-can-define-a-synchronous-function)
    - [or you can define an asynchronous function](#or-you-can-define-an-asynchronous-function)
  - [Define the aggregate id generator function [optional]](#define-the-aggregate-id-generator-function-optional)
    - [you can define a synchronous function](#you-can-define-a-synchronous-function-1)
    - [or you can define an asynchronous function](#or-you-can-define-an-asynchronous-function-1)
  - [Wire up events [optional]](#wire-up-events-optional)
    - [you can define a synchronous function](#you-can-define-a-synchronous-function-2)
    - [or you can define an asynchronous function](#or-you-can-define-an-asynchronous-function-2)
  - [Initialization](#initialization)
  - [Handling a command](#handling-a-command)
    - [or](#or)
    - [more infos, can be useful if testing](#more-infos-can-be-useful-if-testing)
  - [Request domain information](#request-domain-information)
- [Components definition](#components-definition)
  - [Context](#context)
  - [Aggregate](#aggregate)
  - [Command validation](#command-validation)
  - [Pre-Load-Condition](#pre-load-condition)
  - [Pre-Condition](#pre-condition)
  - [Command](#command)
  - [Event](#event)
  - [Business Rule](#business-rule)
  - [Command Handler (Be careful!!!)](#command-handler-be-careful)
- [License](#license)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->
# Workflow

```

        │
       cmd
        │
        ∨
  ╔════════════╗
  ║ validation ║─────────> "rejected"
  ╚════════════╝
        │
       cmd
        │
        ∨
╔═════════════════════╗
║ pre-load-conditions ║─────> "rejected"
╚═════════════════════╝
        │
       cmd
        │
        ∨
╔════════════════╗
║ pre-conditions ║─────> "rejected"
╚════════════════╝
        │
       cmd
        │
        ∨
  ╔════════════╗
  ║ handle cmd ║
  ╚════════════╝
        │
       evt
        │
        ∨
  ╔═══════════╗
  ║ apply evt ║
  ╚═══════════╝
        │
        │
        │
        ∨
╔════════════════╗
║ business rules ║─────> "rejected"
╚════════════════╝
        │
        │
        │
        ∨
   ╔════════╗
   ║ commit ║
   ╚════════╝

```

# Installation

    npm install cqrs-domain

# Usage

	var domain = require('cqrs-domain')({
	  // the path to the "working directory"
	  // can be structured like
	  // [set 1](https://github.com/adrai/node-cqrs-domain/tree/master/test/integration/fixture/set1) or
	  // [set 2](https://github.com/adrai/node-cqrs-domain/tree/master/test/integration/fixture/set2)
	  domainPath: '/path/to/my/files',

	  // optional, default is 'commandRejected'
	  // will be used if an error occurs and an event should be generated
	  commandRejectedEventName: 'rejectedCommand',

	  // optional, default is 800
	  // if using in scaled systems and not guaranteeing that each command for an aggregate instance
	  // dispatches to the same worker process, this module tries to catch the concurrency issues and
	  // retries to handle the command after a timeout between 0 and the defined value
	  retryOnConcurrencyTimeout: 1000,

	  // optional, default is 100
	  // global snapshot threshold value for all aggregates
	  // defines the amount of loaded events, if there are more events to load, it will do a snapshot, so next loading is faster
	  // an individual snapshot threshold defining algorithm can be defined per aggregate (scroll down)
	  snapshotThreshold: 1000,

	  // optional, default is in-memory
	  // currently supports: mongodb, redis, tingodb, azuretable and inmemory
	  // hint: [eventstore](https://github.com/adrai/node-eventstore#provide-implementation-for-storage)
	  eventStore: {
	    type: 'mongodb',
	    host: 'localhost',                          // optional
	    port: 27017,                                // optional
	    dbName: 'domain',                           // optional
	    eventsCollectionName: 'events',             // optional
	    snapshotsCollectionName: 'snapshots',       // optional
	    transactionsCollectionName: 'transactions', // optional
	    timeout: 10000                              // optional
      // authSource: 'authedicationDatabase',        // optional
	    // username: 'technicalDbUser',                // optional
	    // password: 'secret'                          // optional
      // url: 'mongodb://user:pass@host:port/db?opts // optional
	  },

