`walrus`

Source Code: https://github.com/supabase/walrus

Write Ahead Log Realtime Unified Security (WALRUS) is a utility for managing realtime subscriptions to tables and applying row level security rules to those subscriptions.

The subscription stream is based on logical replication slots.

Summary

Managing Subscriptions

User subscriptions are managed through a table

create table realtime.subscription (
    id bigint generated always as identity primary key,
    subscription_id uuid not null,
    entity regclass not null,
    filters realtime.user_defined_filter[] not null default '{}',
    claims jsonb not null,
    claims_role regrole not null generated always as (realtime.to_regrole(claims ->> 'role')) stored,
    created_at timestamp not null default timezone('utc', now()),

    unique (subscription_id, entity, filters)
);

where realtime.user_defined_filter is

create type realtime.user_defined_filter as (
    column_name text,
    op realtime.equality_op,
    value text
);

and realtime.equality_ops are a subset of postgrest ops. Specifically:

create type realtime.equality_op as enum(
    'eq', 'neq', 'lt', 'lte', 'gt', 'gte', 'in'
);

For example, to subscribe to a table named public.notes where the id is 6 as the authenticated role:

insert into realtime.subscription(subscription_id, entity, filters, claims)
values ('832bd278-dac7-4bef-96be-e21c8a0023c4', 'public.notes', array[('id', 'eq', '6')], '{"role", "authenticated"}');

Reading WAL

This package exposes 1 public SQL function realtime.apply_rls(jsonb). It processes the output of a wal2json decoded logical replication slot and returns:

wal: (jsonb) The WAL record as JSONB in the form
is_rls_enabled: (bool) If the entity (table) the WAL record represents has row level security enabled
subscription_ids: (uuid[]) An array subscription ids that should be notified about the WAL record
errors: (text[]) An array of errors

The jsonb WAL record is in the following format for inserts.

{
    "type": "INSERT",
    "schema": "public",
    "table": "todos",
    "columns": [
        {
            "name": "id",
            "type": "int8",
        },
        {
            "name": "details",
            "type": "text",
        },
        {
            "name": "user_id",
            "type": "int8",
        }
    ],
    "commit_timestamp": "2021-09-29T17:35:38Z",
    "record": {
        "id": 1,
        "user_id": 1,
        "details": "mow the lawn"
    }
}

updates:

{
    "type": "UPDATE",
    "schema": "public",
    "table": "todos",
    "columns": [
        {
            "name": "id",
            "type": "int8",
        },
        {
            "name": "details",
            "type": "text",
        },
        {
            "name": "user_id",
            "type": "int8",
        }
    ],
    "commit_timestamp": "2021-09-29T17:35:38Z",
    "record": {
        "id": 2,
        "user_id": 1,
        "details": "mow the lawn"
    },
    "old_record": {
        "id": 1,
    }
}

deletes:

{
    "type": "DELETE",
    "schema": "public",
    "table": "todos",
    "columns": [
        {
            "name": "id",
            "type": "int8",
        },
        {
            "name": "details",
            "type": "text",
        },
        {
            "name": "user_id",
            "type": "int8",
        }
    ],
    "old_record": {
        "id": 1
    }
}

Important Notes:

Row level security is not applied to delete statements
The key/value pairs displayed in the old_record field include the table's identity columns for the record being updated/deleted. To display all values in old_record set the replica identity for the table to full
When a delete occurs, the contents of old_record will be broadcast to all subscribers to that table so ensure that each table's replica identity only contains information that is safe to expose publicly

Error States

Error 400: Bad Request, no primary key

If a WAL record for a table that does not have a primary key is passed through realtime.apply_rls, an error is returned

Ex:

(
    {
        "type": ...,
        "schema": ...,
        "table": ...
    },                               -- wal
    true,                            -- is_rls_enabled
    [...],                           -- subscription_ids,
    array['Error 400: Bad Request, no primary key'] -- errors
)::realtime.wal_rls;

Error 401: Unauthorized

If a WAL record is passed through realtime.apply_rls and the subscription's clams_role does not have permission to select the primary key columns in that table, an Unauthorized error is returned with no WAL data.

Ex:

(
    {
        "type": ...,
        "schema": ...,
        "table": ...
    },                               -- wal
    true,                            -- is_rls_enabled
    [...],                           -- subscription_ids,
    array['Error 401: Unauthorized'] -- errors
)::realtime.wal_rls;

Error 413: Payload Too Large

When the size of the wal2json record exceeds max_record_bytes the record and old_record objects are filtered to include only fields with a value size <= 64 bytes. The errors output array is set to contain the string "Error 413: Payload Too Large".

