Code Conventions

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First off, we thank you for your interest in the Alluxio open source project! We greatly appreciate any contribution; whether it be new features or bug fixes.

If you are a first time contributor to the Alluxio open source project, we strongly encourage you to follow the step-by-step instructions within the Contribution Guide and finish new contributor tasks before making more advanced changes to the Alluxio codebase.

Submitting Code

Submitting changes to Alluxio is done via pull requests. Please read our pull request guidelines for details on how to submit a pull request to the Alluxio repository. Below are some tips for the pull requests.

  • We encourage you to break your work into small, single-purpose patches if possible. It is more difficult to merge in a large change with a lot of disjoint features.
  • We track issues and features in our Github Issues. Open an issue detailing the proposed change or the bug description.
  • Submit the patch as a GitHub pull request.
  • If your pull request aims to solve an existing Github issue, please include a link to the Github issue in the last line of the description field of the pull request, such as Fixes #1234.
  • Please read our pull request guidelines for details.

Coding Style

  • Please follow the style of the existing codebase. We mainly follow the Google Java style, with the following deviations:
    • Maximum line length of 100 characters.
    • Third-party imports are grouped together to make IDE formatting much simpler.
    • Class member variable names should be prefixed with m
      • example: private WorkerClient mWorkerClient;
    • Static variable names should be prefixed with s
      • example: private static String sUnderFSAddress;
  • Bash scripts follow the Google Shell style, and must be compatible with Bash 3.x
  • If you use Eclipse:
    • You can download our Eclipse formatter
    • To organize your imports correctly, configure “Organize Imports” to look like this
  • If you use IntelliJ IDEA:
  • To automatically reorder methods alphabetically, try the Rearranger Plugin, open Preferences, search for rearranger, remove the unnecessary comments, then right click, choose “Rearrange”, codes will be formatted to what you want

  • To verify that the coding standards match, you should run checkstyle before sending a pull request to verify no new warnings are introduced:
$ mvn checkstyle:checkstyle

JavaDoc Style

This codebase follows the Oracle JavaDoc style with the following refinements:

  • All public classes/interfaces should have a class/interface-level comment that describes the purpose of the class/interface.

  • All public members should have a member-level comment the describes the purpose of the member.

/** The number of logical bytes used. */
public final AtomicLong mBytes = new AtomicLong(0);
  • All public methods (including constructors) should use the following format.
 * Does something. This is a method description that uses
 * 3rd person (does something) as opposed to 2nd person (do
 * something).
 * @param param_1 description of 1st parameter
 * ...
 * @param param_n description of nth parameter
 * @return description of return argument (if applicable)
 * @throws exception_1 description of 1st exception case
 * ...
 * @throws exception_n description of nth exception case
  • An exception to the above rule is that @throws doesn’t need to be provided for @Test methods, or for generic exceptions like IOException when there is nothing interesting to document.

  • Only write exception javadoc when you think it will be useful to the developer using the method. There are so many sources of IOException that it’s almost never useful to include javadoc for it. Do not write javadoc for unchecked exceptions like RuntimeException unless it is critical for this method.

  • Getters and setters should omit the method description if it is redundant and only use @param and @return descriptions.

 * @return the number of pages stored
long getPages();
  • Most sentences should start with a capital letter and end with a period. An exception to this style is an isolated sentence; it does not start with a capital letter nor end with a period.
    • GOOD (isolated): this is a short description
    • GOOD (full sentence): This is a short description.
    • GOOD (2 full sentences): This is a slightly longer description. It has two sentences.
    • BAD: this is a short description.
    • BAD: This is a short description
    • BAD: this is a slightly longer description. It has two sentences
  • When writing the description, the first sentence should be a concise summary of the class or method and the description should generally be implementation-independent. It is a good idea to use additional sentences to describe any significant performance implications.
 * The default implementation of a metadata store for pages stored in cache.
public class DefaultMetaStore implements MetaStore {
  • When the @deprecated annotation is added, it should also at least tell the user when the API was deprecated and what to use as a replacement with @see or @link tag.
 * @deprecated as of Alluxio 2.1, replaced by
 *             {@link #newMethodName(int,int,int,int)}
  • When descriptions of @param, @return, @throw exceed one line, the text should align with the first argument after the tag.
@throws FileAlreadyExistsException if there is already a file or directory at the given path
        in Alluxio Filesystem
  • When reference a class name in javadoc, prefer <code>ClassName</code> tags to {@link ClassName}.

