# Holger living inside another program — for idiots
> **Class:** `idiot`. No jargon. What "embedding" means and why it's a marriage, not a
> phone call. The code walk-through is [`embed-guide.md`](embed-guide.md).
## The idea
Holger doesn't have to be a separate server you talk to over the network. Another program
(written in Rust) can **link holger as a library** and run a little package registry right
inside its own process. No socket, no waiting on the network — it just calls holger's
functions directly: *"list the repos," "give me this package's bytes," "store these bytes."*
## Picture it as a marriage
Instead of two programs phoning each other across a network, holger and its host program
live in **one house**. When the host wants to know "what crates are in this registry?", it
just *calls holger's function* — no network hop, no packing the question into a message and
unpacking the answer again. holger hands back the very same data it holds internally. The
program that marries holger this way is **nornir**; its package-browser pane lists what's
inside the registries by calling holger's list function directly.
## The two little registries (the shared rooms)
The reference setup is a **dev pair** — two tiny registries holger can spin up for local
development:
- **`/cache`** — a transparent mirror of crates.io. Ask it for a crate; if it doesn't have
it yet, it fetches it from the internet once and keeps a copy.
- **`/sparring`** — a practice registry for rehearsing a package release before it goes
anywhere real.
These two are the rooms nornir and holger share.
## One bug worth knowing about (already fixed)
When a registry was the *writable* dev kind (like `/cache` and `/sparring`), asking it
"what's inside you?" used to come back **empty** even when it held packages — it was peeking
at the wrong handle. It now re-opens the live view, so the list is correct (commit
`8c9e458`). If you ever see an embedded dev registry look empty, that's the fix you need.
## Why marry instead of phone?
Two reasons. **Speed** — a function call is instant; a network round-trip is not. And
**simplicity** — there's no server to start, no port to open, no certificate to configure.
The host program already owns its own security; holger just rides along inside it. The
trade-off is that only *that one program* can use this registry — if independent machines
need to reach it, run the real `holger-server` instead.