9.2 Levels of Multithread Support

Different DBTools.h++ classes provide different levels of multithread support. Application programs must ensure that DBTools.h++ classes are used appropriately, based on the safety level they provide. The multithread support levels available in DBTools.h++ are:

MT-safe level 2. This is the highest level of multithread support. When an object provides MT-safe level 2 support, application programs can share the object among threads without performing explicit locking around operations. MT-safe level 2 classes contain enough internal locking to maintain their own internal integrity and use appropriate multithread-safe system calls. RWDBDatabase is the only class in DBTools.h++ that provides MT-safe level 2 support.

MT-safe level 1. This is the medium level of multithread support. When an object provides MT-safe level 1 support, application programs can share the object among threads, but they need to perform explicit locking around operations.

Currently RWDBConnection is the only database-dependent class in DBTools.h++ that provides MT Safe level 1 support.

In addition to RWDBConnection, all the database-independent classes that abstract concepts such as SQL expressions, SQL datatypes, SQL phrases, and debugging classes can be used as MT-safe level 1 classes. A list of these classes appears in Table 11, and they are discussed in Section 9.5.

MT-safe level 0. This is the lowest level of multithread support. MT-safe level 0 classes cannot be shared among threads. All the classes in Tools.h++ and DBTools.h++ other than the ones shown in Table 11 should be considered MT-safe level 0 classes.

The DBTools.h++ classes that fall into each level are summarized below. Table 11 lists all MT-safe level 1 and level 2 classes. If a class does not appear there, you should assume that its multithreading support level is MT-safe level 0.

Table 11 -- MT-safe classes in DBTools.h++

MT-safe level	DBTools.h++ classes
level 2	*RWDBDatabase*
level 1	*RWDBAssignment*
	*RWDBBlob*
	*RWDBBoundExpr*
	*RWDBCollectableExpr*
	*RWDBColumn*
	*RWDBConnection*
	*RWDBCriterion*
	*RWDBCritFormDefinition*
	*RWDBCritFuncDef{n}*
	*RWDBDateTime*
	*RWDecimalPortable*
	*RWDBDuration*
	*RWDBExpr*
	*RWDBExprFormDefinition*
	*RWDBExprFuncDef{n}*
	*RWDBForeignKey*
	*RWDBForeignKeyList*
	*RWDBJoinExpr*
	*RWDBNullIndicator*
	*RWDBPhraseBook*
	*RWDBRow*
	*RWDBSchema*
	*RWDBStatus*
	*RWDBTracer*
	*RWDBValue*
level 0	All classes not listed above.

You will find the acquire() and release() functions normally associated with multithreading in the APIs of many MT-safe level 0 classes. Do not use these functions at this time because they slow down performance. We have included the acquire() and release() functions only for the purpose of making classes multithread-ready. These functions will be available if and when the class is promoted to MT-safe level 1. Release notes will provide information on classes that are promoted to a higher multithread-safety level.