You might have seen the recent news reports about the collision between U.S. and Russian communication satellites. The U.S. satellite was one of the Iridium satellites. What wasn’t reported and you probably don’t know is that an object database management system (ODBMS) is an important part of the Iridium system. Even though ODBMSs are a [...]
February 13, 2009
I am now also posting on the Cutter Blog. My initial posting is (The Acronym) SOA is (Perhaps) Dead (at Some Companies); Long Live Services. It is a response to Anne Thomas Manes’ SOA is Dead; Long Live Services on her blog at the Burton Group.
January 9, 2009
The typical definition of an atomic task or process is one that cannot be decomposed further. This is vague and subject to interpretation. The Decomposition Matrix on this site uses a specific definition: A task (for business process diagrams) or a process (for data flow diagrams) is atomic if every input relates to every output [...]
December 3, 2008
My last posting referenced the criteria for a well-formed business process diagram mentioned in Business Process Driven SOA using BPMN and BPEL by Matjaz B. Juric and Kapil Pant. I am going to expand on their criteria to create a more comprehensive definition of a well-formed business process diagram. To start, here are three criteria [...]
November 18, 2008
I recently received two new books on business process modeling. Both books looked interesting because they had great titles. As it turns out, one book is great and the other not so good. The not so good book is Business Process Driven SOA using BPMN and BPEL by Matjaz B. Juric and Kapil Pant. There [...]
October 9, 2008
The Design Decomposition Blog is written by Doug Barry.
The isolation portion of the ACID properties is needed when there are concurrent transactions.
Concurrent transactions are transactions that occur at the same time, such as shared multiple users accessing shared objects. This situation is illustrated at the top of
the figure as activities occurring over time. The safeguards used by a DBMS to prevent conflicts between concurrent transactions are a concept referred to as isolation.
As an example, if two people are updating the same catalog item, it's not acceptable for one person's changes to be "clobbered" when the second person saves a different set of changes. Both users should be able to work in isolation, working as though he or she is the only user.
Each set of changes must be isolated from those of the other users.
An important concept to understanding isolation through transactions is
serializability. Transactions are serializable when the effect on the database is the same whether the transactions are executed in serial order or in an interleaved fashion. As you can see at the top of
the figure, Transactions 1 through Transaction 3 are executing concurrently over time. The effect on the DBMS is that the transactions may execute in serial order based on consistency and isolation requirements. If you look at the bottom of
the figure, you can see several ways in which these transactions may execute. It is important to note that a serialized execution does not imply the first transactions will automatically be the ones that will terminate before other transactions in the serial order.
Degrees of isolation¹:
degree 0 - a transaction does not overwrite data updated by another user or process ("dirty data") of other transactions
degree 1 - degree 0 plus a transaction does not commit any writes until it completes all its writes (until the end of transaction)
degree 2 - degree 1 plus a transaction does not read dirty data from other transactions
degree 3 - degree 2 plus other transactions do not dirty data read by a transaction before the transaction commits
¹ These were originally described as degrees of consistency by Jim
Gray.
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