# peo

[neo4j-browser](https://github.com/neo4j/neo4j-browser), cypher-shell and the Neo4j
drivers on top of PostgreSQL, using [Apache AGE](https://github.com/apache/age) as the
graph engine.

peo is a small Python server that speaks Neo4j's Bolt protocol and translates Cypher
into AGE `cypher()` calls. It can also serve neo4j-browser. A [container
image](#container-image) bundles everything, together with
[pgAdmin](https://www.pgadmin.org/) for working with the same data in SQL.

```
 neo4j-browser (static files)      peo (Python)                     PostgreSQL + AGE
 ┌──────────────────────┐  Bolt  ┌────────────────────────-──┐  SQL  ┌────────────────┐
 │ in your web browser  │───────▶│ Bolt server (TCP + WS)    │──────▶│ cypher('g',$$  │
 │ served on :7474      │ :7687  │ Cypher → AGE translation  │ :5432 │   ... $$)      │
 └──────────────────────┘        │ emulated Neo4j procedures │       └────────────────┘
                                 └─────────────────────────-─┘
```

## Installation

peo needs Python 3.11 or later and a PostgreSQL server with the AGE extension installed
and loaded (`shared_preload_libraries=age`).

```sh
pip install 'peo[binary]'
PEO_PG_HOST=localhost PEO_PG_DATABASE=postgres peo
```

The `binary` extra pulls in psycopg's prebuilt wheels; leave it out to use a psycopg
built against your system's libpq. `python -m peo` works the same as `peo`.

Neo4j clients can now connect to `neo4j://localhost:7687` with a PostgreSQL user name
and password.

peo does not include neo4j-browser itself. To serve it, install the `browser` extra
([Starlette](https://www.starlette.io/) and [uvicorn](https://www.uvicorn.org/)),
build neo4j-browser and point `PEO_BROWSER_DIR` at its `dist` directory; it is then
served on <http://localhost:7474/>. The container image does all of this for you.

```sh
pip install 'peo[binary,browser]'
PEO_PG_HOST=localhost PEO_PG_DATABASE=postgres PEO_BROWSER_DIR=/path/to/neo4j-browser/dist peo
```

## Configuration

peo is configured through environment variables:

| Variable | Default | Purpose |
| --- | --- | --- |
| `PEO_PG_HOST` | `127.0.0.1` | PostgreSQL host. |
| `PEO_PG_PORT` | `5432` | PostgreSQL port. |
| `PEO_PG_DATABASE` | `$POSTGRES_DB`, else `$POSTGRES_USER`, else `postgres` | PostgreSQL database holding the graphs. |
| `PEO_DEFAULT_GRAPH` | `neo4j` | Graph used when a client does not pick a database. |
| `PEO_BOLT_PORT` | `7687` | Port the Bolt server listens on (TCP and WebSocket). |
| `PEO_HTTP_PORT` | `7474` | Port neo4j-browser is served on, when `PEO_BROWSER_DIR` is set. |
| `PEO_BROWSER_DIR` | (unset) | neo4j-browser `dist` directory to serve; needs the `browser` extra. |
| `PEO_ADVERTISED_BOLT_PORT` | `$PEO_BOLT_PORT` | Bolt port advertised to the browser; set it when clients reach the Bolt port on another port. |
| `PEO_ADVERTISED_HOST` | `localhost` | Host reported in `SHOW DATABASES` and to clients that do not send a routing address. |
| `PEO_LOG_LEVEL` | `INFO` | `DEBUG` logs every generated SQL statement. |

## How it works

- **Databases are AGE graphs.** Every AGE graph shows up as a Neo4j database, and
  `:use <name>` switches between them. Clients that do not pick a database get `PEO_DEFAULT_GRAPH`
  (`neo4j`); the container image creates that graph on first start. `CREATE DATABASE x` and `DROP DATABASE x` on the `system`
  database call `create_graph` / `drop_graph`; AGE requires names of at least 3 characters.
- **Queries** are sent to AGE as `SELECT * FROM cypher('<graph>', $$ <query> $$) AS (...)`.
  peo works out the result columns from the final `RETURN` clause and inlines
  query parameters as Cypher literals.
- **Neo4j procedures** that neo4j-browser relies on (`dbms.components`, `SHOW DATABASES`,
  `db.labels`, `db.schema.visualization`, `SHOW PROCEDURES`, ...) are emulated from the
  AGE catalog.
- **Workarounds for AGE**: neo4j-browser's "expand node" query uses a pattern
  comprehension, and its relationship lookup uses `id(n) IN [...]`, which AGE rejects
  for lists of more than one element. peo rewrites both into equivalent Cypher
  that AGE accepts.
- **Authentication** is passed straight through: every Bolt connection opens its own
  PostgreSQL connection with the credentials you log in with.

## Querying the graph from SQL

AGE stores each node label and relationship type as a table in the graph's schema,
with all properties in one `agtype` column, `properties`; `properties ->> 'key'::text`
returns a property as text. For typed columns, add them yourself, either as a view or
as a generated column:

```sql
CREATE VIEW neo4j.people AS
    SELECT id, properties ->> 'name'::text AS name, (properties ->> 'age'::text)::bigint AS age
    FROM neo4j."Person";

ALTER TABLE neo4j."Person"
    ADD COLUMN age bigint GENERATED ALWAYS AS ((properties ->> 'age'::text)::bigint) STORED;
```

PostgreSQL keeps generated columns up to date on every Cypher `CREATE`, `MERGE`, `SET`
and `REMOVE`, and they can be indexed. Some things to keep in mind:

- A value that does not fit the column's type makes the Cypher write fail: with the
  column above, `CREATE (:Person {age: 'old'})` is rejected. A view only fails when it
  is read.
- AGE does not allow dropping columns from its label tables, so a generated column
  cannot be removed again.
- Adding a stored generated column rewrites the table and locks it while doing so.
- AGE has no date or time types, so store those as ISO 8601 strings and cast them in
  SQL.

