The mainframe computer, created in the mid 1960s, is widely used by most large enterprises around the world. The unique differentiator between mainframes and other computers is the hardware architecture. Non mainframe, including minicomputers and microcomputers, use their central processing units (CPUs) for virtually all of their processing, including getting data to and from disk and other peripherals. By contrast, mainframes have a special architecture emphasizing peripheral channels that process all input/ output, leaving the CPU dedicated to processing only data, such as calculating formulas and balances.
In many large companies, much of the day-to-day business data is processed and stored on mainframe systems (and certain minicomputer systems, such as the IBM AS/400) and integrating data from these systems into the data warehouse involves some unique challenges. There are several characteristics of mainframe systems that the ETL team must be familiar with and develop techniques to handle:
The rest of this section discusses these mainframe characteristics and offers techniques for managing them when they are encountered.
Working with COBOL Copybooks
COBOL remains the dominant programming language used on mainframe computers, and the file layout for data is described in COBOL copybooks.A copybook defines the field names and associated data types for a mainframe data file. As with other flat files you encounter in your ETL process, only two data types exist in mainframe flat files: text and numeric. However, numeric values are stored in a variety of ways that you need to understand to accurately process. Likewise, dates are stored simply as strings of numbers (or text) and typically require transformation to be stored in date columns in the data warehouse. Figure below illustrates a 70-byte, fixed length record that describes a simple employee record. Notice that the field names are preceded by level numbers.
A simple copybook that describes an employee record
Nesting of level numbers is used to group related fields. COBOL programs can refer to field names at any defined level. For example, a program can refer to HIRE-DATE to capture the full date of hire or HIRE-YYYY if only the year portion is needed for processing.
Text and numeric data types are denoted using the PIC clauses. PIC X denotes text fields, while PIC 9 means the field is numeric. Field lengths are specified with numbers following the type. For example, the clause PIC 9(4) indicates a four-byte numeric field, whereas PIC X(15) indicates a 15-byte text field. PIC clauses can be coded alternatively by repeating the X or 9 data type indicator, such as PIC 9999 for a four-byte numeric field.
The data file represented in Figure above can easily be transmitted via FTP and loaded into the data warehouse because all of the data is contained in display format. But before you try to transfer this file from the mainframe to the data warehouse platform, you need to take a short lesson on the difference between the familiar ASCII character set used on UNIX and Windows platforms and the EBCDIC character set used on the mainframe.
EBCDIC Character Set
Both the legacy mainframe systems and the UNIX- and Windows-based systems, where most data warehouses reside, are stored as bits and bytes. Each byte is made of eight bits, and each bit represents a binary (base-2) digit. The maximum number that can be represented by a byte made of binary bits is 255 (that is, 28-1). Thus, the number of unique characters (for example, A–Z, a–z, 0–9, punctuation, and special characters) that can be portrayed in a system made up of such bytes is 256 (including character 0).
Converting EBCDIC to ASCII
You might think that since both systems use bits and bytes, data from your mainframe system is readily usable on your UNIX or Windows system. But UNIX and Windows systems use the American Standard Code for Information Interchange (ASCII) character set, whereas mainframes use a different set, known as Extended Binary Coded Decimal Interchange Code (EBCDIC). EBCDIC uses more or less the same characters as ASCII but uses different 8-bit combinations to represent them.
For example, take the lowercase letter a. In ASCII, the letter a is character number 97 (01100001), but in EBCDIC, character number 97 is / (forward slash). In EBCDIC a is character 129 (10000001). In fact, none of the common characters are represented by the same character numbers in ASCII and EBCDIC. To use mainframe data on your UNIX or Windows system, you must first translate it from EBCDIC to ASCII.
Transferring Data between Platforms
Luckily, translating data from EBCDIC to ASCII is quite simple. In fact it’s virtually automatic, assuming you use File Transfer Protocol (FTP) to transfer the data from the mainframe to your data warehouse platform.