	  // optional, default is in-memory
	  // currently supports: mongodb, redis, tingodb, couchdb, azuretable and inmemory
	  // hint settings like: [eventstore](https://github.com/adrai/node-eventstore#provide-implementation-for-storage)
	  aggregateLock: {
	    type: 'redis',
	    host: 'localhost',                          // optional
	    port: 6379,                                 // optional
	    db: 0,                                      // optional
	    prefix: 'domain_aggregate_lock',            // optional
	    timeout: 10000                              // optional
	    // password: 'secret'                          // optional
	  },

	  // optional, default is not set
	  // checks if command was already seen in the last time -> ttl
	  // currently supports: mongodb, redis, tingodb and inmemory
	  // hint settings like: [eventstore](https://github.com/adrai/node-eventstore#provide-implementation-for-storage)
	  deduplication: {
			type: 'redis',
			ttl: 1000 * 60 * 60 * 1, // 1 hour          // optional
			host: 'localhost',                          // optional
			port: 6379,                                 // optional
			db: 0,                                      // optional
			prefix: 'domain_aggregate_lock',            // optional
			timeout: 10000                              // optional
			// password: 'secret'                          // optional
	  }
	});

## Using factory methods for event store or / and aggregate lock in domain definition
You can replace the framework-provided implementation of event store or / and aggregate lock with the one of your own.
To do that, you need to include a factory method in the options object passed to the domain constructor.
Using the factory methods, the example above might become:


	var myGetEventStore = require('./getEventStore.js');
	var myLock = require('./myLock.js');

	var domain = require('cqrs-domain')({
	  domainPath: '/path/to/my/files',
	  commandRejectedEventName: 'rejectedCommand',
	  retryOnConcurrencyTimeout: 1000,
	  snapshotThreshold: 1000,

	  eventStore: function () {
	    return myGetEventStore({
	      host: '127.0.0.1',
	      port: 2113,
	      username: 'admin',
	      password: 'changeit'
	    });
	  },

	  aggregateLock: : function () {
	    return myLock({
	       // ....
	    });
	  },

	  deduplication: : function () {
			return myCommandBumper({
			   // ....
			});
	  }
	});

When using factory methods, the objects they return are required to implement the following public interfaces:

	Event Store:

	  f:  init(function(err));
	  f:  getNewId(function (err, id));
	  f:  on(evtName, function (err));
	  f:  getFromSnapshot(query, revMax, function(err, snapshot, stream));
	  f:  createSnapshot(obj, function (err));
	  f:  setEventToDispatched(evt, function (err));

	Event Stream (returned by getFromSnapshot through the callback):

	  p:  events
	  p:  lastRevision
	  p:  eventsToDispatch
	  f:  addEvents(evts)
	  f:  commit(function (err, stream));

	Aggregate Lock:

	  f: connect(function(err, lock))
	  f: disconnect(function(err))
	  f: getNewId(function(err, id))
	  f: reserve(workerId, aggregateId, function(err))
	  f: getAll(aggregateId, function(err, workerIds))
	  f: resolve(aggregateId, function(err))

	Command Bumper:

	  f: connect(function(err, lock))
	  f: disconnect(function(err))
	  f: getNewId(function(err, id))
	  f: add(key, ttl, function(err, added))

	where:

	  f: function
	  p: property


## Exposed errors
You can use this for example in you custom command handlers.

	require('cqrs-domain').errors.ValidationError
	require('cqrs-domain').errors.BusinessRuleError
	require('cqrs-domain').errors.AggregateDestroyedError
	require('cqrs-domain').errors.AggregateConcurrencyError
	require('cqrs-domain').errors.ConcurrencyError
	require('cqrs-domain').errors.DuplicateCommandError


