On Tue, Sep 21, 2010 at 3:25 PM, Beman Dawes <bdawes@acm.org> wrote:
How do SQLlite or other libraries provide safety against another thread crashing? Do they actually stop the other threads until the b-tree modification completes, have some form of recovery, or some other approach? Do they have protection against another process corrupting the B-tree?
I believe this is how it's done: 1) Put any changes into new or free pages -- don't ever overwrite any old data. 2) Write a list of "free" page indexes -- the indexes of the header and ones that (1) replaced. 3) Fsync. 4) Write a new header to the end of the file. 5) Fsync. If you start an operation and a valid header isn't found at the end of the file, just read in reverse until you find one. This necessitates extra code to encapsulate multiple operations in transactions, because fsync is expensive. You still need to maintain a reader-writer lock on top of the file. -- Cory Nelson http://int64.org