examples_users.rst

User Management

API clients authenticate with JSON Web Tokens. PostgREST does not support any other authentication mechanism directly, but they can be built on top. In this demo we will build a username and password system on top of JWT using only plpgsql.

Future examples such as the multi-tenant blogging platform will use the results from this example for their auth. We will build a system for users to sign up, log in, manage their accounts, and for admins to manage other people's accounts. We will also see how to trigger outside events like sending password reset emails.

Before jumping into the code, a little more about how the tokens work. Every JWT contains cryptographically signed claims. PostgREST cares specifically about a claim called role. When a client includes a role claim PostgREST executes their request using that database role.

How would a client include a role claim, or claims in general? Without knowing the server JWT secret a client cannot create a claim. The only place to get a JWT is from the PostgREST server or from another service sharing the secret and acting on its behalf. We'll use a stored procedure returning type jwt_claims which is a special type causing the server to encrypt and sign the return value.

Storing Users and Passwords

We create a database schema especially for auth information. We'll also need the PostgreSQL extension pgcrypto.

create extension if not exists pgcrypto;

-- We put things inside the basic_auth schema to hide
-- them from public view. Certain public procs/views will
-- refer to helpers and tables inside.
create schema if not exists basic_auth;

Next a table to store the mapping from usernames and passwords to database roles. The code below includes triggers and functions to encrypt the password and ensure the role exists.

create table if not exists
basic_auth.users (
  email    text primary key check ( email ~* '^.+@.+\..+$' ),
  pass     text not null check (length(pass) < 512),
  role     name not null check (length(role) < 512),
  verified boolean not null default false
  -- If you like add more columns, or a json column
);

create or replace function
basic_auth.check_role_exists() returns trigger
  language plpgsql
  as $$
begin
  if not exists (select 1 from pg_roles as r where r.rolname = new.role) then
    raise foreign_key_violation using message =
      'unknown database role: ' || new.role;
    return null;
  end if;
  return new;
end
$$;

drop trigger if exists ensure_user_role_exists on basic_auth.users;
create constraint trigger ensure_user_role_exists
  after insert or update on basic_auth.users
  for each row
  execute procedure basic_auth.check_role_exists();

create or replace function
basic_auth.encrypt_pass() returns trigger
  language plpgsql
  as $$
begin
  if tg_op = 'INSERT' or new.pass <> old.pass then
    new.pass = crypt(new.pass, gen_salt('bf'));
  end if;
  return new;
end
$$;

drop trigger if exists encrypt_pass on basic_auth.users;
create trigger encrypt_pass
  before insert or update on basic_auth.users
  for each row
  execute procedure basic_auth.encrypt_pass();

With the table in place we can make a helper to check passwords. It returns the database role for a user if the email and password are correct.

create or replace function
basic_auth.user_role(email text, pass text) returns name
  language plpgsql
  as $$
begin
  return (
  select role from basic_auth.users
   where users.email = user_role.email
     and users.pass = crypt(user_role.pass, users.pass)
  );
end;
$$;

Password Reset

When a user requests a password reset or signs up we create a token they will use later to prove their identity. The tokens go in this table.

drop type if exists token_type_enum cascade;
create type token_type_enum as enum ('validation', 'reset');

create table if not exists
basic_auth.tokens (
  token       uuid primary key,
  token_type  token_type_enum not null,
  email       text not null references basic_auth.users (email)
                on delete cascade on update cascade,
  created_at  timestamptz not null default current_date
);

In the main schema (as opposed to the basic_auth schema) we expose a password reset request function. HTTP clients will call it. The function takes the email address of the user.

create or replace function
request_password_reset(email text) returns void
  language plpgsql
  as $$
declare
  tok uuid;
begin
  delete from basic_auth.tokens
   where token_type = 'reset'
     and tokens.email = request_password_reset.email;

  select gen_random_uuid() into tok;
  insert into basic_auth.tokens (token, token_type, email)
         values (tok, 'reset', request_password_reset.email);
  perform pg_notify('reset',
    json_build_object(
      'email', request_password_reset.email,
      'token', tok,
      'token_type', 'reset'
    )::text
  );
end;
$$;

This function does not send any emails. It sends a PostgreSQL NOTIFY command. External programs such as a mailer listen for this event and do the work. The most robust way to process these signals is by pushing them onto work queues. Here are two programs to do that:

