Docs: Adds basic frontend data request concepts (#28253)

* Docs: Adds frontend data request docs

* Update contribute/architecture/frontend-data-requests.md

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>

* Update contribute/architecture/frontend-data-requests.md

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>

* Update contribute/architecture/frontend-data-requests.md

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>

* Update contribute/architecture/frontend-data-requests.md

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>

* Docs: changes after PR comments

* Docs: changes after PR comments

* Docs: changes after PR comments

* Update contribute/architecture/frontend-data-requests.md

Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>
Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>
Co-authored-by: achatterjee-grafana <34888589+ashishagarwal06@users.noreply.github.com>

contribute/architecture/README.md

@@ -8,3 +8,6 @@ Learn more about the backend architecture:
 - Part 2: [Communication](communication.md)
 - Part 3: [Database](database.md)
 
+Learn more about the frontend architecture:
+- Part 1: [Data requests](frontend-data-requests.md)
+

contribute/architecture/frontend-data-requests.md

+# Data requests
+
+[BackendSrv](https://grafana.com/docs/grafana/latest/packages_api/runtime/backendsrv) handles all outgoing HTTP requests from Grafana. This document explains the high-level concepts used by `BackendSrv`.
+
+## Canceling requests
+This section describes how canceling requests work in Grafana. While data sources can implement their own cancellation concept, we recommend that you use the method we describe here.
+
+A data request can take a long time to finish. During the time between when a request starts and finishes, the user can change context. For example, the user may navigate away or issue the same request again.
+
+If we wait for canceled requests to complete, it might create unnecessary load on data sources.
+
+Grafana uses a concept called _request cancelation_ to cancel any ongoing request that Grafana doesn't need.
+
+#### Before Grafana 7.2
+Before Grafana can cancel any data request, it has to identify that request. Grafana identifies a request using the property `requestId` [passed as options](https://github.com/grafana/grafana/blob/master/docs/sources/packages_api/runtime/backendsrvrequest.md) when you use [BackendSrv](https://grafana.com/docs/grafana/latest/packages_api/runtime/backendsrv).
+
+The cancellation logic is as follows:
+- When an ongoing request discovers that an additional request with the same `requestId` has started, then Grafana will cancel the ongoing request.
+- When an ongoing request discovers that the special "cancel all requests" `requestId` was sent, then Grafana will cancel the ongoing request.
+
+#### After Grafana 7.2
+Grafana 7.2 introduced an additional way of canceling requests using [RxJs](https://github.com/ReactiveX/rxjs). To support the new cancellation functionality, the data source needs to use the new `fetch` function in [BackendSrv](https://grafana.com/docs/grafana/latest/packages_api/runtime/backendsrv).
+
+Migrating the core data sources to the new `fetch` function [is an ongoing process that you can read about in this issue.](https://github.com/grafana/grafana/issues/27222)
+
+## Request queue
+Depending on how the web browser implements the protocol for HTTP 1.1, it will limit the number of parallel requests, lets call this limit _max_parallel_browser_request_. 
+
+Unless you have configured Grafana to use HTTP2, the browser limits parallel data requests according to the browser's implementation. For more information on how to enable HTTP2, refer to [Configuration](https://grafana.com/docs/grafana/latest/administration/configuration/#protocol).
+
+Because there is a _max_parallel_browser_request_ limit, if some of the requests take a long time, they will block later requests and make interacting with Grafana very slow.
+
+#### Before Grafana 7.2
+Not supported. 
+
+#### After Grafana 7.2
+Grafana uses a _request queue_ to process all incoming data requests in order while reserving a free "spot" for any requests to the Grafana API. 
+
+Since the first implementation of the request queue doesn't take into account what browser the user uses, the _request queue_ limit for parallel data source requests is hard-coded to 5.
+
+> **Note:** Grafana instances [configured with HTTP2 ](https://grafana.com/docs/grafana/latest/administration/configuration/#protocol) will have a hard coded limit of 1000.