Creating a draft version of Basic Concepts section for further review…

… and edits (see documentation PR #9).

concepts/aggregate.md

@@ -0,0 +1,26 @@
+---
+title: Aggregate Concept
+headline: Event Sourcing Concepts
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Aggregate</h2> 
+
+Aggregate is the main building block of a business model. 
+From the application point of view it consists of the following:
+1. Commands which arrive to it. 
+2. Events which appear in response to these commands. 
+3. How these events influence the state of an aggregate.
+
+[Aggregates](http://martinfowler.com/bliki/DDD_Aggregate.html) guarantee consistency of data modifications in response to commands they receive.
+
+Aggregate is the most common case of Command Handler. It modifies its state and produces one or more events in response to a command. These events are used later to restore the state of the aggregate.
+
+In Spine, aggregates are defined as [defined as Java classes](/java/aggregate.md), and their states are [defined as protobuf messages](/biz-model/aggregate-states.md).
+
+
+
+
+

concepts/command-handler.md

@@ -0,0 +1,14 @@
+---
+title: Command Handler Concept
+headline: Command Handler
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Command Handler</h2> 
+
+Command Handler is an object which receives commands, modifies the state of the application, and generates events if the modification was successful. 
+
+For more details, refer to [Java](/java/index.md) section.
+

concepts/commands.md

@@ -0,0 +1,17 @@
+---
+title: Commands Concept
+headline: Commands
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Commands</h2> 
+
+Commands are messages that instruct an entity within Spine framework to perform a certain action. Compared with events, command is not a statement of fact. They are a request, and, thus, can be refused. 
+
+A typical way to convey refusal is to throw an error or failure. 
+
+In Spine, commands are defined as a Protocol buffer messages.
+
+

concepts/event-bus.md

@@ -0,0 +1,12 @@
+---
+title: Event Bus Concept
+headline: Event Bus
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Event Bus</h2> 
+
+Event Bus dispatches incoming events to handlers and provides ways for registering those handlers.
+

concepts/events.md

@@ -0,0 +1,13 @@
+---
+title: Events Concept
+headline: Events
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Events</h2> 
+
+Event is something that happened in the past. All changes to an application state are captured as a sequence of events. Events are the main “database” of the application. 
+
+In Spine, events are defined as Protocol buffer messages. 

concepts/failure.md

@@ -0,0 +1,18 @@
+---
+title: Failure Concept
+headline: Failure
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Failure</h2> 
+
+Failure is a state of business logic which can be handled by the end user. 
+Consider the following examples of  failures: 
+* “credit card validation declined”, 
+* “order cannot be empty”, 
+* “insufficient funds”.
+
+In Spine, failures are defined as Protocol buffer messages. 
+

concepts/identifiers.md

@@ -0,0 +1,18 @@
+---
+title: Identifiers Concept
+headline: Identifier
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Identifiers</h2> 
+
+Spine framework supports the following types of identifiers:
+
+* Integer,
+* Long,
+* String,
+* A generated Java class implementing the `Message`.
+
+We recommend you to select Identifiers for your IDs and encourage using message-based IDs to avoid possible performance issues. 

concepts/index.md

@@ -0,0 +1,20 @@
+---
+title: Basic Concepts Overview 
+headline: Spine Basic Concepts
+bodyclass: docs
+layout: docs
+type: markdown
+sidenav: doc-side-concepts-nav.html
+---
+<h2 class="top">Spine Basic Concepts</h2>
+
+<div id="toc" class="toc hide-block"></div>
+This section introduces key concepts of Spine framework and its parts, helps you get a deeper understanding of how it works and provides some usage notes when creating Spine-based cloud application. 
+
+We assume that you are familiar with the basics of Domain-Driven Design. 
+
+Below you can find a typical Spine Event Engine application architecture; its building blocks are described below.
+
+
+
+

concepts/process-manager.md

@@ -0,0 +1,17 @@
+---
+title: Process Manager Concept
+headline: Process Manager
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Process Manager</h2> 
+
+Process Manager is an independent component that reacts to domain events in a cross-aggregate eventually consistent manner. It serves as a centralized processing unit that maintains the state sequence and defines the next processing step based on intermediate results. 
+
+[Process manager](/java/process-manager.md) can be both Command Handler and Event Handler.
+
+Note: A constructor for  Process Manager can have no parameter at all.  
+
+Tip: We recommend to use process ID when integrating with side services. 

concepts/projection.md

@@ -0,0 +1,13 @@
+---
+title: Projection Concept
+headline: Projection
+bodyclass: docs
+layout: docs
+sidenav: doc-side-concepts-nav.html
+type: markdown
+---
+<h2 class="top">Projection</h2> 
+
+Projection is an Event Handler which transforms multiple events data into a structural representation.
+
+Projections are the main building blocks of the Query side of the application.