re-organize the readme

README.md

@@ -1,7 +1,7 @@
 WebAssembly Micro Runtime
 =========================
 WebAssembly Micro Runtime (WAMR) is a standalone WebAssembly (WASM) runtime designed for a small footprint. It includes:
-- A WebAssembly (WASM) VM core
+- A WebAssembly VM core (namely iwasm)
 - The supporting API's for the WASM applications
 - A mechanism for dynamic management of the WASM application
 
@@ -20,45 +20,85 @@ Current Features of WAMR
 - Supports micro-service and pub-sub event inter-app communication models
 - Easy to extend to support remote FW application management from host or cloud
 
-Architecture
-=========================
-The application manager component handles the packets that the platform receives from external sources through any communication buses such as a socket, serial port or SPI. A packet type can be either a request, a response or an event. The application manager will serve the requests with URI "/applet" and call the runtime glue layer interfaces for installing/uninstalling the application. For other URI's, it will filter the resource registration table and route the request to the internal queue of the responsible application.
-
-- The WebAssembly runtime provides the execution environment for WASM applications.
+Application framework architecture
+===================================
 
-- The messaging layer can support the API for WASM applications to communicate with each other and also the host environment.
+By using the iwasm VM core, we are flexible to build different application frameworks for the specific domains. 
 
-- When ahead of time (AOT) compilation is enabled (TODO), the WASM application could be either WASM or a compiled native binary.
+The WAMR has offered an comprehensive application framework for IoT device usages. The framework solves many common requirements for building a real project:
+- Modular design for multiple runtimes support
+- Inter application communication
+- Remote application management
+- WASM APP programming model and API extension mechanism 
 
-<img src="./doc/pics/architecture.PNG" width="80%" height="80%">
+<img src="./doc/pics/wamr-arch.JPG" width="80%" height="80%">
 
 
 
-Build WAMR Core and run basic WASM applications
+Build WAMR Core and run WASM applications
 ================================================
 
-Please follow the instructions below to build the WAMR core on different platforms.
--Linux
--Zephyr
--Mac
--VxWorks
--AliOS-Things
--Docker
+Please follow the instructions below to build the WAMR core (iwasm) on different platforms:
+- Linux
+- Zephyr
+- Mac
+- VxWorks
+- AliOS-Things
+- Docker
+
+Then you can build some basic WASM applications and run it with the WAMR core. As the WAMR core doesn't include the extended application library, your WASM applications can only use the built-in APIs supported by WAMR.    
+
+See the [doc/building.md](./doc/building.md) for the details.
 
 
 Embed WAMR into software production
 =====================================
 
+WAMR can be built into a standalone executable which takes the WASM application file name as input, and then executes it. In some other situations, the WAMR source code is embedded the product code and built into the final product. 
+
+WAMR provides a set of C API for embedding code to load the WASM module, instantiate the module and invoke a WASM  function from a native call. 
 
+See the [doc/embed_wamr.md](./doc/embed_wamr.md) for the details.
 
 WAMR application programming library
 ===================================
 
+WAMR defined event driven programming model:
+- Single thread per WASM app instance
+- App must implement system callbacks: on_init, on_destrory
+
+
+In general there are a few API classes for the WASM application programming:
+- Built-in API: WAMR core provides a minimal libc API set for WASM APP
+- 3rd party libraries: Programmers can download any 3rd party C/C++ source code and build it together with the WASM APP code
+- WAMR application libraries: 
+  - Timer
+  - Micro service (Request/Response)
+  - Pub/Sub
+  - Sensor
+  - Connection and data transmission
+  - 2D graphic UI (based on littlevgl)
+- User extended native API: extend the native API to the WASM applications
+
+See the [doc/wamr_api.md](./doc/wamr_api.md) for the details.
 
 
 Samples and demos
 =================
 
+The WAMR samples are located in folder [./samples](./samples). A sample usually contains the WAMR runtime build, WASM applications and test tools. The WARM provides following samples:
+- [Simple](./samples/simple/README.md): The runtime integrated most of the WAMR APP libaries and multiple WASM applications are provided for using different WASM API set.
+- [littlevgl](./samples/littlevgl/README.md): Demostrating the graphic user interface application usage on WAMR. The whole littlevgl 2D user graphic library and the UI application is built into WASM application.  
+- [gui](./samples/gui/README.md): Moved the littlevgl library into the runtime and defined a WASM application interface by wrapping the littlevgl API.
+- [IoT-APP-Store-Demo](./test-tools/IoT-APP-Store-Demo/README.md): A web site for demostrating a WASM APP store usage where we can remotely install and uninstall WASM application on remote devices.
+
+
+The graphic user interface sample:
+
+![WAMR samples diagram](./doc/pics/vgl_demo.png "WAMR samples diagram")
+
+
+
 
 Releases, acknowledgment and roadmap
 ====================================

doc/building.md

@@ -0,0 +1,268 @@
+
+Build WAMR Core
+=========================
+Please follow the instructions below to build the WAMR core on different platforms.
+
+Linux
+-------------------------
+First of all please install library dependencies of lib gcc.
+Use installation commands below for Ubuntu Linux:
+``` Bash
+sudo apt install lib32gcc-5-dev g++-multilib
+```
+Or in Fedora:
+``` Bash
+sudo dnf install glibc-devel.i686 
+```
+
+After installing dependencies, build the source code:
+``` Bash
+cd core/iwasm/products/linux/
+mkdir build
+cd build
+cmake ..
+make
+```
+
+Mac
+-------------------------
+Make sure to install Xcode from App Store firstly, and install cmake.
+
+If you use Homebrew, install cmake from the command line:
+``` Bash
+brew install cmake
+```
+
+Then build the source codes:
+```
+cd core/iwasm/products/darwin/
+mkdir build
+cd build
+cmake ..
+make
+```
+
+VxWorks
+-------------------------
+VxWorks 7 SR0620 release is validated.
+
+First you need to build a VSB. Make sure *UTILS_UNIX* layer is added in the VSB.
+After the VSB is built, export the VxWorks toolchain path by:
+```
+export <vsb_dir_path>/host/vx-compiler/bin:$PATH
+```
+Now switch to iwasm source tree to build the source code:
+```
+cd core/iwasm/products/vxworks/
+mkdir build
+cd build
+cmake ..
+make
+```
+Create a VIP based on the VSB. Make sure the following components are added:
+* INCLUDE_POSIX_PTHREADS
+* INCLUDE_POSIX_PTHREAD_SCHEDULER
+* INCLUDE_SHARED_DATA
+* INCLUDE_SHL
+
+Copy the generated iwasm executable, the test WASM binary as well as the needed
+shared libraries (libc.so.1, libllvm.so.1 or libgnu.so.1 depending on the VSB,
+libunix.so.1) to a supported file system (eg: romfs).
+
+Zephyr
+-------------------------
+You need to download the Zephyr source code first and embed WAMR into it.
+``` Bash
+git clone https://github.com/zephyrproject-rtos/zephyr.git
+cd zephyr/samples/
+cp -a <iwasm_dir>/products/zephyr/simple .
+cd simple
+ln -s <iwam_dir> iwasm
+ln -s <shared_lib_dir> shared-lib
+mkdir build && cd build
+source ../../../zephyr-env.sh
+cmake -GNinja -DBOARD=qemu_x86 ..
+ninja
+```
+AliOS-Things
+-------------------------
+1. a developerkit board id needed for testing
+2. download the AliOS-Things code
+   ``` Bash
+   git clone https://github.com/alibaba/AliOS-Things.git
+   ```
+3. copy <iwasm_root_dir>/products/alios-things directory to AliOS-Things/middleware, and rename it as iwasm
+   ``` Bash
+   cp -a <iwasm_root_dir>/products/alios-things middleware/iwasm
+   ```
+4. create a link to <iwasm_root_dir> in middleware/iwasm/ and rename it to iwasm
+   ``` Bash
+   ln -s <iwasm_root_dir> middleware/iwasm/iwasm
+   ```
+5. create a link to <shared-lib_root_dir> in middleware/iwasm/ and rename it to shared-lib
+   ``` Bash
+   ln -s <shared-lib_root_dir> middle/iwasm/shared-lib
+   ```
+6. modify file app/example/helloworld/helloworld.c, patch as:
+   ``` C
+   #include <stdbool.h>
+   #include <aos/kernel.h>
+   extern bool iwasm_init();
+   int application_start(int argc, char *argv[])
+   {
+        int count = 0;
+        iwasm_init();
+       ...
+   }
+   ```
+7. modify file app/example/helloworld/aos.mk
+   ``` C
+      $(NAME)_COMPONENTS := osal_aos iwasm
+   ```
+8. build source code
+   ``` Bash
+   aos make helloworld@developerkit -c config
+   aos make
+   ```
+9. download the binary to developerkit board, check the output from serial port
+
+Docker
+-------------------------
+[Docker](https://www.docker.com/) will download all the dependencies and build WAMR Core on your behalf.
+
+Make sure you have Docker installed on your machine: [macOS](https://docs.docker.com/docker-for-mac/install/), [Windows](https://docs.docker.com/docker-for-windows/install/) or [Linux](https://docs.docker.com/install/linux/docker-ce/ubuntu/).
+
+Build the Docker image:
+
+``` Bash
+docker build --rm -f "Dockerfile" -t wamr:latest .
+```
+Run the image in interactive mode:
+``` Bash
+docker run --rm -it wamr:latest
+```
+You'll now enter the container at `/root`.
+
+
+Build WASM app
+=========================
+You can write a simple ```test.c``` as the first sample.
+
+```C
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv)
+{
+    char *buf;
+
+    printf("Hello world!\n");
+
+    buf = malloc(1024);
+    if (!buf) {
+        printf("malloc buf failed\n");
+        return -1;
+    }
+
+    printf("buf ptr: %p\n", buf);
+
+    sprintf(buf, "%s", "1234\n");
+    printf("buf: %s", buf);
+
+    free(buf);
+    return 0;
+}
+```
+
+There are three methods to build a WASM binary. They are Emscripten, the clang compiler and Docker.
+
+##  Use Emscripten tool
+
+A method to build a WASM binary is to use Emscripten tool ```emcc```.
+Assuming you are using Linux, you may install emcc from Emscripten EMSDK following the steps below:
+
+```
+git clone https://github.com/emscripten-core/emsdk.git
+emsdk install latest
+emsdk activate latest
+```
+source ```./emsdk_env.sh```.
+The Emscripten website provides other installation methods beyond Linux.
+
+Use the emcc command below to build the WASM C source code into the WASM binary.
+``` Bash
+emcc -g -O3 *.c -s WASM=1 -s SIDE_MODULE=1 -s ASSERTIONS=1 -s STACK_OVERFLOW_CHECK=2 \
+                -s TOTAL_MEMORY=65536 -s TOTAL_STACK=4096 -o test.wasm
+```
+You will get ```test.wasm``` which is the WASM app binary.
+
+## Use clang compiler
+
+Another method to build a WASM binary is to use clang compiler```clang-8```.
+
+Add source to your system source list from llvm website, for ubuntu16.04, add following lines to /etc/apt/sources.list:
+
+```Bash
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial main # 7
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main # 8
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main
+```
+
+Download and install clang-8 tool-chain using following commands:
+
+```Bash
+wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+sudo apt-get update
+sudo apt-get install llvm-8 lld-8 clang-8
+```
+
+Create a soft link under /usr/bin:
+
+```Bash
+cd /usr/bin
+sudo ln -s wasm-ld-8 wasm-ld
+```
+
+Use the clang-8 command below to build the WASM C source code into the WASM binary.
+
+```Bash
+clang-8 --target=wasm32 -O3 -Wl,--initial-memory=131072,--allow-undefined,--export=main,
+--no-threads,--strip-all,--no-entry -nostdlib -o test.wasm test.c
+```
+
+You will get ```test.wasm``` which is the WASM app binary.
+
+## Using Docker
+
+The last method availble is using [Docker](https://www.docker.com/). We assume you've already configured Docker (see Platform section above) and have a running interactive shell. Currently the Dockerfile only supports compiling apps with clang, with Emscripten planned for the future.
+
+Use the clang-8 command below to build the WASM C source code into the WASM binary.
+
+```Bash
+clang-8 --target=wasm32 -O3 -Wl,--initial-memory=131072,--allow-undefined,--export=main,
+--no-threads,--strip-all,--no-entry -nostdlib -o test.wasm test.c
+```
+
+You will get ```test.wasm``` which is the WASM app binary.
+
+Run WASM app
+========================
+
+Assume you are using Linux, the command to run the test.wasm is:
+``` Bash
+cd iwasm/products/linux/build
+./iwasm test.wasm
+```
+You will get the following output:
+```
+Hello world!
+buf ptr: 0x400002b0
+buf: 1234
+```
+If you would like to run the test app on Zephyr, we have embedded a test sample into its OS image. You will need to execute:
+```
+ninja run
+```