Ex:

(
    {..., "record": {"id": 1}, "old_record": {"id": 1}}, -- wal
    true,                                  -- is_rls_enabled
    [...],                                 -- subscription_ids,
    array['Error 413: Payload Too Large']  -- errors
)::realtime.wal_rls;

How it Works

Each WAL record is passed into realtime.apply_rls(jsonb) which:

impersonates each subscribed user by setting the appropriate role and request.jwt.claims that RLS policies depend on
queries for the row using its primary key values
applies the subscription's filters to check if the WAL record is filtered out
filters out all columns that are not visible to the user's role

Usage

Given a wal2json replication slot with the name realtime

select * from pg_create_logical_replication_slot('realtime', 'wal2json')

A complete list of config options can be found here:

The stream can be polled with

select
    xyz.wal,
    xyz.is_rls_enabled,
    xyz.subscription_ids,
    xyz.errors
from
    pg_logical_slot_get_changes(
        'realtime', null, null,
        'include-pk', '1',
        'include-transaction', 'false',
        'include-timestamp', 'true',
        'include-type-oids', 'true',
        'write-in-chunks', 'true',
        'format-version', '2',
        'actions', 'insert,update,delete',
        'filter-tables', 'realtime.*'
    ),
    lateral (
        select
            x.wal,
            x.is_rls_enabled,
            x.subscription_ids,
            x.errors
        from
            realtime.apply_rls(data::jsonb) x(wal, is_rls_enabled, subcription_ids, errors)
    ) xyz
where
    xyz.subscription_ids[1] is not null

Or, if the stream should be filtered according to a publication:

with pub as (
    select
        concat_ws(
            ',',
            case when bool_or(pubinsert) then 'insert' else null end,
            case when bool_or(pubupdate) then 'update' else null end,
            case when bool_or(pubdelete) then 'delete' else null end
        ) as w2j_actions,
        coalesce(
            string_agg(
                realtime.quote_wal2json(format('%I.%I', schemaname, tablename)::regclass),
                ','
            ) filter (where ppt.tablename is not null and ppt.tablename not like '% %'),
            ''
        ) w2j_add_tables
    from
        pg_publication pp
        left join pg_publication_tables ppt
            on pp.pubname = ppt.pubname
    where
        pp.pubname = 'supabase_realtime'
    group by
        pp.pubname
    limit 1
),
w2j as (
    select
        x.*, pub.w2j_add_tables
    from
         pub,
         pg_logical_slot_get_changes(
            'realtime', null, null,
            'include-pk', '1',
            'include-transaction', 'false',
            'include-type-oids', 'true',
            'include-timestamp', 'true',
            'write-in-chunks', 'true',
            'format-version', '2',
            'actions', pub.w2j_actions,
            'add-tables', pub.w2j_add_tables
        ) x
)
select
    xyz.wal,
    xyz.is_rls_enabled,
    xyz.subscription_ids,
    xyz.errors
from
    w2j,
    realtime.apply_rls(
        wal := w2j.data::jsonb,
        max_record_bytes := 1048576
    ) xyz(wal, is_rls_enabled, subscription_ids, errors)
where
    w2j.w2j_add_tables <> ''
    and xyz.subscription_ids[1] is not null

Configuration

max_record_bytes

max_record_bytes (default 1 MiB): Controls the maximum size of a WAL record that will be emitted with complete record and old_record data. When the size of the wal2json record exceeds max_record_bytes the record and old_record objects are filtered to include only fields with a value size <= 64 bytes. The errors output array is set to contain the string "Error 413: Payload Too Large".

Ex:

realtime.apply_rls(wal := w2j.data::jsonb, max_record_bytes := 1024*1024) x(wal, is_rls_enabled, subscription_ids, errors)

Installation

The project is SQL only and can be installed by executing the contents of sql/walrus--0.1.sql in a database instance.

Tests

Requires

Postgres 13+
wal2json >= 53b548a29ebd6119323b6eb2f6013d7c5fe807ec

On a Mac:

Install postgres

brew install postgres

Install wal2json

git clone https://github.com/eulerto/wal2json.git
cd wal2json
git reset --hard 53b548a
make
make install

Run the tests, from the repo root.

./bin/installcheck

RFC Process

To open an request for comment (RFC), open a github issue against this repo and select the RFC template.

supabase/walrus

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