JavaScript Journey

Cleared my basics from this site:
https://javascript.info

Made some beginner level codepens using only JS/HTML, no CSS so no expectations for good UI 😛

• JS Clock
• Tip Calculator
• Input Validation
• To Do
• Quiz
• …and more

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DOM specifications: 

https://developer.mozilla.org/en-US/searchBOM specifications:
https://html.spec.whatwg.org/

 

SF development: Tricks to use VMWare effectively

System Specifications:

• Ubuntu 16.04
• VMWare Workstation(licensed)
• At least 16 GB RAM

Problem:
For development purpose, VMWare uses Ubuntu image provided internally that runs four containers namely, tomcat, kafka zookeeper, oracle db. At least 8GB RAM has to be given to VM. While running applications on VM, crashes of the application or process or VM was frequent. Some if the scenarios:

• Using terminal: While running, tomcat frequently crashed when either eclipse opened in the VM or some number of applications are started.
• Code Changes: If you are changing some code and suddenly VM stopped working it will take away the changes too.
• It was consuming a lot of memory to save the screenshot on host and managing them was another task.

Solution:

I installed openssh on host and guest machines. I opened an ssh connection from my host to guest machines using IP of guest machine. I ran containers and any other terminal related tasks from here.

For changes in code, I used remote connection through my Intellij IDEA Ultimate. I was able to get a copy of the code from the VM locally that was mapped to the respective folders inside VM. The downloading part took around 2 hours but it was worth it.
Whenever I needed to change any code, I changed it on host and deployed on guest.

With this solution, the load on VM decreased and the crashing problem was solved!! 🙂

Developing android app using ui5 in phonegap

System: Ubuntu 16.04

Requirements:
npm cli [sudo apt-get install npm]
phonegap cli [link]
gradle [link] I installed 4.1
Android Studio
UI5 SDK or runtime [link]

Step 1: Create a basic template

Make a project folder my-app
cd my-app
phonegap create
The above command will generate a boilerplate code for phonegap application.

Note: Following command can be used to serve the existing code in a web browser
phonegap serve
try opening the URL shown in console.

Step 2: Using ui5 libraries inside your website’s code

Code for your website is inside www/ folder, try opening and playing around.

Copy resources folder from your UI5 SDK/runtime to www/ folder. Now you can include ui5 libraries into your index.html

Step 3: Run your application as an android app

phonegap build
phonegap run android
The above command will produce the app code in platforms/android, open this folder inside Android Studio and run your application in whichever way you want, emulator or through USB debugging.

Boilerplate code for phonegap with ui5 is here.

DZone reader android application

[updates will be done along with the progress, it is still a draft]

It is a personal project to read the DZone rss feed.

Steps I am following:

• Understanding the RSS feed reading in Android
• Creating Navigation drawer according to the feeds available
• Getting data off the feeds
• Parse the XML
• Display

Virtual Memory: Memory Protection

• Read Slide.
• Random offsets are used for security purpose so no one can change the data.
• Kernel space maps to same location in RAM for all programs.

Virtual Memory: Address Translation

• Virtual address bits are decided by ISA (Instruction Set Architecture), and P address bits are based on RAM available. In the cases considered here, there is 32-bit ISA and 256MB RAM.
• Page offsets depend upon frame/page size. Here each page has a size of 4KB.

• This is how an address in RAM is translated.

• When T. address (Page) is on Disk.
• In this case, CPU generates an exception: Page Fault
• Hardware gives control to OS’s Page Fault Exception handler.
• OS decides which page to move out from RAM to replace with this recently accessed one.
• If the page is dirty, it has to be written on the disk too.[ The bit indicates that its associated block of memory has been modified and has not yet been saved to storage. ], then update page table. Now OS jumps to the same instruction what caused fault.
• Page Faults are very expensive.

How Virtual Memory Works?

• Each programs has its on virtual address space and Map(Translator). Map maps to the physical address either in RAM or Disk.
• When address is mapped in Disk, it is first loaded to RAM, also mapping table is updated and then data is loaded.
• This Map is called Page Table and the entry in it is called Page Table Entry.
• WHY WE CALL IT PAGE?
  • Well, if, for each virtual address an entry is used, the size of Page Table alone would be large. Also, note that for each program we have a Page Table.
  • So, each entry instead of handling each V. address, handles chunk of V. addresses(range) and maps that to chunk of P. addresses(range).
  • The point above resolves the problem of large memory usage.
  • We typically have 4kB of pages(1024 words).
  • Bigger the page size, moving from the disk would take more time. Lesser the page size, more memory is needed to store page table.
• A V. address is divided in V. Page number and Page offset. V. Page number is mapped to P. page address and page offset just concatenates with it.

Introduction: Virtual Memory

WHY VIRTUAL MEMORY?

