Hi Daniel, On 02/08/2017 05:07 PM, Daniel Henrique Barboza wrote:
Hi everyone,
I want to start a discussion on https://github.com/kimchi-project/wok/issues/16, "Asynchronous event notification".
As most of you are aware, WoK does not have any sort of push notification. WoK UI works based on a polling strategy using the Notifications API to fetch for any backend notifications. This polling is on a 3 second interval to not overload the server with these requests. The problems with this approach are obvious: the cost of the polling process for both UI and backend, the interval for a backend event to be delivered to the UI and so forth.
- The idea
This work aims to implement an asynchronous strategy to deliver server to client messages. The idea is to use websockets* to establish a socket connection between the UI and the backend. The backend can send any message using this socket and the UI, after receiving it, can act upon immediately.
- Push server implementation
This socket in the backend side would act as a 'push server' that will receive the connections and push the same messages to all of them. Only server to client messages will be sent.
This push server can be implemented in two ways:
* from scratch
* using an external library
One library that seems to do this asynchronous socket implementation is tornado ( https://github.com/tornadoweb/tornado ). It is present in all distros we support and it has Apache 2.0 licensing. I'll experiment with it and see if it helps. I am opened to any other suggestion of libraries that can be used in the push server implementation. If no library is good enough for us, I'll have to implement it from scratch.
I am OK in using it as it is available for Fedora 25, Ubuntu 16.10, openSUSE 42.2 and centOS 7. It would be good to confirm it is also available for Debian 8 as Lucio is working to get the package into the official distro.
- WoK backend design
In WoK backend, my idea is to reuse the 'add_notification' method from the existing Notifications API. When adding a notification, fire a message to the push server and notify all the listeners too.
Could you elaborate more on that? For example, in Kimchi, after adding/starting/deleting a VM we will need to notify all the browsers sessions to update the data in UI. Or on Wok, after enabling/disabling a plugin. How will that be done with the notifications API? From the documentation, the notifications API is as below: * **GET**: Retrieve the full description of the Notification * code: message ID * message: message text already translated * timestamp: first time notification was emitted
This approach has the following advantages:
- it will work out of the box for all backend messages in all plug-ins that uses the 'add_notification' method;
Today, the notifications API is only shown on UI as a warning message to the user. So I think much UI changes will be required.
- we can re-use the same JSON message format of the Notifications API, reducing the amount of UI work we'll have to adapt the existing UIs;
Same I commented above.
- it will be harmless to implement. Given that the push server will send messages to all connected UI endpoints, if no endpoint is connect no message will be sent.
- WoK frontend design
For any tab that wants to receive the push notifications, just connect to the push server via websocket and react to the messages sent - just like it is done today with the notifications API but without the need of sending the GET /notifications messages.
We will need to be careful to not open unnecessary websockets when tab switching. We will need to pay attention to closing up the connections we don't need anymore.
Why do not open just one on Wok and reuse for all the plugins?
I am planning to do a proof of concept of an UI working with this new push server notifications in the 'User Log' tab, together with this backend work. When a new log entry is created, a push notification is sent and the UI would refresh automatically. This implementation would be used as a base for the other tabs/plug-ins.
Let me know what you think!
Daniel
PS: for the record, before deciding to use websockets I've considered using SSE (Server-side Events), a HTML5 standard, but gave up due to lack of SSE support from Microsoft browsers.
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