From 54254e97b9c00260e5d5e590c9c9bc4d09cbfb08 Mon Sep 17 00:00:00 2001 From: Ben Johnson Date: Sat, 24 Oct 2015 13:31:18 -0600 Subject: [PATCH] README: add 'Reading the Source' section This commit adds a section to help people understand the source better while reading it. --- README.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/README.md b/README.md index 0a33ebc..6babdd3 100644 --- a/README.md +++ b/README.md @@ -633,6 +633,56 @@ Here are a few things to note when evaluating and using Bolt: [page-allocation]: https://github.com/boltdb/bolt/issues/308#issuecomment-74811638 +## Reading the Source + +Bolt is a relatively small code base (<3KLOC) for an embedded, serializable, +transactional key/value database so it can be a good starting point for people +interested in how databases work. + +The best places to start are the main entry points into Bolt: + +- `Open()` - Initializes the reference to the database. It's responsible for + creating the database if it doesn't exist, obtaining an exclusive lock on the + file, reading the meta pages, & memory-mapping the file. + +- `DB.Begin()` - Starts a read-only or read-write transaction depending on the + value of the `writable` argument. This requires briefly obtaining the "meta" + lock to keep track of open transactions. Only one read-write transaction can + exist at a time so the "rwlock" is acquired during the life of a read-write + transaction. + +- `Bucket.Put()` - Writes a key/value pair into a bucket. After validating the + arguments, a cursor is used to traverse the B+tree to the page and position + where they key & value will be written. Once the position is found, the bucket + materializes the underlying page and the page's parent pages into memory as + "nodes". These nodes are where mutations occur during read-write transactions. + These changes get flushed to disk during commit. + +- `Bucket.Get()` - Retrieves a key/value pair from a bucket. This uses a cursor + to move to the page & position of a key/value pair. During a read-only + transaction, the key and value data is returned as a direct reference to the + underlying mmap file so there's no allocation overhead. For read-write + transactions, this data may reference the mmap file or one of the in-memory + node values. + +- `Cursor` - This object is simply for traversing the B+tree of on-disk pages + or in-memory nodes. It can seek to a specific key, move to the first or last + value, or it can move forward or backward. The cursor handles the movement up + and down the B+tree transparently to the end user. + +- `Tx.Commit()` - Converts the in-memory dirty nodes and the list of free pages + into pages to be written to disk. Writing to disk then occurs in two phases. + First, the dirty pages are written to disk and an `fsync()` occurs. Second, a + new meta page with an incremented transaction ID is written and another + `fsync()` occurs. This two phase write ensures that partially written data + pages are ignored in the event of a crash since the meta page pointing to them + is never written. Partially written meta pages are invalidated because they + are written with a checksum. + +If you have additional notes that could be helpful for others, please submit +them via pull request. + + ## Other Projects Using Bolt Below is a list of public, open source projects that use Bolt: