contextlocal

Contextlocal is a programming concept describing storage whose values are local to a particular execution context and not shared across different contexts. A context may correspond to a thread, an asynchronous task, or a logical request in a web application. The defining property is that code running within the same context can read and write the contextlocal data, while code running in a different context sees a separate copy.

Contextlocal is typically implemented using context-local storage mechanisms provided by a language or framework. In thread-based environments, this resembles thread-local storage, where each thread has its own separate values. In asynchronous or coroutine-based environments, context-local storage must propagate across await points and task boundaries, often via a context or task-local storage abstraction. Some libraries offer a Local-like proxy that presents a single global-looking API while routing reads and writes to the correct per-context storage.

Common uses include storing data associated with a web request, such as the current user, request ID, or database session; implementing tracing or correlation identifiers that should be visible only within the current request or task; and maintaining per-task configuration without passing these values through call graphs.

Potential challenges include ensuring proper lifecycle management to avoid data leaks across requests or tasks, dealing with propagation rules in asynchronous workflows, and the performance overhead of maintaining per-context state. Contextlocal is a general pattern rather than a single standard, with various language-specific implementations. It complements or substitutes for thread-local storage where tasks or coroutines require isolation of data across concurrent execution paths.