database/sqlite
1
0
Fork 0
mirror of https://github.com/sqlite/sqlite.git synced 2025-08-08 14:02:16 +03:00
Code Activity

Enhance the wal2 header comment in wal.c to explain how the wal-hook is

Browse Source 
invoked in wal2 mode.

FossilOrigin-Name: 118aa7e32a94ad971a955ab60db5bfc5b6241f93734a41ba83ab72ea71bc9aaa
This commit is contained in:
dan
2018-12-22 15:50:00 +00:00
parent c5140b39ce
commit 665f3d209b
3 changed files with 32 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
src/wal.c
View File

@@ -306,6 +306,31 @@
** The first wal file takes the same name as the wal file in legacy wal
** mode systems - "<db>-wal". The second is named "<db>-wal2".
**
** CHECKPOINTS
**
** The "pre-configured size" mentioned above is the value set by
** "PRAGMA journal_size_limit". Or, if journal_size_limit is not set,
** 1000 pages.
**
** There is only a single type of checkpoint in wal2 mode (no "truncate",
** "restart" etc.), and it always checkpoints the entire contents of a single
** wal file. A wal file cannot be checkpointed until after a writer has written
** the first transaction into the other wal file and all readers are reading a
** snapshot that includes at least one transaction from the other wal file.
**
** The wal-hook, if one is registered, is invoked after a write-transaction
** is committed, just as it is in legacy wal mode. The integer parameter
** passed to the wal-hook is the total number of uncheckpointed frames in both
** wal files. Except, the parameter is set to zero if there is no frames
** that may be checkpointed. This happens in two scenarios:
**
** 1. The "other" wal file (the one that the writer did not just append to)
** is completely empty, or
**
** 2. The "other" wal file (the one that the writer did not just append to)
** has already been checkpointed.
**
**
** WAL FILE FORMAT
**
** The file format used for each wal file in wal2 mode is the same as for