Tech Learnings

Rails logging to syslog

noreply@blogger.com (Jigar) — Fri, 01 Aug 2014 21:17:00 +0000

Quick Steps

In config/application.rb create a method called logger under Application class as shown below

def logger
logger ||= Syslog::Logger.new 'Application_Name'
logger.level = ::Logger::WARN # OR ::Logger::INFO, ::Logger::ERROR etc.
return logger
end

Then, in you code use the logger method as shown below
YourModule::Application.logger.info("Some Info")
YourModule::Application.logger.error("Some Error")

To test it out, check contents of the sys log file (In MAC it is /var/log/system.log and in RedHat Linux it is /var/log/messages)

Rails jquery autocomplete box

noreply@blogger.com (Jigar) — Thu, 17 Jul 2014 05:13:00 +0000

Quick steps to add jquery-ui autocomplete box in Rails version 3.2.x site.

Rails comes with jquery.js but does not have jquery-ui which is required for autocomplete box widget.

So to get this working, follow this steps
1. install gem 'jquery-ui-rails’

2. Add this line in application.js - //= require jquery-ui

3. And this one in application.css - *= require jquery-ui

4. Restart the rails server

5. Add following code to the view or helper as per your design
    <script type="text/javascript">
          $(function() {
              // Below is the name of the textfield that will be autocomplete
              $(‘#TEXT_BOX_FIELD_NAME').autocomplete({
                  // This shows the min length of charcters that must be typed before the autocomplete looks for a match.
                  minLength: 1,
                  source: '<%= SOME_VALID_RAILS_PATH(:json) %>',
                  // This updates the textfield when you move the updown the suggestions list, with your keyboard.
                  focus: function(event, ui) {
                      $('#TEXT_BOX_FIELD_NAME').val(ui.item.name);
                      return false;
                  },
                  // Once a value in the drop down list is selected, do the following:
                  select: function(event, ui) {
                         // place the value into the textfield
                          $('#TEXT_BOX_FIELD_NAME').val(ui.item.name);
                          return false;
                  }
              })
              // formats what data is displayed in the dropdown box, and can be customized.
              .data( "uiAutocomplete" )._renderItem = function( ul, item ) {
                return $( "<li></li>" )
                  .append( item.name )
                  .appendTo( ul );
              };
          });
      </script>

Here it is assumed that response from rails controller is in json format and it is an array of hashes which contains “name" as key (that is why item.name is used). Some thing like this [{name:suggestion1},{name:suggesstion2},{name:suggesstion3}]

6. Finally in the controller add an action matching the SOME_VALID_RAILS_PATH and add the code to get the suggestion list. Now to get what user typed use params[:term] ( the key name “term" is some thing used by jquery - not sure on how to change that)

7. Don’t forget to update the routes.rb file for SOME_VALID_RAILS_PATH

References
http://stackoverflow.com/questions/3188157/how-to-set-up-jquery-ui-autocomplete-in-rails
https://github.com/joliss/jquery-ui-rails

Permutations and Combinations

noreply@blogger.com (Jigar) — Sun, 29 Jun 2014 06:13:00 +0000

n = number of possibilities and r = items to choose

Permutations - Order does matter => 123 and 231 are different things

1. Repetition not allowed => 111 and 222 are not allowed

P(n,r) = n! / (n-r)!

2. Repetition allowed => 112 is allowed

Power(n,r) = n raise to r

Combinations - Order does not matter => 123 and 231 are same things

1. Repetition not allowed

C(n,r) = n! / r! * (n-r)! = P(n,r) / r!

2. Repetition allowed

C(n,r) = (n + r - 1)! / r! * (n-1)! = C(n+r-1, r)

Check http://www.mathsisfun.com/combinatorics/combinations-permutations.html

Pass by value concept in Java

noreply@blogger.com (Jigar) — Mon, 14 Apr 2014 08:00:00 +0000

In Java, everything is pass by value. That is a new copy of the parameter* is created when a function is called. If the parameter is primitive type then it is a new copy of that type and straightforward.