## Querying the graph from SQL

AGE stores each node label and relationship type as a table in the graph's schema,
with all properties in one `agtype` column, `properties`; `properties ->> 'key'::text`
returns a property as text. For typed columns, add them yourself, either as a view or
as a generated column:

```sql
CREATE VIEW neo4j.people AS
    SELECT id, properties ->> 'name'::text AS name, (properties ->> 'age'::text)::bigint AS age
    FROM neo4j."Person";

ALTER TABLE neo4j."Person"
    ADD COLUMN age bigint GENERATED ALWAYS AS ((properties ->> 'age'::text)::bigint) STORED;
```

PostgreSQL keeps generated columns up to date on every Cypher `CREATE`, `MERGE`, `SET`
and `REMOVE`, and they can be indexed. Some things to keep in mind:

- A value that does not fit the column's type makes the Cypher write fail: with the
  column above, `CREATE (:Person {age: 'old'})` is rejected. A view only fails when it
  is read.
- AGE does not allow dropping columns from its label tables, so a generated column
  cannot be removed again.
- Adding a stored generated column rewrites the table and locks it while doing so.
- AGE has no date or time types, so store those as ISO 8601 strings and cast them in
  SQL.

## Container image

The container image runs PostgreSQL with AGE, peo, neo4j-browser and pgAdmin in one
container:

```sh
podman build -t peo .
podman run -d --name peo -e POSTGRES_PASSWORD=secret \
    -p 7474:7474 -p 7687:7687 -p 5050:5050 -p 5432:5432 \
    -v peo-data:/var/lib/postgresql -v peo-pgadmin:/var/lib/pgadmin peo
```

| Port | Service | Log in with |
| --- | --- | --- |
| 7474 | neo4j-browser: <http://localhost:7474/> | PostgreSQL user `postgres` and `POSTGRES_PASSWORD` |
| 7687 | Bolt, for Neo4j drivers and cypher-shell (`neo4j://localhost:7687`) | same as above |
| 5050 | pgAdmin: <http://localhost:5050/> | `admin@example.com` and `POSTGRES_PASSWORD` |
| 5432 | PostgreSQL, for `psql` and other SQL clients | same as neo4j-browser |

pgAdmin comes with the bundled PostgreSQL server already registered as "peo";
it asks for the PostgreSQL password the first time you open it. AGE stores each
graph as a schema of the same name, with one table per label.

Any OCI-compatible build tool works. With Docker, pass the file name explicitly:
`docker build -f Containerfile -t peo .`; `docker run` takes the same options as
`podman run`.

Besides peo's own variables above, the image uses these:

| Variable | Default | Purpose |
| --- | --- | --- |
| `POSTGRES_PASSWORD` | (required) | Password of the PostgreSQL superuser, see the [postgres image docs](https://hub.docker.com/_/postgres). `POSTGRES_USER` and `POSTGRES_DB` work too. |
| `PEO_DEFAULT_GRAPH` | `neo4j` | Also the graph created when the database is initialised. |
| `PGADMIN_DEFAULT_EMAIL` | `admin@example.com` | pgAdmin login, created on first start. |
| `PGADMIN_DEFAULT_PASSWORD` | `$POSTGRES_PASSWORD` | pgAdmin login password, set on first start. |
| `PEO_PGADMIN` | `true` | Set to `false` to not start pgAdmin. |
| `PEO_PGADMIN_PORT` | `5050` | Port pgAdmin listens on inside the container. |

Build arguments: `AGE_IMAGE` (base image, default `apache/age:release_PG18_1.8.0`),
`NEO4J_BROWSER_REF` (neo4j-browser commit to build) and `PGADMIN_VERSION` (default `9.18`).

pgAdmin keeps its users and settings in `/var/lib/pgadmin`; changing
`PGADMIN_DEFAULT_*` after the first start has no effect unless that directory is empty.

## Limitations

This is a first version. Things that do not work (yet):

- `RETURN *`, `EXPLAIN` and `PROFILE`.
- Write statistics: write queries report "(no changes, no records)" even when they
  changed the graph.
- Anything AGE does not support, for example multiple labels per node, pattern
  comprehensions, temporal and spatial types, indexes and constraints through Cypher.
- User and role management, `SHOW TRANSACTIONS`, `:sysinfo`.
- TLS (`bolt+s://`, HTTPS).
- Each query's full result is fetched from PostgreSQL before it is streamed to the
  client, so very large results use memory in peo.

## Development

peo depends on `psycopg` 3 and `websockets`, plus Starlette and uvicorn for the
`browser` extra. To work on it, install it in editable mode; `python -m build` (or
`uv build`) builds an sdist and wheel:

```sh
pip install -e '.[binary,browser]'
```

[`tests/smoke_test.py`](tests/smoke_test.py) runs end-to-end checks through the official
Neo4j Python driver against a running instance:

```sh
pip install neo4j
python tests/smoke_test.py neo4j://localhost:7687 postgres secret
```

## License

peo is free software, licensed under the GNU General Public License v3.0 or
later; see [LICENSE](LICENSE). The container image bundles third-party software
under its own licenses, including neo4j-browser (GPL-3.0), Apache AGE
(Apache-2.0), PostgreSQL and pgAdmin (PostgreSQL License). See
[THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md) for the full list and where
each license text and source archive is in the image.

Neo4j is a trademark of Neo4j, Inc.; peo is not affiliated with or endorsed
by Neo4j.