Thursday, August 22, 2013

NoSQL Gets Me Thinking About ACID

This week I attended the NoSQL Now Conference in San Jose, California. This conference focused on NoSQL technology and implementations and a LOT of the discussion focused on transactions and whether or not Big Data and NoSQL databases were at a disadvantage when it comes to their lack of support for ACID (mostly).

You can read all about my experience at this conference on my Data Technology Today blog in the following two posts:

At any rate, though, I got to thinking... and those of you who know me understand that that can be a dangerous thing. Basically, ACID is not a topic that relational folks sit around talking about. It is kind of taken for granted. So I thought it might be a good idea to reinforce the definition of ACID and why it is so important in DB2... and the relational world in general.

ACID is an acronym for atomicity, consistency, isolation, and durability. Each of these four qualities is necessary for a transaction to be designed correctly and deliver data integrity when complete:
  • Atomicity means that a transaction must exhibit “all or nothing” behavior. Either all of the instructions within the transaction happen, or none of them happen. Atomicity preserves the “completeness” of the business process.
  • Consistency refers to the state of the data both before and after the transaction is executed. A transaction maintains the consistency of the state of the data. In other words, after running a transaction, all data in the database is “correct.”
  • Isolation means that transactions can run at the same time. Any transactions running in parallel have the illusion that there is no concurrency. In other words, it appears that the system is running only a single transaction at a time. No other concurrent transaction has visibility to the uncommitted database modifications made by any other transactions. To achieve isolation, a locking mechanism is required.
  • Durability refers to the impact of an outage or failure on a running transaction. A durable transaction will not impact the state of data if the transaction ends abnormally. The data will survive any failures.

Let’s use an example to better understand the importance of ACID transactions to relational database applications. Consider a banking application. Assume that you wish to withdraw $50 from your account with your bank. This business process requires a transaction to be executed. You request the money either in person by handing a slip to a bank teller or by using an ATM. When the bank receives the request, it performs the following tasks, which make up the complete business process. The bank will

  1. Check your account to make sure you have the necessary funds to withdraw the requested amount.
  2. If you do not, deny the request and stop; otherwise continue processing.
  3. Debit the requested amount from your checking account.
  4. Produce a receipt for the transaction.
  5. Deliver the requested amount and the receipt to you.

The transaction that is run to perform the withdrawal must complete all of these steps, or none of these steps, or else one of the parties in the transaction will be dissatisfied. If the bank debits your account but does not give you your money, then you will not be satisfied. If the bank gives you the money but does not debit the account, the bank will be unhappy. Only the completion of every one of these steps results in a “complete business process.” Database developers must understand the requisite business processes and design transactions that ensure ACID properties.

Unit of work (UOW) is another transaction term that describes a physical transaction. A UOW is a series of instructions and messages that, when executed, guarantees data integrity. So a UOW and a transaction are similar in concept. However, a UOW is not necessarily a complete business process—it can be a subset of the business process, and a group of units of work can constitute a single transaction. Each UOW must possess ACID characteristics. In other words, if the transaction were to fail, the state of the data upon failure must be consistent in terms of the business requirements.

To summarize, a transaction—when executed alone, on a consistent database—will either complete, producing correct results, or terminate, with no effect. In either case the resulting condition of the database will be a consistent state.

Hopefully after this discussion it is simple to see why relational databases—with many related tables—rely on ACID properties of transactions to maintain consistency. Of course, the NoSQL world has different use cases and, arguably, can get by with eventual consistency... that is, without ACID. But that is another topic that is, frankly, beyond the scope of this DB2/mainframe-focused blog. 

Hopefully, though, this review of ACID and its importance to data consistency was helpful.

Sunday, August 11, 2013

The Top Ten DB2 Development Best Practices

If you have been reading my blogs lately you know that I have been posting Top Ten lists of various types, and today's post offers up yet another one. This time, the list provides guidance for DB2 developers to keep in mind as they build DB2 applications...

1. Minimize network calls

The fewer number of times you need to make calls across the network the better your program will perform.

2. Minimize passes through the data

Try to read each data item only once. The more times you access the same data the worse performance will be.

3. Put the work into the SQL, not the program

Let SQL do the work... DB2 can optimize SQL better than you can optimize your programming language of choice.