If the parameter is of reference type (object of some class) then new pointer is created which still points to same location. So any changes made to that location will get reflected in calling function.

BUT if we assign some new value to the function parameter then it points to new memory location without disturbing the pointer in calling function. And any changes made thereafter will not get reflected in original variable.

http://stackoverflow.com/questions/5614562/how-to-pass-by-reference-in-java

Comparable and Comparator

noreply@blogger.com (Jigar) — Mon, 10 Mar 2014 08:05:00 +0000

Comparable Interface required for sorting - gives default order of object. If multiple orderings required then use Comparator.

To put elements in Hash, override equals and hashCode also.

public class Person implements Comparable<Person>{

Integer id;

@Override

public int compareTo(Person other){

return id.compareTo(other.id);

}

     private static class ItemComparator implements Comparator<Item>{
       @Override
       public int compare(Item i1, Item i2){
       if(i1 != null & i2 != null)
          return -1*i1.name.compareTo(i2.name);
       else if(i1 != null)
          return 1;
       else if(i2 != null)
          return -1;
       else
          return 0;
       }
     }
}

Probabilty - quick review

noreply@blogger.com (Jigar) — Wed, 19 Feb 2014 07:12:00 +0000

P(X) = (chances of outcome X / Total possible outcomes)

The sum of all probabilities should be equal to 1

Total outcomes possible on N Compound Events (like N Rolls of Dice) =

(Total Possibilities) ^ N (6 ^ 3 for 3 rolls of a dice)

Addition Rule: P(A OR B) = P(A) + P(B) - P(A and B)

Multiplication Rule: P(A and B) = P(A) * P(B) For Independent Events

Probability of at least 2 Heads in 5 Tosses = 1 - P(0 heads) - P(only 1 Head)

P(0 Head) = 1/2 * 1/2 * 1/2 * 1/2 * 1/2 = (1/2) ^ 5

P(only 1 Head) = 5/(2^5) (because there can be 1 Head in either of the 5 possible positions - total 5 such possibilities)

Probability that exactly 10 users are active out of 35

= Choose 10 out of 35 = 35! / (10! * 25!)

Tip: use Venn Diagrams to visualize

Different type of classes in Java

noreply@blogger.com (Jigar) — Thu, 06 Feb 2014 08:58:00 +0000

Nested classes which are static cannot access instance vars and methods of outer class. They are mainly used for packaging convenience.

Inner classes are nested non-static classes which can access even private variables of outer class. Used for logical grouping + hiding the class + more readable code

OuterClass.StaticNestedClass nestedObj = new OuterClass.StaticNestedClass();

OuterClass outerObj = new OuterClass();

OuterClass.InnerClass innerObj = outerObj.new InnerClass();

Local Classes - classes within methods for temp use

Anonymous classes - similar to above but anonymous

Lamba Expressons - kind of anonymous methods

Singleton and Static Class

noreply@blogger.com (Jigar) — Wed, 29 Jan 2014 08:55:00 +0000

Singleton use case - Utility classes like logging, when only one instance required in application of a shared resource.

Singleton is used in favor of Static class (which is just a class with static methods) when there is some data associated with the class.

Implementation options

1. static final Singleton instance = new Singleton() + private constructor

2. double checking + locking + private constructor -> lazy loading

3. public enum Singleton {INSTANCE;}

final and static keywords

noreply@blogger.com (Jigar) — Thu, 16 Jan 2014 08:57:00 +0000

final class - cannot be sub classed

final method - cannot be overridden

final variable - can only be initialized once - either at declare time or in constructor.

