Abstractions, Interfaces, and the Narrow Waist

Interfaces as liberation

An interface is a contract that liberates application writers from low-level details. It represents an abstraction of the resources and services used by applications. In an ideal world, implementations can change without requiring changes to applications.

In practice, this ideal is often undermined by:

• Performance concerns: a naive abstraction may perform poorly on a particular implementation, forcing the application writer to become aware of what lies beneath.
• Mission creep: the interface specification grows over time, adding features that tie applications to specific implementations.
• Specification change: the interface itself is revised, breaking existing applications.

Standard interfaces are everywhere

The narrow waist pattern recurs in many engineered systems:

System	Interface	What sits above	What sits below
National electricity grid	230 V, 50 Hz socket	Appliances	Power stations, renewables, interconnectors
Landline telephone	RJ11 jack, dial tone	Telephones, fax machines, modems	Exchanges, trunks, fibre, microwave links
Money	Currency units, prices	Buyers, sellers, contracts	Central banks, commercial banks, ledgers
TCP/IP	Sockets, ports	Applications	Ethernet, Wi-Fi, 4G, fibre

In each case, the interface decouples the consumers of a service from the providers. New providers can enter below the interface; new consumers can enter above it.

The DBMS logical arrangement

Applications interact only with the DBMS. The DBMS uses the operating system’s file system, which in turn uses device drivers to access physical storage. Some high-performance DBMSs bypass the file system and manage raw storage devices directly.

Storage interface primitives

Primary storage (RAM) provides a simple word-level interface:

write(address, word)
read(address)

Secondary storage (disk/SSD) provides a block-level interface:

write(blkaddress, block)
read(blkaddress) → option(block)
trim(blkaddress)
sync()

• read returns an option: the block might not exist (unlike RAM, where every address always contains something).
• trim tells the device that a block is no longer needed (used by SSDs for garbage collection).
• sync flushes buffered writes to persistent media.

Storage configurations

Configuration	Where data lives	Persistence
In-core	Primary memory (RAM)	Ephemeral — lost on process exit or power loss
Serialised to filesystem	Files managed by the OS	Persistent across restarts
Direct device access	Raw secondary storage, bypassing the file system	Persistent, with full DBMS control over layout

Most general-purpose DBMSs use the serialised-to-filesystem configuration. In-core databases (e.g., Redis) are used where latency is critical and the dataset fits in RAM.

Summary

• An interface is an abstraction that decouples consumers from providers.
• The narrow waist model appears in many engineered systems, not just databases.
• The DBMS sits between user applications and the file system/device drivers.
• Secondary storage differs from primary storage in providing block-level access with optional read results.