BadDB Storage Engine

BadDB is a lightweight binary storage engine for Java. It keeps the code small and explainable, while now supporting enough basic CRUD behavior to be usable for a classroom project or an early proof of concept.

Key Features

• Binary on-disk storage using RandomAccessFile
• In-memory B-Tree index for primary-key lookups
• Insert, read, update, delete, and upsert operations
• Equality-based filtering with select(columnName, value)
• Active-record scanning with getAllRecords()
• Convenience helpers like exists, countRecords, and compact
• Automatic index rebuild when reopening a database file
• Automatic upgrade of legacy v1 files to the newer record-marker format

What Changed

BadDB now stores a small active/deleted marker in each record. That makes a few practical features possible without turning the project into a full database engine:

• updateRecord(...) appends the new version of a row and marks the old one as deleted
• deleteRecord(...) hides a row from future reads while keeping the file format simple
• compact() rewrites the file with only active rows, which is useful after many updates or deletes

This keeps the implementation understandable while making the API much more usable.

Core API

Create a table

Schema schema = new Schema();
schema.addColumn("id", DataType.INT);
schema.addColumn("name", DataType.STRING);
schema.addColumn("grade", DataType.FLOAT);
schema.addColumn("is_active", DataType.BOOLEAN);

Table students = new Table("Students", schema, "students.bad");
students.initialize();

Insert

Record row = new Record(schema.getColumnCount());
row.setValue(0, 101);
row.setValue(1, "Alice Smith");
row.setValue(2, 88.5f);
row.setValue(3, true);

students.insertRecord(row);

Query

Record byId = students.searchByPrimaryKey(101);
List<Record> byName = students.select("name", "Alice Smith");
List<Record> all = students.getAllRecords();
boolean exists = students.exists(101);
int count = students.countRecords();

Update

Record updated = new Record(schema.getColumnCount());
updated.setValue(0, 101);
updated.setValue(1, "Alice Smith");
updated.setValue(2, 91.0f);
updated.setValue(3, true);

students.updateRecord(101, updated);

Delete

students.deleteRecord(101);

Upsert

students.upsertRecord(updated);

Compact

students.compact();

Project Structure

File	Description
src/main/java/org/baddb/model/DataType.java	Enum defining supported types (INT, FLOAT, STRING, BOOLEAN).
src/main/java/org/baddb/model/Column.java	Metadata for one schema column.
src/main/java/org/baddb/model/Schema.java	Collection of columns defining a table layout.
src/main/java/org/baddb/model/Record.java	Holds one row of data.
src/main/java/org/baddb/index/BTreeNode.java	B-Tree node implementation.
src/main/java/org/baddb/index/BTree.java	In-memory key-to-offset index.
src/main/java/org/baddb/storage/DatabaseFileManager.java	Low-level binary file read/write layer.
src/main/java/org/baddb/storage/Table.java	High-level CRUD API.
src/main/java/org/baddb/Main.java	Runnable demo showing the current features.

Compile and Run

Prerequisites

• JDK 8 or higher

Compile

javac -d out (Get-ChildItem -Recurse -Filter *.java src/main/java | ForEach-Object { $_.FullName })

Run

java -cp out org.baddb.Main

Documentation

• sdk_guide.md: quick developer-facing usage guide
• code_explanation.md: beginner-friendly implementation walkthrough
• walkthrough.md: project walkthrough notes