For reference types it means that reference cannot be changed but the value can still be changed. For primitives, value can't be changed.

static class - collection of static vars and methods - useful for utility classes

static methods - can be called without creating instances - cannot access instance vars of same class directly

static vars - only one copy per class - it resides with class in perm area of heap and not with instances.

static block - used to initialize the static vars during class loading (not instantiation)

Java Garbage Collection and Memory

noreply@blogger.com (Jigar) — Fri, 10 Jan 2014 08:49:00 +0000

Local variables allocated in stack. In a function if variable is of type object then its reference is stored in stack.

Objects (reference types) allocated in heap. The variables within an object are stored in heap only.

Static variables allocated in Heap (Perm area) - but again only references and primitive values - the actual object will be in normal heap.

Variables created within a static method are treated in same way as non-static method.

In other words stack memory is kind of private memory of Java Threads, while heap memory is shared among all threads.

http://javarevisited.blogspot.com/2013/01/difference-between-stack-and-heap-java.html#ixzz2q3At99R6

Garbage Collection - Key Points

Main performance improvement can be gained in Heap area.

Heap area is divided into generations.

Why generational? because most objects are short lived

Three generations

1. Young - Eden + survival 0 + survival 1

2. Old

3. Permanent (for classes and methods - code, strings)

Objects first created in Eden. Each minor GC moves surviving object from Eden to Survivor space. And objects in Survivor space keep moving between S0 and S1 until they their age reaches threshold. Once objects are old enough, they are promoted to Old generation.

For responsive applications, major GCs should be minimized

Types of Garbage Collector:

1. Serial GC - for single cores (clients)

2. Parallel GC - Throughput collector - used to increase throughput

3. Concurrent Mark Sweep Collector - better for responsiveness as it runs in parallel with app threads - does not compact memory as others

4. G1 GC - new in Java 7 - Parallel + Concurrent + Incrementally Compacting

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html

using log4net to log errors in file

noreply@blogger.com (Jigar) — Wed, 01 Jan 2014 04:32:00 +0000

Quick steps to use log4net in C# windows application. The error log file name is specified using properties (check step 2)

1. Add following in App.config
<configSections>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net"/>
</configSections>
<log4net>

<appender name="LogFileAppender" type="log4net.Appender.FileAppender">

<file type="log4net.Util.PatternString" value="%property{LogFileName}"/>


<appendToFile value="true"/>


<layout type="log4net.Layout.PatternLayout">
<header value=""/>
<footer value=""/>
<conversionPattern value="%newline%date - %message%newline"/>
</layout>
</appender>

<root>
<level value="ERROR"/>
<appender-ref ref="LogFileAppender"/>
</root>

<logger name="Name_of_Log">

<level value="ALL"/>
<appender-ref ref="LogFileAppender"/>
</logger>
</log4net>

2. Set up following before making any logging call
log4net.GlobalContext.Properties["LogFileName"] = Path.Combine(this.logFolder,Util.ERROR_LOG_FILE_NAME);
log4net.Config.XmlConfigurator.Configure();

3. Instantiate logger
internal static log4net.ILog myLogger = log4net.LogManager.GetLogger("Name_of_Log");

4. Use the logger object
Util.myLogger.Error("Exception is a normal event.", ex);

Iterable and Iterator

noreply@blogger.com (Jigar) — Sun, 29 Dec 2013 09:01:00 +0000

To allow users to iterate (use for each), class has to implement java.lang.Iterable<T> interface which has only one method - public Iterator<T> iterator(){}

In this method return new customIterator object where customIterator is an object of class CustomIterator which implements Iterator<T> interface.

This class has 3 methods:

1. public boolean hasNext()

2. public T next()

3. public void remove() - throw new UnsupportedException()

In the iterator implementation - have a notion of current object in the collection

https://github.com/jigarbjpatel/ds/blob/master/src/LinkedList.java

In C#,

People:IEnumerable{IEnumerator IEnumerable.GetEnumerator(){// return a PeopleEnumerator}}

PeopleEnumerator:IEnumerator{public void Reset()public bool MoveNext()public object Current()

hashCode and equals

noreply@blogger.com (Jigar) — Wed, 11 Dec 2013 08:53:00 +0000

The default implementation of these methods will give you reference equality - in other words, when you compare two references, equals will only return true if they're references to the same object.
You would normally override equals to implement "value equality" - where two distinct objects are deemed equal, usually by virtue of having equal field values themselves. The exact meaning of equality will depend on your design - the two objects could still be distinguishable in other ways, for example.

