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Contributing to the Apache Cassandra Java Driver

Thank you for your interest in contributing!

Table of Contents

Ways to Contribute

There are many ways to contribute, including:

  • Bug Reports: Identify incorrect behavior, inconsistencies, or regressions in the driver. Provide reproduction steps when possible.
  • Feature Requests: Propose improvements or new functionality. Please describe the use case (not just a proposed API).
  • Documentation Improvements: Enhance guides, examples, javadocs, or configuration explanations.
  • Pull Requests: Submit fixes, enhancements, performance improvements, or refactorings.
  • Testing Contributions: Add missing tests, improve coverage, or enhance test infrastructure.
  • Support & Triage: Help evaluate reported issues or contribute to discussions.
  • Verify Releases: Verify the release artifacts work correctly in your environment, when a release is proposed in the mailing list.

Communication

  1. Mailing Lists: Mail to user-subscribe@cassandra.apache.org or dev-subscribe@cassandra.apache.org to join the user@cassandra.apache.org or dev@cassandra.apache.org mailing lists.
  2. Slack: #cassandra-drivers channel in the Apache Software Foundation Slack. You can ask for an invite to the ASF Slack workspace in the mailing lists.
  3. JIRA: https://issues.apache.org/jira/projects/CASSJAVA
  4. GitHub Repository: https://github.com/apache/cassandra-java-driver

Contribution Process

Reporting Issues

All issues must be tracked in Apache JIRA:
https://issues.apache.org/jira/projects/CASSJAVA

When filing an issue:

  • Clearly describe the problem, expected behavior, and actual behavior.
  • Include driver version, Java version, and Cassandra cluster details.
  • Add reproduction steps or a minimal test case if possible.
  • Use the appropriate issue type (Bug, Improvement, New Feature, etc.).
  • Set the correct components (e.g., core, mapper-runtime, quarkus).

Committers will help refine the ticket if needed.

Submitting Changes (Pull Requests)

All code changes require:

  1. A corresponding JIRA ticket unless it's a ninja fix.
    Include the JIRA key in the PR title, e.g.:
    CASSJAVA-40: Driver testing against Java 21

  2. A pull request on GitHub
    Repository: https://github.com/apache/cassandra-java-driver

  3. Tests
    Every fix or feature should include or update tests. PRs without tests are rarely accepted.

  4. Documentation updates
    Update manual, javadocs, examples, or reference docs when applicable.

  5. Passing CI
    PRs must pass all CI jobs unless reviewers explicitly allow exceptions.

  6. Code review
    Committers will review your changes. 2 approvals from committers are required for merging.

  7. Squash When the PR is ready to merge, use git rebase -i to squash your changes into a single commit before merging. The commit message should follow the format of

    <JIRA number> <JIRA title>
patch by <Name>; reviewed by <Name> and <Name>

    For example,

    CASSJAVA-108 Update ESRI (and remove org.json) dependencies
patch by Bret McGuire; reviewed by Bret McGuire and Lukasz Antoniak

Do not mix unrelated changes in one PR—keep contributions focused.

Do not base a PR on another one.

Do not squash commits before the PR is ready to merge.

Development Setup

Prerequisites

  • Java 8+
  • Maven 3.8.1+

Building the Project

  • Ensure Maven is installed and you are using Java 8.
  • Build the project with: 
    mvn clean package -DskipTests
  • If using an IDE like IntelliJ and encountering issues with guava-shaded classes:
    • Run: 
      mvn clean install -DskipTests
    • If IntelliJ uses a different Maven version, use the Maven window in IntelliJ: under Lifecycle, click clean and then install.

