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Development

Developers should read the development section of the website, which covers thing like development philosophy and contribution process.

More information about the writing and building the documentation can be found in the docs module.

Commits and pull requests

Format Git commit messages

When writing a Git commit message, follow these guidelines.

Git merge strategy

Pull requests are usually merged into master using the rebase and merge strategy.

A typical pull request should strive to contain a single logical change (but not necessarily a single commit). Unrelated changes should generally be extracted into their own PRs.

If a pull request does consist of multiple commits, it is expected that every prefix of it is correct. That is, there might be preparatory commits at the bottom of the stack that don't bring any value by themselves, but none of the commits should introduce an error that is fixed by some future commit. Every commit should build and pass all tests.

Commit messages and history are also important, as they are used by other developers to keep track of the motivation behind changes. Keep logical diffs grouped together in separate commits, and order commits in a way that explains the progress of the changes. Rewriting and reordering commits may be a necessary part of the PR review process as the code changes. Mechanical changes (like refactoring and renaming) should be separated from logical and functional changes. E.g. deduplicating code or extracting helper methods should happen in a separate commit from the commit where new features or behavior is introduced. This makes reviewing the code much easier and reduces the chance of introducing unintended changes in behavior.

Code Style

We recommend you use IntelliJ as your IDE. The code style template for the project can be found in the codestyle repository along with our general programming and Java guidelines.

To run checkstyle and other maven checks before opening a PR: ./mvnw validate

In addition to those you should also adhere to the following:

Readability

The purpose of code style rules is to maintain code readability and developer efficiency when working with the code. All the code style rules explained below are good guidelines to follow but there may be exceptional situations where we purposefully depart from them. When readability and code style rule are at odds, the readability is more important.

Consistency

Keep code consistent with surrounding code where possible.

Alphabetize

Alphabetize sections in the documentation source files (both in the table of contents files and other regular documentation files).

Use streams

When appropriate, use the stream API. However, note that the stream implementation does not perform well so avoid using it in inner loops or otherwise performance sensitive sections.

Categorize errors when throwing exceptions.

Categorize errors when throwing exceptions. For example, TrinoException takes an error code as an argument, TrinoException(HIVE_TOO_MANY_OPEN_PARTITIONS). This categorization lets you generate reports so you can monitor the frequency of various failures.

Add license header

Ensure that all files have the appropriate license header; you can generate the license by running mvn license:format.

Prefer String formatting

Consider using String formatting (printf style formatting using the Java Formatter class): format("Session property %s is invalid: %s", name, value) (note that format() should always be statically imported). Sometimes, if you only need to append something, consider using the + operator. Please avoid format() or concatenation in performance critical sections of code.

Avoid ternary operator

Avoid using the ternary operator except for trivial expressions.

Define class API for private inner classes too

It is suggested to declare members in private inner classes as public if they are part of the class API.

Avoid mocks

Do not use mocking libraries. These libraries encourage testing specific call sequences, interactions, and other internal behavior, which we believe leads to fragile tests. They also make it possible to mock complex interfaces or classes, which hides the fact that these classes are not (easily) testable. We prefer to write mocks by hand, which forces code to be written in a certain testable style.

Use AssertJ

Prefer AssertJ for complex assertions.

Use Airlift's Assertions

For thing not easily expressible with AssertJ, use Airlift's Assertions class if there is one that covers your case.

Avoid var

Using var is discouraged.

Prefer Guava immutable collections

Prefer using immutable collections from Guava over unmodifiable collections from JDK. The main motivation behind this is deterministic iteration.

Maintain production quality for test code

Maintain the same quality for production and test code.

Avoid abbreviations

Please avoid abbreviations, slang or inside jokes as this makes harder for non-native english speaker to understand the code. Very well known abbreviations like max or min and ones already very commonly used across the code base like ttl are allowed and encouraged.

Avoid default clause in exhaustive enum-based switch statements

Avoid using the default clause when the switch statement is meant to cover all the enum values. Handling the unknown option case after the switch statement allows static code analysis tools (e.g. Error Prone's MissingCasesInEnumSwitch check) report a problem when the enum definition is updated but the code using it is not.

Additional IDE configuration

When using IntelliJ to develop Trino, we recommend starting with all of the default inspections, with some modifications.

Enable the following inspections:

  • Java | Internationalization | Implicit usage of platform's default charset,
  • Java | Control flow issues | Redundant 'else' (including Report when there are no more statements after the 'if' statement option),
  • Java | Class structure | Utility class is not 'final',
  • Java | Class structure | Utility class with 'public' constructor,
  • Java | Class structure | Utility class without 'private' constructor.

Disable the following inspections:

  • Java | Performance | Call to 'Arrays.asList()' with too few arguments,
  • Java | Abstraction issues | 'Optional' used as field or parameter type,
  • Java | Data flow | Boolean method is always inverted.

Update the following inspections:

  • Remove com.google.common.annotations.Beta from JVM languages | Unstable API usage.

Enable errorprone (Error Prone Installation#IDEA):

  • Install Error Prone Compiler plugin from marketplace,
  • Check the errorprone-compiler profile in the Maven tab

This should be enough - IDEA should automatically copy the compiler options from the POMs to each module. If that doesn't work, you can do it manually:

  • In Java Compiler tab, select Javac with error-prone as the compiler,
  • Update Additional command line parameters and copy the contents of compilerArgs in the top-level POM (except for -Xplugin:ErrorProne) there
    • Remove the XML comments...
    • ...except the ones which denote checks which fail in IDEA, which you should "unwrap"
  • Remove everything from the list under Override compiler parameters per-module

Note that the version of errorprone used by the IDEA plugin might be older than the one configured in the pom.xml and you might need to disable some checks that are not yet supported by that older version. When in doubt, always check with the full Maven build (./mvnw clean install -DskipTests -Perrorprone-compiler).

Language injection in IDE

In order to enable language injection inside Intellij IDEA, some code elements can be annotated with the @org.intellij.lang.annotations.Language annotation. To make it useful, we recommend:

  • Set the project-wide SQL dialect in Languages & Frameworks | SQL Dialects - "Generic SQL" is a decent choice here,
  • Disable inspection SQL | No data source configured,
  • Optionally disable inspection Language injection | Language mismatch.

Even if the IDE does not support language injection this annotation is useful for documenting the API's intent. Considering the above, we recommend annotating with @Language:

  • All API parameters which are expecting to take a String containing an SQL statement (or any other language, like regular expressions),
  • Local variables which otherwise would not be properly recognized by IDE for language injection.

Building the Web UI

The Trino Web UI is composed of several React components and is written in JSX and ES6. This source code is compiled and packaged into browser-compatible Javascript, which is then checked in to the Trino source code (in the dist folder). You must have Node.js and Yarn installed to execute these commands. To update this folder after making changes, simply run:

yarn --cwd core/trino-main/src/main/resources/webapp/src install

If no Javascript dependencies have changed (i.e., no changes to package.json), it is faster to run:

yarn --cwd core/trino-main/src/main/resources/webapp/src run package

To simplify iteration, you can also run in watch mode, which automatically re-compiles when changes to source files are detected:

yarn --cwd core/trino-main/src/main/resources/webapp/src run watch

To iterate quickly, simply re-build the project in IntelliJ after packaging is complete. Project resources will be hot-reloaded and changes are reflected on browser refresh.