If you override equals, you should also override hashCode to be consistent with equals, such that if a.equals(b) is true, then a.hashCode() == b.hashCode(). This will allow instances of your class to be used as keys in hash-based collections (e.g. HashMap) so that you can look up a value based on a key which is equal to the original one, rather than having to use a reference to the exact original key object.

hashCode() is used to get the bucket location in a Map. And then equals() is used to match the keys in the linked list in that bucket.

Proper implementation

@Override

public int hashCode(){

return x.hashCode();

}

@Override

public boolean equals(Object other){

if(other == this) return true;

if(other == null) return false;

Person p = (Person)other;

return p.id == this.id;

}

Aside: difference between == and equals

For primitive types , == compares values but for reference types it compares the references.

equals output depends on implementation of the method in the class - by default it will compare only references.

For C#, http://jigarbjpatel.blogspot.com/2011/02/difference-between-equals-and-in-c.html

Regsitry View on 64-bit windows

noreply@blogger.com (Jigar) — Wed, 16 Oct 2013 00:57:00 +0000

On 64 bit Windows machines, there are different views of the registry depending on process type.
For 32-bit processes, all queries for HKLM\Software\... are redirected to HKLM\Software\WOW6432Node\...
For 64-bit processes, they are not redirected. Hence when developing a 32-bit application on 64-bit machine, you may not get correct results unless you specify which RegistryView you want to use.
More details on are provided at this link (look for Matt's answer)

Overriding versus Hiding Behavior in OOP

noreply@blogger.com (Jigar) — Mon, 26 Aug 2013 22:11:00 +0000

When a method is over riden in child class, and you have an object of type Parent but instance of Child then which method will be called?

class Parent{
public someMethod(){
    }
}

class Child extends Parent{
public someMethod(){
    }
}

Parent obj = new Child();
obj.someMethod();

Well obviously, the child's method will be called.

But things get interesting when you have say a data member with same name in both Parent and Child.

class Parent{
public String someData = "parent";
}

class Child extends Parent{
public String someData = "Child";
}

Parent obj = new Child();
System.out.println(obj.someData);

What would get printed?
It is "parent". This is an example of hiding.

XSLT

noreply@blogger.com (Jigar) — Sat, 10 Mar 2012 13:01:00 +0000

Why XSLT?

We can user XSLT to transform XML data into HTML or XML or Text formats. For example, we can use XSLT to transform raw XML data to Excel Spreadsheet XML.
Applications often use XSLT to convert XML data into HTML or XHTML documents for display as a web page.
It is also used to create output for printing or direct video display, typically by transforming the original XML into XSL Formatting Objects to create formatted output which can then be converted to a variety of formats, including PDF, PostScript and PNG.

What is XSLT?

XSL style sheet basically consists of a set of templates. Each template "matches" some set of elements in the source tree and then describes the contribution that the matched element makes to the result tree
XSLT is a kind of programming language written in XML format. It is a set of rules, each rule applying to some node in source XML file.
XSL = Style Sheets for XML
XSLT = XSL Transformations
XSLT uses XPath to find information in source XML document.
XSL Transformations can be done either on client side (using JavaScript) or on server side (using XslCompiledTransform class).

http://www.w3schools.com/xsl/default.asp

http://nwalsh.com/docs/tutorials/xsl/xsl/slides.html

jQuery KeyPoints

noreply@blogger.com (Jigar) — Wed, 07 Mar 2012 14:32:00 +0000

1. Use vsdoc for jquery to get intellisense in Visual Studio

2. Use CDN for referencing jquery file in internet websites. Check for jQuery object after referencing the CDN file and if it is undefined then use the local version (this is only to ensure that some file is always available)

3. $(document).ready(function {alert('hi');}); - ready function gets called when DOM is loaded but before all images are loaded.

