subrequests

Subrequests are internal requests initiated by a primary request within a software system to obtain data or perform an operation as part of generating a response. Unlike a separate client request, a subrequest is handled within the same process or server instance and often shares the original request’s context, including headers and authentication state.

In web servers, subrequests are used to compose content from multiple internal sources without exposing intermediate results to the client. A main request may trigger a subrequest to an internal route, and the results are integrated into the final response. Systems such as Nginx provide facilities for internal subrequests to enable dynamic content assembly, conditional logic, and content filtering. Apache and other servers offer similar mechanisms through internal redirects or subrequest features.

Key characteristics include potential synchronous or asynchronous execution, inheritance of variables and headers, and configurable timeouts or caching behavior. Subrequests can be nested, may share or override context, and are subject to server limits. Error handling determines whether a failure in a subrequest aborts the parent operation or permits partial results.

Performance considerations are important: subrequests add overhead and can increase latency, especially with deep nesting. They may complicate debugging and resource management, so they are often used for aggregating content or implementing conditional processing rather than for every request. Security and reliability concerns include protecting internal endpoints, limiting data exposure, and ensuring proper permission checks and observability.

Common use cases involve assembling a page from multiple internal sources, while alternatives include external API orchestration, batch requests, or client-side composition via APIs such as GraphQL.