HTTP has a concept of conditional requests, where the result, and even the success of a request, can be controlled by comparing the affected resources with a validator.
These requests are useful for validating cached content, ensuring that it is only fetched if it differs from the copy that is already available to the browser.
Conditional requests are also useful for ensuring the integrity of a document when resuming a download, or preventing lost updates when uploading or modifying a document on the server....Skip to search Skip to select language Article Actions English (US)...(US) Remember language Deutsch Español Français 日本語 한국어 Русский...