4. Selectors - we can select nodes using tagname, id, classname, attribute names, attibute values as shown below

$(selectorExpression) eg. $('div') gives all divs in document
$('div').css('color:red'); - changes color attribute of all divs - no looping required - css is jquery function
$('div').each(function(){alert($(this).innerHTML);}); - loops thru each element
$('#myid') - to select tag with id = myid
$('input[type="text"]') returns all textboxes

5. Always wrap objects with $ to use jquery functions

6. $(':input') gives all input elements - textboxes, dropdowns, textareas, images, buttons...

7. val() returns value of any form element e.g. $(variable).val(). Other useful functions - odd, even, first-child,last-child, contains, * ,^ ...

8. html() function gives inner html of element. - can be used to get and set html. - it works cross browser.

9. codylindley.com - to play with jquery selectors

10. .each(function(index,element){}); - used to iterate thru jquery objects

11. .attr(name,value) - used to change attribute values of html nodes.

12. append,prepend and remove functions to insert/remove DOM objects

13. wrap function wraps html node with another html node

14. .text() function to write/get text of node

15. There are many event shortcuts like .click() which works cross browser. Also it provides event object which again works cross browser

16. bind and unbind functions used to attach and detach events

17. delegate function to hook up events to dynamically added elements

18. .hover function to handle easily mouse enter and leave functions - combine it with toggleClass function to swap CSS classes on an element.

19. .toggle function to toggle between multiple functions.

20. Ajax functions - load, get, getJSON, post, ajax

21. jquery Source Viewer - http://james.padolsey.com/jquery/

Charting Options

noreply@blogger.com (Jigar) — Wed, 29 Feb 2012 12:49:00 +0000

Options to use charts in an ASP.Net application

1. ASP.Net Chart Controls

a. They are easy to use and fulfill almost all normal needs like bar charts, bubble charts, pie charts.

b. Some charts like gauge are not available.

http://www.spoiledtechie.com/post/2008/12/05/The-NEW-ASPNET-Chart-Controls-(with-Screenshots!).aspx

2. Google Charts

a. They are most easy to use.

b. But they render as images loaded from internet. So you always need net connectivity

c. Also some type of charts like gauge do not have standard UI

http://code.google.com/apis/chart/

3. SSRS charts

a. They require SQL Server Reporting Services to be installed.

b. But plus point is that they provide some odd chards like Gauge charts

c. They are also well integrated with ASP.Net and Visual Studio. We can add .rdlc files to the project and use Report controls liks Chart, Gauge, Matrix etc. And then to show it in aspx page we can use ReportViewer control

http://technet.microsoft.com/en-us/library/dd239351.aspx

4. Jqplot (using jquery)

a. Entirely Javascript driven => no images for chart controls. Rest all options create images

b. Does not have all types of charts

http://www.jqplot.com/tests/

5. Custom controls (using .Net code)

a. We can write custom controls to generate charts by using System.Drawing namespace

b. Good for 1 odd chart control type

6. Other free javascript charting libraries

7. Paid charting controls

Center the Update Progress bar

noreply@blogger.com (Jigar) — Tue, 21 Feb 2012 12:46:00 +0000

We need to take care of scroll bar also when trying to center the progress bar html.

function centerUPB() {

var df=document.getElementById('<%#UpdateProgress1.ClientID %>');

dfs=df.style;

dfs.position='absolute' ;

if (window.innerWidth) {

df.style.left = (window.innerWidth / 2 + document.body.scrollLeft - 50) + "px";

df.style.top = (window.innerHeight / 2 + document.body.scrollTop) + "px";

