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AGENTS.md

This file provides guidance to coding agents working with this repository.

Project Overview

Apache Traffic Server (ATS) is a high-performance HTTP/HTTPS caching proxy server written in C++20. It uses an event-driven, multi-threaded architecture with a sophisticated plugin system.

Key Technologies:

  • Language: C++20
  • Build System: CMake (migrated from autotools in v10)
  • Testing: Catch2 (unit tests) + AuTest Python framework (end-to-end tests)
  • Protocols: TLS, HTTP/1.1, HTTP/2, HTTP/3 (via Quiche)

Project Structure

Core sources live in src/ (for example src/proxy, src/iocore, src/traffic_server). Public headers are in include/. Built-in plugins are in plugins/ and plugins/experimental/. End-to-end tests are in tests/, especially tests/gold_tests/. Build system files are in cmake/ plus the top-level CMakeLists.txt, and docs are in doc/. Third party libraries that we include locally are in lib/.

Codex Agent Quick Start

Use this sequence for most tasks:

# Configure and build a dev tree.
cmake --preset dev
cmake --build build-dev

# Run a focused unit test or test suite.
ctest --test-dir build-dev -j4

# Format before commit.
cmake --build build-dev -t format

For environments using the default preset naming:

cmake --preset default
cmake --build build-default
ctest --test-dir build-default
cmake --build build-default -t format

If this file drifts from current project conventions, run /init to re-bootstrap agent guidance, then keep repository-specific commands and rules below.

Personal Preferences

If .codex/AGENTS.local.md exists, load it for user-specific style preferences and working conventions. This file is optional and should stay untracked.

Key CMake Options

  • BUILD_EXPERIMENTAL_PLUGINS=ON - Enable experimental plugins
  • ENABLE_QUICHE=ON - QUIC/HTTP3 support
  • ENABLE_CRIPTS=ON - Cripts scripting API
  • BUILD_REGRESSION_TESTING=ON - Enable legacy test suite
  • ENABLE_ASAN=ON - Configure ASan instrumentation
  • BUILD_TESTING=ON - Enable building of the Catch2 tests

Testing

Unit Tests (Catch2)

Built automatically with the project. Run via ctest:

ctest --test-dir build -j4

Running a Single Unit Test

Unit tests are built into executables. Find the test binary and run it directly:

# Unit tests are typically in build/src/<component>/
./build/src/tscore/test_tscore

End-to-End Tests (AuTest)

Enable autests during configuration:

cmake -B build -DENABLE_AUTEST=ON
cmake --build build
cmake --install build

Run all autests:

cmake --build build -t autest

Run specific test(s):

cd build/tests
./autest.sh --sandbox /tmp/sbcodex --clean=none -f <test_name_without_test_py>

For example, to run cache-auth.test.py:

./autest.sh --sandbox /tmp/sbcursor --clean=none -f cache-auth

To run multiple tests efficiently, pass the -j option.

cd build/tests
./autest.sh -j4 --sandbox /tmp/sbcodex --clean=none -f 'header_rewrite*'

Most end-to-end test coverage is in tests/gold_tests/. The CI system uses the Docker image ci.trafficserver.apache.org/ats/fedora:43 (Fedora version updated regularly).

Writing Autests

New tests should use the Test.ATSReplayTest() approach, which references a replay.yaml file that describes the test configuration and traffic patterns using the Proxy Verifier format. This is simpler, more maintainable, and parseable by tools.

For complete details on writing autests, see:

Development Workflow

Code Formatting

Always format code before committing:

cmake --build build -t format

Before Commit Checklist

  • Keep scope tight and avoid unrelated file churn.
  • Run formatter: cmake --build build -t format (or build-dev when used).
  • Run targeted tests for changed areas (or explain why tests were skipped).
  • Keep commit message body wrapped to 72 characters per line.
  • Describe why the change was needed, not only what changed.

Editing Safety

  • Do not revert unrelated working tree changes.
  • Prefer minimal diffs over broad refactors unless requested.
  • Do not edit generated artifacts unless the task explicitly requires it.
  • When touching behavior, add or update tests when practical.

Git Workflow

  • Branch off master for almost all PRs
  • PRs must pass all Jenkins CI jobs before merging
  • Use the GitHub PR workflow (not Jira)
  • Set appropriate labels: Backport, WIP, etc.

Commit Message Format

When an agent creates commits, use a short imperative summary line (under 50 characters when practical) plus concise body text (1-3 sentences) focused on "why" rather than only "what". If the PR fixes an issue, add 'Fixes: #<issue_number>'. Keep commit message body lines to 72 characters.

Architecture Overview

Core Components

I/O Core (src/iocore/):

  • eventsystem/ - Event-driven engine (Continuations, Events, Processors)
  • cache/ - Disk and RAM cache implementation
  • net/ - Network layer with TLS and QUIC support
  • dns/ - Asynchronous DNS resolution
  • hostdb/ - Internal DNS cache
  • aio/ - Asynchronous I/O

Proxy Logic (src/proxy/):

  • http/ - HTTP/1.1 protocol implementation
  • http2/ - HTTP/2 support
  • http3/ - HTTP/3 implementation
  • hdrs/ - Header parsing and management
  • logging/ - Flexible logging system
  • http/remap/ - URL remapping and routing

Management (src/mgmt/):

  • JSONRPC server for runtime configuration and management
  • Configuration file parsing and validation

Base Libraries (src/):

  • tscore/ - Core utilities and data structures
  • tsutil/ - Core utilities (metrics, debugging, regex, etc.)
  • api/ - Plugin API implementation
  • tscpp/api/ - C++ API wrapper for plugins
  • cripts/ - Cripts scripting framework for plugins

