Senior Software Engineer

ACID is a set of properties of database transactions intended to guarantee validity even in the event of errors, power failures, etc.
ACID stands for : 
1. Atomicity : All statements of a transaction must succeed completely, or fail completely in each and every situation, including power failures, errors and crashes. Example - Debiting and crediting in a money transfer transaction, both must happen either together or not at all.
2. Consistency : The database must remain in a consistent state after any transaction. Data in the database should not have any changes other than intended after the transaction completion.
3. Isolation : Isolation ensures that concurrent execution of transactions leaves the database in the same state that would have been obtained if the transactions were executed sequentially.
4. Durability :Durability guarantees that once a transaction has been committed, it will remain committed even in the case of a system failure which actually means recording the completed transactions (or their effects) in non-volatile memory.

In DBMS, a database transaction is a logical unit of processing that includes one or more database access operations. Transactions in a database represent real-world events occurring within an organization.

1. SQL databases are primarily called Relational Databases while NoSQL databases are primarily called as Non-relational or distributed database. 
2. Traditional RDBMS uses SQL syntax and queries to analyze and get the data for further insights. NoSQL database system consists of various kind of database technologies. These databases were developed in response to the demands presented for the development of the modern application.
3. SQL databases are table based databases while NoSQL databases can be document based, key-value pairs, graph databases
4. SQL databases have a predefined schema. NoSQL databases use dynamic schema for unstructured data.
5. SQL databases are vertically scalable while NoSQL databases are horizontally scalable. 
6. Examples of sql db are Oracle, Postgres, and MS-SQL and examples of NonSQL db are MongoDB,Neo4j, Cassandra.

Semaphore is a non-negative integer variable used as a signaling mechanism. They are used to solve the critical section problem by using two atomic operations, wait and signal that are used for process synchronization. The wait operation decrements the value of its argument S, if it is positive. If S is negative or zero, then no operation is performed. The signal operation increments the value of its argument S. There are two kinds of semaphores :
1. Counting Semaphores : A counting semaphore is a semaphore that has multiple values of the counter. The value can range over an unrestricted domain.
2. Binary Semaphores : A Binary Semaphore is a semaphore whose integer value range over 0 and 1.

Multitasking in Java can be achieved by process based multitasking and thread based multitasking. 
Process based multitasking : Executing various jobs concurrently where each job is a separate independent operation, is process-based multi-tasking. It is best suited at OS level. Example : Executing an excel spreadsheet while running a word processor.
Thread based multitasking :Executing several tasks simultaneously where each task is a separate independent part (thread) of the same program is Thread-based multitasking. It is best suited at program level. Eg: Formatting a text in a word processor and printing it at the same time. A thread can be defined in the following 2 ways:
1. By extending thread class
2. By implementing runnable interface

Tip 1 : Make all necessary assumptions required to create the database schema.
Tip 2 : Identify all the entities and their attributes which will be involved in a food ordering database schema. For eg : Menu, Customer, Chefs, Orders, Payment etc. 
Tip 3 : Identify the relationships between all the entities and mark the primary and foreign keys in each table.