}

else

{

dfs.left = (document.documentElement.offsetWidth / 2 + document.body.scrollLeft) - 50 + "px";

dfs.top = (document.documentElement.offsetHeight / 2 + document.body.scrollTop) + "px

}

window.onload = centerUPB ;

window.onresize = centerUPB ;

window.onscroll = centerUPB ;

ASP.Net Chart Control

noreply@blogger.com (Jigar) — Tue, 24 Jan 2012 15:20:00 +0000

Key Points

1. Namespace to use - System.Web.UI.DataVisualization.Charting

2. UI Control for charting – asp:Chart

3. Important tags within chart control are Titles, Legends, Series & ChartAreas

4. Main Classes to use

a. Series – to provide series of data values using which a chart will be prepared

i. Points collection

ii. Chart Type used to specify which type of chart to render (bar, pie…)

b. DataPoint – to define individual data point in a chart

i. Map Area Attributes property used to define any HTML event (like mouse over) for the map (chart) area defined by the DataPoint

ii. Url – URL target of the map area defined by the DataPoint

iii. Color – to set color for the defined map area

iv. Legend Text – Text of item represented by Data Point in the legend

v. Axis Label – X-axis label for the data point

Best way to learn is to install its samples from here

Order By Particular values in database

noreply@blogger.com (Jigar) — Tue, 10 Jan 2012 15:44:00 +0000

Let’s assume you have a table which contains a U.S. State Code and instead of sorting the result alphabetically based on the U.S. State Code you want it sorted by certain states, like CA, FL, TX, NY in that order. This can be accomplished using the CHARINDEX string function:

SELECT *
FROM [dbo].[Customers]
ORDER BY CHARINDEX([State], 'CA-FL-TX-NY')

We can use functions like CHARINDEX in other databases also to achieve custom order by. (In MYSQL it is done by using Order by Field)

Abandon Session on Browser Close Event

noreply@blogger.com (Jigar) — Mon, 02 Jan 2012 09:11:00 +0000

To abandon session on browser close, we need to send an Ajax request to server (which will kill current session) on close of browser tab/window.

For this we can use onBeforeUnload event of html body for sending Ajax request.

But the above event gets called every time there is a postback or request for some other page. Now in that case, we should not be killing session. For this we can use a global variable in javascript to identify whether we should kill the session or not.

var g_isPostBack = false;

Then do following

1. Identify all postbacks using following 2 methods (2 methods because there are 2 types of postbacks in ASP.Net – one using __doPostBack and other using WebForm_OnSubmit

a. Put this line in Page_Load event of every page (or master page) - this is for WebForm_OnSubmit function

Page.ClientScript.RegisterOnSubmitStatement(this.GetType(), "OnSubmitScript", "g_isPostBack = true;");

b. Override __doPostBack function

addToPostBack = function (func) {

var old__doPostBack = __doPostBack;

if (typeof __doPostBack != 'function') {

__doPostBack = func;

} else {

__doPostBack = function (t, a) {

if (func(t, a)) old__doPostBack(t, a);

}

};

//Call this function on body onload event

function windowOnLoad() {

addToPostBack(function (t, a) {

g_isPostBack = "true";

return true;

});

}

2. For all GET requests (which can be triggered using either anchor tags or something else), we need to set the global flag to true so that on unload of page, we do not kill session.

