bsloane1650 commented on a change in pull request #17: Design notes on schema compiler space/speed issue URL: https://github.com/apache/incubator-daffodil-site/pull/17#discussion_r375333404
########## File path: site/dev/design-notes/term-sharing-in-schema-compiler.adoc ########## @@ -0,0 +1,562 @@ +:page-layout: page +:keywords: schema-compiler performance alignment optimization +// /////////////////////////////////////////////////////////////////////////// +// +// 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.] +// It is acceptable to start each sentence on a new line, but then wrap the lines to a reasonable length. +// It's the starting on a new line that matters. +// +// This makes textual diffs of this file useful in a similar way to the way they work for code. +// +// ////////////////////////////////////////////////////////////////////////// + +== Term Sharing in the Schema Compiler + +=== Introduction + +The DFDL language has a composition property known as _referential transparency_. +It is not supposed to matter whether you lift a part of the schema out +and create a reusable group, reusable type, or reusable element from +it. + +Because DFDL (version 1.0) does not allow recursive definitions of any +kind, it is theoretically possible to implement this by treating all +reusable definitions in the language like macros. +I.e., inline-expand all definitions at their point of use. +This will work for schemas up to some size. However, it leads to an +unacceptable expansion in schema compilation time and space, as the +size of the schema once all these inline substitutions have been +performed can be exponentially larger than the original +non-substituted schema. + +To avoid this undesirable combinatorial explosion, the Daffodil schema +compiler arranges to achieve the same macro-like semantics, while +still sharing the core parts of the reusable components so they need +only be compiled once for each unique way the component is used. + +This is also part of eventually enabling an extension of the DFDL +language to allow recursive definitions. + +The dfdl:alignment property is one of the largest contributors to +complexity in sharing objects by the schema compiler. + +Alignment will be used as the example in this design note to +illustrate the schema compiler behavior. + +=== DSOM Term Objects + +DSOM is the Daffodil Schema Object Model. +Within this model, the entities from a DFDL schema that actually have +representations in the data stream are called _terms_ and they are +represented by the subclasses of the class Term. +There are terms which have no representation at all such as computed elements. + +For this discussion we are concerned only with terms that have representation. + +The classes in Daffodil that are used for concrete terms are: Review comment: "concrete term" is never defined. ---------------------------------------------------------------- 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. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services