• Not enough RAM-> CRASH
  • MIPS provides 32 bit address space(4GB memory), suppose we have only 1GB RAM. How to access address out of 1GB range?
• Secure address space-> CORRUPT
  • Two programs can write on same address
• Holes in the address space-> OUT OF SPACE
  • Say there are 3 programs like-> p1 needs 1GB, p2 needs 2GB and p3 need 1GB. In out 4GB address space p1 and p2 fit in, respectively. Say p1 is terminated. We have 2 address spaces of 1GB above and below of p2.
    Although we have enough space but we cannot allocate it to p3 because of Memory Fragmentation.

HOW IT SOLVES?

• By mapping the virtual memory space given to the program to actual memory space.
• In this, way all the programs have different VS.

WHAT IS VIRTUAL MEMORY?

• It is an indirection from virtual address spaces generated by program to some address from where the required data can be accessed.

AGAIN, HOW IT SOLVES OUT 3 PROBLEMS?

• Not enough RAM-> CRASH
  • Map has the mapping to disk also, whenever some address that is accessed that’s not in RAM, Map goes to the Disk, takes that data and pages out some other data to Disk to put this new, recently accessed data into RAM.
    Along with this, Map updates it self for the address whose data has been paged out.
• Holes in the address space-> OUT OF SPACE
  • Considering the same situation as above, both p2 and p3(as well p1) would have their Maps, now p3’s address space can be mapped to 2 chunks of 1GB.
• Secure address space-> CORRUPT
  • If 2 programs are accessing same address to write, Map of each can prevent them to write on same address.
  • To prevent complete isolation though, programs can have same mappings.

 

DbMS: SQL Keys

• Super Key
• Candidate Key
• Primary Key
• Natural Key/Business Key/Domain Key
• Surrogate Key
• Unique Key
• Foreign Key
• Secondary Key
• Simple Key

Super Key: Super key is the set of attributes in a table that can identify a row uniquely.

Candidate Key: Candidate key is minimal super key. If any of the attribute, in case of two or more attributes, is removed from the candidate key then uniqueness breaks. N0 candidate key can be NULL. A table can have a multiple candidate keys.

Primary Key: It is one of the candidate keys. Primary key cannot be NULL.

Natural Key:  Natural key is a key composed of columns that actually have a logical relationship to other columns within a table. For example, in case of database of a society, Name, Address is a natural key.

Surrogate Key: Surrogate means substitute, in case natural key is not a good choice, surrogate key is used. Usually column(s) are added for uniqueness of a tuple. For example an ID is provided to every row and this ID is used as primary key. Opposite to natural key, surrogate key does not have logical relationship to other values in a tuple.

Unique Key: (Confused) It is a candidate key. Used to prevent duplicate values in a column. It can be NULL. Multiple unique keys can exist. Unique values can be deleted.

Foreign Key: In the context of relational databases, a foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table.In simpler words, the foreign key is defined in a second table, but it refers to the primary key in the first table. Foreign key enforces referential integrity:

• Update to a table having a foreign can happen if the corresponding key exist in the other table.
• Cascade Delete
• Cascade Update

Secondary Key: Candidate key(s), not selected as primary key is/are secondary key.

Simple Key: Candidate key with a single attribute is a simple key.

DbMS: Joins

Joins are used to merge data based on predicate logic, of two tables that results in a temporary table;

Types:

1. Outer Joins
   1. Left Outer Join
   2. Right Outer Join
   3. Full Outer Join
2. Inner Join

Consider tables for examples:

Employee	Location
EmpID EmpName 13 Jason 8 Alex 3 Ram 17 Babu 25 Johnson	EmpID EmpLoc 13 San Jose 8 Los Angeles 3 Pune, India 17 Chennai, India 39 Bangalore, India

Outer Join: Iterates over the all rows of both or one of the tables.

In left outer join, according to the query, it iterates over the key of left table provided in SQL query and matches the same in the right one. Tables will result in:

SQL:
select * from employee left outer join location on employee.empID = location.empID; OR
select * from employee left join location on employee.empID = location.empID;

Employee.EmpID	Employee.EmpName	Location.EmpID	Location.EmpLoc
13	Jason	13	San Jose
8	Alex	8	Los Angeles
3	Ram	3	Pune, India
17	Babu	17	Chennai, India
25	Johnson	NULL	NULL

In right outer join, same happens for the right table:

SQL:
select * from employee right outer join location on employee.empID = location.empID; OR
select * from employee right join location on employee.empID = location.empID;

Employee.EmpID	Employee.EmpName	Location.EmpID	Location.EmpLoc
13	Jason	13	San Jose
8	Alex	8	Los Angeles
3	Ram	3	Pune, India
17	Babu	17	Chennai, India
NULL	NULL	39	Bangalore, India

Inner Join: Here if match happens only then join happens.

SQL:
select * from employee inner join location on employee.empID = location.empID; OR
select * from employee, location where employee.empID = location.empID;

Employee.EmpID	Employee.EmpName	Location.EmpID	Location.EmpLoc
13	Jason	13	San Jose
8	Alex	8	Los Angeles
3	Ram	3	Pune, India
17	Babu	17	Chennai, India

 

Souirce: http://www.programmerinterview.com