="g_isPostBack=true;">

3. Handle Refersh event – which means if user presses F5 key or presses browser refresh button then set the global flag to true to ensure that there is no postback. To identify F5 key use following code:

document.onkeydown = keyisPressed;

function keyisPressed(e) {

var evt = e || window.event;

if (evt.keyCode == 116) {

isF5Pressed = true;

}

Sample code for abandonSession method which gets called on body onBeforeUnload event

function abandonSession() {

if (isF5Pressed == true) {

g_isPostBack = true;

}

if (g_isPostBack != true) {

var url = document.forms[0].action;

var params = "lol=1";

var myajaxrequest = new ajaxRequest();

myajaxrequest.open("POST", url, true);

//Send the proper header information along with the request

myajaxrequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

myajaxrequest.setRequestHeader("Content-length", params.length);

myajaxrequest.setRequestHeader("Connection", "close");

myajaxrequest.onreadystatechange = function () {//Call a function when the state changes.

if (myajaxrequest.readyState == 4 && myajaxrequest.status == 200) {

// alert(myajaxrequest.responseText);

}

myajaxrequest.send(params);

}

function ajaxRequest() {

var activexmodes = ["Msxml2.XMLHTTP", "Microsoft.XMLHTTP"] //activeX versions to check for in IE

if (window.ActiveXObject) { //Test for support for ActiveXObject in IE first (as XMLHttpRequest in IE7 is broken)

for (var i = 0; i < activexmodes.length; i++) {

try {

return new ActiveXObject(activexmodes[i])

}

catch (e) {

//suppress error

}

else if (window.XMLHttpRequest) // if Mozilla, Safari etc

return new XMLHttpRequest()

else

return false

}

Oracle Schemas, Tablespaces, Datafiles

noreply@blogger.com (Jigar) — Tue, 20 Dec 2011 08:56:00 +0000

A schema is the set of objects (tables, views, indexes, etc.) that belongs to a user account. It is often used as another way to refer to an Oracle account. It is collection of database objects, including logical structures such as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links. A schema has the name of the user who controls it.

Other types of objects are also stored in the database but are not contained in a schema:

Contexts
Directories
Profiles
Roles
Tablespaces
Users
Rollback segments

Does each user has its own space of tables - schema?

Yes.

The "schema" is the collection of all objects owned by a particular user. So if user SCOTT creates two tables, EMP and DEPT, and a view EMP_RPT, and a procedure GET_MY_EMP, those objects (tables, views, procedures) collectively make up the SCOTT schema.

Those objects will be physically stored in a tablespace. Schema objects do not have a one-to-one correspondence to physical files on disk that store their information. However, Oracle stores a schema object logically within a tablespace of the database. The data of each object is physically contained in one or more of the tablespace’s data files.

A tablespace is a named collection of data files. So tablespace USERS will be made up of one or more data files. A specific datafile can belong to one and only one tablespace. If a tablespace has more than one data file, oracle will manage those files as a collection invisible to the application - much like the OS or disk subsystem handles striping across multiple physical disks.

A specific object in the SCOTT schema can exist in only one tablespace, but not all objects of the schema have to be in the same tablespace. Likewise a tablespace can contain objects from multiple schemas => There is no relationship between schemas and tablespaces: a tablespace can contain objects from different schemas, and the objects for a schema can be contained in different tablespaces.

Can one user access tables of other users?

Yes.

The user should get the required privileges from the other user or
should have the select any table privilege or
should possess a dba role.

We can use Synonyms to select from table across schemas without using schema identifiers. But the user still needs required privileges to access other user’s objects. Synonyms just help to avoid writing schema name before object names.

A way to grant privileges to tables in one schema to other user

PROCEDURE GRANT_TABLES
IS
BEGIN
  FOR tab IN (SELECT table_name
  FROM   all_tables
  WHERE  owner = this_user) LOOP
EXECUTE IMMEDIATE 'GRANT SELECT, INSERT, UPDATE, DELETE ON '||tab.table_name||' TO other_user';
  END LOOP;
END;

Tablespaces and Datafiles

Assign Tablespaces to users

Users cannot create objects in a tablespace (even it's their default tablespace) unless they have a quota on it (or UNLIMITED TABLESPACE privilege). Some examples:

Grant user Scott access to use all space in the tools tablespace:

ALTER USER Scott QUOTA UNLIMITED ON tools;

Prevent user Scott from using space in the system tablespace:

ALTER USER Scott QUOTA 0 ON system;

Do not create objects in the SYSTEM tablespace. The system tablespace is reserved for the data dictionary.
Don't create tablespaces, with hundreds of small datafiles, these files needs to be checkpointed, resulting is unnecessary processing.

References

http://www.orafaq.com/wiki/Database_Concepts_and_Architecture

http://download.oracle.com/docs/cd/B19306_01/server.102/b14220/intro.htm#sthref70

http://www.adp-gmbh.ch/ora/admin/creatingdbmanually.html

http://download.oracle.com/docs/cd/B10500_01/appdev.920/a96590/toc.htm

http://download.oracle.com/docs/cd/B10500_01/server.920/a96524/c11schem.htm

http://www.adp-gmbh.ch/ora/misc/users_roles_privs.html

Difference between oracle database and an instance

noreply@blogger.com (Jigar) — Thu, 15 Dec 2011 08:44:00 +0000

The terms instance and database are closely related, but don't refer to the same thing.

The database is the set of files where application data (the reason for a database) and meta data is stored.

An instance is the software (and memory) that Oracle uses to manipulate the data in the database. In order for the instance to be able to manipulate that data, the instance must open the database. A database can be opened (or mounted) by more than one instance; however, an instance can open at most one database.

There can be more than one Oracle instance on a single machine. In order to be able to distinguish these instances, Oracle uses a SID (System Identifier) which is a string.

On Windows, each instance requires a Windows service.

ResolveUrl and ResolveClientUrl

noreply@blogger.com (Jigar) — Mon, 21 Nov 2011 18:29:00 +0000

The .NET Control class methods ResolveUrl and ResolveClientUrl both take a relative URL as a parameter and return relative URLs for client browser use. So what's the difference between them?

In simple terms, ResolveClientUrl returns a path relative to the current page, and ResolveUrl returns a path relative to the site root. Both methods are particularly useful when passing in a relative URL prefaced with the tilde (~) to indicate the application root.

Let's say we have an image in a layout directory, and a page which needs to pass a usable relative URL for the image to the client browser.

Image http://www.andornot.com/layout/images/andornotLogo.gif

Page http://www.andornot.com/Products/Default.aspx.

Here is what we would get if we called the two methods from our Default.aspx page.

Page.ResolveUrl("~/layout/images/andornotLogo.gif")

"/layout/images/andornotLogo.gif"

Page.ResolveClientUrl("~/layout/images/andornotLogo.gif")

"../layout/images/andornotLogo.gif"

· ResolveUrl creates the URL relative to the root.

· ResolveClientUrl creates the URL relative to the current page => So ResolveClientUrl will give correct url depending on folder in which our page is.

According to the MSDN documentation:

ResolveClientUrl

A fully qualified URL to the specified resource suitable for use on the browser.

Use the ResolveClientUrl method to return a URL string suitable for use by the client to access resources on the Web server, such as image files, links to additional pages, and so on.

ResolveUrl

The converted URL.

If the relativeUrl parameter contains an absolute URL, the URL is returned unchanged. If the relativeUrl parameter contains a relative URL, that URL is changed to a relative URL that is correct for the current request path, so that the browser can resolve the URL.

For example, consider the following scenario:

A client has requested an ASP.NET page that contains a user control that has an image associated with it.

The ASP.NET page is located at /Store/page1.aspx.

The user control is located at /Store/UserControls/UC1.ascx.

The image file is located at /UserControls/Images/Image1.jpg.

If the user control passes the relative path to the image (that is, /Store/UserControls/Images/Image1.jpg) to the ResolveUrl method, the method will return the value /Images/Image1.jpg.

http://www.andornot.com/blog/post/ResolveUrl-vs-ResolveClientUrl.aspx

http://stackoverflow.com/questions/1874636/what-is-the-difference-between-resolveurl-and-resolveclienturl