Friday, October 02, 2015

Integrating Morphia and Dropwizard

Recently i had the pleasure of using mongodb's document object mapper framework morphia and the REST webservice framework dropwizard (well it is more than simply for REST but that is the primary purpose of it).

Dropwizard comes with the following essential thirdparty libraries:

  1. Jackson - for JSON parsing
  2. Jetty - as embedded light weight Java EE web container
  3. Jersey - JAX-RS 2.0 implementation
  4. and some more... 
Morphia has annotations for POJO model classes that can be persisted in mongodb and is a nice framework that simplifies the development of a mongodb client application. At the same time we can use the same POJO model classes for REST endpoint implementations as well. So there is no need to have separate DTO and entity classes. The same POJO model classes are annotated as entities (using morphia's annotations) and as DTO (using Jackson's annotations).

A simple example for such a model class is given below:

@Entity(value = "students" , noClassnameStored = true, concern = "SAFE")
public class Student extends BaseModel {
    @Id    private String id = new ObjectId().toHexString();

    @Reference    private User user;

    @Reference    private List courseRefs;

    Date lastUpdated = new Date();

    @PrePersist    void prePersist() {
        lastUpdated = new Date();

    public Student(User user) {
        this.user = user;
        this.courseRefs = new ArrayList<>();

    public Student() {

    @JsonProperty    public Date getLastUpdated() {
        return lastUpdated;

    public void setLastUpdated(Date lastUpdated) {
        this.lastUpdated = lastUpdated;

    @JsonProperty    public List getCourseRefs() {
        return courseRefs;

    public void setCourseRefs(List courseRefs) {
        this.courseRefs = courseRefs;

    @JsonProperty    public User getUser() {
        return user;

    public void setUser(User user) {
        this.user = user;

    @JsonProperty    public String getId() {
        return id;

    public void setId(String id) { = id != null ? new ObjectId(id).toHexString() : new ObjectId().toHexString();

    @Override    public String toString() {
        return "Student{" +
                "id=" + id +
                ", user=" + user +
                ", courseRefs=" + courseRefs +
                ", lastUpdated=" + lastUpdated +

Morphia annotations are:

Jackson annotations used are:

Additionally with Dropwizard we also get the bean validations through Hibernate Validator framework which can be used in the POJO above to annotate the fields in the model with @NotNull or @MinLength... etc.

One important learning in this integration was to map the mongodb _id (ObjectID BSON type) to a string id in the model but set it to the hex string value from the _id field. Getting this to work without the use of frameworks like MongoJack or Katharsis keeps the code simple to comprehend and yet meets the requirements.

Using VNC with Ubuntu on VirtualBox

  1. Create an Ubuntu 64 bit VM on VirtualBox with following specs:
    1. CPU - 1
    2. Memory - 1GB
    3. HDD - 20GB (VDI, Dynamic allocation)
    4. Network - NAT with Port forwarding - TCP, 5900 (on local host) forwarded to 5900 (on VM)
  2. Download Ubuntu desktop 15.04 and install it on the VM.
  3. Once installation completes, search for Desktop Sharing applet. 
  4. Enable sharing, provide password.
  5. Now launch a VNC viewer and specify the IP of the parent host on which VirtualBox is installed with port 5900.
    1. If connecting from same host where virtual box is installed then you may use localhost:5900. 
  6. It will prompt for password then connect.

Tuesday, September 22, 2015

AngularJS Introduction

AngularJS 1.5

Directive is a marker on an HTML tag that tells Angular to run or reference some Javascript code.

HTML page:
<html ng-app="studentModule">
    <link rel="stylesheet" type="text/css" href="css/bootstrap.min.css" />
    <script type="text/javascript" src="js/angular.min.js"></script>
    <script type="text/javascript" src="js/app.js"></script>
<body class="container" ng-controller="StudentController as s">
    <ul class="list group">
        <li class="list group item" ng-repeat="student in s.students">
                {{student.firstName}} {{student.lastName}}
                <em class="pull-right">{{student.emailId}}</em>
(function() {
  var app = angular.module('studentModule', []);

  app.controller('StudentController', ['$http', function($http){
    var store = this;
    store.students = [];

    $http.get("/api/students").success(function(data) {
        store.students = data;

Here ng-controller is the directive.

Modules: is where we write pieces of our Angular application and define dependencies for our app. Modules can use other modules (or depend on other modules).

