A Tour of PostgREST

Taking the database seriously
Most web frameworks treat
the DB as a dumb store
This helps give them broad
appeal
What if we take a stand
instead?
??
? ?
?

Taking the database seriously
Is PostgreSQL powerful and
flexible enough to replace
the custom API server?
That’s my experiment

The Traditional Web API Stack
Your App
Web Server
Database

The Traditional Web API Stack
Your App
Web Server
Database
PostgREST
A no-configuration canonical
mapping from DB to HTTP

Talk Overview
The Traditional API Server (brief)
Live demo of PostgREST
What’s the SQL? How did
it do that?

The Traditional App
Handmade Nested
Routes
Controllers
Imperative code
ORM
Logic divorced from
data

What’s in an app?
HTTP request handling
Authentication
Authorization
Request Parsing
Request Validation
Database Communication
Database Response Handling
HTTP Response Building
With error handling woven
throughout...

Maintaining bespoke APIs gets old
“Most APIs look the
same, some have
icing, some have
fondant, some are
vanilla, some
chocolate. At the
core they’re all still
cakes.” -- Jett
Durham

Problem 1: Boilerplate
Want to add a new route?
Create model
Add each CRUD action
Check permissions
Support filtering, pagination
Special routes for joining data

Problem 2: No Single Source of Truth
Constraints are removed
from DB
No longer enforced
continuously + uniformly
Imperative code means
human must write docs
Authorization is per-
controller rather than

Problem 3: Hierarchy
Your info is relational, your routes
hierarchical
Say projects have parts and vice
versa.
Need routes for parts by project
and project by parts?
Other people recognize the
problem, hence GraphQL

Demo Time!
We’ll use the Pagila example database
It was ported from MySQL “Sakila”
It’s a DVD store with films, rentals, customers, payments,
categories, actors etc

Security - Roles for Authorization
Anonymous Authenticator User(s)

Security - JWT for Authentication
YES
NO

Security - Roles in SQL
CREATE ROLE authenticator NOINHERIT LOGIN;
CREATE ROLE anon;
CREATE ROLE worker;
GRANT anon, worker TO authenticator;

Switching to a role
BEGIN ISOLATION LEVEL READ COMMITTED READ WRITE;
SET LOCAL ROLE 'worker';
SET LOCAL "postgrest.claims.id" = 'jdoe';
-- ...
COMMIT;

Row-Level Security
PostgreSQL 9.5+ allows restricting access to individual rows
ALTER TABLE posts ENABLE ROW LEVEL SECURITY;
drop policy if exists authors_eigenedit on posts;
create policy authors_eigenedit on posts
using (true)
with check (
author = basic_auth.current_email()
);

External Actions
You can’t do everything
inside SQL
How do you
● Send an email?
● Call a 3rd party
service?
LISTEN / NOTIFY

How to version the API?
So far OK but… but I don’t
want to couple the internal
schema with an API!
How to encapsulate true
schema?
How to version specific
endpoints?

Use database schemas
Internal Schema V1
table1
table2
table3
view2
proc
view2

HTTP Interface is Flexible
postgrest --schema v1
postgrest --schema v2,v1
postgrest --schema v3,v2,v1
Accept: application/json;
version=2
OR
GET /v2/...

Use the schema search-path
SET search_path TO v2, v1;

How does it work inside?
Warning: Boring / Cool

Generating the payload in 100% SQL
WITH pg_source AS
(SELECT "public"."festival".* FROM "public"."festival")
SELECT
(SELECT pg_catalog.count(1) FROM "public"."festival") AS total_result_set,
pg_catalog.count(t) AS page_total,
NULL AS header,
array_to_json(array_agg(row_to_json(t)))::character VARYING AS body
FROM
(SELECT * FROM pg_source LIMIT ALL OFFSET 0) t

Adding a filter
WITH pg_source AS
(SELECT "public"."festival".* FROM "public"."festival"
WHERE "public"."festival"."name" LIKE '%fun%'::UNKNOWN)
SELECT
(SELECT pg_catalog.count(1) FROM "public"."festival"
WHERE "public"."festival"."name" LIKE '%fun%'::UNKNOWN) AS total_result_set,
NULL AS header,
array_to_json(array_agg(row_to_json(t)))::character varying AS body
FROM
Optimistic cast

Or without a global count
WITH pg_source AS
(SELECT "public"."festival".* FROM "public"."festival")
SELECT
NULL AS total_result_set,
NULL AS header,
FROM

