docs/source/extension-guide/checklist.md

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(extension_checklist)=

# Extension author checklist

The rules in this section, gathered into one list to run through before you
publish. Each links to the page that explains it.

## Protocol

- [ ] **Every getter's capsule name matches its method name.**
      `__datafusion_<thing>__` returns a capsule named `datafusion_<thing>`.
      → {ref}`extension_capsule_protocol`
- [ ] **Your getter does not inspect its argument.** Pass it to
      `ffi_logical_codec_from_pycapsule` and move on. It is not always a
      session, and when it is, it is not the Python `SessionContext` wrapper.
      → {ref}`extension_getter_argument`
- [ ] **You never construct a `SessionContext` inside your library.** Take
      what the FFI constructors need off the argument you were handed. A
      context built inline is already dropped by the time the capsule is used.
      → {ref}`extension_getter_argument`
- [ ] **You do not depend on the `datafusion-python` crate.**
      → {ref}`extension_why_ffi`

## Codecs

- [ ] **Your codec claims narrowly.** Downcast to your own types. Claiming a
      broad category takes nodes from every library installed after you and
      makes your library order-sensitive for everyone downstream.
      → {ref}`extension_codec_order`
- [ ] **You declare `__datafusion_codec_id__` if you might rename the class.**
      The default id is the exporting class's import path, so a rename stops
      older plans decoding. → {ref}`extension_codec_ids`
- [ ] **Name-only decoders check `name` before trusting `buf`.** An empty
      payload has no id to route on, so your `try_decode_udf` can be called
      with another library's function name and an empty buffer.
      → {ref}`extension_codecs`
- [ ] **You round-trip a plan in a test and assert *your* codec did the work.**
      Both being installed does not mean your node reached you.
      → {ref}`extension_codec_order`

## Bundles and planners

- [ ] **You ship a bundle, not loose pieces**, if you have codecs, functions,
      or a planner.
      → {ref}`extension_bundles`
- [ ] **Your bundle is configuration-only.** Fresh components on every call,
      no cached bound components, no retaining the context passed in, no
      registering anything on it — declare what you contribute instead.
      → {ref}`extension_bundles_transaction`
- [ ] **Your codecs are objects exposing the getter, not bare capsules.**
      `with_extensions` refuses a capsule, because there would be nothing to
      name the codec by. → {ref}`extension_bundles_codecs_are_objects`
- [ ] **Your planner hook wraps `fallback` and delegates to it.** Ignoring it
      replaces every layer beneath you, which is legal but not composable.
      → {ref}`extension_bundles`
- [ ] **Your planner hook returns `None`, not `fallback`, when it has nothing
      to contribute.** Returning `fallback` installs the session's own planner
      as a foreign one and adds an FFI hop that was not there.
      → {ref}`extension_bundles`
- [ ] **You read the host's codec chains in the planner hook, not the extension
      hook.** Phase one runs before anything is installed.
      → {ref}`extension_bundles_two_phases`
- [ ] **If you also offer the low-level path**, document that codecs go in
      before a layered planner. → {ref}`planner_codec_rebinding`

## Packaging and documentation

- [ ] **You state which `datafusion` version your release requires.** A
      mismatch raises an `ImportError` on import, which is a good failure — but
      only if your users know what to install. → {ref}`extension_version_mismatch`
- [ ] **You tell your users to keep a context alive** for as long as anything
      derived from it is in use. This is the rule most likely to arrive as a
      bug report against your library. → {ref}`extension_sessions`
- [ ] **Your production codec serializes durable metadata**, not a
      process-local token. The examples in this repository use tokens to make
      ownership observable; that is a demonstration, not a pattern.
      → {ref}`extension_codec_durable_metadata`
- [ ] **You have integration tests across a real FFI boundary.** The two
      example crates in this repository are the pattern: build the cdylib,
      install the wheel, then exercise it from Python.