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JavaScript Services

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Nikos Vasilakis edited this page Nov 13, 2015 · 2 revisions

Overview

WebUI front-end applications are often supposed to receive PUSH notifications from backend. In Xenon this problem is solved via ability to register remote Xenon services attached through WebSocket connection to a node. Such services may be invoked by any other services and they may be used as subscribers for other services updates as well.

WebSocket Connection

Xenon host provides common pure JavaScript library for establishing and managing WebSocket connections to a Xenon node. This library is available at:

/user-interface/resources/com/vmware/dcp/common/WebSocketService/ws-service-lib.js

The above library defines a WSL object which defines WebSocketConnection class. An instance of this class may be created using the following constructor:

var connection = new WSL.WebSocketConnection("/core/ws-endpoint")

In-Browser JavaScript Xenon services

Since In-Browser JavaScript Xenon services cannot have regular public URIs, an auto-generated ephemeral URIs are used to identify such services. These URIs are assigned on request by a Xenon node. These URIs are only valid until WebSocket connection is alive and service is not stopped. In order to create a new JavaScript Xenon service, it is necessary to provide handleRequest(op) method which encapsulates service implementation and may handle any type of REST request.

Service creation involves a remote call to Xenon node for an ephemeral service URI, so service creation result is only available as a callback. Here is an example of a service creation:

connection.createService(
    function (op) {
        try {
            // ### Handle operation here ###
        } finally {
            op.complete();
        }
    },
    function (wss) {
        // ### Handle created WebSocketService here ###
    });

Operation object

Operation object has id, action, statusCode and body fields which correspond to the same fields of com.vmware.dcp.common.Operation class. One of methods complete(), fail(exception) or fail(exception, body) should be used to complete received operation.

WebSocketService object

WebSocketService object has uri field which is set to ephemeral service URI accessible from within Xenon cluster. It also has subscribe, unsibscribe and stop methods. stop methods un-registers the servers. Other two methods are described below.

JavaScript observers

It is possible to use wss.uri explicitly to subscribe JavaScript service on updates for any other observable Xenon service. However in case when used closes browser page, observed service cannot know that observer has gone forever and subscription record gets stuck indefinitely in subscribers list.

WebSocketService object subscribe(uri) and unsubscribe(uri) methods can be used to register and unregister the service as an observer for other service. uri argument is a URI for subscriptions sub-service. Subscriptions registered with subscribe(uri) method will be automatically removed when WebSocket connection is closed or service is stopped.

Example

dcp-host project carries an example JavaScript service which subscribes to examples service factory updates. Demo page is available at: http://localhost:8000/user-interface/resources/com/vmware/dcp/ui/UiService/features/ws/ws-demo.html

Links to all added example service documents are displayed whenever new example documents are created.

Minimalistic JavaScript Xenon service example is provided below:

<html>
<head>
    <title>WebSocket Service Demo</title>
    <script type="text/javascript"
            src="/user-interface/resources/com/vmware/dcp/ui/UiService/libs/jQuery/jquery-2.1.0.js"></script>
    <script type="text/javascript"
            src="/user-interface/resources/com/vmware/dcp/common/WebSocketService/ws-service-lib.js"></script>
    <script type="text/javascript"
            src="/user-interface/resources/com/vmware/dcp/ui/UiService/features/ws/ws-demo.js"></script>
</head>
<body>
    <h1>Created example objects</h1>
    <div id="objects-created"></div>
</body>
</html>
var wsEndpoint = "/core/ws-endpoint";
var exampleServiceSubscriptions = "/core/examples/subscriptions";

new WSL.WebSocketConnection(wsEndpoint).createService(
    function (op) {
        try {
            $("#objects-created").append('<a href="' + op.body.documentSelfLink + '">' + op.body.name +
                ' (' + op.body.documentSelfLink + ')</a><br/>');
        } finally {
            op.complete();
        }
    },
    function (wss) {
        wss.subscribe(exampleServiceSubscriptions);
    }
);

WebSocket communication protocol

WebSocket communication protocol is describe in com.vmware.dcp.common.http.netty.NettyHttpClientRequestHandler JavaDoc.

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