## Catch connect and disconnect events

	// eventStore
	domain.eventStore.on('connect', function() {
	  console.log('eventStore connected');
	});

	domain.eventStore.on('disconnect', function() {
	  console.log('eventStore disconnected');
	});

	// aggregateLock
	domain.aggregateLock.on('connect', function() {
	  console.log('aggregateLock connected');
	});

	domain.aggregateLock.on('disconnect', function() {
	  console.log('aggregateLock disconnected');
	});

	// commandBumper
	domain.commandBumper.on('connect', function() {
	  console.log('commandBumper connected');
	});

	domain.commandBumper.on('disconnect', function() {
	  console.log('commandBumper disconnected');
	});


	// anything (eventStore or aggregateLock or commandBumper)
	domain.on('connect', function() {
	  console.log('something connected');
	});

	domain.on('disconnect', function() {
	  console.log('something disconnected');
	});


## Define the command structure
The values describes the path to that property in the command message.

	domain.defineCommand({
	  // optional, default is 'id'
	  id: 'id',

	  // optional, default is 'name'
	  name: 'name',

	  // optional, default is 'aggregate.id'
	  // if an aggregate id is not defined in the command, the command handler will create a new aggregate instance
	  aggregateId: 'aggregate.id',

	  // optional, only makes sense if contexts are defined in the 'domainPath' structure
	  context: 'context.name',

	  // optional, only makes sense if aggregates with names are defined in the 'domainPath' structure
	  aggregate: 'aggregate.name',

	  // optional, but recommended
	  payload: 'payload',

	  // optional, if defined the command handler will check if the command can be handled
	  // if you want the command to be handled in a secure/transactional way pass a revision value that matches the current aggregate revision
	  revision: 'revision',

	  // optional, if defined the command handler will search for a handle that matches command name and version number
	  version: 'version',

	  // optional, if defined theses values will be copied to the event (can be used to transport information like userId, etc..)
	  meta: 'meta'
	});


## Define the event structure
The values describes the path to that property in the event message.

	domain.defineEvent({
	  // optional, default is 'correlationId'
	  // will use the command id as correlationId, so you can match it in the sender
	  correlationId: 'correlationId',

	  // optional, default is 'id'
	  id: 'id',

	  // optional, default is 'name'
	  name: 'name',

	  // optional, default is 'aggregate.id'
	  aggregateId: 'aggregate.id',

	  // optional, only makes sense if contexts are defined in the 'domainPath' structure
	  context: 'context.name',

	  // optional, only makes sense if aggregates with names are defined in the 'domainPath' structure
	  aggregate: 'aggregate.name',

	  // optional, default is 'payload'
	  payload: 'payload',

	  // optional, default is 'revision'
	  // will represent the aggregate revision, can be used in next command
	  revision: 'revision',

	  // optional
	  version: 'version',

	  // optional, if defined the values of the command will be copied to the event (can be used to transport information like userId, etc..)
	  meta: 'meta',

	  // optional, if defined the commit date of the eventstore will be saved in this value
	  commitStamp: 'commitStamp'
	});


## Define the id generator function [optional]
### you can define a synchronous function

	domain.idGenerator(function () {
	  var id = require('node-uuid').v4().toString();
	  return id;
	});

### or you can define an asynchronous function

	domain.idGenerator(function (callback) {
	  setTimeout(function () {
	    var id = require('node-uuid').v4().toString();
	    callback(null, id);
	  }, 50);
	});


## Define the aggregate id generator function [optional]
### you can define a synchronous function

	domain.aggregateIdGenerator(function () {
	  var id = require('node-uuid').v4().toString();
	  return id;
	});

### or you can define an asynchronous function

	domain.aggregateIdGenerator(function (callback) {
	  setTimeout(function () {
	    var id = require('node-uuid').v4().toString();
	    callback(null, id);
	  }, 50);
	});


## Wire up events [optional]
### you can define a synchronous function

	// pass events to bus
	domain.onEvent(function (evt) {
	  bus.emit('event', evt);
	});

### or you can define an asynchronous function

	// pass events to bus
	domain.onEvent(function (evt, callback) {
	  bus.emit('event', evt, function ack () {
	    callback();
	  });
	});


