With the incredible growth of the Internet, a major concern has been how secure the Internet is, especially when you're sending sensitive information through it. There’s a whole lot of information that we don't want other people to see, such as:
• Credit-card information
• Social Security numbers
• Private correspondence
• Personal details
• Sensitive company information
• Bank-account information
Information security is provided on computers and over the Internet by a variety of methods. The most popular forms of security all rely on is encryption, the process of encoding information in such a way that only the person (or computer) with the key can decode it.
Most computer encryption systems belong in one of two categories:
• Symmetric-key encryption
• Public-key encryption
In symmetric-key encryption, each computer has a secret key (code) that it can use to encrypt a packet of information before it is sent over the network to another computer. Symmetric-key encryption is essentially the same as a secret code that each of the two computers must know in order to decode the information. The code provides the key to decoding the message.
Public-key encryption uses a combination of a private key and a public key. The private key is known only to your computer, while the public key is given by your computer to any computer that wants to communicate securely with it. To decode an encrypted message, a computer must use the public key, provided by the originating computer, and its own private key.
To implement public-key encryption on a large scale, such as a secure Web server might need, requires a different approach. This is where digital certificates come in. A digital certificate is basically a bit of information that says that the Web server is trusted by an independent source known as a certificate authority. The certificate authority acts as a middleman that both computers trust. It confirms that each computer is in fact who it says it is, and then provides the public keys of each computer to the other.
A popular implementation of public-key encryption is the Secure Sockets Layer (SSL). Originally developed by Netscape, SSL is an Internet security protocol used by Internet browsers and Web servers to transmit sensitive information. You will notice that the "http" in the address line is replaced with "https," and you should see a small padlock in the status bar at the bottom of the browser window.
In fact, sending information over a computer network is often much more secure than sending it any other way.
Wednesday, April 9, 2008
Voice over Internet Protocol (VOIP):
Voice over Internet Protocol (VoIP) is a protocol optimized for the transmission of voice through the Internet or other packet switched networks. VoIP is often used abstractly to refer to the actual transmission of voice (rather than the protocol implementing it). This latter concept is also referred to as IP telephony, Internet telephony, voice over broadband, broadband telephony, and broadband phone. The last two are arguably incorrect because telephone-quality voice communications are, by definition, narrowband.
Voice-over-IP systems carry telephony signals as digital audio, typically reduced in data rate using speech data compression techniques, encapsulated in a data packet stream over IP.
There are two types of PSTN-to-VoIP services: Direct inward dialing (DID) and access numbers. DID will connect a caller directly to the VoIP user, while access numbers require the caller to provide an extension number for the called VoIP user.
Some cost savings are due to utilizing a single network to carry voice and data, especially where users have underused network capacity that can carry VoIP at no additional cost. VoIP to VoIP phone calls are sometimes free, while VoIP calls connecting to public switched telephone networks (VoIP-to-PSTN), may have a cost that is borne by the VoIP user.
Corporate customer telephone support often use IP telephony exclusively to take advantage of the data abstraction. The benefit of using this technology is the need for only one class of circuit connection and better bandwidth use. Companies can acquire their own gateways to eliminate third-party costs, which is worthwhile in some situations.
Tuesday, April 8, 2008
V6R1 RPG Enhancements:
Following are the enhancements made by IBM to its all new V6R1 RPG. - This is downright intriguing.
Ø Increased Sizes
The maximum amount of storage that can be occupied by a data structure, array, or stand-alone field is now 16M. Yes, the maximum size of a character field has increased from 64K to 16M (16,773,104 bytes to be exact). This enhancement means that a whole lot of dynamic memory management procedures and pointer/user space procedures are about to disappear. It also means that operations such as XML-INTO are now more functional and less cumbersome to use.
D LongFld S a Len(25000000)
D BigDS DS a Len(60000000)
D BigArray1 S a Len(10000000)
D Dim(6)
D BigArray2 S 1a Dim(50000000)
D BigVarying S a Len(25000000) Varying(4)
D DummyPtr S *
DummyPtr = %Addr(BigVarying : *Data);
You can use the LEN keyword to define a large length.
· Since only seven positions are available to specify the length of a field in the D spec, you use the LEN keyword to specify a length greater than 9,999,999.
· The LEN keyword may also be applied to a data structure.
· The LEN keyword may also be applied to an array.
· Larger variable sizes also mean you can have a larger number of elements in an array as long as the total storage for the array does not exceed 16M. The same applies to multiple-occurrence data structures.