Var app = angular.module('store', []) -- define a new module named 'store' and with no dependencies (so empty array) -- this goes into a separate app.js

-- ng-app directive will run the module named "store" when the document loads. Ng-app also makes the entire html page an angular application which means the contents within can now use the features of the angular framework - like writing expression such as:

I am {{4 + 6 }}

Will translate to:

I am 10

Similarly we can have string expressions: {{ "hello" + " world" }}

We can have filters in the expression by using pipe to get the o/p of expression as input of a filter and thus achieve filtered result:
{{ product.price | currency }}

Or in general: {{ data* | filter: options*}}





Sample 1:

Controllers: where we define our app's behavior.



The purpose of the controller is to get the JSON data (from gem object in this case) over to html page block where the ng-controller is defined. This way we can get the JSON data into a javascript client side object in some way (say by querying REST endpoint) and then populate the div block in the html page with the data - and this is facilitated by the controller's property ("product" in this case) which is set to JSON javascript object (gem in this case).


Built-in validations for common input types:
Email, url, number (max, min),


Monday, September 21, 2015
1:27 PM
Attach application module to a page
Attach a controller function to the page
Repeat a section of the page for each item in an array
Display or not display a section based on an expression
Will set to the classname when condition specified is true.
Ng-class="active:tab === 1"
Ng-model - binds the form elements to properties -- this is how we create a live preview.
Ng-submit - specify method to invoke when form is submitted.

Ng-include - include html snippet

Custom Directives - makes html more readable.. Same can be achieved using ng-include directive by including the duplicating snippet of html code from another html file. But custom directives will be like new tags -


Tuesday, September 22, 2015
12:02 AM
App.js - root module "store" is defined here that is referred in our single page app's html page (with ng-app directive). It will have dependencies on all other sub-modules.

var app = angular.module("store", ["products-directives"])

Products.js - can be another module consisting of custom directives (which also might encapsulate controllers) pertaining to products in the store.

And so on, there could be other modules which are referred by products module or by the main store module.

Tuesday, September 22, 2015
12:06 AM
Services give controllers additional functionality.
  1. Fetching JSON from webservice - $http
  2. Logging message for Javascript console - $log
  3. Filtering an array - $filter


$http({ method: 'GET',  url: '/products.json' });
$http.get('/products.json', {apiKey: 'myApiKey'… other query params});

Returns a promise object: .success() or .error()

$'/api/students',  {param: 'value'});


Monday, September 21, 2015

Flask introduction

Flask is a python microframework for building web applications or for rapid prototyping one.

sudo easy_install virtualenv
python3 -m venv venv --- create a venv directory for the project
source venv/bin/activate -- activate the virtual env.
pip install flask -- install flask

The idea for virtual environment is to import packages like flask only in a copy of the python
Virtual environment and not pollute the global python installation.

An example flask application:

from flask import Flask, render_template
app = Flask(__name__)

def index():
    return render_template('index.html')

def user(name):
    return render_template('user.html', name=name)

if __name__ == '__main__':


Hello, {{ name }}!


It will start the web server

It should o/p Hello, watsh in browser.

Sunday, September 20, 2015

Mac OSX: can't start Git - Intellij IDEA complains

The solution is: in terminal, run 'sudo xcodebuild -license' and then agree to the terms.

Friday, September 18, 2015


MongoDB is a NoSQL document-oriented database which stores data as BSON (Binary Script Object Notation) documents.

Salient features of MongoDB are:
  1. Document Oriented - aggregates the data in minimal number of documents.
  2. Ad hoc queries - like regular expression search, by ranges or field is supported.
  3. Indexing - any field in the document can be indexed.
  4. Replication - high availability is supported by maintaining replicas of data in more than one replica set member. The data is eventually consistent between the replica members.
  5. Load Balancing - uses sharding (a shard is a master with one or more slaves) to distribute the data split into ranges (based on shard key) between multiple shards.
  6. File storage - supports storing a file not as a single document but split across multiple shards - GridFS feature of MongoDB comes built in and is used by NGNIX and lighthttpd.
  7. Aggregation - is similar to SQL GROUP BY clause.
  8. Capped collections - can be used to store data in insertion order and once specified size is reached, behaves like a circular queue. Similar to RRD.
  9. Server side Javascript execution - is supported.

