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Barrister Contact Demo - JS Impl

To install Barrister for node.js, see: https://github.com/coopernurse/barrister-js

In this tutorial we'll use the barrister-js bindings to write a Node.js client and server and a browser based client for a fictitous Contact Service.

The parent directory contains the plain IDL file and its JSON representation. Please refer to these files as we go along:

  • ../contact.idl - IDL file
  • server.js - Implementation of the IDL using Node.js and Express
  • client.js - Client program that demonstrates how to call the service, trap errors, and use batches. Uses Step to ensure the requests are chained in a synchronous manner.
  • static/client.html - Browser based client that uses the same API as client.js, just packaged a bit differently so it can be loaded in the browser.

Running the example

# clone the repo
git clone git://github.com/coopernurse/barrister-demo-contact.git

# install dependencies
cd barrister-demo-contact/js
npm install express step

# start server in background
node server.py &

# run the client - should get a bunch of output 
node client.py

Then in your browser, try loading:

Back at the terminal:
fg      (to foreground the server)
ctrl-c  (to quit the server)

How it works

First we have to write our interface definition, which is in contact.idl. An IDL file may contain many interfaces, but this example only has one:

interface ContactService {
    put(contact Contact) string
    get(contactId string, userId string) Contact
    getAll(userId string) []Contact
    delete(contactId string, userId string) bool

Notice that you can use structs as parameters or return types. [] denotes an array. The type follows the identifier.

Structs can reference other structs or enums, in addition to primitive types. Notice that the Contact has an array of Phone objects:

struct Contact {
  contactId string
  userId    string
  firstName string
  lastName  string
  email     string
  phones    []Phone
  address   Address   [optional]

The [optional] string tells Barrister that address is not required. By default all struct fields are required, and Barrister will automatically reject requests with missing fields.

The []Phone field is not optional, but empty arrays are permitted. A missing phone field, or one set to null would be considered invalid.

Understanding the implementation

Take a moment to read the client.js and server.js code. The comments in the code will hopefully clarify how things work. Rather than repeat those comments here, we'll focus on the less obvious bits.

Creating a Client

When consuming a Barrister service from Node, you'll create a Client and load the contract.

Here's how that would look in Node.js:

var barrister = require('barrister');
var client = barrister.httpClient("http://localhost:3000/contact");
client.enableTrace();   // log to console.log.  optional.
client.loadContract(function(err) {
    if (err) { throw err; }
    // contract successfully loaded -- you can now use client to make requests

And in a browser:

<!-- used to make the POST request, but that can be overridden (see below) -->
<script type="text/javascript" src="jquery-1.6.4.min.js"></script>

<!-- For browsers that don't include the JSON global object
     See: https://github.com/douglascrockford/JSON-js -->
<script type="text/javascript" src="json2.min.js"></script>

<!-- Barrister itself.  A non-minified version is also available -->
<script type="text/javascript" src="barrister.browser.min.js"></script>

<script type="text/javascript">

  jQuery(document).ready(function() {
    var client = Barrister.httpClient("/contact");
    client.loadContract(function(err) {
        if (err) { 
            alert("Unable to load contract: " + err);
        else {
            // contract successfully loaded -- you can now use client to make requests

A few notes about the browser implementation:

  • jQuery is used to make the POST request inside Barrister, but it can be swapped out by passing a function to Barrister.httpClient() instead of a string. See the src/browser_footer.js file for the default implementation and notes

Making requests

To invoke a function, create a proxy object for the interface you wish to call. The proxy will have functions hung off it that match the functions for the interface in the IDL.

For example, to use the ContactService:

// assume we already initialized the client per the above example, then:
var clientService = client.proxy("ContactService");

// now we can call functions on that service:
//   params to function should match the params in the IDL for that function
//   with an additional callback function at the end, which is called with the result
// callback gets an error (null if request succeeds) and the result (null if request fails)
// result type will match the result type for the function in the IDL
clientService.put(contact, function(err, contactId) {
    if (err) {
        // err is a JSON-RPC error with 'code', 'message', and (optional) 'data' keys
        console.log("error in put()  code=" + err.code + " msg=" + err.message);
    else {
        console.log("successfully put() contact with id: " + contactId);

Malformed requests

One of Barrister's selling points is that it helps ensure that requests and responses match the types defined in the IDL so that you don't have to manually deal with that validation in your application.

You can see an example of this in client.js - putInvalidContact. In that function we create with invalid property (firstName field is misnamed as first). When we call put() an error is returned. If you look at server.js there's no validation code in put() that would reject the request in that case. The error is generated by Barrister automatically.

Client request validation

By default clients will validate their own requests, which eliminates the need for a server round trip in the case of malformed request data. If you wish to disable client request validation:

// create client
var client = Barrister.httpClient("/contact");

// turn off validation
client.validateRequest = false;

Error handling

If a RPC call fails, the proxy function will provide an error object containing code, message, and (optionally) data fields.

code is an integer that indicates the error type. The JSON-RPC 2.0 spec defines some built in error codes for things like malformed JSON, invalid request params, etc. Barrister supports these codes and returns them if request parsing or validation fails.

message is a human readable description of the error. data is an opaque entity that provides more error specific information. It is not included by default and the meaning is application specific (meaning whatever your server sends is what will be here).

In your server code you may define your own error codes. In our IDL we define 3 different codes that are meaningful to us. For example, error 102 means "limit exceeded" which we return if a user tries to add more than 10 contacts. Stick to postive integers in your error codes to avoid conflicts with the JSON-RPC built in codes.

See server.js for some examples of returning custom errors.

Batch Requests

Barrister allows you to batch multiple requests in a single call, which can speed things up if your requests are small and the HTTP/network overhead becomes noticeable. To use a batch:

// create a batch from the client
var batch = client.startBatch();

// create a proxy from the batch
var contactService = batch.proxy("ContactService");

// call functions on the proxy
// NOTE: you do NOT pass a callback at this point, since
//       no network request is being made here.

// send the batch - this takes the callback
// err is null if there are no transport errors, but 
// results could contain elements with RPC errors
// results[i].error will be set if the request with offset i had an error
// otherwise, results[i].result will be set, which contains the successful
// return value of batch request i
batch.send(function(err, results) {        
    var i, o;
    if (err) {
        console.log("Error on batch request: " + JSON.stringify(err));
    else {
        console.log("Got " + results.length + " result(s)");
        for (i = 0; i < results.length; i++) {
            o = results[i];
            if (o.error) {
                console.log(" method=" + o.method + " params=" + o.params + 
                            " err code=" + o.error.code + " err msg=" + o.error.message);
            else {
                console.log("  method=" + o.method + " params=" + o.params +
                            " result=" + o.result);

More information

IDL Syntax

The main Barrister docs explain how to write an IDL file and run the barrister tool to convert it to a json file.