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+# iModels for Developers
+
+This document is an attempt to give a quick overview of iModels from a developer perspective and to provide links to more detailed documentation and actual code.
+
+## Overview
+
+### The Data
+
+iModels are designed to store BIM/CAD Engineering data in a format that is ideal for editing applications.  It standardizes the data stored so common concepts are represented the same way across different editing applications. This standardization is accomplished using a 'conceptual schema' called [BIS](https://www.itwinjs.org/bis/guide/intro/overview/) that expresses taxonomy, data structure and relationships for modeling real-world Entities.
+
+### The File
+
+Under the hood, an iModel is a standard SQLite file.  Two standard SQLite extensions are used to create a change history and ease working with large files.
+
+- The [Session Extension](https://sqlite.org/sessionintro.html) to record changes to the database and package them into 'changesets'
+- The [Cloud Backed SQLite VFS](https://sqlite.org/cloudsqlite/doc/trunk/www/index.wiki) to allow access to the database without first downloading the entire database.
+
+### The Tables
+
+While iModels physically store data using a SQL persistence schema, they expose that data as BIS entities. The ECSQL language allows you to use SQL syntax to query data in an iModel based on its conceptual schema rather than in terms of the underlying database structure. The data tables in an iModel are defined by the imported BIS schemas. Additionally, the database includes metadata tables that store these schemas and define how the persistence layer maps to the logical layer.
+
+
+### Beyond the file
+
+iModels are stored as a seed SQLite file plus a sequence of SQLite changesets making a linear and immutable change history. Thus, it is always possible to recreate a version of an iModel by applying changesets in order up to the revision desired.
+
+The change history can be managed via the [iModels API](https://developer.bentley.com/apis/imodels-v2/overview/) and enables operations like [Forks](https://developer.bentley.com/apis/imodels-v2/operations/fork-imodel/) and [Clones](https://developer.bentley.com/apis/imodels-v2/operations/clone-imodel/).
+
+### Editing
+
+It is possible to edit an iModel as a standalone SQLite file and produce no change history.  To produce a change history, the iModel must be managed by the iModel Hub [using the iModel APIs mentioned above](https://developer.bentley.com/apis/imodels-v2/overview/).  When managed by the Hub, the user is able to checkout a [**Briefcase**](https://developer.bentley.com/apis/imodels-v2/operations/acquire-imodel-briefcase/) and use [**Locks**](https://developer.bentley.com/apis/imodels-v2/operations/get-imodel-locks/) on elements to avoid data conflicts.
+
+These locks are optional, and enable conflict-free editing by restricting who can modify an Element and its children. Briefcases ensure Element ids are unique by appending a briefcase id to the sequential id specified for the next element.
+
+As multiple users edit an iModel, they will push changesets with their changes and pull in changesets with others' changes. A briefcase must be at the tip of the change history to push a changeset. If it is not, incoming changesets need to be pulled, and local changes are rebased on top of them. Once conflicts are resolved, the local changes are pushed as one or more changesets.
+
+Forks create an independent copy of the change history, creating an iModel which can be edited independently from the source iModel. Changes must be merged back using [iModel Transformation](https://www.itwinjs.org/learning/transformer/) which works at the BIS Element level rather than with SQLite changesets.
+
+### BIS Basics
+
+[Base Infrastructure Schemas](https://github.com/itwin/bis-schemas) (BIS) is an open source ecosystem of modular and coordinated schemas.  The [BisCore](https://www.itwinjs.org/bis/domains/biscore.ecschema/) schema defines the iModel file format, and all other schemas derive from the base classes BisCore defines.  BisCore defines [**Elements**](https://www.itwinjs.org/bis/guide/fundamentals/element-fundamentals/) which store all data and [**Models**](https://www.itwinjs.org/bis/guide/fundamentals/model-fundamentals/) which contain all Elements.  Elements may also be extended via [**ElementAspects**](https://www.itwinjs.org/bis/guide/fundamentals/elementaspect-fundamentals/) which are considered conceptually to be part of the Element.  Individual Elements are connected via [**Relationships**](https://www.itwinjs.org/bis/guide/fundamentals/relationship-fundamentals/) to build assemblies, systems, networks, hierarchies or other arbitrary connections.
+
+BIS enables modeling real world objects from multiple [**Perspectives**](https://www.itwinjs.org/bis/guide/intro/modeling-with-bis/). Different Elements are used to represent the real world object in each perspective. For example, a pump can have a **functional perspective** defining the requirements of a functional pump (e.g. flow rate or pressure), a **physical perspective** defining the pump location in space (e.g. location, geometry, materials), and an **analytical perspective** defining the attributes needed for flow simulation.
+
+### Code Basics
+
+The [ECDb](https://github.com/iTwin/imodel-native/tree/main/iModelCore/ECDb) library underpins the iModel file format and uses [ECSchemas](https://www.itwinjs.org/bis/ec/) as its data modeling language.  ECSchemas can be represented in [XML](https://github.com/iTwin/bis-schemas/blob/master/System/xsd/ECSchemaXML3.2.xsd) or [JSON](https://github.com/iTwin/bis-schemas/blob/master/System/json_schema/ec32/ecschema.schema.json) and loaded using TypeScript ([ecschema-metadata](https://github.com/iTwin/itwinjs-core/tree/master/core/ecschema-metadata)) or C++ ([ECObjects](https://github.com/iTwin/imodel-native/tree/main/iModelCore/ecobjects)) libraries.  The C++ [iModelPlatform](https://github.com/iTwin/imodel-native/tree/main/iModelCore/iModelPlatform) layer adds basic logic that understands and enforces BisCore concepts.  This internal API is exposed to TypeScript via a [NAPI interface](https://github.com/iTwin/imodel-native/tree/main/iModelCore/iModelPlatform) consumed by the iTwin.js [core-backend](https://github.com/iTwin/itwinjs-core/tree/master/core/backend) package.
+