Merge pull request #12 from pdoerner/complete-before-start-headers

Add Nexus-Operation-Id and Nexus-Link headers to callback definition

Author: pdoerner

Diff for: SPEC.md

@@ -103,7 +103,7 @@ timeout for a single HTTP request. Format of this header value is number + unit,
 milliseconds, `s` for seconds, and `m` for minutes.
 
 The `Nexus-Link` header field can be added to associate resources with the start request. A handler may attach these
-links as metadata to underlying resources to provide end-to-end observabililty. See the [`Nexus-Link`](#nexus-link)
+links as metadata to underlying resources to provide end-to-end observability. See the [`Nexus-Link`](#nexus-link)
 section for more information.
 
 #### Request Body
@@ -306,6 +306,10 @@ For invoking a callback URL:
 - Issue a POST request to the caller-provided URL.
 - Include any callback headers supplied in the originating StartOperation request, stripping away the `Nexus-Callback-`
   prefix.
+- Include the `Nexus-Operation-Id` header, `Nexus-Operation-Start-Time` and any `Nexus-Link` headers for resources
+  associated with this operation to support completing asynchronous operations before the response to StartOperation is
+  received. `Nexus-Operation-Start-Time` should be in a valid HTTP format described [here](https://www.rfc-editor.org/rfc/rfc5322.html#section-3.3).
+  If is omitted, the time the completion is received will be used as operation start time.
 - Include the `Nexus-Operation-State` header.
 - If state is `succeeded`, deliver non-empty results in the body with corresponding `Content-*` headers.
 - If state is `failed` or `canceled`, content type should be `application/json` and the body must have a serialized