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Java Forum / General / February 2007IBM-recommended method not thread-safe?
On the following page: http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407... IBM discusses a database "surrogate key" generation function, along with a listing in Java (Appendix A at the end of the web page). However it seems the function is not thread-safe! Suppose for a certain Table X, the SURROGATEKEYVALUE is currently set to 100, and INCREMENT is set to 1. If 2 threads, A and B, enter getSurrogateKey at nearly the same time It is possible for the following sequence to occur: 1. Thread A executes: updateKeyStmt.execute(); As a result, SURROGATEKEYVALUE is now set to 101. 2. Thread B executes: updateKeyStmt.execute(); As a result, SURROGATEKEYVALUE is now set to 102. 3. Thread A executes: ResultSet rs = getKeyStmt.executeQuery(); This will return 102. 4. Thread B executes: ResultSet rs = getKeyStmt.executeQuery(); This will also return 102 ! Am I missing something here? > On the following page: > [quoted text clipped - 29 lines] > > Am I missing something here? You need to use an atomic query that updates the data at the same time as returning the value. "Larry" <larry.grant...@gmail.com> wrote: >> On the following page: >> [quoted text clipped - 29 lines] >> >> Am I missing something here? > You need to use an atomic query that updates the data at the same time > as returning the value. The problem is likely database concurrency rather than thread concurrency. Try wrapping your transactions in COMMIT / ROLLBACK. - Lew > On the following page: > [quoted text clipped - 4 lines] > listing in Java (Appendix A at the end of the web page). However it > seems the function is not thread-safe! > Am I missing something here? No - you are correct. The code is not thread safe as is. To do that getSurrogateKey should be made synchronized to be threadsafe in a single node config and use some database synchronization technique like a transaction with transaction islation level serializable or switch the statements and use SELECT ... FOR UPDATE (I can not remember if DB2 supports that) to be safe in a multi node config. But if you look at the Singleton method, then you get a clear indication that there is something very wrong with that code. Arne > > On the following page: > [quoted text clipped - 9 lines] > > The code is not thread safe as is. So what is it doing on an official IBM site? >>> On the following page: >>> http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407... [quoted text clipped - 8 lines] > > So what is it doing on an official IBM site? I do not know. Maybe the article is written by a DB2 expert not a Java expert. Maybe an early draft of the code got posted instead of the final. Mistakes happen in IBM also. Arne > >>> On the following page: > >>>http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407... [quoted text clipped - 20 lines] > > Arne The feedback page lists Java and EJB among the author's specialties. In any event, I have emailed my concerns to the author and also left feedback on the page. It seems that the page was put up in 2004, so it's still strange that the error is still there. > The feedback page lists Java and EJB among the author's specialties. > In any event, I have emailed my concerns to the author and also left > feedback on the page. It seems that the page was put up in 2004, so > it's still strange that the error is still there. Maybe it's not ;-) When I try to visit: http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407 I get an error: Our apologies... The page you requested cannot be displayed (both today, and when the OP originally mentioned it). -- chris >> The feedback page lists Java and EJB among the author's specialties. >> In any event, I have emailed my concerns to the author and also left [quoted text clipped - 13 lines] > > (both today, and when the OP originally mentioned it). That is just because the URL was cut short in the original post. The correct URL is: http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407zhang/index.html (I hope it goes through without being truncated) Arne > > http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407 [...] > That is just because the URL was cut short in the original post. > > The correct URL is: http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0407zhang/index.html Thank you. But how did you manage to guess the full URL ? Other people seemed to be able to read the article too, but while it's common enough to add a missing .html or index.htm to the end of an URL, in this case the missing bit was zhang/index.html Am I alone in lacking the clairvoyant skills needed to reconstruct it ? -- chris > But how did you manage to guess the full URL ? Other people seemed to be able > to read the article too, but while it's common enough to add a missing .html or [quoted text clipped - 3 lines] > > Am I alone in lacking the clairvoyant skills needed to reconstruct it ? I Googled on: surrogate key +site:ibm.com (the term "surrogate key" were in the original post) Arne > > Am I alone in lacking the clairvoyant skills needed to reconstruct it ? > [quoted text clipped - 3 lines] > > (the term "surrogate key" were in the original post) How disappointing ! You mean you didn't sacrifice even /one/ black goat ? -- chris >>> Am I alone in lacking the clairvoyant skills needed to reconstruct it ? >> I Googled on: >> >> surrogate key +site:ibm.com > > How disappointing ! You mean you didn't sacrifice even /one/ black goat ? I not even crossed my fingers when I hit the Google Search button. Arne [me:] > > How disappointing ! You mean you didn't sacrifice even /one/ black > > goat ? > > I not even crossed my fingers when I hit the Google Search button. :) -- chris Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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