EWD 3 Training Course Part 7: Applying the QEWD Messaging Pattern

Copyright © 2016 M/Gateway Developments Ltd
EWD 3 Training Course
Part 7
Applying the QEWD
Messaging Pattern
Rob Tweed
Director, M/Gateway Developments Ltd
Twitter: @rtweed

Pattern of a QEWD Application
• Generate an event in the browser
• Send a message to the QEWD back-end
• At the back-end, a handler function for that
message type processes the message and
usually creates a response message
• Response message returned to browser where
a handler function processes it and usually
modifies the User Interface

Let’s Apply these One by One

Pattern of a QEWD application
• Generate an event in the browser, eg:
– Click a button
– Change a form field value
– Move the mouse over a particular area
– etc…
• Need to be able to trigger a handler
function when the required event occurs

Let's Create an Event
• We'll:
– Add a button to index.html
– Add a click event handler for the button in
app.js

Add button to index.html
<html>
<head>
<title>Demo ewd-xpress application</title>
</head>
<body>
<script src="//ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js"></script>
<script src="/socket.io/socket.io.js"></script>
<script src="/ewd-client.js"></script>
<script src="app.js"></script>
<button id="testBtn">Click Me</button>
<div id="content">
Content goes here
</div>
</body>
</html>

Add click handler to app.js
$(document).ready(function() {
EWD.on('ewd-registered', function() {
$('#testBtn').on('click', function(e) {
console.log('button was clicked!');
});
});
EWD.start('demo1', $, io);
});
jQuery event handler
Initially just to make sure the click event is handled
Note how we define the handler within the
document.ready() function and before we
start EWD

Try it out

• Sending a message from the browser to the QEWD back-end:
– Use the EWD.send() function
– Messages must have a type property
– The type property's value is up to you to define
• Any string value
– EWD.send() function not available for use until registration
completes
– EWD.send() automatically adds the session token to the
message
• Its behaviour is protected within a closure and cannot be modified
• Note that manual socket.io messaging is unavailable to user

Edit app.js
var message = {type: 'testButton'};
EWD.send(message);
});
});
});
Simplest message possible for now
Just a type property

Try it out
• Nothing appears to happen in the browser
when you click the button!
• But check the Command Prompt or
terminal window in which you're running
the QEWD Node.js process…

Back-end log
• The message was received by the master
process and passed to a worker:
• Notice that the session token has been
added to the message by the EWD.send()
function
worker 2052 received message: {"type":"testButton",
"token":"4fb0efcd-458c-4afd-a053-8a19600867c4"}

Back-end log
• Worker process reported an error
• QEWD was unable to find and load a
module containing message handlers for
this application ('demo1')
– That's because we haven't written one yet!
Unable to load handler module for: demo1: Error: Cannot find module 'demo1'

Back-end log
• This error message was returned to
master process by the worker
• The master process will have forwarded
this message to the browser
– Nothing displayed in the browser though!
master process received response from worker 2052:
{"type":"testButton","finished":true,"message":{"error":"Unable to load
handler module for: demo1","reason":{"code":"MODULE_NOT_FOUND"}}}

Displaying messages in browser
• Normally it's up to you to handle message
responses and display them in the UI
• During development it's useful to see a
trace of all messages sent and received in
the JavaScript console
• To do this, set EWD.log = true;

Edit app.js
EWD.log = true;
EWD.send(message);
});
});
});

Now the messages show up

Fixing this error
• EWD.send() is working fine
– It's sending the message to the back-end as
expected
• How do we fix the error at the back-end
and how do we handle the incoming
message there?

• At the back-end, you must define a handler function for
each of your message types
• Each handler function processes the message for the
specified type, and usually creates a response message
• All the handler functions for any one application are
defined within a single Node.js module
– That module can, of course, be split into multiple sub-modules if
required

• So, for each application you define a back-end module
• By default, the module name is the same as the application name
– Eg, in our case: demo.js
• You can over-ride this and map an application name to a module name/path,
if you prefer
• Your application module defines a hander function for each of your
application's message types
– Typically a handler function will do any or all of the following:
• Read from / write to a database
• Read from/ write to the user's QEWD Session
• Invoke legacy application functions
• Invoke 3rd-party Node.js modules
• Make web/REST service calls to other local or remote services
– Each of your handler functions has full read/write access to your
embedded Global Storage database:
• Cache, GT.M or Redis

Back-end Application module pattern
• Default name / path:
– Windows:
• C:qewdnode_modules{applicationName}.js
– Linux / Raspberry Pi
• ~/qewd/node_modules/{applicationName}.js

Back-end Application module pattern
module.exports = {
handlers: {
messageType1: function(messageObj, session, send, finished) {
// do something with the incoming message
// create a response object
// use the finished() function to end the processing and
// release the worker
finished(responseObj);
},
messageType2: function(messageObj, session, send, finished) {
// etc….
finished(responseObj);
}
};

Create our back-end module
• C:qewdnode_modulesdemo1.js or
• ~/qewd/node_modules/demo1.js
module.exports = {
handlers: {
testButton: function(messageObj, session, send, finished) {
console.log('*** handling the button click message!');
finished({
ok: 'testButton message was processed successfully!'
});
}
}
};

Create our back-end module
module.exports = {
handlers: {
finished({
});
}
};
This was the value of the type property
in our EWD.send() function
EWD.send({type: 'testButton'});

Save Module and Try it out

And check out the back-end log
worker 2000 received message:
{"type":"testButton","token":"527bd51f-7d6a-4dd7-8
27d-c1ea8dfa1f43"}
*** handling the button click message!
master process received response from worker 2000:
{"type":"testButton","finished":true,"message":{"ok":"testButton
message was processed successfully!","ewd_application":"demo1"}}
*** handleMessage response
{"type":"testButton","finished":true,"message":{"ok":
"testButton message was processed
successfully!","ewd_application":"demo1"}}
sending to socket /#5NdGRfkMn17-vboPAAAC
Master process has finished processing response from worker
process 2000 which is back in available pool
Came from this line:
In our handler function

Got an error?
• Check back-end log:
– "Unable to load module" message?
• Means you have a syntax error in the module
• How to locate it?
– Use the Node.js REPL

Using the Node.js REPL
C:UsersRob Tweed> cd qewd
C:qewd> node
> var x = require('demo1')
C:qewdnode_modulesdemo1.js:6
ok: 'testButton message was processed successfully!''
^
SyntaxError: Unexpected token ILLEGAL
at exports.runInThisContext (vm.js:53:16)
at Module._compile (module.js:373:25)
at Object.Module._extensions..js (module.js:416:10)
at Module.load (module.js:343:32)
at Function.Module._load (module.js:300:12)
at Module.require (module.js:353:17)
at require (internal/module.js:12:17)
at repl:1:9
at REPLServer.defaultEval (repl.js:262:27)
at bound (domain.js:287:14)
>
2 X Ctrl & C to exit from REPL

Back-end message handler resources
• Within a back-end message handler
function you have access to everything
you need:
– testButton: function(messageObj, session, send, finished) {..}
– Incoming message: messageObj
– QEWD session: session.data (read/write access)
– Global Storage database (read/write/execute access):
• this.db gives access to the cache.node APIs,
– eg this.db.function() to execute a function / procedure
• this.documentStore gives access to ewd-document-store JavaScript /
Document database abstraction of your Global Storage database

Using the QEWD Session
module.exports = {
handlers: {
session.data.$('foo').value = 'bar';
finished({
});
}
}
};

• At the back-end, a handler function for the specific message type
processes the message and usually creates a response message
– Response messages are JSON objects
– If using web-sockets, you can send more than 1 message
• send() function for intermediate messages. Worker remains unavailable
• finished() function for final message. Worker released to available pool
– Response messages have a type property
• By default, the type is the same as that of the original incoming message from the
browser
• Optionally, when using the send() function for intermediate messages, you can over-
ride the type
• The type of the finished() message cannot be over-ridden – it is always the same as
the incoming message
– A Handler function MUST end by invoking the finished() function
• Releases the ewd-qoper8 worker process back to the available pool
– Response messages are automatically sent to the originating client or
browser

