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What is the difference between primary, partition and clustering key in Cassandra ?

Editor — Tue, 13 Jul 2021 05:53:14 +0000

Primary Key:

In Cassandra, a table contain a number of rows.
Each row is represent by a primary key, also called the row key.
There are a number of columns in a row but the number of columns can vary in different rows.
For example, a table contain a one row that have three columns whereas another row in the same table can have 10 columns.
It is also important to note that in Cassandra, both column names and values have binary types.
That means column names can have binary values, such as strings, timestamps, or an integer, etc.
This is different from SQL databases, where each row in a SQL table has a fixed number of columns, and column names can only be text.

Create table person (student_id int primary key, fname text, lname text, 
                     dateofbirth timestamp, email text, phone text);

Partition Key:

The use of a partition key is to determine the partition in the cluster that stores that row.
When data is read or written from the cluster, a function called Partitioned.It is used to calculate the hash value of the partition key.
This hash value is used to calculate the partition in the row.
For example, rows whose partition key values range from 1000 to 1234 may reside in node A, and rows with partition key values range from 1235 to 2000 may reside in node B.
If a row having a partition key that contain hash value is 1233 then it will be stored in node A.

Clustering Key

The use of the clustering key is to store row data in a categorize order.
The arrangement of data is based on columns, which are involved in the clustering key.
This arrangement makes it well ordered to recover data using the clustering key.

Example: school system

To make these concepts clear, we will consider to Create a key space with replication strategy ‘SimpleStrategy’ and replication_factor 1.

reate keyspace Students_Details with replication = {‘class’ : ‘SimpleStrategy’, ‘replication_factor’:1};

Now switch to the students_details keyspace:

use students_details;

Check the number of tables present in the keyspace:

students_details> desc TABLES;

We will create a table, student , that contains general information about any student.

create table student (stuid int, avg_marks float, description text, 
                      primary key (stuid));

Type the following insert statements to enter some data into this table.

insert into student (stuid, avg_marks, description) values (1,25.5,’student 1′);
insert into student (stuid, avg_marks, description) values (2,35.5,’student 2′);

To view the details just inserted.

students_details> select * from student;

What is the Cassandra coefficient ?

Editor — Tue, 13 Jul 2021 05:12:36 +0000

Cassandra coefficient:

The four components of the Cassandra Coefficient are
1. The warning, the threat or risk in question,
2. the decision makers or audience who must react,
3. The predictor or possible Cassandra, and
4. The critics who disparage or reject the warning.

The Cassandra coefficient is the framework and it offered as a tool and all decision-makers to separate a legitimate warning from a Chicken Little.

There is a lot of commonality amongst actual Cassandras.

Technical experts in their field,
Very driven by data,
First principle/orthogonal thinkers,
Have a high sense of personal management, and
Many of them had offputting identity / high tension

What is secondary index in Cassandra ?

Editor — Tue, 13 Jul 2021 05:03:39 +0000

Cassandra secondary index

A primary index is global, although a secondary index is local.
To query a users by their secondary indexed value or their email every machine has to query its self target of users.
One query, five reads from disk.

Using a secondary index

Using SQL,after defining a table you can create an index on a column.
You can also index a collection column.
Secondary indexes are used to query a table using a column that is not general query table.
Secondary indexes are difficult to use and can impact performance greatly.

The index table is stored on every node in a cluster, so a query involving a secondary index if multiple nodes are accessed it can quickly become a performance nightmare.
A general rule of thumb is to index a column with low cardinality of few data.
Before creating an index, be aware of when and when not to create an index.

What is a seed node in Apache Cassandra ?

Editor — Tue, 13 Jul 2021 04:49:22 +0000

Apache Cassandra seed node:

Seeds are used to discover the cluster in the start up period.
The seed node designation has no purpose other than bootstrapping the gossip process for new nodes joining the cluster.
Seed nodes could not be a single point of failure, nor they have do any other purpose in cluster operations except the bootstrapping of nodes.
In cluster formation, nodes are verify “join” and each other.
However, they do not join any node which respects the protocol.Risks:old partitioned replicas, different clusters, even malicious nodes, so on. So a cluster can be defined by some starting nodes contain at clear addresses and it can be reference for that cluster for any new nodes to join in trust-table way.
After some time the seed nodes can go away, then the cluster will keep on.