4. Unlearn the “flat file” mentality

SQL is a set-based language. Each SQL statement can operate on multiple rows of data at once. And joins are more efficient than opening multiple cursors and performing "master file" processing logic on them.

5. Be sure data type and length match in predicates

Even though DB2 no longer automatically degrades non-matching predicates to Stage 2, it is still a best practice to match the data type and length for columns and host variables that participate in predicates.

6. Know your Stage 1, Stage 2, and Indexable predicates

...and avoid Stages 3 and 4!

7. Document your code

Without proper documentation application maintenance becomes difficult... especially if you used any SQL tuning tricks/techniques.

8. Always check the SQLCODE or SQLSTATE

If you don't check for an error you may be processing with bad, incorrect, or missing data.

9. Analyze your access paths (and tune your SQL in test)

If you do not examine the access paths formulated by the DB2 optimizer for your SQL then how do you know how efficient (or not) your code is (or will be).

10. Avoid Bachelor Programming Syndrome

Be sure to code COMMITs in your application programs

Monday, August 05, 2013

COPYTOCOPY: The Forgotten Little DB2 Utility?

The COPYTOCOPY utility was introduced some time ago now, way back in DB2 for z/OS Version 7. The purpose of the COPYTOCOPY utility is to make additional image copies of currently existing image copy data sets. But many DBAs "out there" keep pluggiong away, almost unaware that COPYTOCOPY even exists. So let's spend a little time talking about it today.

The primary benefit of COPYTOCOPY is to reduce the amount of time required to run the COPY utility. Remember that the COPY utility can be used to take up to four image copies with a single execution of the utility. But with COPYTOCOPY available, instead of using COPY to make four image copy backups, the DBA can use COPY to make a single image copy, and then run COPYTOCOPY to make additional image copies. The COPY utility will take less time to create a single image copy backup than it will to take multiple image copy backups. And the combination of COPY plus COPYTOCOPY can, at times perhaps, be used to increase availability.

Individual data and index partitions are treated as distinct target objects by the COPYTOCOPY utility. Any other utilities operating on different partitions of the same table space or index space can be run concurrently with COPYTOCOPY.

The following utilities can not be run concurrently on the same database object as the COPYTOCOPY utility:
  •        COPY
  •        LOAD
  •        MERGECOPY
  •        MODIFY
  •        RECOVER
  •        REORG INDEX

Furthermore, COPYTOCOPY is flexible enough to run against any DB2 image copy data set. This includes inline copies made during the execution of the REORG and LOAD utilities. COPYTOCOPY must start with a primary image copy backup – either the local primary or recovery site primary copy. From that image copy, the COPYTOCOPY utility can make up to three copies of one or more of the following types:
  •        local primary
  •        local backup
  •        recovery site primary
  •        recovery site backup

Copies created by COPYTOCOPY can be used by the RECOVER utility just like regular image copies created using the COPY utility. Both table space and index space copies can be made using the COPYTOCOPY utility. Any DB2 utility process that uses image copy data sets can use the image copy data sets created by COPYTOCOPY. This includes MERGECOPY, UNLOAD, and subsequent runs of COPYTOCOPY. However, keep in mind that image copies created with the CONCURRENT option of the COPY utility are not supported by the COPYTOCOPY utility.

Just like the COPY utility, the COPYTOCOPY utility records information about the image copies that it creates in the SYSIBM.SYSCOPY system catalog table. The COPYTOCOPY utility will insert the values in the DSNAME, GROUP_MEMBER, JOBNAME, AUTHID, DSVOLSER and DEVTYPE columns as appropriate depending on the copies that are being created.

You cannot run COPYTOCOPY to create additional image copies for certain DB2 Catalog (SYSCOPY in DSNDB06) and DB2 Directory (DSNDB01 and SYSUTILX both in DSNDB01) objects.

The COPYTOCOPY utility operates in these distinct phases:
  1.        UTILINIT – Initialization and setup
  2.        CPY2CPY – Copying the image copy
  3.        UTILTERM – Cleanup

TERM and Restart Issues

The use of the TERM command to terminate a COPYTOCOPY step that has abended is not recommended. A current restart should be done instead to allow COPYTOCOPY to pickup where it left off. Terminating COPYTOCOPY in such a situation might cause inconsistencies between the ICF catalog and DB2 catalog when GDGs are used.