1. aweber/pgsql-listen-exchange for RabbitMQ
2. SpiderOak/skeeter for ZeroMQ

For experimentation you don't need that though. Here's a sample Node program that listens for the events and logs them to stdout.

var PS = require('pg-pubsub');

if(process.argv.length !== 3) {
  console.log("USAGE: DB_URL");
  process.exit(2);
}
var url  = process.argv[2],
    ps   = new PS(url);

// password reset request events
ps.addChannel('reset', console.log);
// email validation required event
ps.addChannel('validate', console.log);

// modify me to send emails

Once the user has a reset token they can use it as an argument to the password reset function, calling it through the PostgREST RPC interface.

create or replace function
reset_password(email text, token uuid, pass text)
  returns void
  language plpgsql
  as $$
declare
  tok uuid;
begin
  if exists(select 1 from basic_auth.tokens
             where tokens.email = reset_password.email
               and tokens.token = reset_password.token
               and token_type = 'reset') then
    update basic_auth.users set pass=reset_password.pass
     where users.email = reset_password.email;

    delete from basic_auth.tokens
     where tokens.email = reset_password.email
       and tokens.token = reset_password.token
       and token_type = 'reset';
  else
    raise invalid_password using message =
      'invalid user or token';
  end if;
  delete from basic_auth.tokens
   where token_type = 'reset'
     and tokens.email = reset_password.email;

  select gen_random_uuid() into tok;
  insert into basic_auth.tokens (token, token_type, email)
         values (tok, 'reset', reset_password.email);
  perform pg_notify('reset',
    json_build_object(
      'email', reset_password.email,
      'token', tok
    )::text
  );
end;
$$;

Email Validation

This is similar to password resets. Once again we generate a token. It differs in that there is a trigger to send validations when a new login is added to the users table.

create or replace function
basic_auth.send_validation() returns trigger
  language plpgsql
  as $$
declare
  tok uuid;
begin
  select gen_random_uuid() into tok;
  insert into basic_auth.tokens (token, token_type, email)
         values (tok, 'validation', new.email);
  perform pg_notify('validate',
    json_build_object(
      'email', new.email,
      'token', tok,
      'token_type', 'validation'
    )::text
  );
  return new;
end
$$;

drop trigger if exists send_validation on basic_auth.users;
create trigger send_validation
  after insert on basic_auth.users
  for each row
  execute procedure basic_auth.send_validation();

Editing Own User

We'll construct a redacted view for users. It hides passwords and shows only those users whose roles the currently logged in user has db permission to access.

create or replace view users as
select actual.role as role,
       '***'::text as pass,
       actual.email as email,
       actual.verified as verified
from basic_auth.users as actual,
     (select rolname
        from pg_authid
       where pg_has_role(current_user, oid, 'member')
     ) as member_of
where actual.role = member_of.rolname;
  -- can also add restriction that current_setting('postgrest.claims.email')
  -- is equal to email so that user can only see themselves

Using this view clients can see themselves and any other users with the right db roles. This view does not yet support inserts or updates because not all the columns refer directly to underlying columns. Nor do we want it to be auto-updatable because it would allow an escalation of privileges. Someone could update their own row and change their role to become more powerful.

We'll handle updates with a trigger, but we'll need a helper function to prevent an escalation of privileges.

create or replace function
basic_auth.clearance_for_role(u name) returns void as
$$
declare
  ok boolean;
begin
  select exists (
    select rolname
      from pg_authid
     where pg_has_role(current_user, oid, 'member')
       and rolname = u
  ) into ok;
  if not ok then
    raise invalid_password using message =
      'current user not member of role ' || u;
  end if;
end
$$ LANGUAGE plpgsql;

With the above function we can now make a safe trigger to allow user updates.