doc/embed_wamr.md

@@ -0,0 +1,40 @@
+Embed WAMR into software production
+=====================================
+
+![WAMR embed diagram](./pics/embed.PNG "WAMR embed architecture diagram")
+
+
+A typical WAMR API usage is shown below (some return value checks are ignored):
+``` C
+  static char global_heap_buf[512 * 1024];
+
+  char *buffer;
+  wasm_module_t module;
+  wasm_module_inst_t inst;
+  wasm_function_inst_t func;
+  wasm_exec_env_t env;
+  uint32 argv[2];
+
+  bh_memory_init_with_pool(global_heap_buf, sizeof(global_heap_buf));
+  wasm_runtime_init();
+
+  buffer = read_wasm_binary_to_buffer(…);
+  module = wasm_runtime_load(buffer, size, err, err_size);
+  inst = wasm_runtime_instantiate(module, 0, 0, err, err_size);
+  func = wasm_runtime_lookup_function(inst, "fib", "(i32)i32");
+  env = wasm_runtime_create_exec_env(stack_size);
+
+  argv[0] = 8;
+  if (!wasm_runtime_call_wasm(inst, env, func, 1, argv_buf) ) {
+      wasm_runtime_clear_exception(inst);
+  }
+  /* the return value is stored in argv[0] */
+  printf("fib function return: %d\n", argv[0]);
+
+  wasm_runtime_destory_exec_env(env);
+  wasm_runtime_deinstantiate(inst);
+  wasm_runtime_unload(module);
+  wasm_runtime_destroy();
+  bh_memory_destroy();
+```
+

