Daffodil Developer Guide

This guide is for a software engineer who wants to develop Daffodil code. This guide will help you become familiar with DFDL schemas, DFDL processors, and Daffodil development including its code style, code workflow, development environment, directory organization, documentation, and tests.


The Data Format Description Language (DFDL) is a language used to describe almost all data formats both logically and physically. DFDL is not a data format or a procedural language; rather, it is a data modeling language based on a subset of XML Schema annotated with DFDL properties describing the representation and layout of each element of the schema inside a native text or binary data format. A DFDL schema allows data to be converted between its native data format (physical representation, also called a text or binary file) and a DFDL information set (logical representation, also called an infoset) such as an XML document, EXI document, JSON document, SAX callbacks, or several document object model APIs in memory (JDOM, Scala Node, W3C DOM). When you have a DFDL schema for a native data format, you can pick whichever infoset type is easiest for you or your application to use and tell a DFDL processor to read (“parse”) a text or binary file from its native data format to that infoset type. You or your application can do whatever you need to do with the infoset and then you can use the same DFDL schema and DFDL processor to write (“unparse”) the infoset back to its native text or binary file format again, completing a round trip from native data to infoset to native data again.

Using DFDL avoids inventing a completely new data modeling language, avoids writing any parsing and serialization code (with all the bugs that normally arise from implementing such code procedurally), and makes it much easier to convert any native data format to an infoset, operate on the infoset, and convert an infoset back to its native data format again.

To learn more about DFDL, you can watch two short videos put together by Steve Hanson, co-chair of the DFDL working group, read a slideshow written by Mike Beckerle, co-chair of the DFDL working group, or go through some tutorials written by Roger Costello, chair of the DFTVL working group (Data Format Transformation and Validation Language, a future not-yet-defined language to specify policies for Cross-Domain Solution (CDS) devices).

Apache Daffodil, IBM DFDL, and ESA DFDL4S

The standards organization in which DFDL started, the Open Grid Forum, required 2 implementations in order to move forward with the standardization process. This means that there are two leading DFDL processors, a commercial implementation bundled into IBM Integration Bus called IBM DFDL and an open source implementation hosted by the Apache Software Foundation (ASF) called Apache Daffodil. The European Space Agency also has created a proprietary implementation called ESA DFDL4S, which can be used only with their satellite communication formats and is provided only in the form of binary libraries, not source code.

Among these three DFDL processors, Apache Daffodil is considered the most modern and thorough implementation of the Data Format Description Language Specification. Even so, Apache Daffodil lists some unsupported features of the DFDL specification. IBM DFDL lists some more unsupported features and also lists some implementation-specific limits. These limitations will not prevent you from writing DFDL schemas for almost all data formats, but they will reveal which parts of the DFDL specification are rarely used.

Daffodil Development

The Apache Software Foundation hosts the Apache Daffodil project on the following ASF infrastructure:

  • Daffodil's issue tracker is hosted on JIRA
  • Daffodil's users, dev, and commits mailing list archives are hosted on Apache Pony Mail (subscription is required to send to these lists)
  • Daffodil's source code is hosted on GitHub
  • Daffodil's wiki is hosted on Confluence
  • Daffodil's website is hosted on Apache with static content generated by Jekyll using jekyll-asciidoc and asciidoctor-diagram plugins

A good Daffodil developer gets an account on the Apache Software Foundation‘s JIRA and Confluence servers, subscribes to all three Daffodil mailing lists, and forks Daffodil’s source code repositories to the developer's GitHub account.

Only Daffodil developers given “committer” permission by the Daffodil Project Management Committee have direct write access to Daffodil‘s source code repositories. New Daffodil contributors have to fork the Daffodil source repository to their own GitHub account and then clone their working copy from their fork as described in the Code Contributor Workflow, which has all the steps you need to make a successful code contribution to Daffodil. The same workflow applies to Apache Daffodil’s other two source code repositories, that is, always fork these repositories and clone from your forks before you make any changes to the Daffodil VS Code Extension or Daffodil Website files.

