Error - mongod shell doesn't start, /data/db doesn't exsist

Error - mongod shell doesn't start, /data/db doesn't exsist


If you face this error while starting MongoDB (mongod), most probably there are 2 reasons for this -

a.  /data/db is not exist at all
b. mongod process don't have access on /data/db

Solution :-

The default db path for mongodb is /data/db/.
So when you run:

MongoDB Configuration in Linux

DOWNLOAD the stable version of MongoDB. It will a tar file.

1. Create a folder named 'learn' ( or whatever you want to give )

     $ mkdir -p /learn/mongodb  /app/dbMongo

      # /learn/mongodb  = Holds the MongoDB source

      # /app/dbMongo = Holds the MongoDB database

2. Extract the Mongo DB zip file in /learn folder

  $ tar -xvf MongoDB.tar  /learn/mongodb

3.  change the permission of folder to user who run the db here-  In my case User - atul and Group - atul
  $ chown -R atul:atul  /learn/mongodb  /app/dbMongo

Connect to Oracle DB with Linux Shell Script

In this post, we will see how to connect to Oracle DB in Linux Shell Script. There are some prerequisite for a Linux server to connect to Oracle DB. There should be oracle db driver installed on the Linux server from which you want to connect to Oracle DB. Check whether you can call/connect SQLPLUS from command line.
$ sqlplus
if you are not able to, check whether oracle path is set in .profile file. Lets assume, Oracle DB driver is installed on Server and Sqlplus path is set.

Now, there are some basic steps to connect to Oracle DB-
a) Connect to Oracle DB with UserID and Password
b) Execute the SQLs
c) Store the output in a file or send to standard output (terminal)

Why Data Warehouses are Hard to Deploy ?

Inherent Complexity of Data Warehouse Databases 

Data Warehouse Databases are Large

Since data warehouse databases are constructed by combining selected data from several operational databases, data warehouse databases are inherently large. They are often the largest databases within an organization. The very size of these databases can make them very difficult and expensive to query. 

ETL Testing : Approach

Testing is undoubtedly an essential part of DW life-cycle but it received a few attention with respect to other design phases.

DW testing specification:
 – Software testing is predominantly focused on program code, while Software testing is predominantly focused on program code, while DW testing is directed at data and information DW testing is directed at data and information.  .
 – DW testing focuses on the correctness and usefulness of the information delivered to users information delivered to users
 – Differently from generic software systems, DW testing involves a huge data volume huge data volume, which significantly impacts performance and productivity.
 – DW systems are aimed at supporting any views of data, so the possible number of use scenarios is practically infinite and only few them are known from the beginning.
 – It is almost impossible to predict all the possible types of errors that will be encountered in real operational data.

Oracle Interview Questions - Part-4

76. What is the OPTIMAL parameter?
It is used to set the optimal length of a rollback segment.

77. What is the functionality of SYSTEM table space?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.

78. How will you create multiple rollback segments in a database?
- Create a database, which implicitly creates a SYSTEM rollback segment in a SYSTEM tablespace.
- Create a second rollback segment name R0 in the SYSTEM tablespace.
- Make new rollback segment available (after shutdown, modify init.ora file and start database)
- Create other tablespaces (RBS) for rollback segments.
- Deactivate rollback segment R0 and activate the newly created rollback segments.

Interview Questions : DataWareHouse - Part 4

What are the types of Synonyms?
There are two types of Synonyms Private and Public

What is a Redo Log?
The set of Redo Log files YSDATE, UID, USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

What is an Index Segment?
Each Index has an Index segment that stores all of its data.

Explain the relationship among Database, Table space and Data file?
Each databases logically divided into one or more table spaces one or more data files are explicitly created for each table space.

Datastage Common Errors and Solutions

1.     While running ./NodeAgents.sh start command... getting the following error: “LoggingAgent.sh process stopped unexpectedly”

SOL:   needs to kill LoggingAgentSocketImpl

              Ps –ef |  grep  LoggingAgentSocketImpl   (OR)

              PS –ef |               grep Agent  (to check the process id of the above)

2.     Warning: A sequential operator cannot preserve the partitioning of input data set on input port 0

SOL:    Clear the preserve partition flag before Sequential file stages.

DataStage Server Hang Issues & Resolution

Server hang issue can occurred when

1) Metadata repository database detects a deadlock condition and choose failing job as the victim of the deadlock.
2) Log maintenance is ignored.
3) Temp folders are not maintained periodically.

I will try to explain above three points in detail below:

ETL Job Design Standards - 1

When using an off-the-shelf ETL tool, principles for software development do not change: we want our code to be reusable, robust, flexible, and manageable. To assist in the development, a set of best practices should be created for the implementation to follow. Failure to implement these practices usually result in problems further down the track, such as a higher cost of future development, increased time spent on administration tasks, and problems with reliability.

Although these standards are listed as taking place in ETL Physical Design, it is ideal that they be done before the prototype if possible. Once they are established once, they should be able to be re-used for future increments and only need to be reviewed. 

Listed below are some standard best practice categories that should be identified on a typical project. 

Difference between OLTP and OLAP

OLTP:

Online Transactional Processing databases are functional orientated, they are designed to provide real-time responses from concurrent users and applications. To be more specific, OLTP databases must provide real-time concurrent (multi-threaded) processing of all SQL transaction (writes/updates and reads). Another characteristic of an OLTP database, is the fact that its state (underlying data) is constantly changing. Examples of OLTP are databases that support e-commerce applications.

OLTP databases are highly Normalized relational databases. This means that there is very little or no data redundancy. This ensures data consistency (part of the ACID standard). Normalization is the process of arranging data into logical, organized groups of tables, reducing data repetition or going so far as to completely eliminating it. As a result the data is logically grouped into tables, and these tables form relationships with one another through the use of primary and foreign keys. There are different levels of normalization and OLTP data models usually meet the 3rd Normal Form also known as the Entity Attribute Relationship Model.

Oracle Interview Questions - Part-3

51. What is a database instance? Explain.
A database instance (Server) is a set of memory structure and background processes that access a set of database files. The processes can be shared by all of the users. The memory structure that is used to store the most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.

52. What is Parallel Server?
Multiple instances accessing the same database (only in multi-CPU environments)

Interview Questions : DataWareHouse - Part-1

What is Data Warehousing?
A data warehouse is the main repository of an organization’s historical data, its corporate memory. It contains the raw material for management’s decision support system. The critical factor leading to the use of a data warehouse is that a data analyst can perform complex queries and analysis, such as data mining, on the information without slowing down the operational systems. Data warehousing collection of data designed to support management decision making. Data warehouses contain a wide variety of data that present a coherent picture of business conditions at a single point in time. It is a repository of integrated information, available for queries and analysis.

Logical Data Model

A logical data model describes the data in as much detail as possible, without regard to how they will be physical implemented in the database. Features of a logical data model include:

• Includes all entities and relationships among them.
• All attributes for each entity are specified.
• The primary key for each entity is specified.
• Foreign keys (keys identifying the relationship between different entities) are specified.
• Normalization occurs at this level.

Basics of Teradata Database

What exactly is a Teradata Database?

• Teradata Database is a RDMS (Relational Database Management System) which helps in driving the company’s Data warehouse.
• It provides foundation for the company to grow, compete, evolve business by getting answers to a new generation of questions (typically “what if” queries)
• It supports business growth from gigabytes to whooping hundreds of Terabytes
• Teradata Database is an Open system and compliant with major support from ANSI standards
• It works on UNIX MP-RAS, WINDOWS and LINUX operating systems, connects with other client through Network Attached channel (TCP/IP) and Channel connection for Mainframe.

DATABASE Command in TeraData

When users negotiate a successful logon to Teradata, they are automatically positioned in a default database as defined by the database administrator. When an SQL request is executed, by default, it looks in the current database for all referenced objects.

There may be times when the object is not in the current database. When this happens, the user has one of two choices to resolve this situation. One solution is to qualify the name of the object along with the name of the database in which it resides. To do this, the user simply associates the database name to the object name by connecting them with a period (.) or dot as shown below:

Lock/Unlock Oracle Users

In spite of deleting an oracle user, we can LOCK it. In that way the user won’t be accessible. In future if we want we can UNLOCK it.

create a user named JACK which is LOCKED:
SQL> create user jack identified by jack account lock;
User created.

Teradata Users

 In Teradata, a user is the same as a database with one exception. A user is able to logon to the system and a database cannot. Therefore, to authenticate the user, a password must be established. The password is normally established at the same time that the CREATE USER statement is executed. 

The password can also be changed using a MODIFY USER command.

List the tables in TeraData

For listing all the tables, views and macro , execute any sql from below as per need


select * from dbc.tables;

Oracle Interview Questions - Part-2

26. What is hash cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the row’s cluster key value. All rows with the same hash key value are stores together on disk.

27. When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.