Getting Started with Your First Contribution to Redis

A repository this large needs a clear answer to make at the top, and Redis gives one in nineteen lines. The header comment is honest: the real build logic lives in src/Makefile. SUBDIRS = src sets the default; the BUILD_WITH_MODULES=yes branch appends modules for builds that include Bloom filter, Cuckoo filter, and the other module-backed types listed in the README. The .DEFAULT rule is the dispatcher: any target you pass on the command line is forwarded to each subdir via a per-directory $(MAKE) -C. So make runs all in src; make test runs test in src; make install uses the explicit rule on line 16 for the same effect.

Redis is a single binary, and main() is the only entry point. The first surprise on opening it is that the very first branch is not server startup at all: when compiled with REDIS_TEST, the binary doubles as a unit-test runner. Calling ./redis-server test all walks the static redisTests table and runs each test in process, with optional accuracy, large-memory, and valgrind switches passed as additional arguments. monotonicInit() on line 7828 has to run before any test that depends on Redis dict rehashing timing. Past this block the function falls through into the actual server bootstrap: argument parsing, initServerConfig(), initServer(), then aeMain() on the event loop.

Redis tests are written in Tcl, which is a deliberate inheritance from antirez's first Redis prototype. runtest exists because Tcl ships with version-suffixed binaries on most distributions: tclsh8.6, tclsh9.0, and so on. The script tries each known version in order and remembers the last one it finds. If none exists, it prints a clear error and exits with status 1, which is the behaviour you want on a fresh machine without Tcl installed. The final line passes every command-line argument through ${@} to tests/test_helper.tcl, so any selector argument such as a single test path reaches the test runner unchanged. Two sister scripts, runtest-cluster and runtest-sentinel, follow the same pattern for the cluster and Sentinel suites.

If your first contribution adds a command, the file you write lives in src/commands/. echo.json is the smallest realistic example. arity: 2 means exactly one argument is required; function: "echoCommand" names the C handler that the build wires up for you. command_flags declares runtime guarantees: FAST says the command is bounded constant-time, LOADING and STALE let it run during dataset load and on stale replicas. acl_categories drives ACL rule matching. reply_schema is the contract a client sees on the wire, and is also what the documentation site renders. Adding a command means writing the JSON, writing the matching somethingCommand in C, and letting the build regenerate the dispatch table.

Redis stopped being BSD-licensed in March 2024, and the licence on every contribution changed with it. The first sentence of CONTRIBUTING.md is the one that matters: any code you submit is accepted only under the Redis Software Grant and Contributor License Agreement and falls under the tri-license of RSALv2, SSPLv1, and AGPLv3. The CLA itself, headed at line 7, is what you grant to Redis Ltd.; section 3 (further down the file) is the perpetual copyright licence, and section 4 is the patent grant. Read these before you open a pull request, because the project will not merge code from a contributor who has not accepted them. The trademark and issue-routing notes appear later in the same file.

Once orientation is done, the next file to open is src/ae.c. aeMain() is six lines of executable code and is also the heart of the server: it spins until eventLoop->stop is set, calling aeProcessEvents() with the flags that ask for I/O events plus the before-sleep and after-sleep hooks. There are no threads on the hot path, so commands run serially and the source can be read without tracking concurrent execution. The architecture tour on this Explore page picks up here and walks through how a client byte becomes a parsed command, a dict lookup, and a reply, in roughly twenty-five minutes. Replication, persistence, cluster, streams, sentinel, pub/sub, data structures, and the RESP protocol each have their own deeper tour.