Subscriptions¶
Table of contents
A GraphQL subscription is essentially a query where the client receives an event whenever the value of any field changes upstream. The Hasura GraphQL engine supports subscriptions for all kinds of queries. All the concepts of queries hold true for subscriptions as well.
Convert a query to a subscription¶
You can turn any query into a subscription by simply replacing query
with subscription
as the operation type.
Caveat
Hasura follows the GraphQL spec which allows for only one root field in a subscription.
Use cases¶
Communication protocol¶
Hasura GraphQL engine uses the GraphQL over WebSocket Protocol by the apollographql/subscriptions-transport-ws library for sending and receiving events.
Setting headers for subscriptions with Apollo client
If you are using Apollo Client, headers can be passed to a subscription by setting connectionParams
while
creating the wsLink:
// Create a WebSocket link:
const wsLink = new WebSocketLink({
uri: `<graphql-endpoint>`,
options: {
reconnect: true,
connectionParams: {
headers: {headers-object}
}
}
});
See this for more info on
using connectionParams
.
Cookies and WebSockets¶
The Hasura GraphQL engine will read cookies sent by the browser when initiating a
WebSocket connection. The browser will send the cookie only if it is a secure cookie
(secure
flag in the cookie) and if the cookie has a HttpOnly
flag.
Hasura will read this cookie and use it as headers when resolving authorization (i.e. when resolving the auth webhook).
Cookies, WebSockets and CORS¶
As browsers don’t enforce Same Origin Policy (SOP) for websockets, the Hasura server enforces the CORS rules when accepting the websocket connection.
It uses the provided CORS configuration (as per Configure CORS).
- When it is
*
, the cookie is read and the CORS check is not enforced. - When there are explicit domains, the cookie will only be read if the request originates from one of the listed domains.
- If CORS is disabled, the default behaviour is that the cookie won’t be read
(because of potential security issues). To override the behaviour, you can
use the flag
--ws-read-cookie
or the environment variableHASURA_GRAPHQL_WS_READ_COOKIE
. See GraphQL engine server flags reference for the setting.