UDTs In Cassandra – Simplified!!


In most programming languages viz. Scala or Java we can play with object constructs i.e. we can create our own classes and create instances out of it. A similar construct is also provided by Cassandra and that is known as UDT which stands for User Defined Type.

User-defined types (UDTs) can attach multiple data fields, each named and typed, to a single column. The fields that is used to create user-defined type  may be any valid datatype, including collection or other UDTs. Once UDT is created , that can be used to define a column in a table.

Syntax to define UDT :-

CREATE TYPE student.basic_info (
  first_name text,
  last_name text, 
  nationality text,
  roll_no int,
  address text
);

student is the keyspace name here and we are creating basic_info as a type and this type contains five fields (first_name, last_name, nationality, roll_no, address) with type of each. Now , we can use this basic_info type to define a column in a table. in simple words, we can say we use UDT to store value as object in Cassandra that contains some fields within itself.

How to use UDT type column in a table :- 

CREATE TABLE student.student_stats 
( id int PRIMARY KEY, grade text, basics FROZEN<basic_info>); 

So, as you can see in above section we declared a table with name student_stats with three columns named as (id, grade, basics)  with datatype of each. The last column is UDT itself because the datatype of basics is which we declared as a type i.e. basic_info.

How to insert data in a table with UDT :-

INSERT INTO student.student_stats (id, grade, basics) VALUES (1,'SIXTH',{first_name: 'Kunal', last_name: 'sethi', nationality: 'Indian',roll_no: 101, address: 'Noida'});

This is how we insert data in a UDT column. It looks like we are giving a key value pair to specify which field value we are giving.

udt1

Here, I just want to share one more thing about insert statement that, in Cassandra, we can also insert the data in JSON format as below.
e.g :

INSERT INTO student.student_stats JSON'{"id":3, "basics":{"first_name":"Abhinav", "last_name":"Sinha", "nationality":"Indian", "roll_no":103, "address":"Delhi"}, "grade":"Tenth"}'

Lets take another case in which we will not insert the one of the field values of the UDT. Then the question is, will the value of that field be inserted or not ??

INSERT INTO student.student_stats (id, grade, basics) VALUES (2,'SIXTH', {first_name: 'Anshul', last_name: 'shivhare', nationality: 'Indian',roll_no: 102});

In this insert command, we are not passing the value of address field here so question arises how Cassandra will handle this and the answer to this question is ‘yes’, it will insert this value as a normal value but it will take the address field value as null . Every field value except primary key that we do not pass at the time of insertion, Cassandra will take it as null.

As you can see here address field value as null.

udt2 (1)

 

Now, we will go through with one more example where we will see how to fetch data from a UDT field with Java code example:

public class ReadData {

    public static void main(String args[]) throws Exception {

//queries
        String query = "SELECT * FROM student_stats";


//Creating Cluster object
        Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").withPort(9042).build();

        Session session = cluster.connect("folio_viewer");
        List udtValList = new ArrayList<>();
        List basicInfoList = new ArrayList<>();
        ResultSet result = session.execute(query);
        
        result.forEach(row -> {
            UDTValue udt = row.get("basics", UDTValue.class);
            udtValList.add(udt);
        });

        udtValList.stream().forEach(val -> {
                    BasicInfo basicInfo = BasicInfo.builder().firstName(val.get(0, String.class) != null ?
                            val.get(0, String.class) : "")
                            .lastName(val.get(1, String.class) != null ? val.get(1, String.class) : "")
                            .nationality(val.get(2, String.class) != null ? val.get(2, String.class) : "")
                            .rollNo(val.get(3, Integer.class) != null ? val.get(3, Integer.class) : 0)
                            .address(val.get(4, String.class) != null ? val.get(4, String.class) : "").build();
                    basicInfoList.add(basicInfo);
                }
        );
        basicInfoList.stream().forEach(val -> {
            System.out.println("_______________________________________________");
            System.out.println("FirstName :- " + val.getFirstName());
            System.out.println("LastName :- " + val.getLastName());
            System.out.println("Nationality :- " + val.getNationality());
            System.out.println("Roll Number :- " + val.getRollNo());
            System.out.println("Address :- " + val.getAddress());
            System.out.println("_______________________________________________");
        });
    }
}

In result object, we are getting a ResultSet and then we are performing iterations with the help of foreach and in each iteration we get one row each out of which we are extracting the UDT column basics and then casting that value into a UDTValue object.

UDTValue stores the fields in a sequential manner in the order they are present in a UDT column itself. To retrieve values from the UDTValue object, we just need to give the index number of the corresponding field e.g. val.get(3, Integer.class).

As you can notice from the UDT definition, rollno is the fourth field hence we are using the  index number 3 and the type is int so we are typecasting that particular field using Integer.class .

This is how we can get the data from UDT fields and one more thing to notice in this example is that we used lombok builder() method to create objects.

Hope, this blog will reduce your efforts in implementing UDTs. Related image


knoldus-advt-sticker


 

This entry was posted in Scala. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s