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 Mode |
---|---|
GET, HEAD |
READ ONLY |
POST, PATCH, PUT, DELETE |
READ WRITE |
Stored Procedures additionally depend on the function volatility.
Access Mode |
|||
---|---|---|---|
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 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;
GET /rpc/teapot HTTP/1.1
curl "http://localhost:3000/rpc/teapot" -i
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 generated by requesting Tables and Views or Stored Procedures. All generated queries use prepared statements(db-prepared-statements).
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 RAISE an error inside a function to cause a rollback.
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.
GET /people HTTP/1.1
User-Agent: Mozilla/4.01 (compatible; MSIE 6.0; Windows NT 5.1)
curl "http://localhost:3000/people" -i \
-H "User-Agent: Mozilla/4.01 (compatible; MSIE 6.0; Windows NT 5.1)"