Cassandra is a distributed database that is highly scalable and designed to manage substantial amounts of structured data.
If you are looking for an opportunity to become an expert in Cassandra, you must have a good grip on the concepts and be well trained in the subject. For this, improving your technical skills and knowing your thoughts in and out is essential.
Here are some top Cassandra interview questions from the experts on Cassandra.
These are frequently asked questions that will help you gain a grip on the concepts of the subject. The detailed answers will help you understand Cassandra easily.
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Cassandra is a distributed database management system. It is initially developed at Facebook to improve its performance, and it is a tool made to power the Facebook inbox search feature. Due to its outstanding technical features, Cassandra became very popular and a top-level project.
Cassandra is an open-source, distributed, and decentralized database. It is also used for managing a large amount of structured data which is spread out everywhere.
Cassandra has features that are very beneficial as it is easy to work with; Some of those are high performance, fault tolerance, predictable scaling, distributed database. It has high scores on these parameters, and it is also preferred because it is an open-source distributed and NoSQL database management system.
Cassandra has become the primary choice for many companies when it comes to app development and data management. Even new start-ups are preferring it because of the ease with which an operator can work.
Cassandra is a great application where data is collected at high speed from different kinds of sources. As the internet of things application could use Cassandra. It could also be used in product and retail apps, messaging, social media analytics, and even by a recommendation engine.
Although traditional databases provide you with many other features here are some highlights and benefits only a database like Cassandra have:
|Traditional databases||Cassandra database|
|Data is written in mostly one location.||Data is written in many locations.|
|Data volumes are moderate.||Processing data volumes are high.|
|It can handle only moderate incoming data.||It can handle high incoming data volumes.|
|Supports complex transactions||Supports simple transactions.|
|Lines up for just read scalability.||Supports both read and write scalability.|
Cassandra has become famous for its outstanding technical features. Here are some features you must know:
The components of Cassandra include:
This database supports two main categories of functions:
Scalar functions: Its primary purpose is taking some groups of values and producing an output with it.
Aggregate functions: Its primary function is producing a combined result using selected multiple rows.
They go as follows:
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A node is a basic unit of Cassandra, and it is a system that is part of a cluster. Node is the main area where the data is stored.
And the units of a node is represented as computer/server
A data center is a collection of Cassandra nodes. The data in a data center is stored in the form of a cluster, where the cluster is also referred to as a collection of nodes.
MemTable is a location where data is written and stored temporarily. Data is written in memtable after the data is completed in the commit log.
Memtable is a storage engine in Cassandra. Data in MemTable is classified into a key, and where the data is retrieved using the key as each column category has its own MemTable. When the write memory is full, it deletes the messages automatically.
SSTable also means 'Sorted String Table'. SSTable is a data file in Cassandra, and its main function is to save data that is flushed from memtable. Unlike MemTable, SSTbale doesn't delete any data or lets any further addition once data is written.
In MemTable it doesn't store the data. It temporarily accumulates ‘write data’, but it cannot store it into the disk.
Whereas in SStable, it is used to store the data from Memtable into the Cassandra database. The data stored in SSTable is permanent and cannot be changed.
Cassandra database is a highly available database, and it stores data by evenly dividing the data around its nodes. For this, it uses the Murmur3 partitioning function to distribute given data in nodes evenly.
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The data storage path in Cassandra begins with the memtable where the data is stored temporarily and is also called a commit log. And once committed, the data is periodically flushed and written into SSTable
There are two types of operations carried by Cassandra:
Direct request in Cassandra is a part of the read operation. In this, the coordinator node contacts the replica node.
When the coordinator node contacts replicas, it actually requests those nodes which reply fastest. Then these contacted nodes respond with a digest of the data required.
When the coordinator node sends requests, it checks in the nodes for any outdated data. This data is sent for a background reading and repair and is replaced with the updated data. Read and repair requests, is a method to keep the data updated, and it also makes sure that the requested row is consistent on all replicas.
There are step-by-step operations in writing, which go as follows.
Step1: It is as soon as it receives its request it sends the data to the commit log to save the data.
Step2: Data is inserted upon request and then sent to commit log to save data.
Step3: If the memtable reaches its limit then data is flushed to SSTable.
The CAP theorem, also knowns Brewer's theorem, states that a distributed computer system can't use all its three properties at the same time which are
ACID stands for
Atomicity: This means either your transaction can fail or commit
Consistency: Its definition changes from software to software or an application to application, but its general meaning is that data has to stay consistent.
Isolation: Data has to be isolated and separated from each other
Durability: It assures you that once the database receives data, it should ensure that the data is processed. So it is an advantage if the database fails, then the data will not be lost.
Not every application or software needs this strong consistency, so this is where the base comes into action. The BASE stands for Basically Available Soft-state Eventually-consistent properties.NoSQL databases basically use these models.
Consistency refers to updating and synchronizing a row of Cassandra data in all of its replicas. By offering tunable consistency for a given operation (read/write), helps the application to decide the right consistency of data.
|→ Checkout Cassandra vs MongoDB|
Tunable consistency ensures proper levels of consistency for its reads and writes which is the main reason why Cassandra prefers NoSQL databases.
Although Cassandra comes with built-in tolerance features, it still needs to be monitored for effective results. Here are some tools which Cassandra uses to monitor its databases:
The primary purpose of usage of NoSQL databases is because it provides smooth handling of large data. Its simplicity of design and simplicity in horizontal scaling to clusters and fine control are a few of the reasons why Cassandra uses a NoSQL database.
The primary objectives of NoSQL DB are:
A bloom filter is a tool used by Cassandra. The read path of Cassandra has to go through Memtable and the row cache. A bloom filter is a partition cache, and its role in the read path is to avoid checking every SStable to find one particular data.
Initially, Cassandra required an API to do some of the basic tasks like insert, get and delete. But over time, these basic queries were improved and then named Cassandra Query Language. (CQL).
CQL provides a great set of built-in data types, and it also helps the applications to make their own custom data types. Cassandra is also classified as a NoSQL database.
It is very necessary to provide different types of users with different kinds of roles depending upon their requirements. It ensures the security of database users. and their key roles goes as follows:
A cluster is a collection of nodes. This collection of nodes represents a single system. It is the outermost structure of the ring in Cassandra.
These operations are used to make changes in the Cassandra database.
CRUD stands for
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A keyspace is a part of the cluster which controls the replication of the data in a database. A cluster contains one keyspace per node.
Cassandra keyspace contains 3 types of operations which go as follows:
Column family in Cassandra is defined as the collection of rows in an ordered and systematic way. It is used to represent the stored data in a structured manner. These are contained in a keyspace, at least one column family in a keyspace
There are many characteristics of a column family, and a few of them goes as follows:
A super column in Cassandra is an extraordinary and important column. It has so much value because it has the roadmap to all the sub-columns in the database.
These super columns are used to improve the performance of the database
These are some important Cassandra interview questions for beginners and experienced candidates. I hope these questions will help you get familiarise with the concepts and insights of Cassandra and help you prepare for the interviews as well.
Please comment below if you have any queries.
Ravindra Savaram is a Content Lead at Mindmajix.com. His passion lies in writing articles on the most popular IT platforms including Machine learning, DevOps, Data Science, Artificial Intelligence, RPA, Deep Learning, and so on. You can stay up to date on all these technologies by following him on LinkedIn and Twitter.