The following advice is for Daffodil committers who do have write access to Daffodil's source repositories. If you clone an Apache repository directly to your computer, it is very important you maintain that clone as a reading copy clearly separated from any clone of your forked repository. The only commands you should ever run in your reading copy are git pull, git log ORIG_HEAD..HEAD, and git diff ORIG_HEAD..HEAD in order to see what changes have been made by other developers since your last pull from the Apache repository. Your reading copy should remain an exact copy of the Apache source code repository at all times which you can use for reading or occasionally running diff commands between your reading copy and working copy. To be safe, rename your reading copy from daffodil to daffodil-asf and never edit files or run sbt in it so there will be nothing to push to the Apache repository even if you accidentally run git push from your reading copy.

Code Style

Daffodil code is formatted using Scalafmt. Run sbt scalafmtCheckAll to see if changes to the source code require formatting and then run sbt scalafmtAll to apply the changes. For SBT files use sbt scalafmtSbtCheck and sbt scalafmtSbt.

Daffodil mandates that at least 80% of new code and modified code should be covered by unit tests or TDML tests. Daffodil naming conventions and more are covered in the Confluence pages Code Style Guidelines and Coding for Performance.

Code Workflow

The Code Contributor Workflow has all the steps you need to make a successful code contribution to Daffodil. Here are some more things you should know too.

The Daffodil developers want each pull request to add only one commit to the Apache repository to keep its git history as readable and bisectable as possible. Your pull request should fix a JIRA issue in its entirety (including test cases to cover any modified or new lines) but it should not fix 2 or more unrelated JIRA issues in the same commit either. Your pull request should start with only one commit in it before you ask the developers to review it and your pull request should end with only one commit in it after you receive enough reviewer approvals (two +1‘s for Daffodil and Daffodil VS Code Extension, one +1 for Daffodil Website). If you have received your reviewers’ approval but you have made multiple commits to your pull request due to reviewer suggestions (which is the usual case), you must rebase and squash your pull request back to one commit before a Daffodil committer can merge your code into any of the upstream Apache repositories.

Before you change Daffodil code, you will need to know how to use JIRA as well as GitHub. You will assign a JIRA issue to yourself before you start working on it to avoid redundant effort in case another developer decides to work on the same issue. Then you will mark the issue as resolved after your pull request is merged. Here are the steps you need to perform in JIRA to resolve your issue after your pull request is merged (please don't forget these steps):

  1. Click “Resolve Issue”
  2. Click the “Fix Version/s” dropdown and mark the issue as fixed in the currently unreleased version
  3. Paste “Fixed in commit xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx” into the comment field (replacing the xxx's with the correct hash)
  4. Press the “Resolve” button

That‘s it. You can expect someone else to close the JIRA issue so you don’t need to, and should not, close the issue yourself. Just make sure your pull request has tests if appropriate so that someone can look at your pull request and verify that it resolves the JIRA issue.

Development Environment

You will find build instructions in BUILD.md and README.md. You will find more instructions how to set up your development environment in the Confluence pages Java Setup and IntelliJ IDEA Setup.

Directory Organization

Daffodil is composed of one top-level project and numerous subdirectory-level modules (some of these modules contain only generators, macros, tests and tutorials). Daffodil's source code repository is organized as follows:

