Question

Concurrent transactions is especially important for applications that has thousands or millions of users. Can you imagine how the database works for Facebook? Today I'm focusing on IBM. Below is a link to IBM knowledge center that describes concurrent transactions. Please read this guide and explain how it has helped your understanding of concurrent transactions. https://www.ibm.com/support/knowledgecenter/SSEPGG_9.7.0/com.ibm.db2.luw.apdv.embed.doc/doc/c0006107.html

Answer 1

Concurrent transactions occur when there are multiple users accessing or trying to update same piece of information. In the example of IBM, we talk about contexts which can be exchanged between threads of a same process.
Contexts may be exchanged between threads in a process, but not exchanged between processes. Multiple contexts provide support for concurrent transactions.
To control concurrency it is extremely important that a value commits and aborts performing the operations it was supposed to perform.

Concurrency control is needed to avoid discrepancies, if transactions are executed serially, i.e., sequentially with no overlap in time, no transaction concurrency exists. However, if concurrent transactions with interleaving operations are allowed in an uncontrolled manner, some unexpected, undesirable result may occur like the lost update problem,the dirty read problem and so on.
The IBM example implements threaded applications against a DB2 database, serialization of access to the database is enforced by the database APIs. If one thread performs a database call, calls made by other threads will be blocked until the first call completes, even if the subsequent calls access database objects that are unrelated to the first call. In addition, all threads within a process share a commit scope.
concurrent access is or can be achieved by use of multiple contexts that gain thread safe, concurrent database access on platforms that support multi threading.