# Service Workers * Quite a powerful concept, in particular the offline ability ○ Store site/app data locally so that whatever has been downloaded so far can still be access if you go off line. Potentially and changes you make offline can be retained and uploaded way you get access back - but this can of course have some synchronisation issues! * Push notifications can be sent by the server, if a browser/app registers itself as a client * The service worker can also communicate with the webpage/app using messages. The SW and page/app run as separate entities as a security measure, but the webpage can register with the SW as a client (each tab can do this individually) and then they can communicate via messages. There is a Subscription in the RP code to manage receiving SW messages * If you change the sw\.js code, need to close browser and start again otherwise the old service worker will hang around and stop the new one from controlling the page. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://johnkazer.gitbook.io/core-web-dev-concepts/service-workers.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.