## Initialization

	domain.init(function (err, warnings) {
	  // this callback is called when all is ready...
	  // warnings: if no warnings warnings is null, else it's an array containing errors during require of files
	});

	// or

	domain.init(); // callback is optional


## Handling a command

	domain.handle({
	  id: 'b80ade36-dd05-4340-8a8b-846eea6e286f',
	  name: 'enterNewPerson',
	  aggregate: {
	    id: '3b4d44b0-34fb-4ceb-b212-68fe7a7c2f70',
	    name: 'person'
	  },
	  context: {
	    name: 'hr'
	  },
	  payload: {
	    firstname: 'Jack',
	    lastname: 'Huston'
	  },
	  revision: 0,
	  version: 1,
	  meta: {
	    userId: 'ccd65819-4da4-4df9-9f24-5b10bf89ef89'
	  }
	}); // callback is optional

### or

	domain.handle({
	  id: 'b80ade36-dd05-4340-8a8b-846eea6e286f',
	  name: 'renamePerson',
	  aggregate: {
	    id: '3b4d44b0-34fb-4ceb-b212-68fe7a7c2f70',
	    name: 'person'
	  },
	  context: {
	    name: 'hr'
	  },
	  payload: {
	    firstname: 'Jack',
	    lastname: 'Huston'
	  },
	  revision: 0,
	  version: 1,
	  meta: {
	    userId: 'ccd65819-4da4-4df9-9f24-5b10bf89ef89'
	  }
	}, function (err) {
	  // this callback is called when command is handled successfully or unsuccessfully
	  // err can be of type:
	  // - null
	  // - Error
	  //   {
	  //     name: 'Error',
	  //     message: 'optional message'
	  //   }
	  // - ValidationError
	  //   {
	  //     name: 'ValidationError',
	  //     message: 'some message',
	  //     more: { /* more infos */ }
	  //   }
	  // - BusinessRuleError
	  //   {
	  //     name: 'BusinessRuleError',
	  //     message: 'some message',
	  //     more: { /* more infos */ }
	  //   }
	  // - AggregateDestroyedError
	  //   {
	  //     name: 'AggregateDestroyedError',
	  //     message: 'Aggregate has already been destroyed!',
	  //     more: {
	  //       aggregateId: 'ad10d2c0-6509-4cb0-86d2-76312d930001',
	  //       aggregateRevision: 6
	  //     }
	  //   }
	  // - AggregateConcurrencyError
	  //   {
	  //     name: 'AggregateConcurrencyError',
	  //     message: 'Actual revision in command is "3", but expected is "2"!',
	  //     more: {
	  //       aggregateId: 'ad10d2c0-6509-4cb0-86d2-76312d930001',
	  //       aggregateRevision: 2,
	  //       commandRevision: 3
	  //     }
	  //   }
	});

### more infos, can be useful if testing

	domain.handle({
	  id: 'b80ade36-dd05-4340-8a8b-846eea6e286f',
	  name: 'renamePerson',
	  aggregate: {
	    id: '3b4d44b0-34fb-4ceb-b212-68fe7a7c2f70',
	    name: 'person'
	  },
	  context: {
	    name: 'hr'
	  },
	  payload: {
	    firstname: 'Jack',
	    lastname: 'Huston'
	  },
	  revision: 0,
	  version: 1,
	  meta: {
	    userId: 'ccd65819-4da4-4df9-9f24-5b10bf89ef89'
	  }
	}, function (err, events, aggregateData, metaInfos) {
	  // this callback is called when command is handled successfully or unsuccessfully
	  // err: is the same as described before

	  // events: same as passed in 'onEvent' function
	  // events: in case of no error here is the array of all events that should be published
	  // events: in case of error are the one of these Errors (ValidationError, BusinessRuleError, AggregateDestroyedError, AggregateConcurrencyError)
	  // converted in an event with the event name defined in the options (default is 'commandRejected')

