docs(postserveaction): document remote mode for long-lived post-serve actions (#1219)

Add sections explaining the difference between local (subprocess per request)
and remote (long-lived HTTP server) execution modes, including:
- When to use each mode and why local does not scale under high load
- Step-by-step guide: writing the server, payload format, configuring
  Hoverfly via flag and hoverctl, verifying registration via admin API
- Minimal Python server example with inline comments
- Docker Compose example with service hostname

Closes #1188

Author: lobovit

docs/pages/keyconcepts/postserveaction.rst

@@ -39,4 +39,152 @@ Ways to register a Post Serve Action
     }
 
 
+Choosing Between Local and Remote Execution
+--------------------------------------------
+
+Hoverfly supports two execution modes for post-serve actions. Understanding the
+difference is important when running under significant load.
+
+**Local execution**
+
+Hoverfly forks a new subprocess for every request that triggers the action. The
+binary or script is re-executed from scratch each time:
+
+.. code::
+
+    hoverfly -post-serve-action "my-action python3 /path/to/script.py 0"
+
+This is the easiest option to get started, but it does not scale. At high request
+rates (e.g. 40+ rps with a Python script), you will quickly accumulate dozens of
+concurrent processes — each with its own interpreter startup cost, memory footprint,
+and open connections. This commonly leads to OOMKilled in containerised environments.
+
+**Remote execution (recommended for high throughput)**
+
+Instead of forking a subprocess, Hoverfly makes an HTTP ``POST`` request to a
+server that you run separately. That server stays alive indefinitely — only one
+process, shared across all requests:
+
+.. code::
+
+    hoverfly -post-serve-action "my-action http://localhost:8080/trigger 0"
+
+This means you can use async frameworks such as ``aiohttp`` or ``FastAPI`` in their
+natural form: one running event loop handling many concurrent webhooks efficiently,
+without paying the startup cost on every request.
+
+Running a Remote Post Serve Action: Step by Step
+--------------------------------------------------
+
+**Step 1 — Write your server**
+
+Your server must accept HTTP ``POST`` requests and return ``HTTP 200``. Hoverfly
+considers any other status code a failure and logs an error. Here is a minimal
+Python example:
+
+.. code:: python
+
+    # server.py
+    from http.server import BaseHTTPRequestHandler, HTTPServer
+    import json
+
+    class Handler(BaseHTTPRequestHandler):
+        def do_POST(self):
+            length = int(self.headers.get("Content-Length", 0))
+            payload = json.loads(self.rfile.read(length))
+
+            # payload contains two keys: "request" and "response"
+            # use them to implement your webhook/callback logic
+            print(f"[action] path={payload['request']['path']}")
+
+            self.send_response(200)
+            self.end_headers()
+
+        def log_message(self, format, *args):
+            pass  # suppress default access log noise
+
+    if __name__ == "__main__":
+        print("Listening on :8080")
+        HTTPServer(("", 8080), Handler).serve_forever()
+
+Start it:
+
+.. code::
+
+    python3 server.py
+
+**Step 2 — Understand the payload Hoverfly sends**
+
+On every matching request, Hoverfly POSTs a JSON body to your server containing
+the full request-response pair:
+
+.. code:: json
+
+    {
+        "request": {
+            "path": [{"matcher": "exact", "value": "/api/orders"}],
+            "method": [{"matcher": "exact", "value": "POST"}],
+            "destination": [{"matcher": "exact", "value": "example.com"}],
+            "body": [{"matcher": "exact", "value": ""}],
+            "headers": {}
+        },
+        "response": {
+            "status": 200,
+            "body": "Hello World",
+            "encodedBody": false
+        }
+    }
+
+Your server can read any field from this payload to drive its logic — for example,
+extracting an order ID from the request body to send a webhook.
+
+**Step 3 — Configure Hoverfly to call your server**
+
+Pass the remote URL as the second token in ``-post-serve-action``:
+
+.. code::
+
+    hoverfly -post-serve-action "my-action http://localhost:8080 0" -import simulation.json
+
+The format is: ``"<name> <url> <delay-ms>"``.
+
+- ``my-action`` must match the ``postServeAction`` field in your simulation JSON.
+- The URL must be reachable from Hoverfly at runtime.
+- The delay (in milliseconds) is applied before Hoverfly calls the endpoint.
+
+Alternatively, register it at runtime via hoverctl:
+
+.. code::
+
+    hoverctl post-serve-action set --name my-action --remote http://localhost:8080 --delay 0
+
+**Step 4 — Confirm the action is registered**
+
+.. code::
+
+    curl http://localhost:8888/api/v2/hoverfly/post-serve-action
+
+You should see your action listed with its remote URL.
+
+**Running in Docker Compose**
+
+When both Hoverfly and your action server run as containers, use the service name
+as the hostname. Make sure Hoverfly starts after the action server is ready:
+
+.. code:: yaml
+
+    services:
+      hoverfly:
+        image: spectolabs/hoverfly
+        command: >
+          -post-serve-action "my-action http://postaction:8080 0"
+          -import /app/simulation.json
+        depends_on:
+          - postaction
+
+      postaction:
+        build: ./postaction-server
+        ports:
+          - "8080:8080"
+