Creating CSV body
-- ...
(SELECT string_agg(a.k, ',')
FROM
(SELECT json_object_keys(r)::TEXT AS k
FROM
(SELECT row_to_json(hh) AS r
FROM pg_source AS hh LIMIT 1) s
) a
) || 'n' ||
coalesce(
string_agg(
substring(t::text, 2, length(t::text) - 2), 'n'
), ''
)
-- ...
First row
Column names
Remove quotes

Embedding a relation
WITH pg_source AS
(SELECT "public"."film"."id", row_to_json("director".*) AS "director"
FROM "public"."film"
LEFT OUTER JOIN
(SELECT "public"."director".*
FROM "public"."director") AS "director"
ON "director"."name" = "film"."director")
SELECT
(SELECT pg_catalog.count(1) FROM "public"."film") AS total_result_set,
NULL AS header,
FROM
Embed row as field
Key(s) detected

ELSE
CASE
WHEN t.typelem <> 0::oid AND t.typlen = (-1)
THEN 'ARRAY'::text
WHEN nt.nspname = 'pg_catalog'::name THEN
format_type(a.atttypid, NULL::integer)
ELSE 'USER-DEFINED'::text
END
END::information_schema.character_data AS data_type,
information_schema._pg_char_max_length(information_schema._pg_truetypid(a.*,
t.*), information_schema._pg_truetypmod(a.*,
t.*))::information_schema.cardinal_number AS character_maximum_length,
information_schema._pg_char_octet_length(information_schema._pg_truetypid(a.
*, t.*), information_schema._pg_truetypmod(a.*,
t.*))::information_schema.cardinal_number AS character_octet_length,
information_schema._pg_numeric_precision(information_schema._pg_truetypid(a.
*, t.*), information_schema._pg_truetypmod(a.*,
t.*))::information_schema.cardinal_number AS numeric_precision,
information_schema._pg_numeric_precision_radix(information_schema._pg_truety
pid(a.*, t.*), information_schema._pg_truetypmod(a.*,
t.*))::information_schema.cardinal_number AS numeric_precision_radix,
information_schema._pg_numeric_scale(information_schema._pg_truetypid(a.*,
t.*))::information_schema.cardinal_number AS numeric_scale,
information_schema._pg_datetime_precision(information_schema._pg_truetypid(a
.*, t.*), information_schema._pg_truetypmod(a.*,
t.*))::information_schema.cardinal_number AS datetime_precision,
information_schema._pg_interval_type(information_schema._pg_truetypid(a.*,
t.*))::information_schema.character_data AS interval_type,
NULL::integer::information_schema.cardinal_number AS
interval_precision,
NULL::character varying::information_schema.sql_identifier
AS character_set_catalog,
AS character_set_schema,
AS character_set_name,
SELECT DISTINCT
info.table_schema AS schema,
info.table_name AS table_name,
info.column_name AS name,
info.ordinal_position AS position,
info.is_nullable::boolean AS nullable,
info.data_type AS col_type,
info.is_updatable::boolean AS updatable,
info.character_maximum_length AS max_len,
info.numeric_precision AS precision,
info.column_default AS default_value,
array_to_string(enum_info.vals, ',') AS enum
FROM (
/*
-- CTE based on information_schema.columns to remove the owner filter
*/
WITH columns AS (
SELECT current_database()::information_schema.sql_identifier AS table_catalog,
nc.nspname::information_schema.sql_identifier AS table_schema,
c.relname::information_schema.sql_identifier AS table_name,
a.attname::information_schema.sql_identifier AS column_name,
a.attnum::information_schema.cardinal_number AS ordinal_position,
pg_get_expr(ad.adbin, ad.adrelid)::information_schema.character_data AS column_default,
CASE
WHEN a.attnotnull OR t.typtype = 'd'::"char" AND t.typnotnull THEN 'NO'::text
ELSE 'YES'::text
END::information_schema.yes_or_no AS is_nullable,
CASE
WHEN t.typtype = 'd'::"char" THEN
CASE
WHEN bt.typelem <> 0::oid AND bt.typlen = (-1) THEN 'ARRAY'::text
WHEN nbt.nspname = 'pg_catalog'::name THEN format_type(t.typbasetype,
NULL::integer)
ELSE 'USER-DEFINED'::text
END
Matching up foreign keys

Deleting an item
WITH pg_source AS
(DELETE FROM "test"."items"
WHERE "test"."items"."id" = '1'::unknown
RETURNING "test"."items".*)
SELECT
'' AS total_result_set,
'',
''
FROM
(SELECT 1 FROM pg_source) t

Learning More
Read the Docs
http://postgrest.com

github.com / begriffs / postgrest

A Tour of PostgREST

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