Running Tests

Unit Tests

mvn clean install -DskipTests
mvn test

Integration Tests

  1. Install Cassandra Cluster Manager (CCM) following its README.
  2. MacOS only, for CCM and Simulacron-based tests, enable loopback aliases: 
    for i in {2..255}; do sudo ifconfig lo0 alias 127.0.0.$i up; done

Note: This may slow down networking. To remove the aliases after testing:

for i in {2..255}; do sudo ifconfig lo0 -alias 127.0.0.$i up; done
  1. MacOS Apple Silicon only, for some Cassandra versions, you might need to work around the JNA 5.6.0 version, which is incompatible to Apple Silicon. 
    mvn dependency:get -Dartifact=net.java.dev.jna:jna:5.10.0
cp ~/.m2/repository/net/java/dev/jna/jna/5.10.0/jna-5.10.0.jar ~/.ccm/repository/4.0.19/lib/jna-5.6.0.jar
  2. Run integration tests: 
    mvn clean verify
    To target a specific Cassandra version or distribution: 
    mvn verify -Dccm.version=3.11.0
mvn verify -Dccm.distribution=dse -Dccm.version=6.8.0

Coding Guidelines

General

Do not use static imports. They make things harder to understand when you look at the code someplace where you don't have IDE support, like Github's code view.

Avoid abbreviations in class and variable names. A good rule of thumb is that you should only use them if you would also do so verbally, for example "id" and "config" are probably reasonable. Single-letter variables are permissible if the variable scope is only a few lines, or for commonly understood cases (like i for a loop index).

Keep source files short. Short files are easy to understand and test. The average should probably be around 200-300 lines.

Code Formatting and License Headers

Javadoc

All types in "API" packages must be documented. For "internal" packages, documentation is optional, but in no way discouraged: it's generally a good idea to have a class-level comment that explains where the component fits in the architecture, and anything else that you feel is important.

You don't need to document every parameter or return type, or even every method. Don't document something if it is completely obvious, we don't want to end up with this:

/**
 * Returns the name.
 * 
 * @return the name
 */
String getName();

On the other hand, there is often something useful to say about a method, so most should have at least a one-line comment. Use common sense.

Driver users coding in their IDE should find the right documentation at the right time. Try to think of how they will come into contact with the class. For example, if a type is constructed with a builder, each builder method should probably explain what the default is when you don't call it.

Avoid using too many links, they can make comments harder to read, especially in the IDE. Link to a type the first time it's mentioned, then use a text description ("this registry"...) or an @code block. Don't link to a class in its own documentation. Don't link to types that appear right below in the documented item's signature.

/**
* @return this {@link Builder} <-- completely unnecessary
*/
Builder withLimit(int limit) {

Logging

We use SLF4J; loggers are declared like this:

private static final Logger LOG = LoggerFactory.getLogger(TheEnclosingClass.class);

Logs are intended for two personae:

  • Ops who manage the application in production.
  • Developers (maybe you) who debug a particular issue.

Log levels

  • ERROR: something that renders the driver -- or a part of it -- completely unusable. An action is required to fix it: bouncing the client, applying a patch, etc.
  • WARN: something that the driver can recover from automatically, but indicates a configuration or programming error that should be addressed. For example: the driver connected successfully, but one of the contact points in the configuration was malformed; the same prepared statement is being prepared multiple time by the application code.
  • INFO: something that is part of the normal operation of the driver, but might be useful to know for an operator. For example: the driver has initialized successfully and is ready to process queries; an optional dependency was detected in the classpath and activated an enhanced feature.

Do not log errors that are rethrown to the client (such as the error that you're going to complete a request with). This is annoying for ops because they see a lot of stack traces that require no actual action on their part, because they're already handled by application code.

Similarly, do not log stack traces for non-critical errors. If you still want the option to get the trace for debugging, see the Loggers.warnWithException utility.

The last 2 levels are for developers, to help follow what the driver is doing from a "black box" perspective (think about debugging an issue remotely, and all you have are the logs).

  • TRACE: anything that happens for every user request. Not only request handling, but all related components (e.g. timestamp generators, policies, etc).
  • DEBUG: everything else. For example, node state changes, control connection activity, etc.

Note that DEBUG and TRACE can coexist within the same component, for example the LBP initializing is a one-time event, but returning a query plan is a per-request event.

