---
question: "Can one module read another module's document collections?"
kind: overview
---

> **You are reading the v2.x docs (engine modules). New projects use v3.** [Read this page in v3](/capabilities/build/platform/documents-overview) · [Migrate from 2.x to 3.0](/capabilities/upgrade/migrate-to-3)

# Documents

The document service stores mutable JSON-like records by collection, with declared fields, versions and cursor queries. It is the kernel service `services.document`. Each module gets its own namespace: the collections it creates are invisible to other modules. ABI keeps agent memory and job runs there too.

On ABI 3.x, super admins can browse namespaces, collections and documents in Nexus, under System › Data › Documents. Queries with filters are Python only.

:::note Not Nexus Documents
The Documents screen of a workspace edits rich-text documents, which are kept in Git. It does not read this service.
:::

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## Backends

`services.document` in `config.yaml` picks one adapter:

| Adapter | Storage | Used by |
|---|---|---|
| `sqlite` | A SQLite file (`path`, default `storage/documents.sqlite`) | `config.yaml` of a new project (`abi dev up`), and the engine default |
| `postgresql` | A PostgreSQL database (`dsn`, tables in schema `abi_document`) | `config.local.yaml` (Docker stack) |
| `nats_rpc` (ABI 3.x, NATS) | Another engine's document service, over NATS | Engines that share one store |

The keys of each adapter are in the [Configuration Reference](/user-guide/get-started/configuration#document-and-dataset).

## Key concepts

- **Namespace.** The module's name under `modules:` in `config.yaml`. The engine binds it when the module reads `self.engine.services.document`; you never pass it.
- **Collection.** Declared with `ensure_collection(CollectionSpec(...))` before the first write. A module can have as many as it needs. Declarations add fields; they never change a field's type.
- **Fields.** Declared fields have a type (`string`, `int`, `float`, `bool`, `datetime`, `bytes`, `json`) and can be indexed or unique. Undeclared keys are stored as well. Datetimes must be timezone-aware.
- **Versions.** Every document has a `version`, 1 on create, plus one per write. `if_version` makes a write or a delete conditional on it.
- **Queries.** `find` takes `(field, operator, value)` predicates combined with AND, an optional order and a cursor. Fields are top-level keys.
- **Who else uses it.** Agent conversation checkpoints (namespace `naas_abi_core.services.agent`), and, on ABI 3.x, job run records and the activity log in NATS mode.
- **Dependency.** A module lists `DocumentService` in `ModuleDependencies.services`. Without it, `self.engine.services.document` raises `ValueError`.