Logging Conventions

Alluxio uses SLF4J for logging with typical usage pattern of:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public MyClass {
  private static final Logger LOG = LoggerFactory.getLogger(MyClass.class);

  public void someMethod() {"Hello world");

Note that, each class must use its own logger based on the class name, like LoggerFactory.getLogger(MyClass.class) in above example, so its output can easily be searched for. The location of the output of SLF4J loggers can be found for server logs and application logs.

Best Practice

  1. When applicable, logging should be parameterized, to provide good performance and consistent style.
    // Recommended: Parameterized logging
    LOG.debug("Client {} registered with {}", mClient, mHostname);
    // Not recommended: Non-parameterized logging, hard to read and expensive due to String
    // concatenation regardless of if DEBUG is enabled
      LOG.debug("Client " + mClient + " registered with " + mHostname);
  2. Error Messages Should Be Descriptive. Error messages should include enough detail and context to determine the issue and potential solution. This is important for quick diagnosis or fixes of errors.
    // Recommended: error messages with context
    LOG.error("Client {} failed to register to {}", mClient, mHostname, exception);
    // Not recommended: There is no information on which client failed, or what the issue was
    LOG.error("Client failed to register");
  3. Log messages should be concise and readable. Log messages should be written with readability in mind. Here are some tips for writing good log messages.
    • Log levels INFO and above should be easily human readable
    • Log files should be concise and easy to read, noise reduces the value of logs
    • Keep the amount of additional words to a minimum
    • Clearly indicate if a variable reference is being printed by formatting the output as variable: value
    • Ensure objects being logged have appropriate toString() implementations
  4. Error level logs should have troubleshooting pointers if applicable

Which logging level to use

There are several levels of logging, see detailed explanation of different Levels Here are the guidelines for deciding which level to use.

Error Log Level

Error level logging (LOG.error) indicates system level problems which cannot be recovered from. It should always be accompanied by a stack trace.

// Recommended
LOG.error("Failed to do something due to an exception", e);
// Not recommended: stack trace will not be logged
LOG.error("Failed to do something due to an exception {}", e);

When to Use

  • An unrecoverable event has occurred. The thread will be terminated.
    • Example: Failed to flush to the journal
  • An unrecoverable event has occurred. The process will exit.
    • Example: Master failed to bind to the RPC port

During Exception Handling

  • Error level logging should include the exception and full stack trace
  • The exception should only be logged once, at the boundary between the process and the external caller. For internal services (ie. periodic threads), the exception should be logged at the top level thread loop.

Warn Log Level

Warn level logging (LOG.warn) indicates a logical mismatch between user intended behavior and Alluxio behavior. Warn level logs are accompanied by an exception message. The associated stack trace may be found in debug level logs.

// Recommended
LOG.warn("Failed to do something: {}", e.toString());
// Recommended
LOG.warn("Failed to do something: {}", e);
// Not recommended: this will print out the stack trace
LOG.warn("Failed to do something", e);
// Not recommended: the exception class name is not included
LOG.warn("Failed to do something", e.getMessage());

When to Use

  • Unexpected state has been reached, but the thread can continue
    • Example: Failed to delete the temporary file for buffering writes to S3
  • User intended behavior cannot be achieved, but a default or fallback behavior is in place
    • Example: EPOLL is specified but not available in the Netty library provided, default to NIO
  • A misuse of configuration or API is suspected
    • Example: Zookeeper address is specified but zookeeper is not enabled

During Exception Handling

  • Warn level logging should be used for logical errors
  • Only the exception message should be logged, as the stack trace is irrelevant, suppressed stack traces can be revealed at the debug level
  • The warn level error message should be logged once, at the point where the process makes an explicit deviation from expected behavior

Info Log Level

Info level logging ( records important system state changes. Exception messages and stack traces are never associated with info level logs. Note that, this level of logging should not be used on critical path of operations that may happen frequently to prevent negative performance impact."Master started with address: {}.", address);

When to Use

  • Initialization of a long lived object within the process
    • Example: Master RPC Server begins serving
  • Important logical events in the process
    • Example: A worker registers with the master
  • End of life of a long lived object within the process
    • Example: The Master RPC Server shuts down

During Exception Handling

  • Info level logging is never involved with exceptions

Debug Log Level

Debug level logging (LOG.debug) includes detailed information for various aspects of the Alluxio system. Control flow logging (Alluxio system enter and exit calls) is done in debug level logs. Debug level logging of exceptions typically has the detailed information including stack trace. Please avoid the slow strings construction on debug-level logging on critical path.

