mbeckerle commented on code in PR #90: URL: https://github.com/apache/daffodil-site/pull/90#discussion_r889048489
########## site/extensions.adoc: ########## @@ -0,0 +1,60 @@ +:page-layout: page +:url-asciidoctor: http://asciidoctor.org +:keywords: plugins layering UDF charset +// /////////////////////////////////////////////////////////////////////////// +// +// This file is written in AsciiDoc. +// +// If you can read this comment, your browser is not rendering asciidoc automatically. +// +// You need to install the asciidoc plugin to Chrome or Firefox +// so that this page will be properly rendered for your viewing pleasure. +// +// You can get the plugins by searching the web for 'asciidoc plugin' +// +// You will want to change plugin settings to enable diagrams (they're off by default.) +// +// You need to view this page with Chrome or Firefox. +// +// /////////////////////////////////////////////////////////////////////////// +// +// When editing, please start each sentence on a new line. +// See https://asciidoctor.org/docs/asciidoc-recommended-practices/#one-sentence-per-line[one sentence-per-line writing technique.] +// This makes textual diffs of this file useful in a similar way to the way they work for code. +// +// ////////////////////////////////////////////////////////////////////////// + += DFDL Language Extensions in Daffodil + +Daffodil contains numerous extensions to the DFDL v1.0 language. + +Many of these have been, or will be proposed for inclusion in a future version of the DFDL standard. + +This page provides a central starting point for the documentation of these extension features. + +== About Daffodil Plugins + +To provide some new advanced format capabilities such as checksums, compressed or encoded data regions, and user-defined-functions, DFDL schemas sometimes must use Daffodil-specific extensions and incorporate Daffodil plugins that provide the small algorithmic aspects needed by these formats. + +There are 2 kinds of plugins today supported by Daffodil 3.3.0 + +- Layering Transformer (e.g., unzip/zip, verify/recompute checksums) +- User Defined Function (UDF) (e.g., convert mean-sea-level elevation to height-above-ellipsoid) + +There is one additional kind of plugin that will be supported by Daffodil 3.4.0 + +- Character Set Definitions (e.g., a specific 5-bit charset used only by a certain format) + +One needs to think of plugins as being part of the DFDL schema of a format, not part of Daffodil. + +Different DFDL schemas for different kinds of data will need their own such plugins. +Hence the plugins, like the DFDL schema files themselves, are used in applications as part of a specific data-processing flow. + +Keeping in the spirit of DFDL in describing a format declaratively, plugins need to be very small pieces of code (ex: a character set definition should be 10 lines of code.) + +Plugins are compiled from Java/Scala code and would commonly be packaged into a jar file which may or may not also contain the DFDL schema files. +The loading of the plugin is enabled using a standard Java technique for class loading where a special META-INF file identifies the jar as containing a particular type of plug-in. + +Configuring an application must put these jar files on the CLASSPATH so that the executing instance of Daffodil for a specific configured data processing flow finds them on the class path for the data format(s) that flow is processing. + +For greater assurance/trust, the plugin jars could be digitally signed by their creators, and applications could verify these signatures (using public keys) as a startup condition. Review Comment: Yes. Short term these might lead off to existing wiki pages on confluence, this page being more of just an index to start with. But eventually yes real doc. Plugins may have javadoc/scaladoc to reference as well. I also want to refer to java code examples (currently on OpenDFDL, probably need to come back to an Apache Daffodil examples repo) so people can see the managed dependencies between schemas working by all the schemas transitively showing up in lib_managed directory, etc. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
