CORS and metrics

• CORS
• Metrics

CORS

Browsers restrict cross-origin requests unless the server opts in with the appropriate Access-Control-Allow-Origin header. Presto handles this at the router level: pass an allowedOrigin when you initialize the router with RestRouter.init.

var router = RestRouter.init(
  validate, allowedOrigin = some("https://app.example.com"))

When allowedOrigin is set, Presto:

• automatically registers an OPTIONS handler for every route you add, so CORS preflight requests are answered without any extra code;
• adds Access-Control-Allow-Origin to responses when the request's Origin matches the configured value.

The matching rules are:

• allowedOrigin = some("*") allows any origin and echoes * back.
• Otherwise the request Origin must match the configured value. A matching response also sets Vary: Origin to avoid cache poisoning. The configured value may be given with or without an http:// / https:// scheme.
• A request carrying more than one Origin header is rejected with 400 Bad Request.

Metrics

Presto can expose operational metrics through the nim-metrics library. Metrics collection is a compile-time feature: build with -d:metrics to enable it. Without that flag, all the metrics machinery compiles away to nothing.

Server metrics

The server can record, per endpoint:

• the number of responses, labelled by HTTP status;
• the time taken to prepare each response.

It also maintains global counters for processed, missing (404), and invalid (400) requests.

Per-route recording is opt-in through the metrics variants of the routing macros. Pass a set of RestServerMetricsType values (Status, Response, or the combined RestServerMetrics) to metricsApi:

router.metricsApi(MethodGet, "/items/{id}",
                  {RestServerMetricsType.Status}) do (
    id: int) -> RestApiResponse:
  RestApiResponse.response("item")

# record both status counts and response timing
router.metricsApi(MethodGet, "/report", RestServerMetrics) do () -> RestApiResponse:
  RestApiResponse.response("ok")

A rawMetricsApi variant exists for raw handlers, mirroring rawApi.

Client metrics

On the client, metrics are enabled per procedure with the metrics pragma. The client can record DNS resolution time, connection time, request time, response time, and response status, each labelled by an endpoint name.

# use the endpoint path as the metric label
proc getItem(id: int): RestPlainResponse {.
     rest, endpoint: "/items/{id}", metrics.}

# override the label
proc getItem2(id: int): RestPlainResponse {.
     rest, endpoint: "/items/{id}", metrics: "items_by_id".}

# select which metrics to collect
proc getItem3(id: int): RestPlainResponse {.
     rest, endpoint: "/items/{id}", metrics,
     metricsTypes: {RestClientMetricsType.ResponseTime,
                    RestClientMetricsType.Status}.}

The available RestClientMetricsType values are ResolveTime, ConnectTime, RequestTime, ResponseTime, and Status; RestClientMetricsAllTypes is the full set and is used when metricsTypes is omitted.