An FTP connection requires two nodes—a host and a client. When an FTP connection is made between systems, the FTP client identifies its operating system environment to the FTP host, and the host determines whether any translation is required when transferring data between the two systems. So when an FTP connection is made between a mainframe and a UNIX or Windows system, the FTP host translates mainframe data from EBCDIC to ASCII as it transfers the data. In addition, FTP adds the special line feed and carriage return characters used to designate the end of a line (or record) of data on UNIX and Windows. FTP also translates from ASCII to EBCDIC if the data movement is from UNIX or Windows to the mainframe.
If you receive mainframe data on magnetic tape cartridge or CD-ROM rather than via FTP, you need to explicitly translate the data from EBCDIC to ASCII on the data warehouse system. This translation can be performed using the UNIX dd command with the conv=ascii switch. For Windows, you can obtain a port of the dd—and many other useful UNIX commands— on the Internet. In addition, commercial products that handle charactertranslation duties are available. ETL tool suites all handle this conversion. Most robust tools designed specifically for ETL can convert EBCDIC to ASCII on the fly.
If your source data resides on a mainframe system, it is crucial that your ETL tool have the ability to implicitly convert EBCDIC data to ASCII. If at all possible, you want this to occur on the mainframe to avoid any corruption of low values and packed decimals. If data is received via tape or other media, the translation must occur by the ETL tool in the nonmainframe environment. At a minimum, the ETL tool must automatically execute FTP and process files in stream, passing the data directly from the mainframe through the ETL process to the target data warehouse.
As a final point, although mainframes and UNIX or Windows systems use different character sets, translating data from one system to another is a rather simple task—simple, that is, unless your mainframe data has some other traits that are typical of the mainframe world. The next few sections discuss specific characteristics that mainframe data may possess and recommend strategies for managing them during the ETL process.
Handling Mainframe Numeric Data
When you begin to work with quantitative data elements, such as dollar amounts, counts, and balances, you can see that there’s more to these numbers than meets the eye. For one thing, you won’t typically find decimal points in decimal data, because the decimal points are implied. For example, the value 25,000.01 is stored as 002500001. Worse, the value 2,500,001 is stored the same way. So how does the mainframe COBOL program know that 25,000.01 is meant rather than 2,500,001? It’s in the PIC clause. The next section discusses the importance and power of the PIC clause in COBOL copybooks.
You can see in Figure below that the PIC clause can give the same data value different meaning. To accurately process a numeric value that comes from a legacy mainframe system, you must first transform it to its display format before transmitting it to the data warehouse system; otherwise, your ETL tool has to handle interpreting these mainframe values on the UNIX or Windows platform. To resolve decimals in the numeric values, you might think that you can simply divide the numeric value by the power of ten equal to the number of implied decimal places. And if all numeric values were stored with only the decimal point implied, you’d be right. However, it’s not quite that simple. You also have to consider signed numeric values.
The PIC clause in a COBOL copybook indicates the decimal places of a numeric value
In mainframe data, the signs may come before or after the numeric value. What’s more, the sign may be embedded within the numeric value.
The most common format, zoned numeric, embeds the sign within the last numeric digit as shown in the last two rows of Figure. So, how does A in the last position connote both the digit 1 and the sign +, and likewise, how does J represent both 1 and -? The trick is that the last byte is treated as two separate half-bytes (each containing four bits) and each half-byte is interpreted separately—in hexadecimal, of course!
For positive numbers, the first half-byte is set to C, the hexadecimal value of 1100, and negative numbers are set to D, the hexadecimal value of 1101. The second half-byte is set to the hexadecimal value that corresponds to the desired numeric digit. When you combine the first half-byte—1100 for positive or 1101 for negative—to the second half-byte, you get resulting EBCDIC characters, as seen in Figure below.
Hexadecimal to EBCDIC
By now, you are probably scratching your head trying to figure out how to deal with numeric data from your mainframe system. Well, before you try to solve the problem, there’s still one more twist that you are likely to encounter in most legacy mainframe systems.
Unpacking Packed Decimals
Though at present computer hard disk storage is relatively inexpensive, in the past disk storage was among the most expensive components of the computer system. To save disk space, software engineers devised creative formats to store numeric data using fewer bytes than the digits in the number. The most pervasive of these formats is COMP-3, also known as packednumeric.
