1 *** Throughout this document, a page is deemed to have been synced 2 automatically as soon as it is written when PRAGMA synchronous=OFF. 3 Otherwise, the page is not synced until the xSync method of the VFS 4 is called successfully on the file containing the page. 5 6 *** Definition: A page of the database file is said to be "overwriteable" if 7 one or more of the following are true about the page: 8 9 (a) The original content of the page as it was at the beginning of 10 the transaction has been written into the rollback journal and 11 synced. 12 13 (b) The page was a freelist leaf page at the start of the transaction. 14 15 (c) The page number is greater than the largest page that existed in 16 the database file at the start of the transaction. 17 18 (1) A page of the database file is never overwritten unless one of the 19 following are true: 20 21 (a) The page and all other pages on the same sector are overwriteable. 22 23 (b) The atomic page write optimization is enabled, and the entire 24 transaction other than the update of the transaction sequence 25 number consists of a single page change. 26 27 (2) The content of a page written into the rollback journal exactly matches 28 both the content in the database when the rollback journal was written 29 and the content in the database at the beginning of the current 30 transaction. 31 32 (3) Writes to the database file are an integer multiple of the page size 33 in length and are aligned to a page boundary. 34 35 (4) Reads from the database file are either aligned on a page boundary and 36 an integer multiple of the page size in length or are taken from the 37 first 100 bytes of the database file. 38 39 (5) All writes to the database file are synced prior to the rollback journal 40 being deleted, truncated, or zeroed. 41 42 (6) If a master journal file is used, then all writes to the database file 43 are synced prior to the master journal being deleted. 44 45 *** Definition: Two databases (or the same database at two points it time) 46 are said to be "logically equivalent" if they give the same answer to 47 all queries. Note in particular the the content of freelist leaf 48 pages can be changed arbitarily without effecting the logical equivalence 49 of the database. 50 51 (7) At any time, if any subset, including the empty set and the total set, 52 of the unsynced changes to a rollback journal are removed and the 53 journal is rolled back, the resulting database file will be logical 54 equivalent to the database file at the beginning of the transaction. 55 56 (8) When a transaction is rolled back, the xTruncate method of the VFS 57 is called to restore the database file to the same size it was at 58 the beginning of the transaction. (In some VFSes, the xTruncate 59 method is a no-op, but that does not change the fact the SQLite will 60 invoke it.) 61 62 (9) Whenever the database file is modified, at least one bit in the range 63 of bytes from 24 through 39 inclusive will be changed prior to releasing 64 the EXCLUSIVE lock. 65 66 (10) The pattern of bits in bytes 24 through 39 shall not repeat in less 67 than one billion transactions. 68 69 (11) A database file is well-formed at the beginning and at the conclusion 70 of every transaction. 71 72 (12) An EXCLUSIVE lock must be held on the database file before making 73 any changes to the database file. 74 75 (13) A SHARED lock must be held on the database file before reading any 76 content out of the database file. 77
