With regard to HSQLDB, one thing that it doesn't have as a project that SQLite has is the documentation of a robust testing suite and online documentation of assiduous ACID compliance.
I don't mean to take anything away from HSQLDB. It's meant to serve as an alternative to MySQL not to fopen() as SQLite is intended. One can say that the scope of HSQLDB (all the Java RDBMS's really) is much more ambiitious. Fredt and his group have accomplished an extraordinary achievement with HSQLDB. Even so, doing the Google search "Is HSQLDB ACID compliant" doesn't leave an early adopter feeling as confident as one feels after reading about the testing harnesses on the SQLite website.
At http://sqlite.org/transactional.html
"SQLite is Transactional
A transactional database is one in which all changes and queries appear to be Atomic, Consistent, Isolated, and Durable (ACID). SQLite implements serializable transactions that are atomic, consistent, isolated, and durable, even if the transaction is interrupted by a program crash, an operating system crash, or a power failure to the computer.
We here restate and amplify the previous sentence for emphasis: All changes within a single transaction in SQLite either occur completely or not at all, even if the act of writing the change out to the disk is interrupted by
- a program crash,
- an operating system crash, or
- a power failure.
The claim of the previous paragraph is extensively checked in the SQLite regression test suite using a special test harness that simulates the effects on a database file of operating system crashes and power failures."
At http://sqlite.org/testing.html
"1.0 Introduction
The reliability and robustness of SQLite is achieved in part by thorough and careful testing.
As of version 3.7.14, the SQLite library consists of approximately 81.3 KSLOC of C code. (KSLOC means thousands of "Source Lines Of Code" or, in other words, lines of code excluding blank lines and comments.) By comparison, the project has 1124 times as much test code and test scripts - 91421.1 KSLOC.
1.1 Executive Summary
Three independently developed test harnesses
100% branch test coverage in an as-deployed configuration
Millions and millions of test cases
Out-of-memory tests
I/O error tests
Crash and power loss tests
Fuzz tests
Boundary value tests
Disabled optimization tests
Regression tests
Malformed database tests
Extensive use of assert() and run-time checks
Valgrind analysis
Signed-integer overflow checks"