// Recommended
LOG.debug("Failed to connect to {} due to exception", mAddress, e);

// Recommended: string concatenation is only performed if debug is enabled
if (LOG.isDebugEnabled()) {
    LOG.debug("Failed to connect to address {} due to exception", host + ":" + port, e);
// Not recommended: string concatenation is always performed
LOG.debug("Failed to connect to {} due to exception", host + ":" + port, e);

When to Use

  • Any exit or entry point of the process where execution is handed to an external process
    • Example: A request is made to the under file system

During Exception Handling

  • Debug level logs provide the control flow logging, in the case of an error exit, the exception message is logged
  • For warn level messages which may not always be benign, the stack trace can be logged at the debug level. These debug level stack traces should eventually be phased out.

Trace Log Level

When to Use

  • Any degree of detail within an experimental or new feature

During Exception Handling

  • Up to the developer’s discretion


These are the guidelines for throwing and handling exceptions throughout the Alluxio codebase.

Throwing Exceptions

These are the guidelines for how and when to throw exceptions.

Use unchecked exceptions for events which indicate bugs

Examples: Illegal states, invalid API usage

public void method(String neverSupposedToBeNull) {
  Preconditions.checkNotNull(neverSupposedToBeNull, "neverSupposedToBeNull");

Use checked exceptions for everything else

Examples: File not found, interrupted exception, timeout exceeded

// Recommended: the checked exception should just be propagated
public void handleRawUserInput(String date) throws InvalidDateException {

// Don't do this - it's reasonable for user input to be invalid.
public void handleRawUserInput(String date) throws InvalidDateException {
  try {
  } catch (InvalidDateException e) {
    throw new RuntimeExcepiton("date " + date + " is invalid", e);

Design code to minimize use of checked exceptions

Require callers to validate inputs so that invalid arguments can be unchecked exceptions.

Reuse existing exceptions when appropriate

Try to find an appropriate existing exception before inventing a new one.

Selectively javadoc exceptions

Only write exception javadoc when you think it will be useful to the developer using the method. There are so many sources of IOException that it’s almost never useful to include javadoc for it.


On the wire we represent exceptions with one of 14 status codes, e.g. NOT_FOUND, UNAVAILABLE. Within our server and client code, we represent these exceptions using exception classes corresponding to these statuses, e.g. NotFoundException and UnavailableException. AlluxioStatusException is the superclass for these Java exceptions.

Handling exceptions

These are the guidelines for how to handle exceptions.

Never swallow an unchecked exception

Either log the exception or propagate it.

It is usually wrong to both log and propagate. If every method did this, the same exception would be logged dozens of times, polluting the logs. The responsibility for logging lies with whatever method ends up handling the exception without propagating it.

Do not use Throwables.propagate()

See this explanation on why Throwables.propagate is deprecated.

Never Ignore InterruptedException

An InterruptedException means that another thread has signalled that this thread should die. There are a few acceptable ways to handle an InterruptedException, listed in order of preference.

Actually stop the thread

class MyRunnable implements Runnable {
  public void run() {
    while (true) {
      try {
      } catch (InterruptedException e) {

Directly propagate the exception

public void myMethod() throws InterruptedException {

Reset the interrupted flag and continue

This punts the problem forward. Hopefully later code will inspect the interrupted flag and handle it appropriately.

public void myMethod() {
  try {
  } catch (InterruptedException e) {

Reset the interrupted flag and wrap the InterruptedException

public void myMethod() {
  try {
  } catch (InterruptedException e) {
    throw new RuntimeException(e);

Always release resources, even in the event of an unchecked exception

Assume that any method might throw an unchecked exception and make sure this doesn’t cause resource leaks. We do not stop servers when RPC threads throw RuntimeExceptions. try-finally and try-with-resources blocks can make releasing resources much easier.

// with try-finally
Resource r = acquireResource();
try {
} finally {

// with try-with-resources
try (Resource r = acquireResource()) {

Be careful with exceptions thrown from finally blocks

If both the try block and the finally block throw exceptions, the exception from the try block will be lost and the finally block exception will be thrown. This is almost never desirable since the exception in the try block happened first. To avoid this, either make sure your finally blocks can’t throw exceptions, or use Guava’s Closer.

Using Guava’s Closer

Guava’s Closer is helpful for reducing boilerplate and making exception handling less error-prone, for releasing resources.

Closing multiple Closeables

Closer closer = new Closer();

If both calls to close() throw an exception, the first exception will be thrown and the second exception will be added as a suppressed exception of the first one.

The Closer Idiom

From the Closer javadoc:

Closer closer = Closer.create();
try {
  InputStream in = closer.register(openInputStream());
  OutputStream out = closer.register(openOutputStream());
  // do stuff
} catch (Throwable e) {
  // ensure that any checked exception types other than IOException that could be thrown are
  // provided here, e.g. throw closer.rethrow(e,  CheckedException.class);
  throw closer.rethrow(e);
} finally {

Closing resources only when an exception occurs

mCloser = new Closer();
mCloser.register(new resource1());
try {
} catch (Throwable t) {
  // We want to close resources and throw the original exception t. Any exceptions thrown while
  // closing resources should be suppressed on t.
  try {
    throw mCloser.rethrow(t);
  } finally {

We have a util method for closing a closer and suppressing its exceptions onto an existing exception

mCloser = new Closer();
mCloser.register(new resource1());
try {
} catch (Throwable t) {
  throw CommonUtils.closeAndRethrow(mCloser, t);

Code Health

Use the @Nullable annotation for all methods which may return null

This will improve static analysis of our code so that we can detect potential NullPointerExceptions before they happen.

Use the javax.annotation.Nullable import.

import javax.annotation.Nullable;

public String getName() {
  if (mName == "") {
    return null;
  return mName;

Use the @Nullable annotation for method parameters which are allowed to be null

When a method is specifically designed to be able to handle null parameters, those parameters should be annotated with @Nullable.

Use the javax.annotation.Nullable import.

import javax.annotation.Nullable;

public repeat(@Nullable String s) {
  if (s == null) {
    System.out.println("Hello world");
  } else {

Supply the variable name to Preconditions.checkNotNull

The preconditions check gives a more useful error message when you tell it the name of the variable being checked.

Preconditions.checkNotNull(blockInfo, "blockInfo") // Do this
Preconditions.checkNotNull(blockInfo); // Do NOT do this

Use static imports for standard test utilities

Tests are easier to read when there is less boilerplate. Use static imports for methods in org.junit.Assert, org.junit.Assume, org.mockito.Matchers, and org.mockito.Mockito.

// Change
// to

// It may be necessary to add the import
import static org.junit.Assert.assertFalse;

Unit Testing

Unit Test Goals

  1. Unit tests act as examples of how to use the code under test.
  2. Unit tests detect when an object breaks its specification.
  3. Unit tests don’t break when an object is refactored but still meets the same specification.

How to Write a Unit Test

  1. If creating an instance of the class takes some work, create a @Before method to perform shared setup steps. The @Before method gets run automatically before each unit test. Test-specific setup should be done locally in the tests that need it. In this example, we are testing a BlockMaster, which depends on a journal, clock, and executor service. The executor service and journal we provide are real implementations, and the TestClock is a fake clock which can be controlled by unit tests.
public void before() throws Exception {
  Journal blockJournal = new ReadWriteJournal(mTestFolder.newFolder().getAbsolutePath());
  mClock = new TestClock();
  mExecutorService =
  Executors.newFixedThreadPool(2,"TestBlockMaster-%d", true));
  mMaster = new BlockMaster(blockJournal, mClock, mExecutorService);
  1. If anything created in @Before creates something which needs to be cleaned up (e.g. a BlockMaster), create an @After method to do the cleanup. This method is automatically called after each test.
public void after() throws Exception {
  1. Decide on an element of functionality to test. The functionality you decide to test should be part of the public API and should not care about implementation details. Tests should be focused on testing only one thing.

