File: gdbm.info,  Node: Database consistency,  Next: Recovery,  Prev: Errors,  Up: Top

15 Database consistency
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In the chapters that follow we will cover different aspects of "database
consistency" and ways to maintain it.  Speaking about consistency, it is
important to distinguish between two different aspects of it: structural
and logical consistency.

   "Structural consistency" means that all internal structures of the
database are in good order, contain valid data and are coherent with one
another.  Structural consistency means that the database is in good
shape "technically", but it does not imply that the data it contains are
in any way meaningful.

   "Logical consistency" means that the data stored in the database are
coherent with respect to the application logic.  Usually this implies
that structural consistency is observed as well.

   For as long as the program is free from memory management errors and
each opened database is properly closed before the program terminates,
structural consistency is maintained.  Maintaining logical consistency
is more complex task and its maintenance is entirely the responsibility
of the application programmer.  *Note Crash Tolerance::, for a detailed
discussion.

   Both consistency aspects can suffer as a result of both application
errors that cause the program to terminate prematurely without properly
saving the database, and hardware errors, such as disk failures or power
outages.  When such situations occur, it becomes necessary to "recover
the database".

   In the next chapter we will discuss how to recover structural
consistency of a database.