Postgres: GraphQL Subscriptions

Introduction

A GraphQL subscription is essentially a query where the client receives an update whenever the value of any field changes upstream.

Subscriptions are supported for all kinds of queries. All the concepts of queries hold true for subscriptions as well.

:::info Postgres Compatibility

Hasura works with most Postgres compatible flavours.

:::

:::info Single subscription in each query caveat

Hasura follows the GraphQL spec which allows for only one root field in a subscription. You also cannot execute multiple separate subscriptions in one query. To have multiple subscriptions running at the same time they must be in separate queries.

:::

Types of subscriptions

Live queries

A live query subscription will return the latest result of the query being made and not necessarily all the individual events leading up to the result. By default, updates are delivered to clients every 1 sec.

See more details here.

Streaming subscriptions

A streaming subscription streams the response according to the cursor input by the user. A streaming subscription is different from a live query as it sends individual rows at a time and not the entire result set.

See more details here.

Live query vs streaming subscriptions

Suppose we need to display the messages of a group chat on a page, this can be done either via live queries or streaming subscriptions. Let's see how they can be used and how they differ from each other.

Using live query

With live query, we'll make the following query:
graphql
```
subscription {
  messages(where: { group_id: 1 }, order_by: { created_at: asc }) {
    id
    sender
    receiver
    content
    created_at
    edited_at
  }
}
```
The initial response for this subscription will be all the messages of the group. Let's say the initial response contained 100 messages. Now, if there is one more message sent to the group, then all 101 messages will be sent in a new response.
Using streaming subscriptions

With streaming subscriptions, we'll make the following query:
graphql
```
subscription {
  messages_stream(where: { group_id: 1 }, cursor: { initial_value: { created_at: now } }, batch_size: 10) {
    id
    sender
    receiver
    content
    created_at
    edited_at
  }
}
```
Here, we'll start getting all messages of the group in batches given by batch_size with created_at greater than now.

Following the example of the live query, if we have 100 messages corresponding to the group and only 5 messages with created_at greater than the current value of the cursor maintained by the cursor, then we will get only the 5 messages.

The experimental feature flag remove_empty_subscription_responses can be used to reduce network overhead for projects making heavy use of streaming subscriptions where responses are often empty by only returning non-empty result sets. Note that for streaming subscriptions this shouldn't result in any observable change from the subscriber's point of view, but for live queries null and [] will never be returned which means the subscriber won't receive an initial empty result, for instance, or won't be able to detect when a query goes from returning a certain result to returning no result(s).

Communication protocol

Hasura GraphQL Engine uses the GraphQL over WebSocket Protocol by the apollographql/subscriptions-transport-ws library and the GraphQL over WebSocket Protocol by the graphql-ws library for sending and receiving events. The GraphQL Engine uses the Sec-WebSocket-Protocol header to determine the communication protocol that'll be used. By default, the GraphQL engine will use the apollographql/subscriptions-transport-ws protocol.

:::info 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 behavior is that the cookie won't be read (because of potential security issues). To override the behavior, you can use the flag --ws-read-cookie or the environment variable HASURA_GRAPHQL_WS_READ_COOKIE. See GraphQL Engine server config reference for the setting.