What are WebSockets?
WebSockets provide the ability to control communication between the web server and the browser.
This involves updating content that is shown in the browser.
Normally, communication between the web server and the browser takes place via an HTTP connection.
So that content is updated with an HTTP connection, the browser must send a request to the web server.
This is not required when using WebSockets. With these, the web server sends messages/updates
to the server without the browser having to play an active role.
Therefore, a user action is not required to send messages to the browser or update content in the browser.
Sending messages or updating browser content is triggered automatically by an event.
Events that trigger the sending of messages or the updating of browser content
can take place in Intrexx (internally) or outside of Intrexx (externally).
External events can be the receipt of status and warning messages from machines or emails arriving in an external inbox.
Internal events can be timer events, data group events and user-defined events;
essentially, all events that can be responded to with process event handlers.
See the Use cases
for more information about events.
The messages, which the Intrexx Portal Server sends to the browser, are created in Intrexx.
That are various options for this.
For example, you can create static messages that never change. This can be useful when emails arrive in
an external inbox. In this case, it would be enough if the message "You have new mail"
is displayed in each case. You can also generate dynamic messages that contain
different variables, for example. You can use all functions of Groovy and Velocity to create these messages.
Messages can be simple text messages or be JSON messages.
Topics - Receive messages, make messages available
Messages are not immediately sent from the server to the browser. Instead, the messages are transferred
to a so-called topic first / the topic receives the messages.
You must create topics in Intrexx.
Topics can also receive messages that result from multiple events.
Subscribe to topics
A topic can deliver/send the message it has received to the browser. However, the browser must
subscribe to a topic first.
The subscription determines whether the messages of a topic should be displayed in the browser.
Equally, where and how the message should be displayed on the browser-side.
It can be displayed, for example, in a tooltip or in a static text element.
Furthermore, when the topic subscription begins and if applicable, ends.
Technically, topics represent the object that WebSocket messages
output/make available. A "consumer" can subscribe to a topic.
A topic can be compared to a message channel that can be subscribed to.
Topics also appear in conjunction with the
Intrexx Industrial test MQTT broker
They are used in a similar way there.
Who is allowed to subscribe to topics?
You can grant permissions for topics. In this way, you can control that
only certain users can see messages that are intended for or relevant to them.
The permissions also allow you to control which user can transfer messages to a topic.
WebSockets versus polling
Automatically updating content on websites could previously be done in Intrexx. This could be implemented with
so-called polling. When using polling, the browser queries information from the server in predefined intervals.
In this way, messages from the server are not immediately shown in the browser but always at the time of the query.
At the same time, this increases the server load.
More information about this is available in the "Advanced Techniques" workshop
When using WebSockets, the server actively sends messages. The browser does not
always need to query the server. In this way, the notifications/updates are
immediate. Equally, the server load is less than when using polling.