# Accelerating reverse proxy for dynamic APIs

This design serves teams whose API responses are personalized or transactional and cannot be cached. The application bypasses the cache on those paths, so every call reaches the origin through a connector. Application Accelerator unlocks that bypass and the `OPTIONS`, `POST`, `PUT`, `PATCH`, and `DELETE` methods an API needs. The cacheable paths of the same site keep their cache rules. The design implements the use case [Accelerate websites and APIs with a CDN](/en/documentation/use-cases/improve-performance-and-reliability/accelerate-websites-and-apis-with-a-cdn/).

## Architecture diagram

```mermaid
%%{init: {"layout": "dagre", "themeVariables": {"fontSize": "13px"}, "flowchart": {"nodeSpacing": 12, "rankSpacing": 12, "padding": 6, "wrappingWidth": 70, "minNodeWidth": 40, "useMaxWidth": true}}}%%
flowchart TD
  Client["Client"] -->|"request"| App["application rules"]
  AA["Application Accelerator"] -->|"unlocks Bypass Cache and the methods beyond GET and HEAD"| App
  App -->|"dynamic path: Bypass Cache"| Conn["connector"]
  App -->|"cacheable path: Set Cache Policy"| Cache["Cache"]
  Cache -->|"miss"| Conn
  Conn -->|"every dynamic request, on a keep-alive connection where possible"| Origin["origin"]
  App -.->|"request time and cache status"| RTM["Real-Time Metrics"]
```

The diagram splits one site at the rules of the application. A request on a dynamic path skips the cache and reaches the origin through the connector. The origin's response time is therefore part of every response on that path. A request on a cacheable path tries Cache first and reaches the same connector only on a miss. Nothing on a dynamic path is stored, so that path has no TTL to set and nothing to purge. It also has no copy to serve when the origin fails, so [Keep an application online when an origin fails](/en/documentation/use-cases/improve-performance-and-reliability/keep-an-application-online-when-an-origin-fails/) adds a standby origin behind the same connector to keep those paths answering. The decisions are which paths bypass the cache and how the connector reaches the origin.

### Dataflow

1. A client sends a request to the domain of a workload, and the workload's deployment names the application.
2. The application runs its Request Phase rules in order. A rule that matches a dynamic path, such as `/api/`, carries the **Bypass Cache** behavior.
3. **Bypass Cache** sends the request to the origin through the connector that a **Set Connector** rule names, and Azion stores no copy of the response. A response to a request sent with `Pragma: azion-debug-cache` reports `BYPASS` in its `x-cache` header.
4. The connector reaches the origin with the protocol its settings decide. Where possible, the bypassed request travels on a keep-alive connection to the origin.
5. A request on a cacheable path takes the cache setting that a **Set Cache Policy** rule applies. Cache answers from a valid copy, and a miss reaches the origin through the same connector.
6. Real-Time Metrics records every request to the domain. On a dynamic path, the request time always includes the time the origin takes to answer.

## Components

- **application**: the Platform Resource that holds the per-path rules. A Request Phase rule with **Bypass Cache** matches each dynamic path, and **Set Cache Policy** rules apply cache settings to the other paths. A **Max Age** of `0` does not replace the bypass. It merges parallel requests for one object into one origin request, so two simultaneous callers receive the same answer.
- **Application Accelerator**: the Product that lets the application accept `OPTIONS`, `POST`, `PUT`, `PATCH`, and `DELETE`, beyond the `GET` and `HEAD` it accepts natively. It also unlocks **Bypass Cache**. A request that **Bypass Cache** sends to the origin keeps protocol optimizations and, where possible, a keep-alive connection to it. Its switch belongs to the application and is off on a new one.
- **connector**: the Platform Resource that carries the origin's address and shapes each request to it, for every dynamic request and every cache miss. Its **Transport Protocol Policy** sets the protocol to the origin: forcing HTTPS protects that leg and costs a TLS handshake on every new connection. Without Load Balancer, the connection's timeouts follow platform defaults and are not settings.
- **Cache**: stores the responses of the paths that stay cacheable, under the cache setting each path's rule applies. **Bypass Cache** does not reach the Tiered Cache layer. An application whose cache settings have Tiered Cache on keeps caching objects there for the minimum TTL.
- **Real-Time Metrics**: shows how long the application takes to answer a request, filtered to the domain. The **Requests** dashboard charts **Average Request Time**, and a filter on **Upstream Cache Status** set to `BYPASS` keeps only the bypassed requests. In the GraphQL API, the `upstreamResponseTime` field of [`workloadMetrics`](/en/documentation/devtools/graphql/gql-real-time-metrics-fields/#workloadmetrics) sums the time Azion takes to receive the origin's responses.

## Implementation

- [Accelerate websites and APIs with a CDN](/en/documentation/use-cases/improve-performance-and-reliability/accelerate-websites-and-apis-with-a-cdn/) - builds the caching reverse proxy for a site and its API, bypasses the cache on `/api/`, and compares that design with this one.
- [Configure cache policies for an application](/en/documentation/guides/application-performance/cache-and-purge/cache-settings/) - turns on Application Accelerator, then bypasses the cache for one path or for several paths and a session cookie.
- [Create a cache setting](/en/documentation/guides/application-performance/cache-and-purge/tune-cache-settings/) - creates the cache setting of the cacheable paths and the rule that applies it.
- [Applications quickstart](/en/documentation/platform/applications/quickstart/) - creates the connector to your origin, the application, the rule that sends every request to the connector, and the workload that serves it.
- [Add a custom domain to a workload](/en/documentation/guides/platform/migration/configure-a-domain/) - lists your domain on the workload and points the domain at it.
- [Check the cache status of a response](/en/documentation/guides/application-performance/cache-and-purge/check-page-cache-time/) - reads the `x-cache` header, which reports `BYPASS` on a dynamic path and the cache status on the other paths.

## Related resources

- [Cache variation](/en/documentation/platform/applications/application-accelerator/cache-variation.md#bypass-cache-and-a-ttl-of-0): How Bypass Cache differs from a Max Age of 0, and what each one costs the origin.
- [Application Accelerator settings](/en/documentation/platform/applications/application-accelerator/settings.md#http-methods): The methods and the Rules Engine behaviors that Application Accelerator adds to an application.
- [How Connectors works](/en/documentation/platform/connectors/how-it-works.md#connection-reuse-and-timeouts): The timeout and idle defaults of the connection from Azion to the origin, and when they become settings.
- [Build dashboards](/en/documentation/platform/real-time-metrics/build-dashboards.md#requests): What Average Request Time and the other charts of the Requests dashboard measure.