You cannot use RESTART(PHASE) for a COPYTOCOPY job. It is fine to use RESTART(CURRENT) as long as you avoid using the -TERM UTILITY command to terminate a COPYTOCOPY step. When you use RESTART(CURRENT), COPYTOCOPY will restart from the last commit point with the same image copy data set, so be sure to code a data set disposition of DISP=(MOD,CATLG,CATLG) on your JCL DD statements.

Inline Copy Exception

When using COPYTOCOPY to copy an inline image copy that was made by the REORG utility with the part range option you will need to specify individual DSNUM for the partitions to be copied. The COPYTOCOPY utility does not support part range. COPYTOCOPY will copy only the specified partition data from the input inline image copy data set into the output image copy data set.


To run the COPYTOCOPY utility it is not necessary to provide the explicit data set name of the image copy being copied. Instead, the input to the COPYTOCOPY utility is the name of the table space, index space, or index for which the original copy was made, and an indication of which image copy in the catalog should be copied. To specify this information COPYTOCOPY provides three options:

  1.         FROMLASTCOPY – indicates that the most recent image copy taken for the table space or index space is to be used as input to the COPYTOCOPY utility. The input could be either a full image copy or incremental copy. The utility will retrieve the information from the SYSIBM.SYSCOPY system catalog table.
  2.         FROMLASTFULLCOPY – indicates that the most recent full image copy taken for the object is to be used as the input to COPYTOCOPY job. Once again, this information is obtained by querying the DB2 Catalog.
  3.      FROMLASTINCRCOPY – indicates that the most recent incremental image copy taken for the object is to be used as the input to COPYTOCOPY job. FROMLASTINCRCOPY is not valid for index spaces or indexes. If FROMLASTINCRCOPY is specified for an index space or index, COPYTOCOPY will use the last full image copy that was taken for the index, if one is available. And once again, this information is obtained by querying the DB2 Catalog.

Of course, you may choose instead to specify the data set name for the image copy that is to be copied by the COPYTOCOPY utility. This can be accomplished by using the FROMCOPY clause. But keep in mind that when you are using COPYTOCOPY with a list of objects defined using the LISTDEF statement, the FROMCOPY clause is not valid.

If the FROMCOPY keyword is not used the COPYTOCOPY utility must determine which specific image copy is to be copied. Before COPYTOCOPY can execute it may have to choose between the local site primary copy, local site backup copy, recovery site primary copy, and recovery site backup copy data sets. COPYTOCOPY will search image copies in the following order to determine the input data set to be used:
  •      If you are running COPYTOCOPY at your local site, the search order will be (1) local site primary copy, (2) local site backup copy, (3) recovery site primary copy, (4) recovery site backup copy.
  •       If you are running the utility at your recovery site, the search order will be (1) recovery site primary copy, (2) recovery site backup copy, (3) local site primary copy, then finally (4) local site backup copy.

If the input data set cannot be allocated or opened, the COPYTOCOPY utility will try to use the next image copy data with the same START_RBA value in SYSIBM.SYSCOPY column, in the search order as indicated previously. When the FROMCOPY keyword is used though, only the explicitly specified data set can be used as the input to COPYTOCOPY.

An Example of the COPYTOCOPY Utility

Let’s take a quick look at a sample JCL job step to run the COPYTOCOPY utility. The following code can be run to make a backup local image copy of the table space DSN8S71E in the sample DB2 database DSN8D71A. This will be either a full or incremental image copy, whichever was last run for this object:

//         UTPROC='',
//         SYSTEM='DB2T',DB2LEV=DB2A
//SYSIN    DD *
//         SPACE=(CYL,(15,1)),DISP=(NEW,CATLG,CATLG)
//SYSIN    DD *


Bottom Line

The COPYTOCOPY utility provides a useful new feature to the toolkit of DB2 DBAs. Using COPYTOCOPY to create additional image copies from existing image copies can enhance availability and assist DBAs in creating an optimal backup and recovery plan for their DB2 applications and databases.