Re: computational model of transactions

Since nobody else has mentioned it, I think I should raise a flag about one
of your ideas. See below.
"Marshall" <marshall.spight@xxxxxxxxx> wrote in message
news:1154452230.243215.174250@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I've been thinking about concurrency and transactions. It occurs to me
that much of the difficulty lies with the fact that multiple concurrent
transactions may operate on the same data. I begin to recall some
conversations from years past about "multiple assignment".

It seems to me that much or maybe all of the difficulty with multiple
concurrent transactions operating on the same data would be
eliminated if it wasn't possible for a transaction to read back
the results of its own writes.

In other words, consider the model in which transactions can only
observe the database state that was current at the time the
transaction started.


SQL Server 2005 provides this capability, it's called READ COMMITTED
SNAPSHOT isolation. It accomplishes it by accessing the previous states of
the rows that have been changed in the transaction log. The mechanism is
similar to that used by the Volume Shadow Copy service in Windows 2003 and
many of the open file backup options that have been available for years.

The benefit of this mechanism is that it provides the same level of
protection for the issuing transaction as READ COMMITTED without applying
any shared locks. Fewer locks means less blocking and fewer deadlocks.
Fewer locks means better overall performance. But it's not a silver bullet.
There are times when it is definitely contraindicated. The danger is that
DBAs and developers will use it without fully understanding the
ramifications of using it.

This should never be used in a posting routine because the information read
out is already stale even before the read is completed, so a subsequent
update based on that information would introduce garbage into the database.
It is inevitable that overzealous DBAs and developers will use this to boost
performance and end up with corrupt databases.

There are other arguments against using this in any procedure that issues
updates, but it would take more time and space to give it the attention
needed than is available or warranted in a single post.

So, how much of a burden, at the *logical* level, would this be?
Clearly it is not the same as with SQL transactions. Does it
matter? Is there a use case I'm not thinking of that makes this
problematic? I will admit there have been times where I've
opened up an interactive SQL session, started a transaction,
and typed a whole series of DML, and observed the results
along the way, but I don't think I've ever written *code* that
does anything like that.

Your experiences and thoughts are appreciated.


Marshall



.

Relevant Pages

  • Re: Single database transaction across a multiple HTTP requests?
    ... Either this transaction is poorly designed, or you entire systems of stored ... You will need to change one or the other until you obtain a design that will ... > database engine. ... It is normal practice to issue the locks manually just before performing the ...
    (comp.lang.php)
  • Re: Help - Timing Logic
    ... server application, both of which ran on the same box. ... the client applications and 'lodging' them in the database. ... Another part of the server application was dedicated to retrieving messages ... commit transaction ...
    (microsoft.public.dotnet.languages.vb)
  • Re: Help - Timing Logic
    ... put a trigger on the database table. ... Transaction and Database Locking - look at isolation levels / settings ... Maybe implement a message broker ... ...
    (microsoft.public.dotnet.languages.vb)
  • Re: Help - Timing Logic
    ... build a trigger on the database .. ... Transaction and Database Locking - look at isolation levels / settings ... If you need to implement a locking mechanism / or / logging mechanism / or / a checking mechanism to avoid duplicate messages caused by multi-threading ... ... message broker gets all the necessary messages or message id's from the database to be sent ... ...
    (microsoft.public.dotnet.languages.vb)
  • Re: Simple transaction question ????
    ... in SQL data base through store procedure or function but then sounds more ... 1- I am not used to the SQL syntax in SQL server side, ... 2- Before storing those data in other database table, ... if you have to manage transaction that spawns a single database then ...
    (microsoft.public.dotnet.framework.adonet)
Quantcast