We follow the Google Java Style Guide. See https://github.com/google/google-java-format for IDE plugins. The rules are not configurable.

The build will fail if the code is not formatted. To format all files from the command line, run:

mvn fmt:format -Dformat.validateOnly=false

Some aspects are not covered by the formatter:

• imports: please configure your IDE to follow the guide (no wildcard imports, normal imports in ASCII sort order come first, followed by a blank line, followed by static imports in ASCII sort order).
• braces must be used with if, else, for, do and while statements, even when the body is empty or contains only a single statement.
• implementation comments: wrap them to respect the column limit of 100 characters.
• XML files: indent with two spaces and wrap to respect the column limit of 100 characters.

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.

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) {

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.

The first 3 log levels are for ops:

• 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.

The last 2 levels are for developers:

• DEBUG: anything that would be useful to understand what the driver is doing from a "black box" perspective, i.e. if all you have are the logs.
• TRACE: same thing, but for events that happen very often, produce a lot of output, or should be irrelevant most of the time (this is a bit more subjective and left to your interpretation).

Logs statements start with a prefix that identifies its origin, for example:

• for components that are unique to the cluster instance, just the cluster name: [c0].
• for sessions, the cluster name + a generated unique identifier: [c0|s0].
• for channel pools, the session identifier + the address of the node: [c0|s0|/127.0.0.2:9042].
• for channels, the identifier of the owner (session or control connection) + the Netty identifier, which indicates the local and remote ports: [c0|s0|id: 0xf9ef0b15, L:/127.0.0.1:51482 - R:/127.0.0.1:9042].
• for request handlers, the session identifier, a unique identifier, and the index of the speculative execution: [c0|s0|1077199500|0].

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.

Please don't use java.util.stream in the driver codebase. Streams were designed for data processing, not to make your collection traversals "functional".

Here's an example from the driver codebase (ChannelSet):

DriverChannel[] snapshot = this.channels;
DriverChannel best = null;
int bestScore = 0;
for (DriverChannel channel : snapshot) {
  int score = channel.availableIds();
  if (score > bestScore) {
    bestScore = score;
    best = channel;
  }
}
return best;

And here's a terrible way to rewrite it using streams:

// Don't do this:
DriverChannel best =
    Stream.of(snapshot)
        .reduce((a, b) -> a.availableIds() > b.availableIds() ? a : b)
        .get();

The stream version is not easier to read, and will probably be slower (creating intermediary objects vs. an array iteration, compounded by the fact that this particular array typically has a low cardinality).

The driver never does the kind of processing that the stream API is intended for; the only large collections we manipulate are result sets, and these get passed on to the client directly.

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.

Static imports are permitted in a couple of places:

• AssertJ's assertThat / fail.
• Some Mockito methods, provided that you're already using a non-statically imported method at the beginning of the line. For example:

  // any and eq are statically imported, it's pretty clear that they at least relate to Mockito
  Mockito.verify(intCodec).decodePrimitive(any(ByteBuffer.class), eq(ProtocolVersion.DEFAULT));

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 TestNG's 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...

The build will fail if some license headers are missing. To update all files from the command line, run:

mvn license:format

Ensure pre-commit.sh is executable, then run:

ln -s ../../pre-commit.sh .git/hooks/pre-commit

This will only allow commits if the tests pass. It is also a good reminder to keep the test suite short.

Note: the tests run on the current state of the working directory. I tried to add a git stash in the script to only test what's actually being committed, but I couldn't get it to run reliably (it's still in there but commented). Keep this in mind when you commit, and don't forget to re-add the changes if the first attempt failed and you fixed the tests.

Keep your changes focused. Each commit should have a single, clear purpose expressed in its message. (Note: these rules can be somewhat relaxed during the initial developement phase, when adding a feature sometimes requires other semi-related features).

Resist the urge to "fix" cosmetic issues (add/remove blank lines, etc.) in existing code. This adds cognitive load for reviewers, who have to figure out which changes are relevant to the actual issue. If you see legitimate issues, like typos, address them in a separate commit (it's fine to group multiple typo fixes in a single commit).

Commit message subjects start with a capital letter, use the imperative form and do not end with a period:

• correct: "Add test for CQL request handler"
• incorrect: "Added test for CQL request handler"
• incorrect: "New test for CQL request handler"

Avoid catch-all messages like "Minor cleanup", "Various fixes", etc. They don't provide any useful information to reviewers, and might be a sign that your commit contains unrelated changes.

We don't enforce a particular subject line length limit, but try to keep it short.

You can add more details after the subject line, separated by a blank line. The following pattern (inspired by Netty) is not mandatory, but welcome for complex changes:

One line description of your change
 
Motivation:

Explain here the context, and why you're making that change.
What is the problem you're trying to solve.
 
Modifications:

Describe the modifications you've done.
 
Result:

After your change, what will change.

Like commits, pull requests should be focused on a single, clearly stated goal.

Avoid basing a pull request onto another one. If possible, the first one should be merged in first, and only then should the second one be created.

If you have to address feedback, avoid rebasing your branch and force-pushing (this makes the reviewers' job harder, because they have to re-read the full diff and figure out where your new changes are). Instead, push a new commit on top of the existing history; it will be squashed later when the PR gets merged. If the history is complex, it's a good idea to indicate in the message where the changes should be squashed:

* 20c88f4 - Address feedback (to squash with "Add metadata parsing logic") (36 minutes ago)
* 7044739 - Fix various typos in Javadocs (2 days ago)
* 574dd08 - Add metadata parsing logic (2 days ago)

(Note that the message refers to the other commit's subject line, not the SHA-1.)