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Frequently Asked Questions

What's the Difference Between Mercure and WebSockets?

In a nutshell the WebSocket API is low level, Mercure is a high level. Mercure provides convenient built-in features such as authorization, re-connection, state reconciliation and a presence API ; while with WebSockets, you need to implement them yourself.

Also WebSockets are not designed to leverage HTTP/2+ and are known to be hard to secure. On the other hand Mercure relies on plain HTTP connections and benefits from the performance an security improvement built-in the latest versions of this protocol.

HTTP/2 connections are multiplexed and bidirectional by default (it was not the case of HTTP/1). When using Mercure over a h2 connection (recommended), your app can receive data through Server-Sent Events, and send data to the server with regular POST (or PUT/PATCH/DELETE) requests, with no overhead.

Basically, in most cases Mercure can be used as a modern and easier to use replacement for WebSocket.

What's the Difference Between Mercure and WebSub?

WebSub is a server-to-server only protocol, while Mercure is also a server-to-client and client-to-client protocol.

Mercure has been heavily inspired by WebSub, and we tried to make the protocol as close as possible from the WebSub one.

Mercure uses Server-Sent Events to dispatch the updates, while WebSub use POST requests. Also, Mercure has an advanced authorization mechanism, and allows to subscribe to several topics with only one connection using URI templates.

What's the Difference Between Mercure and Web Push?

The Push API is a simplex protocol mainly designed to send notifications to devices currently not connected to the application. In most implementations, the size of the payload to dispatch is very limited, and the messages are sent through the proprietary APIs and servers of the browsers' and operating systems' vendors.

On the other hand, Mercure is a duplex protocol designed to send live updates to devices currently connected to the web or mobile app. The payload is not limited, and the message goes directly from your servers to the clients.

In summary, use the Push API to send notifications to offline users (that will be available in Chrome, Android and iOS's notification centers), and use Mercure to receive and publish live updates when the user is using the app.

What's the Maximum Number of Open Connections Per Browser?

When using HTTP/2+ (the default for almost all users), the maximum number of simultaneous HTTP streams is negotiated between the server and the client (it defaults to 100). When using HTTP 1.1, this limit is of 6.

By using template selectors and by passing several topic parameters, it's possible to subscribe to an unlimited of topics using a single HTTP connection.

How to Use Mercure with GraphQL?

Because they are delivery agnostic, Mercure plays particularly well with GraphQL's subscriptions.

For instance, the API Platform framework has native support for GraphQL subscriptions thanks to Mercure.

In response to the subscription query, the GraphQL server may return a corresponding topic URL. The client can then subscribe to the Mercure's event stream corresponding to this subscription by creating a new EventSource with a URL like<subscription-id> as parameter.

Updates for the given subscription can then be sent from the GraphQL server to the clients through the Mercure hub (in the data property of the server-sent event).

To unsubscribe, the client just calls EventSource.close().

Also, Mercure can easily be integrated with Apollo GraphQL by creating a dedicated transport.

Cookies are automatically sent by the browser when opening an EventSource connection if the withCredentials property is set to true:

const eventSource = new EventSource('', {
    withCredentials: true