How to Document Ballerina Code

Ballerina has a built-in ballerina flavored markdown documentation framework named Docerina. The documentation framework allows you to write unstructured documents with a bit of structure to enable generating HTML content as API documentation.

Developers can write the documentation inline with the Ballerina source code using the lightweight markdown markup language. They can mark special occurrences such as parameters, return parameters, fields, endpoints within the documentation code using documentation attributes. Once the code is documented, developers can generate a basic HTML version of their Ballerina modules using the ballerina doc command. You are encouraged to have your custom themes and styles, to have a better presentation of your Ballerina documentation.

Ballerina design and usage is aligned with project and module semantics of Ballerina. You can generate documentation for the project modules using the ballerina doc command.


  • Ballerina programmers can place the documentation inline with the source code using documentation syntax.
  • Ballerina records, type definitions, objects, global variables, annotations, endpoints can be documented using the documentation syntax.
  • Fields, parameters, return parameters, endpoints can be marked using documentation attributes.
  • HTML documents can be generated using the ballerina doc command for each Ballerina module and if you have custom handlebars templates, you can use them to generate the HTMLs.

Writing Ballerina Documentation

Ballerina flavored markdown documentation is a first class syntax in the Ballerina language. The # at the beginning of a line denotes a line of documentation. If necessary, you can have multiple lines of documentation, which you can group together.

# <documentation line 1>
# <documentation line 2>
# ...

When you write documentation, you can use the markdown documentation syntax given above. For example:

# Provides the HTTP actions for interacting with an HTTP server. Apart from the standard 
# HTTP methods, `forward()` and `execute()` functions are provided. More complex and 
# specific endpoint types can be created by wrapping this generic ```HTTP``` actions 
# implementation.
# ...

The supported structure of documentation syntax is as follows:

# <description_line_1>
# <description_line_2>
# ...
# + <parameter_name/field_name> - <description_line_1>
#                                 <description_line_2>
# ...
# + <parameter_name/field_name> - <description_line_1>
#                                 <description_line_2>
# ...
# + return - <return_parameter_description_line_1>
#            <return_parameter_description_line_2>

Sample Usage

# Submits an HTTP request to a service with the specified HTTP verb.
# The `submit()` function does not give out a `Response` as the result,
# rather it returns an `HttpFuture`, which can be used to do further 
# interactions with the endpoint.
# Example:
#     ```ballerina
#     HttpFuture future = myMsg.submit("GET", "/test", req);
#     ```
# + httpVerb - The HTTP verb value
# + path - The resource path
# + request - An HTTP outbound request message
# + return - An `HttpFuture` that represents an asynchronous service invocation, 
#            or an `error` if the submission fails
public function submit(@sensitive string httpVerb, string path, Request request) returns HttpFuture|error;

Documenting A Module

A Ballerina module can have a file which describes the module and its usage.

A typical project structure of a Ballerina project is like this:

  Ballerina.toml       # Configuration that defines project intent
  .ballerina/          # Internal cache management and contains the project repository
                       # Project repository contains compiled module binaries

  main.bal             # Part of the “unnamed” module, compiled into a main.balx
                       # You can have many files in the "unnamed" module, 
                       # though unadvisable

  module1/            # The source in this directory will be named “<org-name>/module1”         # Optional, contains descriptive metadata for display at 
                       # Ballerina Central
    *.bal              # In this dir and recursively in subdirs except tests/ and 
                       # resources/
    [tests/]           # Module-specific unit and integration tests
    [resources/]       # Module-specific resources

  [resources/]         # Resources included with every module in the project

  target/              # Compiled executables and other artifacts end up here
     Ballerina.lock    # Generated during build, used to rebuild identical binary

ballerina doc command will read the and append it in the generated HTML file.

Please check HTTP module documentation for a sample HTML that has a content at the top, followed by the other module constructs.

Generating Ballerina Documentation

Ballerina provides a doc command which can be executed against a given Ballerina project. This command will result in generating the Ballerina documentation as HTML files, for all the modules in the project.

First, let’s create a new Ballerina project:

$ mkdir myproject
$ cd myproject
$ ballerina init -i
Create Ballerina.toml [yes/y, no/n]: (y) y
Organization name: (user) y
Version: (0.0.1) 
Ballerina source [service/s, main/m, finish/f]: (s)  
Module for the service: (no module) math
Ballerina source [service/s, main/m, finish/f]: (f) s
Module for the service: (no module) time
Ballerina source [service/s, main/m, finish/f]: (f) f

Ballerina project initialized

Now, let’s generate documentation of the project:

$ ballerina doc


docerina: API documentation generation for sources - [math, time]
docerina: HTML file written: /private/tmp/myproject/target/api-docs/math.html
docerina: HTML file written: /private/tmp/myproject/target/api-docs/time.html
docerina: HTML file written: /private/tmp/myproject/target/api-docs/index.html
docerina: HTML file written: /private/tmp/myproject/target/api-docs/module-list.html

target/api-docs/html folder would contain following files;

$ ls target/api-docs/html/
docerina-theme    index.html    math.html   module-list.html   time.html
  • index.html - contains an index to the ballerina project documentation
  • math.html - contains the content of the module named math
  • time.html - contains the content of the module named time
  • module-list.html - contains the module list, which will be useful to find out the list of modules.
  • docerina-theme - folder contains basic styles shipped by default with the pack.

If you want to generate documentation for a selected Ballerina module, then you can execute the following command from the ballerina project root directory:

$ ballerina doc <module_name>

If you have custom Handlebars templates, you can pass them via the doc command:

$ ballerina doc -t <path_to_templates> <module_name>

For other options, please run ballerina doc --help.