create or replace function
update_users() returns trigger
language plpgsql
AS $$
begin
  if tg_op = 'INSERT' then
    perform basic_auth.clearance_for_role(new.role);

    insert into basic_auth.users
      (role, pass, email, verified)
    values
      (new.role, new.pass, new.email,
      coalesce(new.verified, false));
    return new;
  elsif tg_op = 'UPDATE' then
    -- no need to check clearance for old.role because
    -- an ineligible row would not have been available to update (http 404)
    perform basic_auth.clearance_for_role(new.role);

    update basic_auth.users set
      email  = new.email,
      role   = new.role,
      pass   = new.pass,
      verified = coalesce(new.verified, old.verified, false)
      where email = old.email;
    return new;
  elsif tg_op = 'DELETE' then
    -- no need to check clearance for old.role (see previous case)

    delete from basic_auth.users
     where basic_auth.email = old.email;
    return null;
  end if;
end
$$;

drop trigger if exists update_users on users;
create trigger update_users
  instead of insert or update or delete on
    users for each row execute procedure update_users();

Finally add a public function people can use to sign up. You can hard code a default db role in it. It alters the underlying basic_auth.users so you can set whatever role you want without restriction.

create or replace function
signup(email text, pass text) returns void
as $$
  insert into basic_auth.users (email, pass, role) values
    (signup.email, signup.pass, 'hardcoded-role-here');
$$ language sql;

Generating JWT

As mentioned at the start, clients authenticate with JWT. PostgREST has a special convention to allow your sql functions to return JWT. Any function that returns a type whose name ends in jwt_claims will have its return value encoded. For instance, let's make a login function which consults our users table.

First create a return type:

drop type if exists basic_auth.jwt_claims cascade;
create type basic_auth.jwt_claims AS (role text, email text, exp integer);

The exp field is expiration time of the token expressed in seconds since the epoch. In the function below we choose to make the token valid for one hour.

create or replace function
login(email text, pass text) returns basic_auth.jwt_claims
  language plpgsql
  as $$
declare
  _role name;
  _verified boolean;
  _email text;
  result basic_auth.jwt_claims;
begin
  -- check email and password
  select basic_auth.user_role(email, pass) into _role;
  if _role is null then
    raise invalid_password using message = 'invalid user or password';
  end if;
  -- check verified flag whether users
  -- have validated their emails
  _email := email;
  select verified from basic_auth.users as u where u.email=_email limit 1 into _verified;
  if not _verified then
    raise invalid_authorization_specification using message = 'user is not verified';
  end if;
  select _role as role, login.email as email,
         extract(epoch from now())::integer + 60*60 as exp
    into result;
  return result;
end;
$$;

An API request to login would look like this.

POST /rpc/login

{ "email": "foo@bar.com", "pass": "foobar" }

Response

{
  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJlbWFpbCI6ImZvb0BiYXIuY29tIiwicm9sZSI6ImF1dGhvciJ9.KHwYdK9dAMAg-MGCQXuDiFuvbmW-y8FjfYIcMrETnto"
}

Try decoding the token at jwt.io. (It was encoded with a secret of secret which is the default.) To use this token in a future API request include it in an Authorization request header.

Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJlbWFpbCI6ImZvb0BiYXIuY29tIiwicm9sZSI6ImF1dGhvciJ9.KHwYdK9dAMAg-MGCQXuDiFuvbmW-y8FjfYIcMrETnto

Same-Role Users

You may not want a separate db role for every user. You can distinguish one user from another in SQL by examining the JWT claims which PostgREST makes available in the SQL variable postgrest.claims. Here's a function to get the email of the currently authenticated user.

-- Prevent current_setting('postgrest.claims.email') from raising
-- an exception if the setting is not present. Default it to ''.
ALTER DATABASE your_db_name SET postgrest.claims.email TO '';

create or replace function
basic_auth.current_email() returns text
  language plpgsql
  as $$
begin
  return current_setting('postgrest.claims.email');
end;
$$;

Remember that the login function set the claims email and role. You can modify login to set other claims as well if they are useful for your other SQL functions to reference later.

Permissions

Basic table-level permissions. We'll add an the authenticator role which can't do anything itself other than switch into other roles as directed by JWT.

create role anon;
create role authenticator noinherit;
grant anon to authenticator;

grant usage on schema public, basic_auth to anon;

-- anon can create new logins
grant insert on table basic_auth.users, basic_auth.tokens to anon;
grant select on table pg_authid, basic_auth.users to anon;
grant execute on function
  login(text,text),
  request_password_reset(text),
  reset_password(text,uuid,text),
  signup(text, text)
  to anon;

Conclusion

This section explained the implementation details for building a password based authentication system in pure sql. The next example will put it to work in a multi-tenant blogging API.