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+## Background
+
+Sometimes you need to merge one Git repo into another. This is a common task
+when administrating a monorepo.
+
+Here's a checklist that I follow:
+
+1. Detect leaked secrets.
+1. Rotate leaked secrets.
+1. Purge leaked secrets from repo history.
+1. Create mainline references to branches (for deployments).
+1. Subtree-merge into the target repo.
+1. Format the code.
+1. Celebrate!
+
+## Secrets
+
+**Note:** If you notice any leaked secrets, first and foremost rotate them
+before moving on...
+
+`gitleaks` supports `gitleaks protect`, but that doesn't seem to work for `WRN`
+level leaks, which in my experience often contain sensitive cleartext. We can
+use `git-filter-repo` to purge the cleartext from our repo history.
+
+Let's make a `secrets.txt` file that we can feed `git-filter-repo`:
+
+```shell
+λ gitleaks detect -r /tmp/secrets.json
+λ jq -r 'map_values(.Secret) | .[]' /tmp/secrets.txt
+```
+
+Now for the redacting...
+
+```shell
+λ git-filter-repo --force --replace-text /tmp/secrets.txt
+```
+
+Verify that the secrets were removed.
+
+```shell
+λ rg --hidden '\*\*\*REMOVED\*\*\*'
+λ gitleaks detect -v
+```
+
+Looks good! Let's move on to support the adopted repo's deploy strategy.
+
+## Supporting Deploys
+
+While deploying services when someone pushes to a given branch is a common
+deployment strategy, branch-based deployment don't make a whole lot of sense in
+a monorepo.
+
+When adopting another repo, you'll typically encounter a Github Action
+configuration that contains a section like this:
+
+```yaml
+on:
+  push:
+    - staging
+    - production
+```
+
+In our monorepo, `staging` and `production` don't exist. And I don't think we
+want to support them either. `staging` and `production` are ambiguous in a
+monorepo that hosts multiple services each of which likely having its own notion
+of `staging` and `production`.
+
+Doing "pinned releases" where a service is deployed from a `git` revision from
+the mainline branch works well in these scenarios. In order to support this we
+need to make sure the adopted repo has references to
+
+`git subtree add` asks us to define which branch it should use when grafting the
+repository onto our monorepo. We'll use `main` (or whatever the mainline branch
+is).
+
+In order to support the *current* deployments while migrating to a pinned
+release strategy, we have to ensure that `main` has a commit containing the same
+tree state as `staging` *and* another commit containing the same tree state as
+`production`. Let's do that!
+
+```shell
+λ git checkout main # ensure you're on the main branch
+λ git diff main staging >/tmp/main-to-staging.patch
+λ git diff main production >/tmp/main-to-production.patch
+```
+
+### staging
+
+```shell
+λ git apply /tmp/main-to-staging.patch
+λ git add . && git commit # chore: main -> staging
+λ git revert HEAD
+λ git commit --amend # revert: staging -> main
+```
+
+### production
+
+```shell
+λ git apply /tmp/main-to-production.patch
+λ git add . && git commit # chore: main -> production
+λ git revert HEAD
+λ git commit --amend # revert: production -> main
+```
+
+Now let's check our work:
+
+```shell
+λ git log --oneline
+38f4422 revert: production -> main
+f071a9f chore: main -> production
+02ea731 revert: staging -> main
+308ed90 chore: main -> staging
+```
+
+When we go to support pinned releases we can do something like so:
+
+```json
+{
+  "staging": "308ed90",
+  "production": "f071a9f"
+}
+```
+
+## Subtree Merge
+
+Now the repo is ready to be merged.
+
+```shell
+λ git subtree add --prefix=foo/bar/baz path/to/baz main
+λ git commit --amend # subtree: Dock baz into monorepo!
+```
+
+## Formatting
+
+Some CI enforces code formatting standards, so you may need to run that:
+
+```shell
+λ repofmt
+λ git add . && git commit # chore(fmt): Format the codes
+```
+
+Lastly, if you need the latest monorepo code from `origin/main` before opening a
+pull request, the following should work:
+
+```shell
+λ git fetch origin main && git rebase origin/main --rebase-merges --strategy=subtree
+```