In many mainframe systems, most if not all numeric data is stored in COMP-3 format. COMP-3 format is a simple space-saving technique that uses half-bytes—or nibbles—rather than full bytes to store numeric digits. Each numeric digit can be stored in binary format within the four bits of a nibble. The last nibble of a COMP-3 numeric field stores the sign (positive/ negative) of the numeric value. Using half-bytes to store numeric digits saves nearly half the space used by the display format. But this simple space saving technique throws a wrench into the EBCDIC to ASCII character-set translation.
As a result of this translation conundrum, mainframe data that contains numeric values stored using numeric storage formats such as Zoned Numeric or COMP-3 (not to mention COMP, COMP-1, and COMP-2) cannot simply be translated from EBCDIC to ASCII and then processed on the UNIX orWindows warehouse system.
One of the following techniques must be used to maintain the integrity of mainframe numeric data:
Working with Redefined Fields
Rather than wasting space—remember it used to be expensive—mainframe engineers devised REDEFINES, which allow mutually exclusive data elements to occupy the same physical space. Figure below contains an excerpt from a COBOL Copybook that helps illustrate the concept of REDEFINES in mainframe data files. The excerpt describes the data fields that represent an employee’s wage information. Notice EMPLOYEE-TYPE, which is a one-byte code that indicates whether the employee is exempt or hourly. Also, notice that two separate series of fields carry the wage information for the employee. The field set used depends on whether the employee is exempt or hourly. Exempt employees’ wages are represented in three fields (PAY-GRADE, SALARY, and PAY-PERIOD), which take up a total of eight bytes. Hourly employees use a different set of fields that take up seven bytes (PAY-RATE and JOB-CLASS).
REDEFINES clause in a COBOL copybook
Since an employee is exempt or hourly, never both, only one of the two field sets is ever used at a time. The exempt wage fields occupy positions 72 through 79 in the file, and the hourly wage fields occupy positions 72 though 78. Furthermore, notice that the fields for exempt and hourly wages use different data types even though they occupy the same positions. When reading the employee record, the program must determine how to interpret these positions based on the value of EMPLOYEE-TYPE in position 71. The same positions can have more than one REDEFINES associated with them, so rather than just two possible uses, the same positions can have two, three, or more possible uses. REDEFINES introduce one further complication that renders mere EBCDIC-to-ASCII character-set translation insufficient.
When you encounter multiple REDEFINES in your sources, you should consider making each definition a separate pass of the extract logic over the source data if the subsequent processing is quite different (using Exempt versus Hourly as an example). This would allow you to build separate code lines for each extract rather than one complex job with numerous tests for the two conditions.
Mainframe and COBOL precede relational databases and Edward Codd’s normalization rules. Prior to utilizing relational theory to design databases, repeating groups were handled with mainframe COBOL programs that use an OCCURS clause to define data fields that repeat within a data file. For example, in Figure you can see an area of an employee record that stores information about performance ratings. The record is designed to keep track of up to five performance ratings. But rather than creating the needed fields five times—remember, this precedes relational theory so there won’t be a separate performance rating table with a foreign key that points back to the employee—they are named only once within a special OCCURS field. The OCCURS clause indicates the number of times the fields within it repeat. Essentially, the OCCURS clause defines an array contained within the file. Thus, in the employee record, data for the first performance rating occupies positions 80 to 99, the second rating from 100 to 119, the third from 120 to 139, the fourth from 140 to 159, and the fifth—and last—from 160 to 179.
In most cases, the ETL process needs to normalize any data contained in a OCCURS section of a mainframe file. Even though it is possible to manually program the ETL process to manage the repeating data, it is strongly recommended that you use a robust ETL tool that allows you to use the COBOL copybooks to define inputs or at least allows you to manually define input file arrays in some other way. If your tools do not support input arrays, you are stuck with the toil of writing code to deal with repeating groups within records sourced from your legacy mainframe systems.