	  // aggregateData: represents the aggregateData after applying the resulting events

	  // metaInfos: { aggregateId: '3b4d44b0-34fb-4ceb-b212-68fe7a7c2f70', aggregate: 'person', context: 'context' }
	});


## Request domain information

After the initialization you can request the domain information:

	domain.init(function (err) {
	  domain.getInfo();
	  // ==>
	  // { contexts: [
	  //   {
	  //      "name": "hr",
	  //      "aggregates": [
	  //        {
	  //          "name": "person",
	  //          "version": 3,
	  //          "commands": [
	  //            {
	  //              "name": "enterNewPerson",
	  //              "version": 0,
	  //							"preconditions": [...]
	  //            },
	  //            {
	  //              "name": "unregisterAllContactInformation",
	  //              "version": 2,
	  //							"preconditions": [...]
	  //            },
	  //            {
	  //              "name": "unregisterAllContactInformation",
	  //              "version": 1,
	  //							"preconditions": [...]
	  //            }
	  //          ],
	  //          "events": [
	  //            {
	  //              "name": "enteredNewPerson",
	  //              "version": 3
	  //            },
	  //            {
	  //              "name": "enteredNewPerson",
	  //              "version": 0
	  //            },
	  //            {
	  //              "name": "enteredNewPerson",
	  //              "version": 2
	  //            },
	  //            {
	  //              "name": "unregisteredEMailAddress",
	  //              "version": 0
	  //            },
	  //            {
	  //              "name": "unregisteredPhoneNumber",
	  //              "version": 0
	  //            }
	  //          ],
	  //          "businessRules": [
	  //            {
	  //              "name": "atLeast1EMail",
	  //              "description": "at least one character should be in email address"
	  //            },
	  //            {
	  //              "name": "nameEquality",
	  //              "description": "firstname should never be equal lastname"
	  //            }
	  //          ]
	  //        }
	  //      ]
	  //   }
	  //]}
	});


# Components definition

## Context

	module.exports = require('cqrs-domain').defineContext({
	  // optional, default is the directory name
	  name: 'hr'
	});


## Aggregate

	module.exports = require('cqrs-domain').defineAggregate({
	  // optional, default is last part of path name
	  name: 'person',

	  // optional, default 0
	  version: 3,

	  // optional, default ''
	  defaultCommandPayload: 'payload',

	  // optional, default ''
	  defaultEventPayload: 'payload',

	  // optional, default ''
	  defaultPreConditionPayload: 'payload',

	  // optional, default false
	  // by skipping the history, only the last event will be loaded and defaultly not applyed (just to ensure the revision number increment)
	  skipHistory: true,

	  // optional, default false
	  // only optionally needed when skipHistory is set to true, only the last event will be loaded and applyed
	  applyLastEvent: true
	},

	// optionally, define some initialization data...
	{
	  emails: ['default@mycomp.org'],
	  phoneNumbers: []
	})

	// optionally, define snapshot need algorithm...
	.defineSnapshotNeed(function (loadingTime, events, aggregateData) {
	  // loadingTime is the loading time in ms of the eventstore data
	  // events are all loaded events in an array
	  // aggregateData represents the aggregateData after applying the resulting events
	  return events.length >= 200;
	})

	// optionally, define if snapshot should be ignored
	// if true, the whole event stream will be loaded
	.defineIgnoreSnapshot({
	  version: 0
	}, function (data) {
	  return true;
	})
	//.defineIgnoreSnapshot({
	//  version: 0
	//}, true)
	//.defineIgnoreSnapshot({
	//  version: 0
	//}) // default true

