Glossary

Trace and span

A trace connects an operation’s path; a span describes a segment with timing and context. Verify context propagation between processes or threads rather than merely logging an identifier.

Transaction

A database boundary: changes commit together or roll back. Isolation and constraints determine concurrent safety. An SQL transaction cannot undo a sent HTTP request.

Transaction isolation

Rules governing visibility and interaction of concurrent transactions. MVCC uses data versions; the isolation level determines allowed anomalies.

Transmission Control Protocol

TCP

Transmission Control Protocol

A transport providing an ordered byte stream. A disconnected connection does not reveal whether the business operation committed.

Trust boundary

Where data or authority arrives from another control scope, such as client input or a vendor response. Typing does not remove validation or authorization requirements.

Truthiness

How a condition interprets a value. Python treats zero, None and empty collections as false, although their business meanings can differ. Absence checks should follow the contract.

Type narrowing

Refining a possible type after a check such as typeof in TypeScript. A runtime check provides evidence; an as assertion does not validate data.

TypedDict

A description of expected dictionary keys and types for a Python type checker. It does not generate JSON validation or a domain-object class. Parse and validate network data separately at runtime.

TypeScript

TS

TypeScript

A language extending JavaScript with static type information. Types do not automatically validate incoming JSON at runtime.

TypeScript AI SDK

An AI SDK library for TypeScript application AI features. An optional product dependency, not a replacement for data/cost/quality controls.

TypeScript non-null assertion

A postfix ! tells the TypeScript checker to treat an expression as neither null nor undefined. It adds no runtime validation. Use it only when the code invariant is justified, such as an index staying inside array bounds.

Unicode and text encoding

UTF-8

Unicode Transformation Format, 8-bit

UTF8

Unicode Transformation Format, 8-bit

UTF-16

Unicode Transformation Format, 16-bit

ASCII

American Standard Code for Information Interchange

Unicode describes characters; encodings such as UTF-8 represent them as bytes. Character count need not equal byte count; ASCII covers only a limited character set.

Uniqueness

UNIQUE is a SQL keyword, not an abbreviation.

A database-defined guarantee against duplicate keys. A preceding SELECT alone does not provide it during concurrent INSERTs.

Unit of Work

UoW

Unit of Work

Coordinates object changes and their persistence within one scope. It is not synonymous with an SQL transaction: the database provides guarantees an in-memory copy cannot prove.

Unit test

A check of a small behaviour unit under controlled conditions. Its scope follows the behavioural boundary, not a requirement to mock every dependency.

URL

URL

Uniform Resource Locator

An address identifying a resource and access mechanism. Path, query and fragment have different roles; a user-provided URL is not automatically a safe server-request destination.

User Interface

UI

User Interface

The views, controls and messages through which a person operates a system. A backend should provide a clear error contract without coupling domain rules to a particular screen.

UTC

UTC

Coordinated Universal Time

A common time reference for unambiguous instants. User time zones serve presentation and local rules; a local date string alone is not an instant.

UUID

UUID

Universally Unique Identifier

A standardized 128-bit identifier. Its version affects generation and properties; an unguessable identifier does not replace authorization.

uv

A Python project, dependency and interpreter manager. This centre uses setup to create the pinned .venv rather than global application dependencies.

Uvicorn

An ASGI server running Python applications such as FastAPI. Server operation is distinct from application validation and business logic.

Value object

An object defined by its values rather than separate identity. Money with currency is an example; equality and validation should follow domain rules.

Variable

A name bound to a value or reference. Scope determines where it is accessible; reassignment differs from mutation of the referenced object.

Virtual machine

VM

Virtual Machine

A virtual computer running a guest operating system with resources allocated from its host. Containers share the kernel of the environment they run in; Linux containers on macOS or Windows generally need an additional Linux layer.

Virtual thread pinning

A situation where a virtual thread cannot unmount from its carrier platform thread while blocking. On JDK 21, blocking inside synchronized is one cause; JDK 24 removed that cause. Check the actual JVM version and remaining constraints instead of applying old advice without measuring.

Virtual threads

VT

Virtual Thread

Lightweight JVM-managed threads useful for many tasks waiting on I/O. They do not increase database capacity or accelerate CPU computation itself.

Vitest

A JavaScript/TypeScript test runner executing cases and reporting assertions. It does not itself replace type checking.

VPC

VPC

Virtual Private Cloud

A logically isolated cloud network. Design subnets, routing and traffic rules together; the label private does not prove absence of external access.

VPC endpoint

A private access path to a supported service. Endpoint type affects routing, DNS and policies. Interface endpoints use PrivateLink; not all endpoints use the same mechanism.

Webhook

An HTTP call notifying a recipient of an event in another system. Design signatures, retries, ordering and safe handling of late messages.