· Varying-length fields now require 2 or 4 bytes to store the actual length of the field. A size of 2 is assumed if the specified length is between 1 and 65535; otherwise, a size of 4 is assumed. You can specify either VARYING(2) or VARYING(4) for definitions whose length is between 1 and 65535. For definitions whose length is greater than 65535, VARYING(4) is required.
· The %ADDR BIF has also been enhanced. The optional *DATA parameter may be specified so that %ADDR returns the address of the data portion of a variable-length field.
UCS-2 and Graphic fields can have a maximum length of 8M.
The maximum size of literals has also increased:
· Character literals can now have a length of up to 16380 characters.
· UCS-2 literals can now have a length of up to 8190 UCS-2 characters.
· Graphic literals can now have a length of up to 16379 DBCS characters.
UCS-2 variables can now be initialized with character or graphic literals without using the %UCS2 built-in function. UCS-2 enhancements are available as PTFs back to V5R3.
Ø Files Defined in Subprocedures
Subprocedures have F-specs. This means that a file defined in a subprocedure is local to the subprocedure.
P GetName B
FCustomer IP E K Disk
D GetName PI 50a
D CusNo 10a
D GetCustRec Ds LikeRec(CustomerR)
/Free
Chain Cusno Customer GetCustRec;
Return GetCustRec.CustName;
/End-free
P E
A few points are worth noting:
· Input and output specifications are not generated for local files; therefore, all input and output must be done with result data structures.
· By default, files are automatically opened when the subprocedure is called and automatically closed when the subprocedure ends (either normally or abnormally). Of course, USROPN may be specified for a file so the opening and closing of the file are under your control in the subprocedure.
· You can change the default opening and closing of the file by specifying the STATIC keyword on the F-spec. This means that the storage associated with the file is static and all invocations of the subprocedure will use the same file. If the file is open when the procedure returns, it will remain open for the next call to the procedure.
Ø Templates
The TEMPLATE keyword allows you to define template data structures, stand-alone fields, and files.
· Templates for Data Structures
The concept of a template for data structures is not new. Below is the traditional way of defining and using a virtual template.
D Phone DS Based(DummyPtr) Qualified
D CountryCode 5i 0
D NDDPrefix 5
D AreaCode 5
D Number 9
D Extension 4
D IDDPrefix 5
D HomePhone DS LikeDS(Phone)
D CellPhone DS LikeDS(Phone)
D WorkPhone DS LikeDS(Phone)
/Free
HomePhone.CountryCode = 353;
CellPhone.CountryCode = 353;
WorkPhone.CountryCode = 353;
The new comparable structure being defined with the new TEMPLATE keyword. At first glance, they are fairly similar, the major difference being the use of the INT keyword on the DS definition of PHONE and the INZ(353) for the CountryCode subfield in Phone. The use of the INZ keyword with the template data structure means the same initialization may be applied to any dependent data structures (defined using LIKEDS) by specifying INZ(*LIKEDS).
D Phone DS Template Inz
D CountryCode 5i 0 Inz(353)
D NDDPrefix 5
D AreaCode 5
D Number 9
D Extension 4
D IDDPrefix 5
D HomePhone DS LikeDS(Phone)
D Inz(*LikeDS)
D CellPhone DS LikeDS(Phone)
D Inz(*LikeDS)
D WorkPhone DS LikeDS(Phone)
D Inz(*LikeDS)
· The TEMPLATE keyword may also be applied to a stand-alone field.
A definition defined with a TEMPLATE keyword may only be used as a parameter for the LIKE or LIKEDS keywords or the %SIZE, %LEN, %ELEM, or %DECPOS BIFs; it may not be used as a normal data structure or field.
· Templates for Files
The TEMPLATE keyword may be specified for files. Files defined with the TEMPLATE keyword are not included in the program; the file definition is used only at compile time. The template file can only be used as a basis for defining other files later in the program using the new LIKEFILE keyword. The LIKEFILE keyword also allows you to pass a file as a parameter.
· FCustomer IF E K Disk Template
P GetCustomer B
D GetCustomer PI N
· D customerFile LikeFile(Customer)
· D customerData LikeRec(CustomerR)
D customerKey Like(customerData.CustNo)
D Const
Chain customerKey customerFile customerData;
If %Found(customerFile);
If customerData.Status <> 'D';
Return *On;
EndIf;
EndIf;
Return *Off;
P E
The above shows a portion of a member containing a subprocedure (GetCustomer) that retrieves customer data. This member is compiled and placed in a service program. The main points to note are these (refer to the numbers above):
1. The TEMPLATE keyword is specified for the customer file. This means that the File specification is for reference purposes only, and the file definition may not be used for processing.
2. The LIKEFILE keyword identifies the first parameter as a file with the same characteristics as the Customer file.
3. The second parameter is the customer record that will be returned by the subprocedure.
4. The file parameter name is used to identify the file to be processed (i.e., customerFile not Customer). Since Input and Output specs are not generated for a template file or a file identified by LIKEFILE, you must use a result data structure for the CHAIN operation.
How do you call the GetCustomer subprocedure? The below snippet of a program shows a typical call: The file to be processed (CustFile) is simply passed as the first parameter. The format of the file CustFile must be the same as the Customer file.
FCustFile IF E K Disk
D custData DS LikeRec(CustFileR)
/Free
If GetCustomer(CustFile: CustFileR: 'THISCUST');
// DO cool things with data
EndIf;
The LIKEFILE keyword may also be used on the F-specs. The example below shows two files (CurrCust and OldCust) being defined like the Customer file. The processing options (file type, record addition, record address type, device, etc.) and most (but not all) of the keywords are inherited.
FCustomer IF E K Disk Template Block(*YES)
FCurrCust LikeFile(Customer)
F ExtFile('CURRLIB/CUSTFILE')
FOldCust LikeFile(Customer)
F ExtFile('OLDLIB/CUSTFILE')
There are a few items to bear in mind when using LIKEFILE:
• The parent file must be externally defined.
• Files are implicitly qualified; therefore, resulting data structures are required for input and output operations.
• The parent file must define any blocking requirements.
• Not all keywords are inherited. These are keywords that must be unique for each file (e.g., INDDS, INFDS, INFSR, OFLIND).
• Although the SFILE keyword may be inherited, you still need to define it for dependent files in order to specify a unique RRN field.
Ø Other File Enhancements
There are a few other file related enhancements worth having a look at.
(1) FCustomer IP E K Disk ExtDesc('MYLIB/CUSTFILE')
(2) F ExtFile(*ExtDesc)
(3) F Qualified
FScreens CF E WorkStn
D GetCustRec Ds LikeRec(CustomerR)
(4) D GetDetails E Ds ExtName('MYLIB/SCREENS' :
D Screen1 : *ALL)
/Free
(5) Read Customer.CustomerR GetCustRec;
If GetCustRec.Type = 1;
Eval-Corr GetDetails = GetCustRec;
(6) ExFmt Screen1 GetDetails;
EndIf;
1. You are aware that the EXTFILE keyword allows you to specify the file to be used when a program is called (a built-in override), but EXTFILE does not have any effect at compile time. The EXTDESC keyword allows you to specify the file definition to be referenced at compile time. This provides a means of handling SQL defined tables (where the file and format name are the same) other than renaming the record format.
2. The EXTFILE keyword allows a special value of *EXTDESC, which means that the value specified for the EXTDESC keyword should be used by the EXTFILE. Basically, you are specifying the same value for both the EXTDESC (compile time) and EXTFILE (run time) keywords.
3. The QUALIFIED keyword may be specified for files. This means that all references (except for the RENAME, INCLUDE, IGNORE, and SFILE file keywords) to record format names must be qualified with the file name.
4. The file name specified on the EXTNAME keyword may be a character literal in any of the forms 'LIBRARY/FILE', 'FILE', or '*LIBL/FILE'.
5. As with file specifications in subprocedures, Input and Output specifications are not generated for a qualified file (i.e., external fields from the file are not automatically defined as fields in the program and all I/O to the file must be done with result data structures).
6. A data structure name may be specified as the result for an EXFMT operation. This eases the use of qualified data structures with display files. The *ALL value must be specified on the LIKREC or EXTNAME keyword for the data structure.
Ø No Cycle RPG
If you have delved into the wonderful world of ILE, you are almost certain to have coded a module with the NOMAIN keyword in the control specifications. This means that the module only contains global definitions (F- and D-specs) and subprocedures and that the compiler does not place any RPG cycle code in the module since there is no mainline (a linear module). Since a NOMAIN module does not contain a Program Entry Procedure (PEP), it cannot be compiled as a callable program.
The introduction of the MAIN keyword on the control specification allows you to code a module that may be created as a program but does not contain the RPG cycle. The MAIN keyword allows you to specify the name of the subprocedure to be used as the PEP for the program. The below snippet shows the code in a member named CUST001, the member is compiled using the CRTBNDRPG command. The MAIN keyword identifies the MaintainCustomer subprocedure as the PEP for the program.
H Main(MaintainCustomer)
P MaintainCustomer...
P B
D MaintainCustomer...
D PI
/Free
// Lots of cool code
/End-Free
P E
These are a few of the many other enhancements made in V6R1 RPG.
Ø Increased Sizes
The maximum amount of storage that can be occupied by a data structure, array, or stand-alone field is now 16M. Yes, the maximum size of a character field has increased from 64K to 16M (16,773,104 bytes to be exact). This enhancement means that a whole lot of dynamic memory management procedures and pointer/user space procedures are about to disappear. It also means that operations such as XML-INTO are now more functional and less cumbersome to use.
D LongFld S a Len(25000000)
D BigDS DS a Len(60000000)
D BigArray1 S a Len(10000000)
D Dim(6)
D BigArray2 S 1a Dim(50000000)
D BigVarying S a Len(25000000) Varying(4)
D DummyPtr S *
DummyPtr = %Addr(BigVarying : *Data);
You can use the LEN keyword to define a large length.
· Since only seven positions are available to specify the length of a field in the D spec, you use the LEN keyword to specify a length greater than 9,999,999.
· The LEN keyword may also be applied to a data structure.
· The LEN keyword may also be applied to an array.
· Larger variable sizes also mean you can have a larger number of elements in an array as long as the total storage for the array does not exceed 16M. The same applies to multiple-occurrence data structures.
· Varying-length fields now require 2 or 4 bytes to store the actual length of the field. A size of 2 is assumed if the specified length is between 1 and 65535; otherwise, a size of 4 is assumed. You can specify either VARYING(2) or VARYING(4) for definitions whose length is between 1 and 65535. For definitions whose length is greater than 65535, VARYING(4) is required.
· The %ADDR BIF has also been enhanced. The optional *DATA parameter may be specified so that %ADDR returns the address of the data portion of a variable-length field.
UCS-2 and Graphic fields can have a maximum length of 8M.
The maximum size of literals has also increased:
· Character literals can now have a length of up to 16380 characters.
· UCS-2 literals can now have a length of up to 8190 UCS-2 characters.
· Graphic literals can now have a length of up to 16379 DBCS characters.
UCS-2 variables can now be initialized with character or graphic literals without using the %UCS2 built-in function. UCS-2 enhancements are available as PTFs back to V5R3.
Ø Files Defined in Subprocedures
Subprocedures have F-specs. This means that a file defined in a subprocedure is local to the subprocedure.
P GetName B
FCustomer IP E K Disk
D GetName PI 50a
D CusNo 10a
D GetCustRec Ds LikeRec(CustomerR)
/Free
Chain Cusno Customer GetCustRec;
Return GetCustRec.CustName;
/End-free
P E
A few points are worth noting:
· Input and output specifications are not generated for local files; therefore, all input and output must be done with result data structures.
· By default, files are automatically opened when the subprocedure is called and automatically closed when the subprocedure ends (either normally or abnormally). Of course, USROPN may be specified for a file so the opening and closing of the file are under your control in the subprocedure.
· You can change the default opening and closing of the file by specifying the STATIC keyword on the F-spec. This means that the storage associated with the file is static and all invocations of the subprocedure will use the same file. If the file is open when the procedure returns, it will remain open for the next call to the procedure.
Ø Templates
The TEMPLATE keyword allows you to define template data structures, stand-alone fields, and files.
· Templates for Data Structures
The concept of a template for data structures is not new. Below is the traditional way of defining and using a virtual template.
D Phone DS Based(DummyPtr) Qualified
D CountryCode 5i 0
D NDDPrefix 5
D AreaCode 5
D Number 9
D Extension 4
D IDDPrefix 5
D HomePhone DS LikeDS(Phone)
D CellPhone DS LikeDS(Phone)
D WorkPhone DS LikeDS(Phone)
/Free
HomePhone.CountryCode = 353;
CellPhone.CountryCode = 353;
WorkPhone.CountryCode = 353;
The new comparable structure being defined with the new TEMPLATE keyword. At first glance, they are fairly similar, the major difference being the use of the INT keyword on the DS definition of PHONE and the INZ(353) for the CountryCode subfield in Phone. The use of the INZ keyword with the template data structure means the same initialization may be applied to any dependent data structures (defined using LIKEDS) by specifying INZ(*LIKEDS).
D Phone DS Template Inz
D CountryCode 5i 0 Inz(353)
D NDDPrefix 5
D AreaCode 5
D Number 9
D Extension 4
D IDDPrefix 5
D HomePhone DS LikeDS(Phone)
D Inz(*LikeDS)
D CellPhone DS LikeDS(Phone)
D Inz(*LikeDS)
D WorkPhone DS LikeDS(Phone)
D Inz(*LikeDS)
· The TEMPLATE keyword may also be applied to a stand-alone field.
A definition defined with a TEMPLATE keyword may only be used as a parameter for the LIKE or LIKEDS keywords or the %SIZE, %LEN, %ELEM, or %DECPOS BIFs; it may not be used as a normal data structure or field.
· Templates for Files
The TEMPLATE keyword may be specified for files. Files defined with the TEMPLATE keyword are not included in the program; the file definition is used only at compile time. The template file can only be used as a basis for defining other files later in the program using the new LIKEFILE keyword. The LIKEFILE keyword also allows you to pass a file as a parameter.
· FCustomer IF E K Disk Template
P GetCustomer B
D GetCustomer PI N
· D customerFile LikeFile(Customer)
· D customerData LikeRec(CustomerR)
D customerKey Like(customerData.CustNo)
D Const
Chain customerKey customerFile customerData;
If %Found(customerFile);
If customerData.Status <> 'D';
Return *On;
EndIf;
EndIf;
Return *Off;
P E
The above shows a portion of a member containing a subprocedure (GetCustomer) that retrieves customer data. This member is compiled and placed in a service program. The main points to note are these (refer to the numbers above):
1. The TEMPLATE keyword is specified for the customer file. This means that the File specification is for reference purposes only, and the file definition may not be used for processing.
2. The LIKEFILE keyword identifies the first parameter as a file with the same characteristics as the Customer file.
3. The second parameter is the customer record that will be returned by the subprocedure.
4. The file parameter name is used to identify the file to be processed (i.e., customerFile not Customer). Since Input and Output specs are not generated for a template file or a file identified by LIKEFILE, you must use a result data structure for the CHAIN operation.
How do you call the GetCustomer subprocedure? The below snippet of a program shows a typical call: The file to be processed (CustFile) is simply passed as the first parameter. The format of the file CustFile must be the same as the Customer file.
FCustFile IF E K Disk
D custData DS LikeRec(CustFileR)
/Free
If GetCustomer(CustFile: CustFileR: 'THISCUST');
// DO cool things with data
EndIf;
The LIKEFILE keyword may also be used on the F-specs. The example below shows two files (CurrCust and OldCust) being defined like the Customer file. The processing options (file type, record addition, record address type, device, etc.) and most (but not all) of the keywords are inherited.
FCustomer IF E K Disk Template Block(*YES)
FCurrCust LikeFile(Customer)
F ExtFile('CURRLIB/CUSTFILE')
FOldCust LikeFile(Customer)
F ExtFile('OLDLIB/CUSTFILE')
There are a few items to bear in mind when using LIKEFILE:
• The parent file must be externally defined.
• Files are implicitly qualified; therefore, resulting data structures are required for input and output operations.
• The parent file must define any blocking requirements.
• Not all keywords are inherited. These are keywords that must be unique for each file (e.g., INDDS, INFDS, INFSR, OFLIND).
• Although the SFILE keyword may be inherited, you still need to define it for dependent files in order to specify a unique RRN field.
Ø Other File Enhancements
There are a few other file related enhancements worth having a look at.
(1) FCustomer IP E K Disk ExtDesc('MYLIB/CUSTFILE')
(2) F ExtFile(*ExtDesc)
(3) F Qualified
FScreens CF E WorkStn
D GetCustRec Ds LikeRec(CustomerR)
(4) D GetDetails E Ds ExtName('MYLIB/SCREENS' :
D Screen1 : *ALL)
/Free
(5) Read Customer.CustomerR GetCustRec;
If GetCustRec.Type = 1;
Eval-Corr GetDetails = GetCustRec;
(6) ExFmt Screen1 GetDetails;
EndIf;
1. You are aware that the EXTFILE keyword allows you to specify the file to be used when a program is called (a built-in override), but EXTFILE does not have any effect at compile time. The EXTDESC keyword allows you to specify the file definition to be referenced at compile time. This provides a means of handling SQL defined tables (where the file and format name are the same) other than renaming the record format.
2. The EXTFILE keyword allows a special value of *EXTDESC, which means that the value specified for the EXTDESC keyword should be used by the EXTFILE. Basically, you are specifying the same value for both the EXTDESC (compile time) and EXTFILE (run time) keywords.
3. The QUALIFIED keyword may be specified for files. This means that all references (except for the RENAME, INCLUDE, IGNORE, and SFILE file keywords) to record format names must be qualified with the file name.
4. The file name specified on the EXTNAME keyword may be a character literal in any of the forms 'LIBRARY/FILE', 'FILE', or '*LIBL/FILE'.
5. As with file specifications in subprocedures, Input and Output specifications are not generated for a qualified file (i.e., external fields from the file are not automatically defined as fields in the program and all I/O to the file must be done with result data structures).
6. A data structure name may be specified as the result for an EXFMT operation. This eases the use of qualified data structures with display files. The *ALL value must be specified on the LIKREC or EXTNAME keyword for the data structure.
Ø No Cycle RPG
If you have delved into the wonderful world of ILE, you are almost certain to have coded a module with the NOMAIN keyword in the control specifications. This means that the module only contains global definitions (F- and D-specs) and subprocedures and that the compiler does not place any RPG cycle code in the module since there is no mainline (a linear module). Since a NOMAIN module does not contain a Program Entry Procedure (PEP), it cannot be compiled as a callable program.
The introduction of the MAIN keyword on the control specification allows you to code a module that may be created as a program but does not contain the RPG cycle. The MAIN keyword allows you to specify the name of the subprocedure to be used as the PEP for the program. The below snippet shows the code in a member named CUST001, the member is compiled using the CRTBNDRPG command. The MAIN keyword identifies the MaintainCustomer subprocedure as the PEP for the program.
H Main(MaintainCustomer)
P MaintainCustomer...
P B
D MaintainCustomer...
D PI
/Free
// Lots of cool code
/End-Free
P E
These are a few of the many other enhancements made in V6R1 RPG.
Monday, April 7, 2008
Ins and Outs of Constrains:
Constraints are a function of Referential Integrity, where the database manager ensures the logical consistency of data values between files and the validity of data relationships, based on rules set by you. Impressive as that sounds, it is something you are already doing; except that you are doing it in your application programs. For example you cannot delete the customer if there are dependant invoices on the invoice file, and you do not employ people under the age of sixteen. Those constraints are implemented through logic in your RPG or COBOL programs. As your applications expand and data becomes accessible outside of the traditional green screen, it becomes imperative that these rules are consistent across all interfaces. What better way to implement them then through the database manager?
Constraints are defined for physical files or tables. You can define three types of Constraint: Key, Referential and Check.
How do you define constraints?
You can define constraints using the Add Physical File Constraint (ADDPFCST) command. You can also use the CHGPFCST, RMVPFCST, WRKPFCST, EDTCPCST and DSPCPCST commands.
You can define them in SQL using the CREATE TABLE or ALTER TABLE commands.
Key constraints
Key constraints define unique keys for a table. The end result is an access path, but there is no corresponding logical file. Since DB2 automatically shares access paths, there is no extra overhead if there is already a logical file that defines the access path.
There are two types of Key constraints: unique and primary. A table may have only one primary Key constraint but may have many unique Key constraints.
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/CATEGOR) TYPE(*PRIKEY)
KEY(CATCOD) CST(CategoryPrimaryKey)
Referential constraints
Referential constraints are where you define a constraint between two tables: a parent and a dependant. The parent file must have a primary constraint defined for it.
In this example there is a dependency between the Category file and the Product file. Every product "belongs" to a category. Therefore, you should not be able to delete a category if any products refer to it, and you should not be able to assign a non-existent category to a product. Think how you would manage this in an application -- a logical over the product file that you use to check for existing records in the Category maintenance program and the Product maintenance program checks the Category file to make sure the category code is valid. (But you can bypass all of that with DFU.)
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/PRODUCT) TYPE(*REFCST)
KEY(CATCOD) CST(CategoryProductRestriction)
PRNFILE(ALLTHAT1FL/CATEGOR) PRNKEY(CATCOD)
DLTRULE(*RESTRICT) UPDRULE(*RESTRICT)
The possible delete rules are as follows:
• RESTRICT -- Record cannot be deleted if there are dependent records.
• CASCADE -- It's OK to delete a parent, but all dependent records are deleted as well.
• SET NULL -- Null-capable fields, in the dependent key, are set to null.
• SET DEFAULT -- Fields in the dependent key are set to their default values.
• NO ACTION -- A record cannot be deleted if there are dependent records; however, triggers will be fired before checking Referential constraints.
Check constraints
Check constraints allow you to define validation for columns in a table. The nearest to this in DDS is the COMP, RANGE and VALUES keywords, but they apply only to display files. Check constraints are maintained on the database.
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/PRODUCT) TYPE(*REFCST)
KEY(CATCOD) CST(Right_Price)
CHKCST('SELLPR >= LNDCST')
Referential Integrity is a powerful tool for us to use in our applications and provide a means of ensuring data integrity outside of our application.
Constraints are defined for physical files or tables. You can define three types of Constraint: Key, Referential and Check.
How do you define constraints?
You can define constraints using the Add Physical File Constraint (ADDPFCST) command. You can also use the CHGPFCST, RMVPFCST, WRKPFCST, EDTCPCST and DSPCPCST commands.
You can define them in SQL using the CREATE TABLE or ALTER TABLE commands.
Key constraints
Key constraints define unique keys for a table. The end result is an access path, but there is no corresponding logical file. Since DB2 automatically shares access paths, there is no extra overhead if there is already a logical file that defines the access path.
There are two types of Key constraints: unique and primary. A table may have only one primary Key constraint but may have many unique Key constraints.
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/CATEGOR) TYPE(*PRIKEY)
KEY(CATCOD) CST(CategoryPrimaryKey)
Referential constraints
Referential constraints are where you define a constraint between two tables: a parent and a dependant. The parent file must have a primary constraint defined for it.
In this example there is a dependency between the Category file and the Product file. Every product "belongs" to a category. Therefore, you should not be able to delete a category if any products refer to it, and you should not be able to assign a non-existent category to a product. Think how you would manage this in an application -- a logical over the product file that you use to check for existing records in the Category maintenance program and the Product maintenance program checks the Category file to make sure the category code is valid. (But you can bypass all of that with DFU.)
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/PRODUCT) TYPE(*REFCST)
KEY(CATCOD) CST(CategoryProductRestriction)
PRNFILE(ALLTHAT1FL/CATEGOR) PRNKEY(CATCOD)
DLTRULE(*RESTRICT) UPDRULE(*RESTRICT)
The possible delete rules are as follows:
• RESTRICT -- Record cannot be deleted if there are dependent records.
• CASCADE -- It's OK to delete a parent, but all dependent records are deleted as well.
• SET NULL -- Null-capable fields, in the dependent key, are set to null.
• SET DEFAULT -- Fields in the dependent key are set to their default values.
• NO ACTION -- A record cannot be deleted if there are dependent records; however, triggers will be fired before checking Referential constraints.
Check constraints
Check constraints allow you to define validation for columns in a table. The nearest to this in DDS is the COMP, RANGE and VALUES keywords, but they apply only to display files. Check constraints are maintained on the database.
The same constraint could be defined on green screen using the following command:
ADDPFCST FILE(ALLTHAT1FL/PRODUCT) TYPE(*REFCST)
KEY(CATCOD) CST(Right_Price)
CHKCST('SELLPR >= LNDCST')
Referential Integrity is a powerful tool for us to use in our applications and provide a means of ensuring data integrity outside of our application.
Friday, April 4, 2008
How does RPG talk to a browser?
Please refer
http://search400.techtarget.com/tip/0,289483,sid3_gci1043521,00.html
http://search400.techtarget.com/tip/0,289483,sid3_gci1043521,00.html
Thursday, April 3, 2008
Multi Threading:
Multithreading is a general purpose programming technique that reduces the complexity and overhead of concurrent programming. Multithreading is the process by which identical processes run in as many threads as needed, accessing the same file in any mode.
One can simulate this multithreading concept in reading the same file and processing records from it based on an arbitrary set of criteria in a program running in many threads. For a file having "n" records which is split across "m" jobs the processing time drops by about n/m time fraction.
Processing time would depend on many factors (such as number of processors, number of other jobs etc.) and we cannot say that processing time would reduce to n/m times. All we can say is that processing time would reduce considerably.
The idea is to split the number of records in the file by "m", a predefined number that indicates the number of threads that need to be run that can be decided based on the size of the file. It decides the number of records for each thread and gets the relative record number slot for each thread needed. Say there are 100 records and we have 5 threads. Then thread 1 gets rrn 1 to 20, thread 2 gets 21 to 40 and so on. The last job gets the full rrn slot needed for it or a slot smaller than that if the value of the number of records / number of jobs is not a round number.
In a nutshell, we run multiple jobs in parallel in such a way that each job is allocated a unique set of records for processing as opposed to having one job processing all the records.
One can simulate this multithreading concept in reading the same file and processing records from it based on an arbitrary set of criteria in a program running in many threads. For a file having "n" records which is split across "m" jobs the processing time drops by about n/m time fraction.
Processing time would depend on many factors (such as number of processors, number of other jobs etc.) and we cannot say that processing time would reduce to n/m times. All we can say is that processing time would reduce considerably.
The idea is to split the number of records in the file by "m", a predefined number that indicates the number of threads that need to be run that can be decided based on the size of the file. It decides the number of records for each thread and gets the relative record number slot for each thread needed. Say there are 100 records and we have 5 threads. Then thread 1 gets rrn 1 to 20, thread 2 gets 21 to 40 and so on. The last job gets the full rrn slot needed for it or a slot smaller than that if the value of the number of records / number of jobs is not a round number.
In a nutshell, we run multiple jobs in parallel in such a way that each job is allocated a unique set of records for processing as opposed to having one job processing all the records.
Wednesday, April 2, 2008
String Searching Algorithm:
String searching algorithms, sometimes called string matching algorithms, are an important class of string algorithms that try to find a place where one or several strings (also called patterns) are found within a larger string or text.
Let Σ be an alphabet (finite set). Formally, both the pattern and searched text are concatenations of elements of Σ. The Σ may be a usual human alphabet (for example, the letters A through Z in English). Other applications may use binary alphabet (Σ = {0,1}) or DNA alphabet (Σ = {A,C,G,T}) in bioinformatics.
In practice how the string is encoded can affect the feasible string search algorithms. In particular if a variable width encoding is in use then it is slow (time proportional to N) to find the Nth character. This will significantly slow down many of the more advanced search algorithms. A possible solution is to search for the sequence of code units instead, but doing so may produce false matches unless the encoding is specifically designed to avoid it.
Naïve String Search:
The simplest and least efficient way to see where one string occurs inside another is to check each place it could be, one by one, to see if it's there. So first we see if there's a copy of the needle in the first few characters of the haystack; if not, we look to see if there's a copy of the needle starting at the second character of the haystack; if not, we look starting at the third character, and so forth.
Finite State automation based search:
In this approach, we avoid backtracking by constructing a deterministic finite automaton that recognizes strings containing the desired search string. These are expensive to construct—they are usually created using the powerset construction—but very quick to use. This approach is frequently generalized in practice to search for arbitrary regular expressions.
Index methods
Faster search algorithms are based on preprocessing of the text. After building a substring index, for example a suffix tree or suffix array, the occurrences of a pattern can be found quickly. As an example, a suffix tree can be built in Θ(m) time, and all z occurrences of a pattern can be found in O(m + z) time (if the alphabet size is viewed as a constant).
Let Σ be an alphabet (finite set). Formally, both the pattern and searched text are concatenations of elements of Σ. The Σ may be a usual human alphabet (for example, the letters A through Z in English). Other applications may use binary alphabet (Σ = {0,1}) or DNA alphabet (Σ = {A,C,G,T}) in bioinformatics.
In practice how the string is encoded can affect the feasible string search algorithms. In particular if a variable width encoding is in use then it is slow (time proportional to N) to find the Nth character. This will significantly slow down many of the more advanced search algorithms. A possible solution is to search for the sequence of code units instead, but doing so may produce false matches unless the encoding is specifically designed to avoid it.
Naïve String Search:
The simplest and least efficient way to see where one string occurs inside another is to check each place it could be, one by one, to see if it's there. So first we see if there's a copy of the needle in the first few characters of the haystack; if not, we look to see if there's a copy of the needle starting at the second character of the haystack; if not, we look starting at the third character, and so forth.
Finite State automation based search:
In this approach, we avoid backtracking by constructing a deterministic finite automaton that recognizes strings containing the desired search string. These are expensive to construct—they are usually created using the powerset construction—but very quick to use. This approach is frequently generalized in practice to search for arbitrary regular expressions.
Index methods
Faster search algorithms are based on preprocessing of the text. After building a substring index, for example a suffix tree or suffix array, the occurrences of a pattern can be found quickly. As an example, a suffix tree can be built in Θ(m) time, and all z occurrences of a pattern can be found in O(m + z) time (if the alphabet size is viewed as a constant).
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