├── .asf.yaml                   - Enables/disables parts of GitHub's UI
├── .gitattributes              - Excludes KEY from source releases
├── .gitignore                  - Ignores auto-generated files from source control
├── .sbtopts                    - Tells sbt to use 4GB of memory
├── BUILD.md                    - Lists Daffodil's build requirements
├── KEYS                        - Lists public keys of Daffodil developers
├── LICENSE                     - Licenses Daffodil under the Apache License
├── NOTICE                      - Lists notices affecting LICENSE
├── README.md                   - Explains how to get started
├── build.sbt                   - Tells sbt how to build Daffodil
├── containers/
   └── release-candidate       - Defines Docker image used to build release
├── daffodil-cli/
   ├── README.md               - Explains how to start Daffodil
   ├── bin.LICENSE             - Contains licenses of Daffodil and subcomponents
   ├── bin.NOTICE              - Contains notices of Daffodil and subcomponents
   ├── build.sbt               - Tells sbt how to build CLI and installers
   └── src/                    - Contains CLI source code, tests, scripts, etc.
├── daffodil-codegen-c/         - Contains Daffodil's C code generator
├── daffodil-core/              - Contains Daffodil's schema compiler
├── daffodil-io/                - Contains Daffodil's input/output/charsets
├── daffodil-japi/
│   ├── build.sbt               - Tells sbt how to generate javadocs
│   └── src/                    - Contains Daffodil's Java API
├── daffodil-lib/               - Contains Daffodil's utilities
├── daffodil-macro-lib/         - Defines Daffodil's Scala 2.x macros
├── daffodil-propgen/           - Contains generators to generate more source code
├── daffodil-runtime1/          - Contains Daffodil's Scala parser
├── daffodil-runtime1-layers/   - Contains Daffodil's Scala layers 
├── daffodil-runtime1-unparser/ - Contains Daffodil's Scala unparser
├── daffodil-sapi/              - Contains Daffodil's Scala API
├── daffodil-schematron/
   ├── README.md               - Explains how to validate with Schematron rules
   └── src/                    - Contains Daffodil's Schematron validator
├── daffodil-tdml-lib/          - Contains Daffodil's TDML runner
├── daffodil-tdml-processor/    - Contains Daffodil's Scala & C TDML implementations
├── daffodil-test/              - Contains Daffodil's TDML tests
├── daffodil-test-ibm1/         - Contains more TDML tests from IBM & Tresys
├── daffodil-udf/
   ├── README.md               - Explains how to implement user defined functions
   └── src/                    - Contains Daffodil's UDF interfaces
├── project/
│   ├── Dependencies.scala      - Defines third party dependencies
│   ├── Rat.scala               - Lists files to be ignored by license audit tool
│   ├── build.properties        - Defines which sbt version to use
│   └── plugins.sbt             - Adds third party plugins to sbt
├── test-stdLayout/             - Contains TDML tests laid out in different ways
└── tutorials/                  - Contains Daffodil's TDML test tutorials

When compiled into a command line interface, Daffodil is composed of a script and a lib directory containing all Daffodil jars and third-party dependency jars needed by the Daffodil CLI. Daffodil's command line interface is released in binary form as follows:

├── LICENSE          - Contains licenses of Daffodil and subcomponents
├── NOTICE           - Contains notices of Daffodil and subcomponents
├── README.md        - Explains how to start Daffodil
├── bin/
   ├── daffodil     - Starts Daffodil on Linux
   └── daffodil.bat - Starts Daffodil on Windows
└── lib/             - Contains Daffodil jars and third party jars

When an application uses Daffodil‘s Java or Scala API, it will need to put fewer Daffodil jars and third-party dependency jars on its classpath (omitting the jars needed only by Daffodil’s CLI). The best way for a Java or Scala application to use Daffodil is to get Daffodil‘s Java API from Maven or get Daffodil’s Scala API from sbt:

libraryDependencies += "org.apache.daffodil" %% "daffodil-sapi" % "3.4.0"

When you use Daffodil's C code generator to generate C code from a DFDL schema, the newly created C code directory is organized as follows:

├── Makefile              - Contains build and test targets
├── libcli/
   ├── cli_errors.c      - Implements CLI error messages
   ├── daffodil_getopt.c - Implements CLI options
   ├── daffodil_main.c   - Starts the CLI
   ├── stack.c           - Pushes XML nodes on a stack
   ├── xml_reader.c      - Reads an infoset from a XML file
   └── xml_writer.c      - Writes an infoset to a XML file
├── libruntime/
   ├── errors.c          - Implements error messages
   ├── generated_code.c  - Implements functions to create/use infoset
   ├── infoset.c         - Implements functions to walk infoset
   ├── parsers.c         - Implements functions to read binary data
   └── unparsers.c       - Implements functions to write binary data
└── tests/
    ├── bits.c            - Tests integers with different lengths
    └── extras.c          - Avoids an undefined reference when linking

You can use the Makefile to build a Daffodil executable (c/daffodil) and run c/daffodil with similar parse and unparse options as Daffodil‘s Scala command line interface. If you want to embed the generated C code into your own C application, you can call only libruntime to read and write binary data into C structs in memory or call both libruntime and libcli (replacing daffodil_main.c with your application’s main.c) to read and write an XML infoset as well as binary data into C structs.