Event-Driven Architecture

ATS uses a Continuation-based programming model:

  • All async operations use Continuations (callback objects)
  • Events are dispatched through an event system
  • Multiple thread pools handle different event types
  • Non-blocking I/O throughout

Configuration Files

Primary configs (installed to /etc/trafficserver/ by default):

  • records.yaml - Main configuration (formerly records.config)
  • remap.config - URL remapping rules
  • plugin.config - Plugin loading configuration
  • ip_allow.yaml - IP access control
  • ssl_multicert.config - TLS certificate configuration
  • sni.yaml - SNI-based routing
  • storage.config - Cache storage configuration

Configuration can be modified at runtime via:

  • traffic_ctl config reload (for some settings)
  • JSONRPC API

Plugin Development

Plugin Types

  • Global plugins - Loaded in plugin.config, affect all requests
  • Remap plugins - Loaded per remap rule, affect specific mapped requests

Key Plugin APIs

Headers in include/ts/:

  • ts.h - Main plugin API
  • remap.h - Remap plugin API
  • InkAPIPrivateIOCore.h - Advanced internal APIs

Plugin Hooks

Plugins register callbacks at various points in request processing:

  • TS_HTTP_READ_REQUEST_HDR_HOOK
  • TS_HTTP_SEND_REQUEST_HDR_HOOK
  • TS_HTTP_READ_RESPONSE_HDR_HOOK
  • TS_HTTP_SEND_RESPONSE_HDR_HOOK
  • Many more (see ts.h)

Example Plugins

  • example/plugins/ - Simple example plugins
  • plugins/ - Stable core plugins
  • plugins/experimental/ - Experimental plugins

Common Development Patterns

When Modifying HTTP Processing

  1. Understand the state machine: src/proxy/http/HttpSM.cc - HTTP State Machine
  2. Hook into the appropriate stage via plugin hooks or core modification
  3. Use HttpTxn objects to access transaction state
  4. Headers are accessed via HDRHandle and field APIs

When Adding Configuration

  1. Define record in src/records/RecordsConfig.cc
  2. Add validation if needed
  3. Update documentation in doc/admin-guide/
  4. Configuration can be read via REC_ APIs

When Working with Cache

  • Cache is content-addressable, keyed by URL (modifiable via plugins)
  • Cache operations are async, continuation-based
  • Cache has RAM and disk tiers
  • Cache code is in src/iocore/cache/

When Debugging

  1. Enable debug tags in records.yaml: 
    proxy.config.diags.debug.enabled: 1
proxy.config.diags.debug.tags: http|cache
  2. Check logs in /var/log/trafficserver/:
    • diags.log - Debug output
    • error.log - Errors and warnings
  3. Use traffic_top for live statistics
  4. Use traffic_ctl for runtime inspection

Debug Controls in Code:

static DbgCtl dbg_ctl{"my_component"};
SMDebug(dbg_ctl, "Processing request for URL: %s", url);

Important Conventions

License Headers

  • Always add Apache License 2.0 headers to the top of new source and test files
  • This includes .cc, .h, .py, and other code files
  • Follow the existing license header format used in the codebase

Code Style

  • .editorconfig and .clang-format is the source of truth for baseline formatting rules.
  • C++20 standard (nothing from C++23 or later)
  • Use RAII principles
  • Prefer smart pointers for ownership
  • Don't use templates unless needed and appropriate
  • Run cmake --build build -t format before committing
  • Source code line length: 132 characters maximum
  • Don't add comments where the code documents itself, don't comment AI interactions

C++ Formatting (Mozilla-based style):

  • Indentation: 2 spaces for C/C++
  • Braces: Linux style (opening brace on same line)
  • Pointer alignment: Right (Type *ptr, not Type* ptr)
  • Variable declarations: Add empty line after declarations before subsequent code
  • Avoid naked conditions (always use braces with if statements)

Naming Conventions:

  • CamelCase for classes: HttpSM, NetVConnection
  • snake_case for variables and functions: server_entry, handle_api_return()
  • UPPER_CASE for macros and constants: HTTP_SM_SET_DEFAULT_HANDLER
  • Private member variables have the m_ prefix.

Modern C++ Patterns (Preferred):

// GOOD - Modern C++20
auto buffer = std::make_unique<MIOBuffer>(alloc_index);
for (const auto &entry : container) {
  if (auto *vc = entry.get_vc(); vc != nullptr) {
    // Process vc
  }
}

// AVOID - Legacy C-style
MIOBuffer *buffer = (MIOBuffer*)malloc(sizeof(MIOBuffer));

Python Code Style (for tests and tools)

  • Python 3.11+ with proper type annotations
  • 4-space indentation, never TABs

Memory Management

  • Custom allocators supported (jemalloc, mimalloc)
  • Use ats_malloc family for large allocations
  • IOBuffers for network data (zero-copy where possible)

Threading

  • Event threads handle most work
  • DNS has dedicated threads
  • Disk I/O uses thread pool
  • Most code should be async/event-driven, not thread-based

Error Handling

  • Return error codes for recoverable errors
  • Use ink_release_assert for unrecoverable errors
  • Log errors appropriately (ERROR vs WARNING vs NOTE)

Key Files for Understanding Architecture

  • include/iocore/eventsystem/Continuation.h - Core async pattern
  • src/proxy/http/HttpSM.cc - HTTP request state machine
  • src/iocore/net/UnixNetVConnection.cc - Network connection handling
  • src/iocore/cache/Cache.cc - Cache implementation
  • src/proxy/http/remap/RemapConfig.cc - URL remapping logic
  • include/ts/ts.h - Plugin API

Resources