Log prefix shows origin, e.g.:
[s0|90232530|0] (session name | hash code of the CqlRequestHandler instance | number of request attempts)

Tests run with the configuration defined in src/test/resources/logback-test.xml. The default level for driver classes is WARN, but you can override it with a system property: -DdriverLevel=DEBUG. A nice setup is to use DEBUG when you run from your IDE, and keep the default for the command line.

When you add or review new code, take a moment to run the tests in DEBUG mode and check if the output looks good.

Don't abuse the stream API

The java.util.stream API is often used (abused?) as a "functional API for collections":

List<Integer> sizes = words.stream().map(String::length).collect(Collectors.toList());

The perceived advantages of this approach over traditional for-loops are debatable:

  • readability: this is highly subjective. But consider the following:
    • everyone can read for-loops, whether they are familiar with the Stream API or not. The opposite is not true.
    • the stream API does not spell out all the details: what kind of list does Collectors.toList() return? Is it pre-sized? Mutable? Thread-safe?
    • the stream API looks pretty on simple examples, but things can get ugly fast. Try rewriting NetworkTopologyReplicationStrategy with streams.
  • concision: this is irrelevant. When we look at code we care about maintainability, not how many keystrokes the author saved. The for-loop version of the above example is just 5 lines long, and your brain doesn't take longer to parse it.

The bottom line: don't try to "be functional" at all cost. Plain old for-loops are often just as simple.

Never assume a specific format for toString()

Only use toString() for debug logs or exception messages, and always assume that its format is unspecified and can change at any time.

If you need a specific string representation for a class, make it a dedicated method with a documented format, for example toCqlLiteral. Otherwise it's too easy to lose track of the intended usage and break things: for example, someone modifies your toString() method to make their logs prettier, but unintentionally breaks the script export feature that expected it to produce CQL literals.

toString() can delegate to toCqlLiteral() if that is appropriate for logs.

Concurrency annotations

We use the JCIP annotations to document thread-safety policies.

Add them for all new code, with the exception of:

  • enums and interfaces;
  • utility classes (only static methods);
  • test code.

Make sure you import the types from net.jcip, there are homonyms in the classpath.

Nullability annotations

We use the Spotbugs annotations to document nullability of parameters, method return types and class members.

Please annotate any new class or interface with the appropriate annotations: @NonNull, @Nullable. Make sure you import the types from edu.umd.cs.findbugs.annotations, there are homonyms in the classpath.

Coding Guidelines for Tests

Static imports are permitted in a couple of places:

  • All AssertJ methods, e.g.: 
    assertThat(node.getDatacenter()).isNotNull();
fail("Expecting IllegalStateException to be thrown");
  • All Mockito methods, e.g.: 
    when(codecRegistry.codecFor(DataTypes.INT)).thenReturn(codec);
verify(codec).decodePrimitive(any(ByteBuffer.class), eq(ProtocolVersion.DEFAULT));
  • All Awaitility methods, e.g.: 
    await().until(() -> somethingBecomesTrue());

Test methods names use lower snake case, generally start with should, and clearly indicate the purpose of the test, for example: should_fail_if_key_already_exists. If you have trouble coming up with a simple name, it might be a sign that your test does too much, and should be split.

We use AssertJ (assertThat) for assertions. Don't use JUnit assertions (assertEquals, assertNull, etc).

Don't try to generify at all cost: a bit of duplication is acceptable, if that helps keep the tests simple to understand (a newcomer should be able to understand how to fix a failing test without having to read too much code).

Test classes can be a bit longer, since they often enumerate similar test cases. You can also factor some common code in a parent abstract class named with "XxxTestBase", and then split different families of tests into separate child classes. For example, CqlRequestHandlerTestBase, CqlRequestHandlerRetryTest, CqlRequestHandlerSpeculativeExecutionTest...

Unit tests

They live in the same module as the code they are testing. They should be fast and not start any external process. They usually target one specific component and mock the rest of the driver context.

Integration tests

They live in the integration-tests module, and exercise the whole driver stack against an external process, which can be either one of:

  • Simulacron: simulates Cassandra nodes on loopback addresses; your test must "prime" data, i.e. tell the nodes what results to return for pre-determined queries.

    For an example of a Simulacron-based test, see NodeTargetingIT.