MongoDB is one of the top performing NoSQL database. Benchmarks have reported MongoDB performance to be better than some other NoSQL DBs by as much as 25x.
Eventual Consistency - eventually all nodes in the cluster of NoSQL DB will have the same data as data may not be propagated if the network breaks down or the node goes down but eventually when node is up and the network is working then data will be consistent across all shards.
1. Collections - set of documents
2. Document - BSON document with dynamic schema - means that documents in the same collection do not need to have the same set of fields or structure, and common fields in a collection's documents may hold different types of data.
Important mongo shell commands:
>show dbs
>show collections
>{"name":"tutorials point"}) --- will insert a movie document in movie collection.
>db.createCollection("mycol", { capped : true, autoIndexID : true, size : 6142800, max : 10000 } )
RDBMS Where Clause Equivalents in MongoDB
To query the document on the basis of some condition, you can use following operations
OperationSyntaxExampleRDBMS Equivalent
Equality{:}db.mycol.find({"by":"tutorials point"}).pretty()where by = 'tutorials point'
Less Than{:{$lt:}}db.mycol.find({"likes":{$lt:50}}).pretty()where likes < 50
Less Than Equals{:{$lte:}}db.mycol.find({"likes":{$lte:50}}).pretty()where likes <= 50
Greater Than{:{$gt:}}db.mycol.find({"likes":{$gt:50}}).pretty()where likes > 50
Greater Than Equals{:{$gte:}}db.mycol.find({"likes":{$gte:50}}).pretty()where likes >= 50
Not Equals{:{$ne:}}db.mycol.find({"likes":{$ne:50}}).pretty()where likes != 50
AND condition:
>db.mycol.find({key1:value1, key2:value2}).pretty()
OR condition:
      $or: [
      {key1: value1}, {key2:value2}
'where likes>10 AND (by = 'tutorials point' OR title = 'MongoDB Overview')'
>db.mycol.find("likes": {$gt:10}, $or: [{"by": "tutorials point"}, {"title": "MongoDB Overview"}] }).pretty()
>db.mycol.update({'title':'MongoDB Overview'},{$set:{'title':'New MongoDB Tutorial'}})

Replace old document with new data:
      "_id" : ObjectId(5983548781331adf45ec7), "title":"Tutorials Point New Topic", "by":"Tutorials Point"
>db.mycol.remove({'title':'MongoDB Overview'})
>db.COLLECTION_NAME.remove(DELETION_CRITERIA,1) -- removes the first match
>db.mycol.remove() - remove all documents in the collection.

Projection: selectively display fields and not all fields of a document.
>db.mycol.find({},{"title":1,_id:0}) -- will display title and not _id (which is always displayed otherwise)
Sort: 1 is used for ascending order while -1 is used for descending order.
Index: Here key is the name of filed on which you want to create index and 1 is for ascending order. To create index in descending order you need to use -1.
> db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$sum : 1}}}])
select by_user, count(*) from mycol group by by_user -- how many tutorials are written by each user (grouped by a user)

There is a list available aggregation expressions.
$sumSums up the defined value from all documents in the collection.db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$sum : "$likes"}}}])
$avgCalculates the average of all given values from all documents in the collection.db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$avg : "$likes"}}}])
$minGets the minimum of the corresponding values from all documents in the collection.db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$min : "$likes"}}}])
$maxGets the maximum of the corresponding values from all documents in the collection.db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$max : "$likes"}}}])
$pushInserts the value to an array in the resulting document.db.mycol.aggregate([{$group : {_id : "$by_user", url : {$push: "$url"}}}])
$addToSetInserts the value to an array in the resulting document but does not create duplicates.db.mycol.aggregate([{$group : {_id : "$by_user", url : {$addToSet : "$url"}}}])
$firstGets the first document from the source documents according to the grouping. Typically this makes only sense together with some previously applied “$sort”-stage.db.mycol.aggregate([{$group : {_id : "$by_user", first_url : {$first : "$url"}}}])
$lastGets the last document from the source documents according to the grouping. Typically this makes only sense together with some previously applied “$sort”-stage.db.mycol.aggregate([{$group : {_id : "$by_user", last_url : {$last : "$url"}}}])
Replication: client application always interact with primary node and primary node then replicate the data to the secondary nodes.
  • All write operations goes to primary
  • Reads can happen from any of primary or secondary nodes.
Setting up replica set:
mongod --port "PORT" --dbpath "YOUR_DB_DATA_PATH" --replSet "REPLICA_SET_INSTANCE_NAME"
Backup/Restore MongoDB data: