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  - [Dynamic MQTT Node](designs/dynamic-mqtt-node.md)
  - [Exportable Subflow](designs/exportable-subflow/README.md)
  - [Node Timeout API](designs/timeout-api.md)
+ - [Metrics Usage](designs/metrics-usage.md)
 
 #### In-progress
 
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+---
+state: draft
+---
+
+# metrics usage
+
+## Summary
+
+Metrics logs are useful for gathering and estimating execution statistics of Node-RED.  This design note summarizes metrics usage scenarios and expected enhancements on them.
+
+
+## Authors
+
+ - @HiroyasuNishiyama
+ - @k-toumura
+
+## Details
+
+### Supported Metrics Logs
+
+Currently, Node-RED runtime and its core nodes support following metrics logs.
+
+|      | Runtim/Node         | Name                                                   |
+| ---- | ------------------- | ------------------------------------------------------ |
+| 1    | Runtime (all nodes) | `send`, `receive`                                      |
+| 2    | `Function` node     | `duration`                                             |
+| 3    | `Httprequest` node  | `duration.millis`, `size.bytes`                        |
+| 4    | `Http in` node      | `response.time.millis`, `response.content-length.size` |
+
+### Usage Scenarios and Enhancements
+
+Following table lists possible usage scenarios and enhancements of metrics logs:
+
+|      | Purpose                                       | Used Metrics                                                 | Metrics Usage                                                | Desc.                                                        |
+| ---- | --------------------------------------------- | ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------------------ |
+| 1    | Capture Elapsed Time                          | `send`, `receive`                                            | *<elapsed time>*=*<time of recv>*-*<time of send>*           | \- current implementation (1.0-) can not be used for this purpose because of async message send [(Issue #2444)](https://github.com/node-red/node-red/issues/2444) <br />- processing between `recv` and `send` must be synchronous or new metrics that logs transfer of control is required |
+| 2    | Trace Message Processing                      | *send*, *receive*, **done**                                  | use message ID to create directed graph to capture execution path | - current implementation do not record end of message processing ([Issue #2446])[https://github.com/node-red/node-red/issues/2446],<br />\- `send` and *done* must contain ID of contributing input messages |
+| 3    | Statistics on Incoming/Outgoing  HTTP request | - `duration.millis`, `size.bytes`<br />- `response.time.millis`, `response.content-length.size` | use metrics logs to know statistics on HTTP requests         |                                                              |
+| 4    | Processing time of `Function` node            | `duration`                                                   | use metrics log to know processing time of `Function` node   | - similar problem to #1 can occur if function body is not synchronous. |
+
+### Node coding conventions
+
+#### for '1. Capture Elapsed Time'
+
+To measure an elapsed time for asynchronous operation, each node should emit following logs:
+1. When the node receives a message and then put further operation for the message to asynchronous function.
+2. When the asynchronous operation is started or completed.
+```
+node.on("input", function(msg, send, done) {
+    // handle a message, but not complete its processing.
+    // e.g. put it on a queue, or schedule timeout function.
+    setTimeout(function(msg, send, done) {
+        // start asynchronous operation
+        node.metric("asyncStart");      // 2.
+        send(msg);
+        done();
+        node.metric("asyncEnd");        // 2.
+    }, 100000);
+    node.metric("receiveEnd");          // 1.
+```
+
+#### for '2. Trace Message Processing'
+
+When a node creates message(s) which is related to received message (e.g. split node),
+or the node aggregate multiple messages (e.g. join node), the node should emit a log to correlate each messages.
+```
+node.metric("correlate", msg, {"correlate": anotherMsgId});  // anotherMsgId contains an id of the related message
+```
+
+### Example metric log output and interpretations
+
+#### for '1. Capture Elapsed Time' and '2. Trace Message Processing'
+In a following flow, join node("D") gathers three messages from upstream nodes (inject node "A","B", and "C").
+<img src="./metrics-sample-flow.png">
+
+Example metric logs:
+```
+14:21:51 - [metric] {"level":99,"nodeid":"D","event":"node.join.receive","msgid":"Y","timestamp":1579670511863}
+14:21:51 - [metric] {"level":99,"nodeid":"D","event":"node.join.receiveEnd","msgid":"Y"}
+14:21:53 - [metric] {"level":99,"nodeid":"D","event":"node.join.receive","msgid":"Z","timestamp":1579670513508}
+14:21:53 - [metric] {"level":99,"nodeid":"D","event":"node.join.receiveEnd","msgid":"Z"}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.receive","msgid":"X","timestamp":1579670516424}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.receiveEnd","msgid":"X"}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.correlate","msgid":"X", "value":"Z","timestamp":…}
+14:21:56 - [metric] {“level”:99,“nodeid”:“D”,“event”:“node.join.correlate","msgid":"X","value":"Y","timestamp":…}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.send","msgid":"X","timestamp":1579670516425}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.done","msgid":"X",”timestamp”:…}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.done","msgid":“Y",”timestamp”:…}
+14:21:56 - [metric] {"level":99,"nodeid":"D","event":"node.join.done","msgid":“Z",”timestamp”:…}
+```
+- By "receive" and "receiveEnd" logs, we can measure the elapsed time for input message processing.
+- By "correlate" logs, we can infer the message "X" is related with "Y" and "Z".
+- By "done" logs, we can infer that the process of handling message "X", "Y", "Z" in node "D" had completed. 
+
+## History
+
+ - 2020-02-09 - initial note
+ - 2020-03-25 - add metric log usage for done/receiveEnd/asyncStart/asyncEnd
+