学步园

1．What are isolation levels?

Isolation levels provide a degree of control of the effects one transaction can have on another concurrent transaction.

Table 1

Isolation levels are not part of the EJB specification. They can only be set on the resource manager either explicitly on the Connection (for bean managed persistence) or via the application server specific configuration.

The EJB specification indicates that isolation level is part of the Resource Manager.As the transaction isolation level increases, likely performance degradation follows, as additional locks are required to protect data integrity. If the underlying data does not require such a high degree of integrity, the isolation level can be lowered to improve performance.

2．How will you control two concurrent transactions accessing a database?

You can use isolation levels. An isolation level defines how concurrent transactions accessing a relational database are isolated from each other for read purpose. These isolation levels can prevent one or more of the phenomena that happen during concurrent transactions:

• Dirty reads: A transaction reads uncommitted changes from another transaction.

• Nonrepeatable reads: A transaction reads a row in a database table, a second transaction changes the same row and the first transaction re-reads the row and gets a different value.

• Phantom reads: A transaction executes a query, returning a set of rows that satisfies a search condition and a second transaction inserts another row and the first re-executes the same query and get an additional record returned.

Table 2

You should use a highest possible isolation level that gives acceptable performance. It is basically a tradeoff between data integrity and performance. For example the isolation level “TRANSACTION_SERIALIZABLE” attribute guarantees the highest level of data integrity but adversely affects performance because even simple reads must wait in line.