update doc

Author: eeliu

testapps/libevent_http_server/image-1.png

@@ -1,3 +1,19 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2020 NAVER Corp
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License.  You may obtain a copy
+// of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+// License for the specific language governing permissions and limitations under
+// the License.
+////////////////////////////////////////////////////////////////////////////////
+
 #include "pp_ev_http.h"
 #include "event2/http.h"
 #include <malloc.h>
@@ -158,8 +174,6 @@ static int pp_evhttp_make_request(struct evhttp_connection *evcon,
     req->cb = pp_sub_request_done;
     req->cb_arg = ctx;
 
-    // todo add pinpoint-header
-
     struct evkeyvalq *output_headers;
     output_headers = evhttp_request_get_output_headers(req);
 
@@ -318,3 +332,5 @@ void pp_free_http_client(pp_http_client_t *client) {
     free(client);
   }
 }
+
+//@author eeliu

testapps/libevent_http_server/image.png

@@ -1,5 +1,19 @@
 #pragma once
-
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2020 NAVER Corp
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License.  You may obtain a copy
+// of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+// License for the specific language governing permissions and limitations under
+// the License.
+////////////////////////////////////////////////////////////////////////////////
 #include <event2/buffer.h>
 #include <event2/bufferevent.h>
 #include <event2/event.h>
@@ -97,3 +111,5 @@ void pinpoint_set_agent_helper(const char *app_name, const char *app_id,
       pinpoint_end_trace(node);                                                \
     }                                                                          \
   }
+
+//@author eeliu

testapps/libevent_http_server/pp_ev_http.c

@@ -3,7 +3,8 @@
 ## Command
 
 ```
-$ mkdir build && cd build && make -j
+$ mkdir build && cd build && cmake .. && make -j
+$ # curl http://localhost:8080/http-client
 ```
 
 ### Thanks
@@ -21,3 +22,93 @@ $ mkdir build && cd build && make -j
 
 ![call stack ](call_stack.png)
 
+## Introduction for pinpoint on asynchronous framework
+
+> what's problem in asynchronous framework (event driver)?
+
+In synchronous, we gets trace by it's called order, that is the logical order when handling request, but in asynchronous framework, trace are driven by event, it's the event order. So, we must find a way to recover the logical order. 
+
+### Synchronous call flow
+
+![alt text](image.png)
+
+### Asynchronous call flow
+
+>  Red line: user A logical flow \
+   Blue line: user B logical flow
+
+  ![alt text](image-1.png)
+
+Here is the question, how to separate A and B from mixed call chains ?
+  
+>Inspired by nginx
+
+Pass `ctx` to everywhere 
+
+```c
+#define CTX_MATIC_NUM 0x1357325l
+typedef struct {
+  int magic_num_;
+  NodeID current_id_;  // current trace ID
+  void *ori_arg_;
+  void (*on_complete_cb_origin_)(struct evhttp_request *, void *);
+  void *on_complete_cb_arg_origin_;
+
+  void (*http_client_request_cb_)(struct evhttp_request *, void *);
+  void *http_client_request_done_cb_arg_;
+  ...
+} pp_request_context_t;
+```
+
+If you want to trace `foo` function,
+
+1. add `ctx` into parameter list
+
+|Old | Now|
+|----|----|
+|void foo(int a, ...)| void foo(void* ctx, int a,...)|
+
+2. wrapper `foo` function
+
+```c
+void foo(void *ctx,int a, ...);
+
+void pp_foo(void *ctx,int a, ...){
+    ...
+    NodeID trace_id = pinpoint_start_trace(ctx_.current_id_);
+    foo(ctx,a);
+    // end current trace and update current_id_
+    ctx_.current_id_ = pinpoint_end_trace(trace_id);
+}
+...
+call_pp_foo();
+...
+```
+
+3. what if needs to register a callback function?
+
+Before
+``` 
+// some like
+async_http_request(.. request_done_call_cb,request_done_call_cb_arg...);
+
+```
+Now
+```
+cxt->origin_request_done_call_cb = request_done_call_cb;
+ctx->origin_request_done_call_cb_arg = request_done_call_cb_arg
+int pp_request_done_call_cb(arg){
+    cxt->origin_request_done_call_cb(ctx->origin_request_done_call_cb_arg);
+}; 
+
+async_http_request(ctx,.. pp_request_done_call_cb,ctx,...);
+
+```
+
+While, there are many unknown cases in below list. If you have any question, [create an issue 🙋‍♂️ ](https://github.com/pinpoint-apm/pinpoint-c-agent/issues/new).
+
+
+
+
+
+