4 ms·

I have to admit that I only recently got familiar with logs. I was designing a B+Tree[1] in Python for fun and was struggling to make it survive crashes: a sing

I have to admit that I only recently got familiar with logs. I was designing a B+Tree[1] in Python for fun and was struggling to make it survive crashes: a single insertion in a tree often results in multiple page writes which is not atomic. The solution to this problem is simple and elegant with a Write-Ahead Log. Every page write is appended to the log and only merged back into the tree file when it's sure that the log is safely written to storage. SQLite has an extensive documentation of its WAL file format, which is great for learning. [1] https://github.com/NicolasLM/bplustree https://github.com/NicolasLM/bplustree [2] https://www.sqlite.org/fileformat.html#the_write_ahead_log https://www.sqlite.org/fileformat.html#the_write_ahead_log

rakoo 9y ago
If you're interested in those, there are at least two other designs you should have a look at: - couchdb uses a single file for each db, which means the write ahead log _is_ the storage. The atomicity is guaranteed by saying that the latest root is the valid root. If writes are interrupted, everything since the last root is invalid and discarded upon restart. A simple design that just works, although it tends to be wasteful and requires frequent compactions - lmdb (https://en.m.wikipedia.org/wiki/Lightning_Memory-Mapped_Database https://en.m.wikipedia.org/wiki/Lightning_Memory-Mapped_Data...) uses copy on write to make sure space is properly used, and atomicity is provided by sharing only the strict minimal pieces of information, so small in fact atomicity is guaranteed by the os. Follow the different links in the wikipedia page, there's a lot of interesting stuff