Transactions

After User Impersonation, every request to an API resource runs inside a transaction. The sequence of the transaction is as follows:

BEGIN; -- <Access Mode> <Isolation Level>
-- <Transaction-scoped settings>
-- <Main Query>;
END;

Access Mode

The access mode on Tables and Views is determined by the HTTP method.

HTTP Method	Access Method
GET, HEAD	READ ONLY
POST, PATCH, PUT, DELETE	READ WRITE

Stored Procedures additionally depend on the function volatility.

	Access Method
HTTP Method	VOLATILE	STABLE	IMMUTABLE
GET, HEAD	READ ONLY	READ ONLY	READ ONLY
POST	READ WRITE	READ ONLY	READ ONLY

Modifying the database inside READ ONLY transactions is not possible. PostgREST uses this fact to enforce HTTP semantics in GET and HEAD requests.

Note

The volatility marker is a promise about the behavior of the function. PostgreSQL will let you mark a function that modifies the database as IMMUTABLE or STABLE without failure. But, because of the READ ONLY transaction the function will fail under PostgREST.

The OPTIONS method method doesn’t start a transaction, so it’s not relevant here.

Isolation Level

Every transaction uses the PostgreSQL default isolation level: READ COMMITTED. Unless you modify default_transaction_isolation for an impersonated role or function.

Using Impersonated Role Settings, change the isolation level for all the role’s requests with:

ALTER ROLE webuser SET default_transaction_isolation TO 'repeatable read';

Or to change the isolation level per function call.

CREATE OR REPLACE FUNCTION myfunc()
RETURNS text as $$
  SELECT 'hello';
$$
LANGUAGE SQL
SET default_transaction_isolation TO 'serializable';

Transaction-Scoped Settings

PostgREST uses settings tied to the transaction lifetime. These can be used to get data about the HTTP request. Or to modify the HTTP response.

You can get these with current_setting

-- request settings use the ``request.`` prefix.
SELECT
  current_setting('request.<setting>', true);

And you can set them with set_config

-- response settings use the ``response.`` prefix.
SELECT
  set_config('response.<setting>', 'value1' ,true);

Request Headers, Cookies and JWT claims

PostgREST stores the headers, cookies and headers as JSON. To get them:

-- To get all the headers sent in the request
SELECT current_setting('request.headers', true)::json;

-- To get a single header, you can use JSON arrow operators
SELECT current_setting('request.headers', true)::json->>'user-agent';

-- value of sessionId in a cookie
SELECT current_setting('request.cookies', true)::json->>'sessionId';

-- value of the email claim in a jwt
SELECT current_setting('request.jwt.claims', true)::json->>'email';

Note

The role in request.jwt.claims defaults to the value of db-anon-role.

Request Path and Method

The path and method are stored as text.

SELECT current_setting('request.path', true);

SELECT current_setting('request.method', true);

Request Role and Search Path

Because of User Impersonation, PostgREST sets the standard role. You can get this in different ways:

SELECT current_role;

SELECT current_user;

SELECT current_setting('role', true);

Additionally it also sets the search_path based on db-schemas and db-extra-search-path.

Response Headers

You can set response.headers to add headers to the HTTP response. For instance, this statement would add caching headers to the response:

-- tell client to cache response for two days

SELECT set_config('response.headers',
  '[{"Cache-Control": "public"}, {"Cache-Control": "max-age=259200"}]', true);

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Cache-Control: no-cache, no-store, must-revalidate

Notice that the response.headers should be set to an array of single-key objects rather than a single multiple-key object. This is because headers such as Cache-Control or Set-Cookie need repeating when setting many values. An object would not allow the repeated key.

Note

PostgREST provided headers such as Content-Type, Location, etc. can be overriden this way. Note that irrespective of overridden Content-Type response header, the content will still be converted to JSON, unless you also set raw-media-types to something like text/html.

Response Status Code

You can set the response.status to override the default status code PostgREST provides. For instance, the following function would replace the default 200 status code.

create or replace function teapot() returns json as $$
begin
  perform set_config('response.status', '418', true);
  return json_build_object('message', 'The requested entity body is short and stout.',
                           'hint', 'Tip it over and pour it out.');
end;
$$ language plpgsql;

HTTPCurl

GET /rpc/teapot HTTP/1.1

HTTP/1.1 418 I'm a teapot

{
  "message" : "The requested entity body is short and stout.",
  "hint" : "Tip it over and pour it out."
}

If the status code is standard, PostgREST will complete the status message(I’m a teapot in this example).

Main query

The main query is produced by requesting the API resources.

Transaction End

If the transaction doesn’t fail, it will always end in a COMMIT. Unless db-tx-end is configured to ROLLBACK in any case or conditionally with Prefer: tx=rollback. This can be used for testing purposes.

Aborting transactions

Any database failure(like a failed constraint) will result in a rollback of the transaction. You can also do a RAISE inside a function to cause a rollback.

Raise errors with HTTP Status Codes

You can return non-200 HTTP status codes by raising SQL exceptions. For instance, here’s a saucy function that always responds with an error:

CREATE OR REPLACE FUNCTION just_fail() RETURNS void
  LANGUAGE plpgsql
  AS $$
BEGIN
  RAISE EXCEPTION 'I refuse!'
    USING DETAIL = 'Pretty simple',
          HINT = 'There is nothing you can do.';
END
$$;

Calling the function returns HTTP 400 with the body

{
  "message":"I refuse!",
  "details":"Pretty simple",
  "hint":"There is nothing you can do.",
  "code":"P0001"
}

One way to customize the HTTP status code is by raising particular exceptions according to the PostgREST error to status code mapping. For example, RAISE insufficient_privilege will respond with HTTP 401/403 as appropriate.

For even greater control of the HTTP status code, raise an exception of the PTxyz type. For instance to respond with HTTP 402, raise ‘PT402’:

RAISE sqlstate 'PT402' using
  message = 'Payment Required',
  detail = 'Quota exceeded',
  hint = 'Upgrade your plan';

Returns:

HTTP/1.1 402 Payment Required
Content-Type: application/json; charset=utf-8

{
  "message": "Payment Required",
  "details": "Quota exceeded",
  "hint": "Upgrade your plan",
  "code": "PT402"
}

Pre-Request

The pre-request is a function that can run after the Transaction-Scoped Settings are set and before the Main query. It’s enabled with db-pre-request.

This provides an opportunity to modify settings or raise an exception to prevent the request from completing.

Setting headers via pre-request

As an example, let’s add some cache headers for all requests that come from an Internet Explorer(6 or 7) browser.

create or replace function custom_headers()
returns void as $$
declare
  user_agent text := current_setting('request.headers', true)::json->>'user-agent';
begin
  if user_agent similar to '%MSIE (6.0|7.0)%' then
    perform set_config('response.headers',
      '[{"Cache-Control": "no-cache, no-store, must-revalidate"}]', false);
  end if;
end; $$ language plpgsql;

-- set this function on postgrest.conf
-- db-pre-request = custom_headers

Now when you make a GET request to a table or view, you’ll get the cache headers.

HTTPCurl

GET /people HTTP/1.1
User-Agent: Mozilla/4.01 (compatible; MSIE 6.0; Windows NT 5.1)