From Ingres Community Wiki

Ingres Architecture Overview

Overview of the Ingres Source Code

• Source Code Architecture
• Ingres Catalogs
• Adding New Functions to the Ingres Relational Database Management System

Common Infrastructure Specifications

Components shared across the Ingres Product Family

• Ingres Compatability Library (CL)
• Abstract Data Facility
• General Communications API (GCA)/General Communications Facility(GCF) API External Interface Specification

RDBMS Specifications

Ingres Database

• A Short Excursion Through The Ingres Back-End
• Query Execution Facility
• Optimizer Facility
• R-Tree: A Geospatial Index for Ingres
• High Availability Option for Windows Clusters

Protocol Specifications

• GCM Specification - Underlying Management Protocol for Ingres Management Facility (IMF)
• Data Access Messaging Protocol Specification - Protocol used between JDBC/.NET DP and Data Access Server (DAS)

Additional Information

• Additional Ingres Specifications available here.
• Still Confused about the Ingres Architecture - ask at Contributor's Forum