  • CCM: launches actual Cassandra nodes locally. The ccm executable must be in the path.

    You can pass a -Dccm.version system property to the build to target a particular Cassandra version (it defaults to 3.11.0). -Dccm.directory allows you to point to a local installation -- this can be a checkout of the Cassandra codebase, as long as it's built. See CcmBridge in the driver codebase for more details.

    For an example of a CCM-based test, see PlainTextAuthProviderIT.

Categories

Integration tests are divided into three categories:

Parallelizable tests

These tests can be run in parallel, to speed up the build. They either use:

  • dedicated Simulacron instances. These are lightweight, and Simulacron will manage the ports to make sure that there are no collisions.
  • a shared, one-node CCM cluster. Each test works in its own keyspace.

The build runs them with a configurable degree of parallelism (currently 8). The shared CCM cluster is initialized the first time it's used, and stopped before moving on to serial tests. Note that we run with parallel=classes, which means methods within the same class never run concurrent to each other.

To make an integration test parallelizable, annotate it with @Category(ParallelizableTests.class). If you use CCM, it must be with CcmRule.

For an example of a Simulacron-based parallelizable test, see NodeTargetingIT. For a CCM-based test, see DirectCompressionIT.

Serial tests

These tests cannot run in parallel, in general because they require CCM clusters of different sizes, or with a specific configuration (we never run more than one CCM cluster simultaneously: it would be too resource-intensive, and too complicated to manage all the ports).

The build runs them one by one, after the parallelizable tests.

To make an integration test serial, do not annotate it with @Category. The CCM rule must be CustomCcmRule.

For an example, see DefaultLoadBalancingPolicyIT.

Note: if multiple serial tests have a common "base" class, do not pull up CustomCcmRule, each child class must have its own instance. Otherwise they share the same CCM instance, and the first one destroys it on teardown. See TokenITBase for how to organize code in those cases.

Isolated tests

Not only can those tests not run in parallel, they also require specific environment tweaks, typically system properties that need to be set before initialization.

The build runs them one by one, each in its own JVM fork, after the serial tests.

To isolate an integration test, annotate it with @Category(IsolatedTests.class). The CCM rule must be CustomCcmRule.

For an example, see HeapCompressionIT.

About test rules

Do not mix CcmRule and SimulacronRule in the same test. It makes things harder to follow, and can be inefficient (if the SimulacronRule is method-level, it will create a Simulacron cluster for every test method, even those that only need CCM).

Use the @BackendRequirement annotation to restrict the backend type and version. Specify the Cassandra Distribution, CASSANDRA/DSE/HCD, and the version requirement. For example, @BackendRequirement(type = BackendType.CASSANDRA, minInclusive = "2.2")

Class-level rules

Rules annotated with @ClassRule wrap the whole test class, and are reused across methods. Try to use this as much as possible, as it's more efficient. The fields need to be static; also make them final and use constant naming conventions, like CCM_RULE.

When you use a server rule (CcmRule or SimulacronRule) and a SessionRule at the same level, wrap them into a rule chain to ensure proper initialization order:

private static final CcmRule CCM_RULE = CcmRule.getInstance();
private static final SessionRule<CqlSession> SESSION_RULE = SessionRule.builder(CCM_RULE).build();

@ClassRule
public static final TestRule CHAIN = RuleChain.outerRule(CCM_RULE).around(SESSION_RULE);
Method-level rules

Rules annotated with @Rule wrap each test method. Use lower-camel case for field names:

private CcmRule ccmRule = CcmRule.getInstance();
private SessionRule<CqlSession> sessionRule = SessionRule.builder(ccmRule).build();

@ClassRule
public TestRule chain = RuleChain.outerRule(ccmRule).around(sessionRule);

Only use this for:

  • CCM tests that use @BackendRequirement restrictions at the method level (ex: BatchStatementIT).
  • tests where you really need to restart from a clean state for every method.
Mixed

It's also possible to use a @ClassRule for CCM / Simulacron, and a @Rule for the session rule. In that case, you don't need to use a rule chain.