Unfortunately, it is not possible to search or edit all of Daffodil's documentation in one single place. Daffodil documentation is split between the Daffodil Confluence wiki, the Daffodil source repository, and the Daffodil site repository. Confluence has a page tree on the left side so you can find all the Confluence pages by expanding the page tree. The first link below is the only link you need to find or search all the pages in the wiki. However, there are a lot of pages in the wiki so here are some of the most important pages worth reading as well (sorted in alphabetical order).

Some of the Daffodil website pages don't seem to have any links pointing to them from other website pages. So that you can read all of the website pages in your browser more easily, here are direct links to every website page (sorted in alphabetical order).


The majority of Daffodil tests are written inside TDML files with corresponding Scala unit tests which allow your IDE or sbt to run these tests within the TDML files. Daffodil also has some pure Scala unit tests for testing smaller parts of Daffodil at a lower level. Both kinds of tests are important, although new Daffodil tests tend to be written in TDML and focus on testing issues with Daffodil's processing of DFDL schemas and parsing data or unparsing data. You can run both kinds of tests with sbt using this command:

sbt test

As mentioned in sbt Tips and Tricks, you can replace test with testOnly and arguments to run a smaller number of tests. At the most extreme, you can run only one test method in one Scala test file, or you can use either wildcards or multiple arguments to filter which tests are run. It is easier to type such testOnly commands at the sbt prompt since you will need to enclose the entire testOnly command including its arguments in quotes if you type it at the terminal's command line prompt. Nevertheless, here are examples of how to enclose a testOnly command and its arguments in quotes on the command line:

sbt "daffodil-test/testOnly org.apache.daffodil.section05.facets.TestNulChars"
sbt "daffodil-test-integration/testOnly org.apache.daffodil.cliTest.TestCLIUdfs -- --tests=test_noUdfsLoaded_regular_schema"

TDML tests are helpful for Daffodil developers because they allow a developer to write a TDML file with a schema, a test case, and test data which isolates a complex data format issue so the developer can work on that issue more easily. The developer is free to decide whether to define the schema and test data inline in the TDML file or leave the schema and test data in separate files which the TDML file reads. For easier development, you can run TDML tests in the same directory without needing your IDE or sbt using this command:

daffodil test nums.tdml

The developer also can choose which TDML implementation Daffodil should use to run a TDML test, either Daffodil‘s Scala backend or Daffodil’s C code generator, simply by passing an option to Daffodil‘s test command or passing a TDMLImplementation instance to Daffodil’s TDML Runner class in a Scala test class. The developer can use a TDML test to work on a part of DFDL which needs to be supported by the C code generator and keep running (or debugging) the TDML test until the generated C code works successfully:

daffodil test -I daffodilC nums.tdml

Daffodil also has command line interface tests in the daffodil-cli and daffodil-test-integration modules. Slower CLI tests that require forking should go in the latter module. If a developer changes any part of Daffodil's command line interface, the developer should test that change in one of these modules. You can run all CLI tests using these sbt commands:

sbt daffodil-cli/test
sbt daffodil-test-integration/test

Daffodil also has a GitHub Actions continuous integration workflow which builds the codebase and runs all the tests every time a pull request is changed or merged into the repository. The pull request will display whether the CI checks succeeded or failed, so Daffodil developers will have to become familiar with troubleshooting these CI checks whenever their pull request fails a CI check. For example, one of the CI checks generates a test coverage analysis report and fails if the pull request has less than 80% coverage. Another CI check generates a SonarCloud quality report too. Here are current and past workflow runs on GitHub, the current quality report on SonarCloud, and the current test coverage report on Codecov: