*** Throughout this document, a page is deemed to have been synced automatically as soon as it is written when PRAGMA synchronous=OFF. Otherwise, the page is not synced until the xSync method of the VFS is called successfully on the file containing the page. *** Definition: A page of the database file is said to be "overwriteable" if one or more of the following are true about the page: (a) The original content of the page as it was at the beginning of the transaction has been written into the rollback journal and synced. (b) The page was a freelist leaf page at the start of the transaction. (c) The page number is greater than the largest page that existed in the database file at the start of the transaction. (1) A page of the database file is never overwritten unless one of the following are true: (a) The page and all other pages on the same sector are overwriteable. (b) The atomic page write optimization is enabled, and the entire transaction other than the update of the transaction sequence number consists of a single page change. (2) The content of a page written into the rollback journal exactly matches both the content in the database when the rollback journal was written and the content in the database at the beginning of the current transaction. (3) Writes to the database file are an integer multiple of the page size in length and are aligned to a page boundary. (4) Reads from the database file are either aligned on a page boundary and an integer multiple of the page size in length or are taken from the first 100 bytes of the database file. (5) All writes to the database file are synced prior to the rollback journal being deleted, truncated, or zeroed. (6) If a master journal file is used, then all writes to the database file are synced prior to the master journal being deleted. *** Definition: Two databases (or the same database at two points it time) are said to be "logically equivalent" if they give the same answer to all queries. Note in particular the content of freelist leaf pages can be changed arbitarily without effecting the logical equivalence of the database. (7) At any time, if any subset, including the empty set and the total set, of the unsynced changes to a rollback journal are removed and the journal is rolled back, the resulting database file will be logical equivalent to the database file at the beginning of the transaction. (8) When a transaction is rolled back, the xTruncate method of the VFS is called to restore the database file to the same size it was at the beginning of the transaction. (In some VFSes, the xTruncate method is a no-op, but that does not change the fact the SQLite will invoke it.) (9) Whenever the database file is modified, at least one bit in the range of bytes from 24 through 39 inclusive will be changed prior to releasing the EXCLUSIVE lock. (10) The pattern of bits in bytes 24 through 39 shall not repeat in less than one billion transactions. (11) A database file is well-formed at the beginning and at the conclusion of every transaction. (12) An EXCLUSIVE lock must be held on the database file before making any changes to the database file. (13) A SHARED lock must be held on the database file before reading any content out of the database file.