> For the complete documentation index, see [llms.txt](https://docs.onside.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.onside.io/sdk/core-concepts.md). # Core Concepts - [Authentication & User Account](https://docs.onside.io/sdk/core-concepts/authentication.md): How OnsideKit handles authentication: explicit and on-demand login, standard and external login flows, reading session state, and logging out. - [External Login](https://docs.onside.io/sdk/core-concepts/external-login.md): Sign in with your app's account identifier, configure fallback login, and handle the purchase restrictions for external profiles. - [The Payment Queue & Transactions](https://docs.onside.io/sdk/core-concepts/payment-queue.md): How the OnsidePaymentQueue processes transactions, why a transaction observer is required, the transaction lifecycle, and why finishing transactions is mandatory. - [Threading & Object Lifetime](https://docs.onside.io/sdk/core-concepts/threading-and-retention.md): OnsideKit's main-actor threading model and the lifetime rules for request objects, observers, and delegates. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://docs.onside.io/sdk/core-concepts.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. 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.