- #Database deadlock exception how to#
- #Database deadlock exception software#
- #Database deadlock exception code#
#Database deadlock exception code#
By the time that a database deadlock is resolved, the damage to the performance of the DCA is done, since rolling back transactions, issuing exceptions inside the DCA, and executing defensive code within these exception handlers to retry these transactions incur a significant performance penalty.
#Database deadlock exception how to#
Unfortunately, this solution is only partially effective even though it is widely used as part of the defensive programming practice, where programmers should write special database deadlock exception-handling code that should repeat aborted transactions - searching for ``database deadlock exception'' on the Web yields close to 2,500 web pages, many of which instruct programmers how to handle database deadlock exceptions for different databases. Doing so effectively resolves the database deadlock, but this resolution causes performance degradation, since DCAs should repeat the rolled back transactions to ensure functional correctness. That is, once a deadlock occurs, the database rolls back one of the transactions that is involved in the circular wait. Currently, database deadlocks are typically detected within database engines using special algorithms that analyze whether transactions hold resources in cyclic dependencies, and these database engines resolve database deadlocks by forcibly breaking the hold-and-wait cycle.
#Database deadlock exception software#
In enterprise systems, database deadlocks may appear when a new transaction is issued by a DCA to a database that is already used by some other legacy DCA, thus making the process of software evolution error-prone, expensive, and difficult. A condition for observing database deadlock is that a database should simultaneously service two or more transactions that come from one or more DCAs, and these transactions contain SQL statements that share the same resources (e.g., tables or rows). Database deadlocks occur within database engines and not within DCAs that use these databases. In general, deadlocks occur when two or more threads of execution lock some resources and wait on other resources in a circular chain, i.e., in a hold-and-wait cycle. The responsibility of database engines is to provide layers of abstractions to guarantee Atomicity, Consistency, Isolation, and Durability (ACID) properties however, these guarantees do not include freedom from database deadlocks. When DCAs use the same database at the same time, concurrency errors are observed frequently, and these errors are known as database deadlocks, which is one of the reasons for major performance degradation in these applications. The Problem of Database Deadlocks Many organizations and companies deploy Database-centric applications (DCAs), which use databases by sending transactions to them - atomic units of work that contain SQL statements - and obtaining data that result from execution of these SQL statements. The entire code, results, and video can be obtained from here. Our tool prevented all existing database deadlocks in these applications and increased their throughputs by approximately up to three orders of magnitude.
We build a tool that implements our approach and we experimented with three applications. For a realistic case of over 1,000 SQL statements, all hold-and-wait cycles are detected in less than 15 seconds. To prevent database deadlocks automatically, we created a novel approach and we rigorously evaluated it. Since applications often share the same databases concurrently, database deadlocks routinely occur in these databases resulting in major performance degradation in these applications. Summary Many organizations deploy applications that use databases by sending Structured Query Language (SQL) statements to them and obtaining data that result from executions of these statements.
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