Alternative Inventory Backends

Kapitan's inventory is pluggable: you can swap the engine that loads and resolves it without rewriting a single class, target, or parameter. Three backends ship with Kapitan, and they differ in three things that matter when you choose one — speed, interpolation syntax, and which features they support.

reclass (default): the original Kapitan inventory (see reclass). Bundled — nothing to install.
reclass-rs: a Rust drop-in replacement, much faster (see reclass-rs). Optional extra.
omegaconf: an OmegaConf-based alternative with custom resolvers (see omegaconf). Optional extra, experimental.

Which backend should I use?

Stay on reclass unless you have a reason to switch. It is the default, the most mature, and supports every Kapitan inventory feature.
Switch to reclass-rs when inventory rendering is slow — large inventories with heavy parameter interpolation in class includes. It is a drop-in: same syntax, no inventory changes, just faster (benchmarks). Check the feature gaps first.
Choose omegaconf only if you specifically want its resolver toolkit (${merge:}, ${escape:}, conditionals, custom Python resolvers). It uses different interpolation syntax, so it is not a drop-in — existing inventories need migration. It is experimental.

Comparison

	`reclass` (default)	`reclass-rs`	`omegaconf`
Install	bundled	`pip install kapitan[reclass-rs]`	`pip install kapitan[omegaconf]`
Maturity	stable, default	stable	experimental (pre-release dep)
Implementation	Python	Rust	Python
Speed	baseline	much faster on large inventories (benchmarks)	comparable to reclass
Interpolation syntax	colon paths `${a:b:c}`	colon paths `${a:b:c}` (same as reclass)	dotted paths `${a.b.c}` + resolvers
Drop-in for reclass?	—	✅ yes, same syntax	❌ no, needs migration
List merge (class include)	extend / concatenate	extend / concatenate	extend + de-duplicate (`EXTEND_UNIQUE`)
Custom resolvers	❌	❌	✅ `${merge:}`, `${escape:}`, `${if:}`, user-defined, …
Inventory queries (`$[...]`, exports)	✅	❌ not supported	❌ not supported

List-merge semantics differ

reclass / reclass-rs concatenate lists on class inclusion (duplicates kept). omegaconf de-duplicates (EXTEND_UNIQUE) during class merge — but its ${merge:...} resolver keeps duplicates (EXTEND). If you switch backends, a list that previously had repeated entries may change. Verify list-valued parameters after migrating.

Switching backend

Pick the backend by either:

(recommended) setting it in the .kapitan config file:

global:
  inventory-backend: reclass-rs

or passing --inventory-backend=<backend> on the kapitan command line.

Feature support

reclass is the reference: it supports the full feature set.

reclass-rs is a drop-in, but a few reclass options are not implemented yet (known gaps, sourced from the upstream reclass-rs project — may change between versions):

inventory queries ($[...] expressions)
allow_none_override: false (effectively pinned to true; Kapitan's default is true, so the default case works)
storage types other than yaml_fs (yaml_git, mixed)

It also has minor behavioral divergences in non-compatible modes (e.g. null in nested references, dots in compose_node_name) — see the upstream README. If a feature you rely on is missing, open an issue upstream.

omegaconf supports a different feature set — see Differences from Reclass for what is and isn't supported, plus its resolver catalogue.