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How to Return Binary Data from a Node Service

Description

A node service can return binary data to an Xbasic script using optional attachments sent using the sendResponse() callback function.

Discussion

If you use Node with you Xbasic scripts in your Alpha Anywhere applications, binary data can be sent to and received from Node. Sending binary data may be necessary if the service loads or generates a file, such as an image, that needs to be returned to the calling script.

A node service takes three parameters: a packet that contains data passed to the service, a response which is used to send data from the service, and a sendResponse function that takes two parameters: the response object and an optional attachments JSON object that contains one or more binary objects to return to the calling script.

For example, consider the example below which loads an image using the Node File System library 'fs' and returns the image data with the response using the attachments object:

exports.handler = function(packet,response,sendResponse) {
    var fs = require('fs');
    var fn = packet.filename;

    function _cb(error,data) {
        if (error) {
            response.error = error;
            sendResponse(response,null);
        } else {
            var attachments = [{name: fn, data: data}];
            response.result = true;
            sendResponse(response,attachments);
        }
    };

    fs.readFile(fn,_cb);
}
If the node service does not call sendResponse(), either because a JavaScript error occurs or a callback fails to execute, the service will not terminate until the Node timeout is reached. This can create the impression that an Xbasic script has become unresponsive.

Each entry in the attachments JSON object can define the following properties:

name

A string that uniquely identifies the data. E.g. "image1". Not required, but recommended.

data

The binary or text data.

mimetype

Optional. Specifies the MIME type of the data. E.g. "image/png".

The node service in the example above requires a filename passed into the packet parameter. It can be called from Xbasic as follows:

dim filename as c = "c:/moiveimages/4772.JPG"

dim pIn as p
pIn.filename = filename

dim pOut as p
pOut = node_request_result("getBinary", pIn)

if (pOut.result = .T.) then
    dim attachments as Helper::NodeAttachments
    attachments = pOut.attachments

    dim data as Helper::NodeAttachment
    data = attachments.attachments[1]
    
    encoded = base64encode(data.data)
    
    Context.Response.Write("<p>Loaded " + data.name + "</p>")
    Context.Response.Write("<img src=\"data:image/jpg;base64,"+encoded+"\">")
else 
    ' an error occurred
    Context.Response.Write("An error occurred when calling 'getBinary'. Details below:<br>")
    Context.Response.Write("<br>" + stritran(properties_enum(pOut.error,"R;O=p=v"),crlf(),"<br/>"))
end if

The Xbasic script above can be inserted into an .a5w page. The script calls the getBinary node service, passing in an image. If the request succeeds, the binary image data is base64 encoded and displayed in the .a5w page. If the request fails, it prints all of the properties and values of the pOut.error object. For example, if the file doesn't exist, the Xbasic script prints the following information:

An error occurred when calling 'getBinary'. Details below:

Errno=-4058
Code="ENOENT"
Syscall="open"
Path="c:\moiveimages\4772.JPG"

To learn more, watch the video below.

Calling a Node Service that Returns Binary Data

When you call a Node service from Xbasic, the service can return both text and binary data. In this video we show how a Node Service can return binary data.

Download the getBinary.js Node Service shown above

2017-02-28

See Also