Read and Write path in Cassandra

Prerequisites –

Write Path Execution in Cassandra :

In Cassandra, while writing data, writes are written to any node in the cluster (coordinator).
when any user will insert data, it means they write the data first to commit log then to memtable.
When any user will write the data, every write will include a timestamp.
Once memtable starts getting full then it is flushed to disk periodically (SSTable).
After that a new memtable is created in memory.
In the case of write path execution, deletes are special write cases which are called a tombstone.

Inserting Data :
In case of inserting data in Cassandra, we will create a keyspace and then create a table and then insert data into the table.

Example –

// Creating a keyspace
create keyspace UniersityData
replication = {'class': 'SimpleStrategy', 'replication_factor' : '3' };

// Creating a table and declaring the columns
create table CSE_Student(
student_id int,
name text,
email text,
primary key(student_id)
);

// Using the newly created keyspace
use UniersityData;

// Inserting values in the table for all the columns
Insert into CSE_student(student_id, name, email) 
values(012345, 'Ashish', 'ashish@gmail.com');

Insert into CSE_student(student_id, name, email) 
values(012346, 'Abi', 'abi@gmail.com');

Insert into CSE_student(student_id, name, email) 
values(012347, 'Rana', 'rana@gmail.com');

Insert into CSE_student(student_id, name, email) 
values(012348, 'Aayush', 'aayush@gmail.com');

Insert into CSE_student(student_id, name, email) 
values(012349, 'harsh', 'haarsh@gmail.com');

Read Path Execution :

In Cassandra while reading data, any server may be queried which acts as the coordinator.
when we want to access read data then we contact nodes with requested key.
In a data center, on each node, data is pulled from SStable and is merged.
In Cassandra, while considering read consistency, we can check –
Consistency < ALL performs read repair in the background (read_repair_chance).

Reading Data :
Write a cqlsh query to read data from CSE_student and give output for the same.

select * 
from CSE_student;

Output :

index	student_id	name	email
0	12345	Ashish	ashish@gmail.com
1	12346	Abi	abi@gmail.com
2	12347	Rana	rana@gmail.com
3	12348	Aayush	aayush@gmail.com
4	12349	harsh	haarsh@gmail.com

Article Tags :

DBMS

Apache