	// optionally, define conversion algorithm for older snapshots
	// always convert directly to newest version...
	// when loaded a snapshot and it's an older snapshot, a new snapshot with same revision but with newer aggregate version will be created
	.defineSnapshotConversion({
	  version: 1
	}, function (data, aggregate) {
	  // data is the snapshot data
	  // aggregate is the aggregate object

	  aggregate.set('emails', data.emails);
	  aggregate.set('phoneNumbers', data.phoneNumbers);

	  var names = data.name.split(' ');
	  aggregate.set('firstname', names[0]);
	  aggregate.set('lastname', names[1]);
	})
	// optionally, define idGenerator function for new aggregate ids
	// sync
	.defineAggregateIdGenerator(function () {
	  return require('node-uuid').v4().toString();
	});
	// or async
	.defineAggregateIdGenerator(function (callback) {
	  setTimeout(function () {
	    var id = require('node-uuid').v4().toString();
	    callback(null, id);
	  }, 50);
	});


## Command validation
All command schemas are json schemas. Hint [http://jsonary.com/documentation/json-schema/](http://jsonary.com/documentation/json-schema/)

Internally the [tv4](http://geraintluff.github.io/tv4/) module is used for validation. Additionaly you can extend the tv4 instance with other functionality like [tv4-formats](https://github.com/ikr/tv4-formats), so you can easily use format constraints (i.e. 'email') for your 'string'-types.
To extend tv4 just catch the validator before having initialized the domain:


    domain.extendValidator(function (validator) {

      // own formats
      validator.addFormat(require('tv4-formats'));
      validator.addFormat('mySpecialFormat', function (data) {
        if (data === 'special') {
          return null;
        }
        return 'wrong format for special';
      });

      // or other schemas
      validator.addSchema({ 'mySharedSchema'; { /* the schema json */ } });
      validator.addSchema('myOtherSharedSchema', { /* the schema json */ });

      // or replace the core valitator
      validator.validator(function (options, schema) {
        // options.schemas => all schemas
        // options.formats => all formats

        return function (cmdDataToValidate) {
          if (everythingIsOk) {
            return null;
          } else {
            return new require('cqrs-domain').errors.ValidationError('command not valid', { 'because': 'of this' });
          }
        };
      });

    });


Each command schema title should match the command name. Example: [enterNewPerson.json](https://github.com/adrai/node-cqrs-domain/blob/1.0/test/integration/fixture/set1/hr/person/validationRules/enterNewPerson.json)

To support multiple versions look at: [unregisterAllContactInformation.json](https://github.com/adrai/node-cqrs-domain/blob/v2.1.5/test/integration/fixture/set1/hr/person/validationRules/unregisterAllContactInformation.json#L10)

or: [unregisterAllContactInformation_v1.json](https://github.com/adrai/node-cqrs-domain/blob/v2.2.0/test/integration/fixture/set1/hr/person/validationRules/unregisterAllContactInformation_v1.json#L3)
[unregisterAllContactInformation_v2.json](https://github.com/adrai/node-cqrs-domain/blob/v2.2.0/test/integration/fixture/set1/hr/person/validationRules/unregisterAllContactInformation_v2.json#L3)


You can also have an hierarchical command extension look at:

- [command](https://github.com/adrai/node-cqrs-domain/blob/1.0/test/integration/fixture/set1/hr/person/validationRules/enterNewPerson.json)
- [aggregate](https://github.com/adrai/node-cqrs-domain/blob/1.0/test/integration/fixture/set1/hr/person/command.json)
- [context](https://github.com/adrai/node-cqrs-domain/blob/1.0/test/integration/fixture/set1/hr/command.json)
- [general](https://github.com/adrai/node-cqrs-domain/blob/1.0/test/integration/fixture/set1/command.json)

## Pre-Load-Condition
Can be used to perform some business rules before handling the command. Contrary to Pre-Conditions, these rules are applied BEFORE the aggregate is loaded.

This allows you to (for example) run checks against external information by using closures.

> **Tip:** Pre-load conditions are especially useful when you have checks that you want to run on an aggregate, but when it is OK for those checks to run on potentially stale data (eg a view model). By doing these checks before the aggregate is locked, you avoid creating a locking bottleneck at the aggregate level, and can keep your aggregate smaller because the information for those checks is externalized to the domain. This helps for performance if the domain you are in is performance critical, and helps you keep focus on the real, strongly consistent invariants in your domain.

A Command can have multiple pre-load-conditions.

    var externalDataLoader = require('some_file');

	module.exports = require('cqrs-domain').definePreLoadCondition({
	  // the command name
	  // optional, default is file name without extension,
	  // if name is '' it will handle all commands that matches the appropriate aggregate
	  // if name is an array of strings it will handle all commands that matches the appropriate name
	  name: 'checkSomeViewModel',

	  // optional, default 0
	  version: 2,

	  // optional, if not defined it will use what is defined as default in aggregate or pass the whole command
	  payload: 'payload',

	  // optional
	  description: 'firstname should always be set',

	  // optional, default Infinity, all pre-conditions will be sorted by this value
	  priority: 1
	}, function (data, callback) {
	  // data is the command data
	  // callback is optional, if not defined as function argument you can throw errors or return errors here (sync way)

      if (externalDataLoader.get(data.id) !== data.value) {
        return callback('not allowed');
        // or
        // return callback(new Error('not allowed'));
        // or
        // return callback(new Error()); // if no error message is defined then the description will be taken
        // or
        // return callback(new require('cqrs-domain').BusinessRuleError('not allowed', { /* more infos */ }));
      }

	  callback(null);

	  // or if callback is not defined as function argument
	  // if (externalDataLoader.get(data.id) !== data.value)
    //   return 'not allowed';
    //   // or
    //   // return new Error('not allowed');
    //   // or
    //   // return new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error('not allowed');
    //   // or
    //   // throw new require('cqrs-domain').BusinessRuleError('not allowed', { /* more infos */ });
    // }
	});


## Pre-Condition
Can be used to perform some business rules before handling the command. The aggregate is locked and loaded before the pre-condition is applied.

A Command can have multiple pre-conditions.

	module.exports = require('cqrs-domain').definePreCondition({
	  // the command name
	  // optional, default is file name without extension,
	  // if name is '' it will handle all commands that matches the appropriate aggregate
	  // if name is an array of strings it will handle all commands that matches the appropriate name
	  name: 'unregisterAllContactInformation',

	  // optional, default 0
	  version: 2,

	  // optional, if not defined it will use what is defined as default in aggregate or pass the whole command
	  payload: 'payload',

	  // optional
	  description: 'firstname should always be set',

	  // optional, default Infinity, all pre-conditions will be sorted by this value
	  priority: 1
	}, function (data, aggregate, callback) {
	  // data is the command data
	  // aggregate is the aggregate object
	  // callback is optional, if not defined as function argument you can throw errors or return errors here (sync way)

	  if (!agg.has('firstname')) {
	    return callback('not personalized');
	    // or
	    // return callback(new Error('not personalized'));
	    // or
	    // return callback(new Error()); // if no error message is defined then the description will be taken
	    // or
	    // return callback(new require('cqrs-domain').BusinessRuleError('not personalized', { /* more infos */ }));
	  }
	  callback(null);

	  // or if callback is not defined as function argument
	  // if (!agg.has('firstname')) {
    //   return 'not personalized';
    //   // or
    //   // return new Error('not personalized');
    //   // or
    //   // return new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error('not personalized');
    //   // or
    //   // throw new require('cqrs-domain').BusinessRuleError('not personalized', { /* more infos */ });
    // }
	});


## Command
Collect all needed infos from aggregate to generate your event(s).

Move checks out of here, the correct places are "business rules", "pre-conditions" or "pre-load consitions"!

Do NOT manipulate the aggregate here!

	module.exports = require('cqrs-domain').defineCommand({
	  // optional, default is file name without extension
	  name: 'enterNewPerson',

	  // optional, default 0
	  version: 1,

	  // optional, if not defined it will use what is defined as default in aggregate or pass the whole command
	  payload: 'payload',

	  // optional, default undefined
	  // if true, ensures the aggregate to exists already before this command was handled
	  // if false, ensures the aggregate to not exists already before this command was handled
	  existing: true
	}, function (data, aggregate) {
	  // data is the command data
	  // aggregate is the aggregate object

	  // if (aggregate.get('someAttr') === 'someValue' && aggregate.has('special')) { ... }

	  aggregate.apply('enteredNewPerson', data);
	  // or
	  // aggregate.apply('enteredNewPerson', data, version);
	  // or
	  // aggregate.apply({
	  //   event: 'enteredNewPerson',
	  //   payload: data
	  // });
	})

	// if defined it will load all the requested event streams
	// useful if making bigger redesigns in domain and you need to handle a command on a new aggregate
	.defineEventStreamsToLoad(function (cmd) {
	  return [{ // order is new to old
	    context: 'hr',
	    aggregate: 'mails',
	    aggregateId: cmd.meta.newAggId
	  },{
	    context: 'hr',
	    aggregate: 'persons',
	    aggregateId: cmd.aggregate.id
	  }];
	});

## Event
This is the place where you should manipulate your aggregate.

	module.exports = require('cqrs-domain').defineEvent({
	  // optional, default is file name without extension
	  name: 'enteredNewPerson',

	  // optional, default 0
	  version: 3,

	  // optional, if not defined it will use what is defined as default in aggregate or pass the whole event...
	  payload: 'payload'
	},
	// passing a function is optional
	function (data, aggregate) {
	  // data is the event data
	  // aggregate is the aggregate object

	  aggregate.set('firstname', data.firstname);
	  aggregate.set('lastname', data.lastname);
	  // or
	  // aggregate.set(data);
	});


## Business Rule

	module.exports = require('cqrs-domain').defineBusinessRule({
	  // optional, default is file name without extension
	  name: 'nameEquality',

	  // optional
	  description: 'firstname should never be equal lastname',

	  // optional, default Infinity, all business rules will be sorted by this value
	  priority: 1
	}, function (changed, previous, events, command, callback) {
	  // changed is the new aggregate object
	  // previous is the old aggregate object
	  // events is the array with the resulting events
	  // command the handling command
	  // callback is optional, if not defined as function argument you can throw errors or return errors here (sync way)

	  if (changed.get('firstname') === changed.get('lastname')) {
	    return callback('names not valid');
	    // or
	    // return callback(new Error('names not valid'));
	    // or
	    // return callback(new Error()); // if no error message is defined then the description will be taken
	    // or
	    // return callback(new require('cqrs-domain').BusinessRuleError('names not valid', { /* more infos */ }));
	  }
	  callback(null);

	  // or if callback is not defined as function argument
	  // if (changed.get('firstname') === changed.get('lastname')) {
    //   return 'names not valid';
    //   // or
    //   // return new Error('names not valid');
    //   // or
    //   // return new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error(); // if no error message is defined then the description will be taken
    //   // or
    //   // throw new Error('names not valid');
    //   // or
    //   // throw new require('cqrs-domain').BusinessRuleError('names not valid', { /* more infos */ });
    // }
	});


## Command Handler (Be careful!!!)
Is your use case not solvable without a custom command handling? Sagas? Micro-Services?

	module.exports = require('cqrs-domain').defineCommandHandler({
	  // optional, default is file name without extension
	  name: 'enterNewSpecialPerson',

	  // optional, default 0
	  version: 1,

	  // optional, if not defined it will pass the whole command...
	  payload: 'payload'
	}, function (aggId, cmd, commandHandler, callback) {
	  // aggId is the aggregate id
	  // cmd is the command data

	  commandHandler.loadAggregate(aggId, function (err, aggregate, stream) {
	    if (err) {
	      return callback(err);
	    }

	    callback(null, [{ my: 'special', ev: 'ent' }]);

	//    // check if destroyed, check revision, validate command
	//    var err = commandHandler.verifyAggregate(aggregate, cmd);
	//    if (err) {
	//      return callback(err);
	//    }
	//
	//    // call api or emit a command or whatever...
	//    // and at the end perhaps you call: commandHandler.handle(cmd, callback);
	  });
	});


[Release notes](https://github.com/adrai/node-cqrs-domain/blob/master/releasenotes.md)

# License

Copyright (c) 2016 Adriano Raiano

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.