  2. Give your test a name that describes what functionality it’s testing. The functionality being tested should ideally be simple enough to fit into a name, e.g. removeNonexistentBlockThrowsException, mkdirCreatesDirectory, or cannotMkdirExistingFile.

  public void detectLostWorker() throws Exception {
  1. Set up the situation you want to test. Here we register a worker and then simulate an hour passing. The HeartbeatScheduler section enforces that the lost worker heartbeat runs at least once.
// Register a worker.
long worker1 = mMaster.getWorkerId(NET_ADDRESS_1);
ImmutableMap.of("MEM", 100L),
ImmutableMap.of("MEM", 10L),

// Advance the block master's clock by an hour so that the worker appears lost.
mClock.setTimeMs(System.currentTimeMillis() + Constants.HOUR_MS);

// Run the lost worker detector.
HeartbeatScheduler.await(HeartbeatContext.MASTER_LOST_WORKER_DETECTION, 1, TimeUnit.SECONDS);
HeartbeatScheduler.await(HeartbeatContext.MASTER_LOST_WORKER_DETECTION, 1, TimeUnit.SECONDS);
  1. Check that the class behaved correctly:
// Make sure the worker is detected as lost.
Set<WorkerInfo> info = mMaster.getLostWorkersInfo();
assertEquals(worker1, Iterables.getOnlyElement(info).getId());
  1. Repeat from step #3 until the class’s entire public API has been tested.


  1. The tests for src/main/java/ should go in src/test/java/
  2. Tests do not need to handle or document specific checked exceptions. Prefer to simply add throws Exception to the test method signature.
  3. Aim to keep tests short and simple enough so that they don’t require comments to understand.

Patterns to avoid

  1. Avoid randomness. Edge cases should be handled explicitly.
  2. Avoid waiting for something by calling Thread.sleep(). This leads to slower unit tests and can cause flaky failures if the sleep isn’t long enough.
  3. Avoid using Whitebox to mess with the internal state of objects under test. If you need to mock a dependency, change the object to take the dependency as a parameter in its constructor (see dependency injection)
  4. Avoid slow tests. Mock expensive dependencies and aim to keep individual test times under 100ms.

Managing Global State

All tests in a module run in the same JVM, so it’s important to properly manage global state so that tests don’t interfere with each other. Global state includes system properties, Alluxio configuration, and any static fields. Our solution to managing global state is to use JUnit’s support for @Rules.

Changing Alluxio configuration during tests

Some unit tests want to test Alluxio under different configurations. This requires modifying the global Configuration object. When all tests in a suite need configuration parameters set a certain way, use ConfigurationRule to set them.

public ConfigurationRule mConfigurationRule = new ConfigurationRule(ImmutableMap.of(
    PropertyKey.key1, "value1",
    PropertyKey.key2, "value2"));

For configuration changes needed for an individual test, use Configuration.set(key, value), and create an @After method to clean up the configuration changes after the test:

public void after() {

public void testSomething() {
  Configuration.set(PropertyKey.key, "value");

Changing System properties during tests

If you need to change a system property for the duration of a test suite, use SystemPropertyRule.

public SystemPropertyRule mSystemPropertyRule = new SystemPropertyRule("propertyName", "value");

To set a system property during a specific test, use the SystemPropertyRule#toResource() method to get a Closeable for a try-catch statement:

public void test() {
  try (Closeable p = new SystemPropertyRule("propertyKey", "propertyValue").toResource()) {
    // Test something with propertyKey set to propertyValue.

Other global state

If a test needs to modify other types of global state, create a new @Rule for managing the state so that it can be shared across tests. One example of this is TtlIntervalRule.

MacOS System Settings for Unit Tests

Sometimes you will need to play with a few system settings in order to have the unit tests pass locally. A common setting that may need to be set is ulimit.

In order to increase the number of files and processes allowed on MacOS, run the following

$ sudo launchctl limit maxfiles 32768 32768
$ sudo launchctl limit maxproc 32768 32768

It is also recommended to exclude your local clone of Alluxio from Spotlight indexing. Otherwise, your Mac may hang constantly trying to re-index the file system during the unit tests. To do this, go to System Preferences > Spotlight > Privacy, click the + button, browse to the directory containing your local clone of Alluxio, and click Choose to add it to the exclusions list.