Where the dependency maintainer provides a tarball, prefer that over the automatically generated Github tarball. Github generated tarball SHA256 values can change when Github change their tar/gzip libraries breaking builds. Maintainer provided tarballs are more stable and the maintainer can provide the SHA256.

This is the preferred style of adding dependencies that use Bazel for their build process.

1. Define a new Bazel repository in bazel/repositories.bzl, in the envoy_dependencies() function.
2. Reference your new external dependency in some envoy_cc_library via the deps attribute.
3. bazel test //test/...

This is the preferred style of adding dependencies that use CMake for their build system.

1. Define a the source Bazel repository in bazel/repositories.bzl, in the envoy_dependencies() function.
2. Add an envoy_cmake rule to bazel/foreign_cc/BUILD. This will reference the source repository in step 1.
3. Reference your new external dependency in some envoy_cc_library via the name bound in step 1 deps attribute.
4. bazel test //test/...

Python dependencies should be added via pip and rules_python. The process is:

1. Define a pip_install() pointing at your target requirements.txt in bazel/repositories_extra.bzl

2. Add a requirements("<package name>") in the BUILD file that depends on this package.

You can use tools/config_validation/BUILD as an example for this flow. See also the rules_python documentation for further references.

1. Update the corresponding entry in the repository locations file.
2. bazel test //test/...

An external dependency built by genrule repository or native Bazel could be overridden by specifying Bazel option --override_repository to point to a local copy. The option can used multiple times to override multiple dependencies. The name of the dependency can be found in the repository locations file. The path of the local copy has to be absolute path.

For repositories built by envoy_cmake() in bazel/foreign_cc/BUILD, it is necessary to populate the local copy with some additional Bazel machinery to support --override_repository:

1. Place an empty WORKSPACE in the root.
2. Place a BUILD file with filegroup(name = "all", srcs = glob(["**"]), visibility = ["//visibility:public"]) in the root.

For all external dependencies, overriding with a local copy as described in the previous section is a useful tool.

Below we describe specific tips for obtaining additional debug for specific dependencies:

• libevent: add "EVENT__ENABLE_VERBOSE_DEBUG": "on", to cache_entries in the event target in bazel/foreign_cc/BUILD for verbose tracing of libevent processing.

• nghttp2: set ENVOY_NGHTTP2_TRACE in the environment and run at -l trace.

• QUICHE: set ENVOY_QUICHE_VERBOSITY=n in the environment to display verbose logs up to level n.

Usually Bazel downloads all dependencies during build time. But there is a possibility to prefetch dependencies and point Bazel to them by using --distdir option and providing a path to directory which contains tarballs with exactly the same name and the same SHA256 sum that are defined in repositories definitions.

For example, let's assume that your distdir location is $HOME/envoy_distdir. To prefetch boringssl which is defined in bazel/repository_locations.bzl as:

boringssl = dict(
    # Use commits from branch "chromium-stable-with-bazel"
    sha256 = "d1700e0455f5f918f8a85ff3ce6cd684d05c766200ba6bdb18c77d5dcadc05a1",
    strip_prefix = "boringssl-060e9a583976e73d1ea8b2bfe8b9cab33c62fa17",
    # chromium-70.0.3538.67
    urls = ["https://github.com/google/boringssl/archive/060e9a583976e73d1ea8b2bfe8b9cab33c62fa17.tar.gz"],
),

$HOME/envoy_distdir needs to contain 060e9a583976e73d1ea8b2bfe8b9cab33c62fa17.tar.gz file.

Then Envoy needs to be built with the following command:

bazel build --distdir=$HOME/envoy_distdir //source/exe:envoy