SourceItem Interface

A SourceItem identifies an entity in an external source and the state of the content of the entity. The data provided by the SourceItem is stored in the iModel as the "provenance" of the entity. This provenance data enables users and applications to trace an element in an iModel back to its native, external source. Provenance also enables a connector to track and synchronize changes to the entity.

Document, external source, and scope

The "document" is the data source that is being read by the connector. This could be a file, a managed document, or a database connection. A document is represented in the iModel by a RepositoryLink element. The "external source" represents either the document as a whole or an area within the document. (Also see ExternalSourceGroup.) The external source is represented by an ExternalSource element. See https://www.itwinjs.org/learning/provenence-in-imodels/. The "scope" identifies either the ExternalSource or an area within in.

Kind

A value that the connector uses to classify the entity.

Id

The ID that was assigned to the entity by the external system that creates and manages it. This ID must be stable -- a given entity must have the same ID each time it is synchronized.

Entity identification

The (kind, scope, id) triple must uniquely identify the entity in the outside world. This triple is what allows the iModel to track the entity.

Entity change detection

The version and/or checksum values capture the state of the content of the entity. These values are stored in the iModel. They allow the Synchronizer to detect if the entity has changed since the last synchronization. See below for details on change-detection.

Persistence

The properties of a SourceItem are stored in the ExternalSourceAspect of the Element in the iModel that represents the entity. The ExternalSourceAspect tracks the external source of the element. https://www.itwinjs.org/bis/domains/biscore.ecschema/#externalsourceaspect

Change-detection using version and checksum

The connector must call Synchronizer.detectChanges on each entity to determine if there are changes that must be written to the iModel or not.

The SourceItem's version and/or checksum values must capture the entity's current state in the external source. The ExternalSourceAspect records the state of the entity as of the last synchronization.

Synchronizer.detectChanges compares the SourceItem's current state with the stored state recorded by the aspect. If they match, the entity is unchanged and should be skipped. If they do not match, the entity has changed, and the connector should convert the entity and then call Synchronizer.updateIModel.

Change-detection is based on version and/or checksum values that capture the state of the content of each entity in a summary form. In principle, Synchronizer could detect changes by letting the connector convert an entity to BIS format and then comparing the results with the Elements and Aspects currently in the iModel. That would be a waste of time in the common case where most entities are unchanged. To avoid this, Synchronizer uses version and/or checksum values instead. This approach minimizes the cost of up-front change-detection. The connector can compute these summary values directly from the raw source data, without using its conversion logic. One or both of these values may even be directly available in the source data.

A SourceItem must have a version or a checksum, and it can have both.

1. If a SourceItem defines both version and checksum, then the entity is considered to be unchanged if the version is unchanged. If the version has changed, then the checksum is checked.
2. If a SourceItem defines version only, the entity is considered to have changed if the item's version has changed.
3. If a SourceItem defines checksum only, the entity is considered to have changed if the item's checksum has changed.

Methods

Name	Description
checksum(): undefined | string Optional	The optional cryptographic hash (any algorithm) of the source entity's content.

Properties

Name	Type	Description
id	string	The unique identity of the entity (relative to its scope and kind) in the external source.
kind	string | undefined	Indicates what kind of thing this is in the external source.
scope	string | undefined	Identifies an element in the iModel that represents an area of the external source.
source	string | undefined
version	string | undefined	An optional value that is typically a version number or a pseudo version number like last modified time.

Defined in

• Synchronizer.ts Line 96