Onboarding engineers to a large codebase

In a large codebase, a new engineer's first months are mostly spent finding things out: where the code for X lives, who owns it, why it's built that way, and what will break if they touch it. Most of that knowledge is in senior engineers' heads, and each question costs someone else twenty minutes. Good onboarding moves that knowledge somewhere a new person can find it by themselves.

Set a target that isn't "understands everything"

Nobody understands all of a large codebase, including the people who've been there five years. Give new engineers concrete milestones instead:

• Day 1: laptop set up, repo builds, tests run, they can open a PR.
• Week 1: a small change merged and deployed to production.
• Month 1: owns a small feature or bug fix in one area, start to finish.
• Month 3: reviews others' PRs in that area and is in the on-call rotation, with backup.

Shipping something real in week one matters more than any document. It forces the new person through the build, review, CI and deploy, and it tells them the team trusts them.

Make setup a script, not a wiki page

If setup takes more than an hour, it's a bug. Put it in a script or a dev container and treat failures as build failures. Ask each new hire to fix whatever broke for them: they're the only people who still see the gaps. After three or four hires, setup is usually solid.

Give them a map, not a tour

A two-hour architecture walkthrough on day one is forgotten by day three. What sticks is a short written map they can come back to:

• One diagram: services, data stores, queues, and who calls whom.
• The entry points: HTTP routes, workers, scheduled jobs.
• A table of top-level directories, one line each, with an owner.
• The five or so conventions that aren't obvious: how errors are handled, where business logic goes, how feature flags work.

Keep it next to the code, in the repo, so it's reviewed when the code changes. A CODEOWNERS file does double duty: it routes reviews and tells newcomers who to ask.

Curate the first tasks

"Good first issue" labels usually collect the tasks nobody wanted. Pick first tasks on purpose instead: each should touch a different layer (an API endpoint, a background job, a UI change), be small enough to finish in a day or two, and have a named reviewer who's expecting it. The goal is to learn the paths through the system, not the specific feature.

Pair on reading, not just on writing

An hour of a senior engineer tracing a real request with the new person, out loud, beats a week of reading alone. The useful part isn't the code; it's the commentary: "this looks odd but it's there because of the 2024 double-charge incident", "don't trust this cache, it's invalidated in three places". Record those remarks. They're the most valuable onboarding material you have and they're almost never written down.

Teach them to find the "why" themselves

The question new engineers ask most is some version of "why is it like this?" Every answer that goes through a person is an interruption and doesn't scale. Teach the self-serve path early:

• Blame with -w -C -C, then follow the commit to its pull request and read the review.
• git log -L for the full history of a function; the pickaxe for values that moved.
• Check linked tickets, ADRs and postmortems.

We've written these up as why is this code like this? and git blame alternatives. Share them with new hires in their first week. For the new hire's own side of this, see how to understand a legacy codebase fast.

Make the team write down reasons as it goes

Onboarding is slow mostly because of decisions that were never recorded. A few habits change that without much process:

• PR descriptions that explain why, not just what. A template with a "Why" section helps.
• Short ADRs for decisions that will surprise someone later: one page, what was chosen, what was rejected, why.
• Comments with links on anything odd: # see #1482: 3 retries doubled charges.
• Answer questions in public channels, so the next person can search for the answer.

Measure it

Track time to first merged PR, time to first on-call shift, and ask every new hire at 30 days: "What took you longest to find out?" The answers show you exactly which document to write next. If the same question comes up from three hires, it belongs in the repo.

Where Enhanciar fits

Enhanciar is built for the "why is it like this?" questions. It reads your commits, pull requests, review comments and docs, and answers in plain language with the file:line and PR each answer comes from, so a new engineer can check the source instead of interrupting a senior one. Their AI coding agents can ask the same questions over MCP. It's in early access.

Join the waitlist at enhanciar.in →

Related guides

• Why is this code like this? How to find the reason behind any line
• How to understand a legacy codebase fast
• Git blame alternatives: how to find why code changed

FAQ

How long does it take to onboard an engineer to a large codebase?

Expect a first merged change in week one, ownership of a small area in about a month, and reviewing and on-call in around three months. Nobody understands a whole large codebase, so aim for milestones rather than full understanding.

What should be in engineering onboarding documentation?

A scripted local setup, one architecture diagram, entry points, a table of top-level directories with owners, the non-obvious conventions, and links to ADRs. Keep it in the repo so it is reviewed with code changes.

What are good first tasks for a new engineer?

Small, deliberately chosen tasks that touch different layers, such as an API endpoint, a background job and a UI change, each with a named reviewer and finishable in a day or two.

How do new engineers find out why code was written a certain way?

Teach them to use git blame with -w -C -C, git log -L and the pickaxe to find the commit, then read the pull request review and linked ticket. Encourage the team to write the reason in PR descriptions and code comments.