Apache Kudu - Contributing to Apache Kudu

Contributing to Apache Kudu

Contributing Patches Using Gerrit

The Kudu team uses Gerrit for code review, rather than Github pull requests. Typically, you pull from Github but push to Gerrit, and Gerrit is used to review code and merge it into Github.

See the Gerrit Tutorial for an overview of using Gerrit for code review.

Initial Setup for Gerrit

Sign in to Gerrit using your Github username.
Go to Settings. Update your name and email address on the Contact Information page, and upload a SSH public key. If you do not update your name, it will show up as "Anonymous Coward" in Gerrit reviews.
If you have not done so, clone the main Kudu repository. By default, the main remote is called origin. When you fetch or pull, you will do so from origin.
```
git clone https://github.com/apache/kudu
```
Change to the new kudu directory.
Add a gerrit remote. In the following command, substitute <username> with your Github username.
```
git remote add gerrit ssh://<username>@gerrit.cloudera.org:29418/kudu
```
Run the following command to install the Gerrit commit-msg hook. Use the following command, replacing <username> with your Github username.
```
gitdir=$(git rev-parse --git-dir); scp -p -P 29418 <username>@gerrit.cloudera.org:hooks/commit-msg ${gitdir}/hooks/
```
Be sure you have set the Kudu repository to use pull --rebase by default. You can use the following two commands, assuming you have only ever checked out master so far:
```
git config branch.autosetuprebase always
git config branch.master.rebase true
```
If for some reason you had already checked out branches other than master, substitute master for the other branch names in the second command above.

To submit a patch, first commit your change (using a descriptive multi-line commit message if possible), then push the request to the gerrit remote. For instance, to push a change to the master branch:

git push gerrit HEAD:refs/for/master --no-thin

or to push a change to the gh-pages branch (to update the website):

git push gerrit HEAD:refs/for/gh-pages --no-thin

While preparing a patch for review, it’s a good idea to follow generic git commit guidelines and good practices.

The --no-thin argument is a workaround to prevent an error in Gerrit. See https://code.google.com/p/gerrit/issues/detail?id=1582.

Consider creating Git aliases for the above commands. Gerrit also includes a command-line tool called git-review, which you may find helpful.

Gerrit will add a change ID to your commit message and will create a Gerrit review, whose URL will be emitted as part of the push reply. If desired, you can send a message to the kudu-dev mailing list, explaining your patch and requesting review.

After getting feedback, you can update or amend your commit, (for instance, using a command like git commit --amend) while leaving the Change ID intact. Push your change to Gerrit again, and this will create a new patch set in Gerrit and notify all reviewers about the change.

When your code has been reviewed and is ready to be merged into the Kudu code base, a Kudu committer will merge it using Gerrit. You can discard your local branch.

Abandoning a Review

If your patch is not accepted or you decide to pull it from consideration, you can use the Gerrit UI to Abandon the patch. It will still show in Gerrit’s history, but will not be listed as a pending review.

Reviewing Patches In Gerrit

You can view a unified or side-by-side diff of changes in Gerrit using the web UI. To leave a comment, click the relevant line number or highlight the relevant part of the line, and type 'c' to bring up a comment box. To submit your comments and/or your review status, go up to the top level of the review and click Reply. You can add additional top-level comments here, and submit them.

To check out code from a Gerrit review, click Download and paste the relevant Git commands into your Git client. You can then update the commit and push to Gerrit to submit a patch to the review, even if you were not the original reviewer.

Gerrit allows you to vote on a review. A vote of +2 from at least one committer (besides the submitter) is required before the patch can be merged.

Code Style

Get familiar with these guidelines so that your contributions can be reviewed and integrated quickly and easily.

In general, Kudu follows the Google C++ Style Guide, with the following exceptions:

Notes on C++ 11

Kudu uses C++ 11. Check out this handy guide to C++ 11 move semantics and rvalue references: https://www.chromium.org/rvalue-references

We aim to follow most of the same guidelines, such as, where possible, migrating away from foo.Pass() in favor of std::move(foo).

Limitations on `boost` Use

boost classes from header-only libraries can be used in cases where a suitable replacement does not exist in the Kudu code base. However:

Do not introduce dependencies on boost classes not already depended on. We are actively trying to remove boost from the Kudu code base, and new dependencies make that work more difficult.
Do not use boost in any public headers for the Kudu C++ client, because boost commonly breaks backward compatibility, and passing data between two boost versions (one by the user, one by Kudu) causes serious issues.

Line length

The Kudu team allows line lengths of 100 characters per line, rather than Google’s standard of 80. Try to keep under 80 where possible, but you can spill over to 100 or so if necessary.

Pointers

Smart Pointers and Singly-Owned Pointers

Generally, most objects should have clear "single-owner" semantics. Most of the time, singly-owned objects can be wrapped in a gscoped_ptr<> or in a unique_ptr which ensures deletion on scope exit and prevents accidental copying. gscoped_ptr is similar to C++11’s unique_ptr in that it has a release method and also provides emulated move semantics (see gscoped_ptr.h for example usage).

If an object is singly owned, but referenced from multiple places, such as when the pointed-to object is known to be valid at least as long as the pointer itself, associate a comment with the constructor which takes and stores the raw pointer, as in the following example.

  // 'blah' must remain valid for the lifetime of this class
  MyClass(const Blah* blah) :
    blah_(blah) {
  }

If you use raw pointers within STL collections or inside of vectors and other containers, associate a comment with the container, which explains the ownership semantics (owned or un-owned). Use utility code from gutil/stl_util.h, such as STLDeleteElements or ElementDeleter, to ease handling of deletion of the contained elements.

