Tutorial: LevelDB

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LevelDB^{View Repo} is a fast key-value storage library written at Google. Think of it like a simple database where you store pieces of data (values) associated with unique names (keys). It’s designed to be very fast for both writing new data and reading existing data, and it reliably stores everything on disk. It uses a log-structured merge-tree (LSM-tree) design to achieve high write performance and manages data in sorted files (SSTables) across different levels for efficient reads and space management.

flowchart TD
    A0["DBImpl"]
    A1["MemTable"]
    A2["Table / SSTable & TableCache"]
    A3["Version & VersionSet"]
    A4["Write-Ahead Log (WAL) & LogWriter/LogReader"]
    A5["Iterator"]
    A6["WriteBatch"]
    A7["Compaction"]
    A8["InternalKey & DBFormat"]
    A0 -- "Manages active/immutable" --> A1
    A0 -- "Uses Cache for reads" --> A2
    A0 -- "Manages DB state" --> A3
    A0 -- "Writes to Log" --> A4
    A0 -- "Applies Batches" --> A6
    A0 -- "Triggers/Runs Compaction" --> A7
    A1 -- "Provides Iterator" --> A5
    A1 -- "Stores Keys Using" --> A8
    A2 -- "Provides Iterator via Cache" --> A5
    A3 -- "References SSTables" --> A2
    A3 -- "Picks Files For" --> A7
    A4 -- "Recovers MemTable From" --> A1
    A4 -- "Contains Batch Data" --> A6
    A5 -- "Parses/Hides InternalKey" --> A8
    A6 -- "Inserts Into" --> A1
    A7 -- "Builds SSTables" --> A2
    A7 -- "Updates Versions Via Edit" --> A3
    A7 -- "Uses Iterator for Merging" --> A5

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Table of contents

• Table, SSTable & TableCache
• MemTable
• Write-Ahead Log (WAL)
• DBImpl
• WriteBatch
• Version & VersionSet
• Iterator
• Compaction
• InternalKey & DBFormat