• Response message is returned to the browser
where a handler function processes it and
usually modifies the User Interface
– Two methods for handling responses within the
browser:
• Callback function as 2nd argument of EWD.send() function
– EWD.send(messageObj, function(responseObj) {…});
• Pub/Sub mechanism, specific to a message type:
– EWD.on(messageType, function(responseObj) {…});
– UI modification using JavaScript
• Up to you which framework, if any, you use

Edit app.js
EWD.log = true;
EWD.send(message, function(messageObj) {
console.log('Response received: ' + JSON.stringify(messageObj.message.ok));
});
});
});
});
Using the EWD.send() callback function to
handle the response

browser:
• We'll just use jQuery since it's already loaded

Edit app.js
EWD.log = true;
$('#content').text(messageObj.message.ok);
});
});
});
});
Change the text inside the <div id="content"> tag

browser:
– Mainly used for messages you'll send frequently from back-end
– Particularly intermediate messages using send() function

Intermediate messages
module.exports = {
handlers: {
send({
type: 'intermediate',
foo: 'bar',
date: new Date().toString()
});
finished({
});
}
};
Overrides the
incoming message
type

Intermediate messages
• C:ewd3node_modulesdemo1.js or
• C:ewd3node_modulesdemo1.js
module.exports = {
handlers: {
send({
type: 'intermediate',
foo: 'bar',
date: new Date().toString()
});
finished({
});
}
};
Must still invoke the
finished() function
to tell ewd-qoper8
to release the worker
back to the available
pool

Edit app.js
EWD.log = true;
EWD.on('intermediate', function(responseObj) {
$('#content').text(responseObj.message.date);
});
$('#content').append('<br /> ' + messageObj.message.ok);
});
});
});
});
Handle instances of the 'intermediate' message
Using Pub/Sub handler

Edit app.js
EWD.log = true;
EWD.on('intermediate', function(responseObj) {
$('#content').text(responseObj.message.date);
});
$('#content').append('<br /> ' + messageObj.message.ok);
});
});
});
});
Combine the responses within the <div id="content"> tag

Application Module Mapping
• By default, the back-end handler module
for a QEWD application is named the
same as the application, eg:
– Our application is named demo1
– By default, QEWD will assume that the
handler functions for this application are in a
module named demo1.js ie:

• You can over-ride this default behaviour by
defining a module map in your QEWD
startup file
• Application module mapping is done in
your QEWD start-up file
– Define a config property named moduleMap
• This is an object / hash mapping one or more
Application names to corresponding module
names/paths

• Example:
var config = {
managementPassword: 'keepThisSecret!',
serverName: 'My QEWD Server',
port: 8080,
poolSize: 2,
database: {
type: 'gtm'
},
moduleMap: {
demo1: 'myDemoApp'
}
};
var qewd = require('qewd').master;
qewd.start(config);

• Example:
var config = {
port: 8080,
poolSize: 2,
database: {
type: 'gtm'
},
moduleMap: {
demo1: 'myDemoApp'
}
};
qewd.start(config);
This tells QEWD that in
order to handle messages
for the demo1 application,
it must load a module using
require('myDemoApp')

• Example:
var config = {
port: 8080,
poolSize: 2,
database: {
type: 'gtm'
},
moduleMap: {
demo1: '/path/to/myDemoApp'
}
};
qewd.start(config);
This tells QEWD that in
order to handle messages
for the demo1 application,
it must load a module using
require('/path/to/myDemoApp')

• Example:
var config = {
port: 8080,
poolSize: 2,
database: {
type: 'gtm'
},
moduleMap: {
demo1: '/path/to/myDemoApp',
finance: 'myFinanceApp'
}
};
qewd.start(config);
You can map as many
Applications as needed

• QEWD Application back-end modules can
therefore be defined as standard Node.js
modules
– Can be published to NPM
– Can be installed from NPM

EWD 3 Training Course Part 7: Applying the QEWD Messaging Pattern

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