Using std::auto_ptr is strictly disallowed because of its difficult and bug-prone semantics. Besides, std::auto_ptr is declared deprecated since C++11.

Smart Pointers for Multiply-Owned Pointers:

Although single ownership is ideal, sometimes it is not possible, particularly when multiple threads are in play and the lifetimes of the pointers are not clearly defined. In these cases, you can use either std::shared_ptr or Kudu’s own scoped_refptr from gutil/ref_counted.hpp. Each of these mechanisms relies on reference counting to automatically delete the referent once no more pointers remain. The key difference between these two types of pointers is that scoped_refptr requires that the object extend a RefCounted base class, and stores its reference count inside the object storage itself, while shared_ptr maintains a separate reference count on the heap.

The pros and cons are:

shared_ptr

can be used with any type of object, without the object deriving from a special base class
part of the standard library and familiar to most C++ developers
supports the weak_ptr use cases:
- a temporary ownership when an object needs to be accessed only if it exists
- break circular references of shared_ptr, if any exists due to aggregation
you can convert from the shared_ptr into the weak_ptr and back
if creating an instance with std::make_shared<>() only one allocation is made (since C++11; a non-binding requirement in the Standard, though)
if creating a new object with shared_ptr<T> p(new T) requires two allocations (one to create the ref count, and one to create the object)
the ref count may not be near the object on the heap, so extra cache misses may be incurred on access
the shared_ptr instance itself requires 16 bytes (pointer to the ref count and pointer to the object)

scoped_refptr

only requires a single allocation, and ref count is on the same cache line as the object
the pointer only requires 8 bytes (since the ref count is within the object)
you can manually increase or decrease reference counts when more control is required
you can convert from a raw pointer back to a scoped_refptr safely without worrying about double freeing
since we control the implementation, we can implement features, such as debug builds that capture the stack trace of every referent to help debug leaks.
the referred-to object must inherit from RefCounted
does not support the weak_ptr use cases

Since scoped_refptr is generally faster and smaller, try to use it rather than shared_ptr in new code. Existing code uses shared_ptr in many places. When interfacing with that code, you can continue to use shared_ptr.

Function Binding and Callbacks

Existing code uses boost::bind and boost::function for function binding and callbacks. For new code, use the Callback and Bind classes in gutil instead. While less full-featured (Bind doesn’t support argument place holders, wrapped function pointers, or function objects), they provide more options by the way of argument lifecycle management. For example, a bound argument whose class extends RefCounted will be incremented during Bind and decremented when the Callback goes out of scope.

See the large file comment in gutil/callback.h for more details, and util/callback_bind-test.cc for examples.

`CMake` Style Guide

CMake allows commands in lower, upper, or mixed case. To keep the CMake files consistent, please use the following guidelines:

built-in commands in lowercase

add_subdirectory(some/path)

built-in arguments in uppercase

message(STATUS "message goes here")

custom commands or macros in uppercase

ADD_KUDU_TEST(some-test)

GFlags

Kudu uses gflags for both command-line and file-based configuration. Use these guidelines to add a new gflag. All new gflags must conform to these guidelines. Existing non-conformant ones will be made conformant in time.

Name

The gflag’s name conveys a lot of information, so choose a good name. The name will propagate into other systems, such as the Configuration Reference.

The different parts of a multi-word name should be separated by underscores. For example, fs_data_dirs.
The name should be prefixed with the context that it affects. For example, webserver_num_worker_threads and cfile_default_block_size. Context can be difficult to define, so bear in mind that this prefix will be used to group similar gflags together. If the gflag affects the entire process, it should not be prefixed.
If the gflag is for a quantity, the name should be suffixed with the units. For example, tablet_copy_idle_timeout_ms.
Where possible, use short names. This will save time for those entering command line options by hand.
The name is part of Kudu’s compatibility contract, and should not change without very good reason.

Default value

Choosing a default value is generally simple, but like the name, it propagates into other systems.

The default value is part of Kudu’s compatibility contract, and should not change without very good reason.

Description

The gflag’s description should supplement the name and provide additional context and information. Like the name, the description propagates into other systems.

The description may include multiple sentences. Each should begin with a capital letter, end with a period, and begin one space after the previous.
The description should NOT include the gflag’s type or default value; they are provided out-of-band.
The description should be in the third person. Do not use words like you.
A gflag description can be changed freely; it is not expected to remain the same across Kudu releases.

Testing

All new code should have tests.: Add new tests either in existing files, or create new test files as necessary.
All bug fixes should have tests.: It’s OK to fix a bug without adding a new test if it’s triggered by an existing test case. For example, if a race shows up when running a multi-threaded system test after 20 minutes or so, it’s worth trying to make a more targeted test case to trigger the bug. But if that’s hard to do, the existing system test should be enough.
Tests should run quickly (< 1s).: If you want to write a time-intensive test, make the runtime dependent on KuduTest#AllowSlowTests, which is enabled via the KUDU_ALLOW_SLOW_TESTS environment variable and is used by Jenkins test execution.
Tests which run a number of iterations of some task should use a gflags command-line argument for the number of iterations.: This is handy for writing quick stress tests or performance tests.
Commits which may affect performance should include before/after perf-stat(1) output.: This will show performance improvement or non-regression. Performance-sensitive code should include some test case which can be used as a targeted benchmark.

Documentation

See Documentation Style Guide for guidelines about contributing to the official Kudu documentation.