Roaring B-tree Format

The RBF format represents a Roaring bitmap whose containers are stored in the leafs of a b-tree. This allows the bitmap to be efficiently queried & updated.

File Format

The RBF file is divided into equal 8KB pages. Each page after the meta page is numbered incrementally from 1 to 2^31.

Pages can be one of the following types:

Meta page: contains header information.
Branch page: contains pointers to lower branch & leaf pages.
Leaf page: contains array and RLE container data.
Bitmap page: contains bitmap container data.

All integer values are little endian encoded.

Page header

Every page type except the bitmap page contains the following header:

[4] page number
[4] flags (indicates the type of the page)

Meta page

The meta page contains the following header:

[4]  magic (\xFFRBF)
[4]  flags
[4]  page count
[8]  wal ID
[4]  root records pgno
[4]  freelist pgno

Root Records page

A list of all b-tree names & their respective root page numbers are stored in root record pages. Once a bitmap root is created, it is never moved so the root record pages only need to be rewritten when creating, renaming, or deleting a b-tree. If records exceed the size of a page then they are overflowed to additional pages.

[4] page number
[4] flags
[4] overflow pgno
[*] bitmap records

Each bitmap record is represented as:

[4] pgno
[2] name size
[*] name

All bitmap records are loaded into memory when the file is opened.

Branch page

The branch page contains the following header:

[4] page number
[4] flags
[2] cell count
[*] cell index (2 * cell count)
[*] padding for 4-byte alignment

Each cell is formatted as:

[8] highbits
[4] flags
[4] page number

Leaf page

The leaf page contains the following header:

[4] page number
[4] flags
[2] cell count
[*] cell index (2 * cell count)

The leaf page contains a series of cells with the header of:

[8] highbits
[4] flag
[4] child count
[*] array or RLE data or Handle (a pageno) to Bitmap Data

Bitmap Data page

The data for the bitmap data page takes up the entire 8KB.

Proof of Concept Notes

The following are notes made that are temporary for the RBF format. This will change as development progresses:

Transaction support is deferred
WAL support is deferred

Pattern of branch splits when adding data in ascending sorted order.

In this example, fan-out is restricted to 2 to make the drawings easy and the splits obvious. The data leaves are only allowed one roaring.Container key (ckey) in this example.

Each frame adds the next datum: A,B,C,D,E,... in order.

The letter represent data leaves, while numbers represent branch pages. The one exception is the first frame where the root is a leaf with data. Every frame after has normal branch at the root.

NB: there are only three places pages get written on addition: a) writeRoot b) putLeafCell c) putBranchCells

add A: root 3 A

add B: root 3 4 5 A B

add C: this sequence of updates occurs

putLeafCell writes leaf B to page 5
putLeafCell writes leaf C to page 6
putBranchCells writes branch page 7 with children 4,5
putBranchCells writes branch page 8 with child 6
writeRoot writes branch cells to pgno 3, children: 7,8

root 3 7 8 4 5 6 A B C

add D: root
3 7 8 4 5 6 9 A B C D

gernest/rbf