Git: main commands

1. git status
Основной инструмент, используемый для определения, какие файлы в каком состоянии находятся
2. git add
Для того чтобы начать отслеживать (добавить под версионный контроль) новый файл, используется команда
3. git commit

Spring: questions

1. What are the main Spring features?

• AOP support 
• Lightweight framework 
• container 
• Inversion of control implementation 

Spring

1. What is Spring

• lightweight jar libraries
• container (manages lifecycle of objects) (there is no need to use new for creating objects. All objects are in container)
• framework (there are a lot of classes which can help with data base, services)
• Dependency Injection (Inversion of Control) (objects get their their dependencies and do not create it)
• AOP (Aspect Oriented Programming)

Examples with xpath and Css (ID, Name, Text and Links)

1. With ID : - //input[@id='email'] or  //*[@id='email']

   css - input#email or #email

2. With Name - //input[@name='email'] or  //*[@name='email']
    css - input[name=email] or [name=email]

Hadoop installation notes

Cluster planning:

• Small cluster (2-10 nodes): Clusters of three or more machines typically use a dedicated NameNode/ResourceManager, and all other nodes are workers. 
• Medium cluster (10-40 nodes): separate machines for master, usually dedicated edge node 
• Large cluster (> 40 nodes): occupies multiple racks, requires individual planning

Storage layer for HDFS:

• Cloud and virtualization solutions are very popular in the enterprise world 
• With Hadoop, focus on using JDOD (just bunch of disks) instead of SAN (storage area network) 
• Hadoop heterogeneous storage was introduced in Hadoop 2.5

Commodity hardware:

Balanced configuration for one worker recommended by vendors:

• 2-4 8-cores CPU 
• 128 GB RAM 
• 12 hard drives (1 or 2 TB each)

HDFS Architecture

• Individual machines are known as nodes 
• A cluster can have as few as one node, as many as several thousands
• Two types of nodes: NameNode and DataNode 
• More nodes = better performance 
• HDFS is a filesystem written in Java 
• The NameNode daemon must be running at all times If the NameNode stops, the cluster becomes inaccessible 
• The NameNode holds all of its metadata in RAM for fast access 
• A separate daemon known as the Secondary NameNode takes care of some housekeeping tasks for the NameNode
• Although files are split into 64MB or 128MB blocks
• Blocks are stored as standard files on the DataNodes, in a set of directories specified in Hadoop’s configuration files 
• Without the metadata on the NameNode, there is no way to access the files in the HDFS cluster
• When a client application wants to read a file: It communicates with the NameNode to determine which blocks make up the file, and which DataNodes those blocks reside on. It then communicates directly with the DataNodes to read the data

Git: how to restore deleted no commited file

If the deletion has not been committed, the command below will restore the deleted file in the working tree.

$ git checkout -- <file>