Sometimes programmers use OCCURS to store different facts in an array, rather than storing the same fact N times. For example, suppose O-DATE occurs four times. The first date is CREATE, the second is SHIP, the third is ACKNOWLEDGE, and the fourth is PAYMENT. So in this case you don’t normalize this OCCURS data but rather create discrete fields for each position in the array.
To ensure data integrity, model data that results from a COBOL OCCURS clause in a normalized fashion—a master table and child table—in the staging area of the data warehouse. It’s good practice to stage this data in separate tables because the result of the process most likely loads data into a fact table and a dimension, two separate dimensions, or two separate fact tables. We find that in these situations it makes sense to set data down to settle before integrating it with the data warehouse.
Managing Multiple Mainframe Record Type Files
The concept of multiple record type files is touched upon in the section that discusses REDEFINES. The main difference between REDEFINES as discussed earlier and what we’re introducing now is that instead of having just a small portion of a record contain multiple definitions, the entire record has multiple definitions. Multiple record types are often used to span a single logical record across two or more physical records. Figure below contains an extract of a COBOL copybook that illustrates the concept of redefining an entire record.
Recognizing multiple record types within the same file
In Figure above, the REDEFINES clause applies to the entire record. So now, instead of the file carrying only an employee’s basic information, it also carries an employee’s job history with the company as well. In this file, every employee has at least two records: one EMP-RECORD and one JOBRECORD. When an employee transfers to a new job, a new JOB-RECORD is added to the file. So an employee’s total job history is contained in two or more records on the file: one EMP-RECORD and one or more JOBRECORD( s).
In this file, the physical order of the records is critically important because the JOB-RECORDs do not have any information to link them to their corresponding EMP-RECORDs. The JOB-RECORDs for an employee follow immediately after his or her EMP-RECORD. So to accurately process the job history of an employee, you must treat two or more physically adjacent records as one logical record.
The benefit of using multiple record types is—once again—to save space. The alternative, without using relational theory, is to have extremely wide, space-wasting records to carry all data in a single record. If, for example, you want to track job history for up to five prior positions, you have to add 255 bytes to each employee record (the base EMP-RECORD, plus five occurrences of JOB-RECORD fields (5 × 51 bytes). But the number of job history field segments is situational—it depends on how many jobs an employee has held.
By using multiple record types, the mainframe system can store job history records only as needed, so employees with only one job require only one JOB-RECORD (70 bytes including FILLER), saving 185 bytes on the file. Furthermore, you are no longer limited to a fixed number of jobs in file. An unlimited number of 70-byte JOB-RECORDs can be added for each employee.
Our employee example has only two record types, but multiple REDEFINES can be used to create any number of record types that can combine into a single logical record. If we expand the employee example, you might imagine a third record type to carry information about the employee’s fringe benefits and a fourth type to carry information about the employee’s family dependents.
The ETL process must manage multiple record types by retaining the values from the first physical record in a set—which is only the first part of the logical record—in memory variables so they can be joined to the rest of the data that follows in subsequent records.
Handling Mainframe Variable Record Lengths
In the previous section, we discuss how information related to a single entity is spanned across two or more records using multiple record types. A variable record length is another approach used in mainframe files to store situational information. Rather than storing the job history of an employee in separate JOB-RECORDs, each job is stored in an OCCURS job history segment. Furthermore, as illustrated in Figure below, instead of the record having a fixed number of such segments, the number of segments varies between 0 and 20, based on the numeric value in the DEPENDING ON JOB -HISTORY-COUNT clause. Each additional employee job, up to a maximum of 20, adds 50 bytes to the length of the record.
Variable record lengths in a COBOL copybook using the DEPENDING ON
Variable-length records that use DEPENDING ON clauses in the copybook make straightforward EBCDIC-to-ASCII character-set translation ineffective. The following is a list of ways to mitigate the risk of creating corrupt data from variable-length records during the ETL process.
Extracting from IMS, IDMS, Adabase, and Model 204
If you use any of these systems, you will need special extractors. For starters, there are ODBC gateways to each of these.
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