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This document describes a proposed API for the Chandler sharing framework to allow individual parcels to support backward and forward-compatible sharing, even when their domain model changes between parcel versions and the clients doing the sharing do not all have the same version of the parcel installed.

The proposed API does this by allowing parcel developers to specify "sharing schemas" for their items. A sharing schema is a kind of logical transmission format, that breaks items down into simple records containing elementary data types that are easy to store or transmit for use by other programs.

Sharing schemas defined using this API will also be used to implement "dump and reload" including schema evolution during upgrades or downgrades. As a parcel's item schema changes, its sharing schema(s) must be modified so that data produced by previous versions of the parcel can still be imported. A parcel can also optionally provide support for exporting data in such a way that it can be read by older versions.

Typically, a parcel will provide its own sharing schema for the Kinds and Annotations it contains. However, it's also possible for a parcel to define one or more sharing schemas for other parcels that it depends on.

Parcel developers define a sharing schema by defining one or more record types (using the @sharing.recordtype decorator), and one or more sharing.Schema subclasses. The record types define the format of the data to be shared, and the sharing.Schema classes provide code that convert items to records and vice versa. The sharing.Schema base class will provide many utility methods to automatically handle common mapping patterns, so that most schemas will include relatively little code.