doc/wamr_api.md

@@ -0,0 +1,316 @@
+
+WASM application library
+========================
+In general, there are 3 classes of API's important for the WASM application:
+- Built-in API's: WAMR provides a minimal API set for developers.
+- 3rd party API's: Programmer can download and include any 3rd party C source code and add it into their own WASM app source tree.
+- Platform native API's: WAMR provides a mechanism to export a native API to the WASM application.
+
+
+Built-in application library
+---------------
+Built-in API's include Libc API's, Base library and Extension library reference.
+
+**Libc API's**<br/>
+This is a minimal set of Libc API's for memory allocation, string manipulation and printing. The header file is located at ```lib/app-libs/libc/lib_base.h```. The current supported API set is listed here:
+``` C
+void *malloc(size_t size);
+void *calloc(size_t n, size_t size);
+void free(void *ptr);
+int memcmp(const void *s1, const void *s2, size_t n);
+void *memcpy(void *dest, const void *src, size_t n);
+void *memmove(void *dest, const void *src, size_t n);
+void *memset(void *s, int c, size_t n);
+int putchar(int c);
+int snprintf(char *str, size_t size, const char *format, ...);
+int sprintf(char *str, const char *format, ...);
+char *strchr(const char *s, int c);
+int strcmp(const char *s1, const char *s2);
+char *strcpy(char *dest, const char *src);
+size_t strlen(const char *s);
+int strncmp(const char * str1, const char * str2, size_t n);
+char *strncpy(char *dest, const char *src, unsigned long n);
+```
+
+**Base library**<br/>
+Basic support for communication, timers, etc is available. You can refer to the header file ```lib/app-libs/base/wasm_app.h``` which contains the definitions for request and response API's, event pub/sub API's and timer API's. Please note that these API's require the native implementations.
+The API set is listed below:
+``` C
+typedef void(*request_handler_f)(request_t *) ;
+typedef void(*response_handler_f)(response_t *, void *) ;
+
+// Request API's
+bool api_register_resource_handler(const char *url, request_handler_f);
+void api_send_request(request_t * request, response_handler_f response_handler, void * user_data);
+void api_response_send(response_t *response);
+
+// Event API's
+bool api_publish_event(const char *url,  int fmt, void *payload,  int payload_len);
+bool api_subscribe_event(const char * url, request_handler_f handler);
+
+struct user_timer;
+typedef struct user_timer * user_timer_t;
+
+// Timer API's
+user_timer_t api_timer_create(int interval, bool is_period, bool auto_start, void(*on_user_timer_update)(user_timer_t
+));
+void api_timer_cancel(user_timer_t timer);
+void api_timer_restart(user_timer_t timer, int interval);
+```
+
+**Library extension reference**<br/>
+Currently we provide several kinds of extension library for reference including sensor, connection and GUI.
+
+Sensor API: In the header file ```lib/app-libs/extension/sensor/sensor.h```, the API set is defined as below:
+``` C
+sensor_t sensor_open(const char* name, int index,
+                     void(*on_sensor_event)(sensor_t, attr_container_t *, void *),
+                     void *user_data);
+bool sensor_config(sensor_t sensor, int interval, int bit_cfg, int delay);
+bool sensor_config_with_attr_container(sensor_t sensor, attr_container_t *cfg);
+bool sensor_close(sensor_t sensor);
+```
+Connection API: In the header file `lib/app-libs/extension/connection/connection.h.`, the API set is defined as below:
+``` C
+/* Connection event type */
+typedef enum {
+    /* Data is received */
+    CONN_EVENT_TYPE_DATA = 1,
+    /* Connection is disconnected */
+    CONN_EVENT_TYPE_DISCONNECT
+} conn_event_type_t;
+
+typedef void (*on_connection_event_f)(connection_t *conn,
+                                      conn_event_type_t type,
+                                      const char *data,
+                                      uint32 len,
+                                      void *user_data);
+connection_t *api_open_connection(const char *name,
+                                  attr_container_t *args,
+                                  on_connection_event_f on_event,
+                                  void *user_data);
+void api_close_connection(connection_t *conn);
+int api_send_on_connection(connection_t *conn, const char *data, uint32 len);
+bool api_config_connection(connection_t *conn, attr_container_t *cfg);
+```
+GUI API: The API's is list in header file ```lib/app-libs/extension/gui/wgl.h``` which is implemented based open soure 2D graphic library [LittlevGL](https://docs.littlevgl.com/en/html/index.html). Currently supported widgets include button, label, list and check box and more wigdet would be provided in future.
+
+The mechanism of exporting native API to WASM application
+=======================================================
+
+The basic working flow for WASM application calling into the native API is shown in the following diagram:
+
+![WAMR WASM API ext diagram](./pics/extend_library.PNG "WAMR WASM API ext architecture diagram")
+
+
+WAMR provides the macro `EXPORT_WASM_API` to enable users to export a native API to a WASM application. WAMR has implemented a base API for the timer and messaging by using `EXPORT_WASM_API`. This can be a point of reference for extending your own library.
+``` C
+static NativeSymbol extended_native_symbol_defs[] = {
+    EXPORT_WASM_API(wasm_register_resource),
+    EXPORT_WASM_API(wasm_response_send),
+    EXPORT_WASM_API(wasm_post_request),
+    EXPORT_WASM_API(wasm_sub_event),
+    EXPORT_WASM_API(wasm_create_timer),
+    EXPORT_WASM_API(wasm_timer_set_interval),
+    EXPORT_WASM_API(wasm_timer_cancel),
+    EXPORT_WASM_API(wasm_timer_restart)
+};
+```
+
+![#f03c15](https://placehold.it/15/f03c15/000000?text=+) **Security attention:** A WebAssembly application should only have access to its own memory space. As a result, the integrator should carefully design the native function to ensure that the memory accesses are safe. The native API to be exported to the WASM application must:
+- Only use 32 bits number for parameters
+- Should not pass data to the structure pointer (do data serialization instead)
+- Should do the pointer address conversion in the native API
+- Should not pass function pointer as callback
+
+Below is a sample of a library extension. All code invoked across WASM and native world must be serialized and de-serialized, and the native world must do a boundary check for every incoming address from the WASM world.
+
+
+<img src="./pics/safe.PNG" width="90%">
+
+
+Steps for exporting native API
+==========================
+
+WAMR implemented a framework for developers to export API's. Below is the procedure to expose the platform API's in three steps:
+
+**Step 1. Create a header file**<br/>
+Declare the API's for your WASM application source project to include.
+
+**Step 2. Create a source file**<br/>
+Export the platform API's, for example in ``` products/linux/ext_lib_export.c ```
+``` C
+#include "lib_export.h"
+
+static NativeSymbol extended_native_symbol_defs[] =
+{
+};
+
+#include "ext_lib_export.h"
+```
+
+**Step 3. Register new API's**<br/>
+Use the macro `EXPORT_WASM_API` and `EXPORT_WASM_API2` to add exported API's into the array of ```extended_native_symbol_defs```.
+The pre-defined MACRO `EXPORT_WASM_API` should be used to declare a function export:
+``` c
+#define EXPORT_WASM_API(symbol)  {#symbol, symbol}
+```
+
+Below code example shows how to extend the library to support `customized()`:
+
+``` 
+//lib_export_impl.c
+void customized()
+{
+   // your code
+}
+
+
+// lib_export_dec.h
+#ifndef _LIB_EXPORT_DEC_H_
+#define _LIB_EXPORT_DEC_H_
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void customized();
+
+#ifdef __cplusplus
+}
+#endif
+#endif
+
+
+// ext_lib_export.c
+#include "lib_export.h"
+#include "lib_export_dec.h"
+
+static NativeSymbol extended_native_symbol_defs[] =
+{
+    EXPORT_WASM_API(customized)
+};
+
+#include "ext_lib_export.h"
+```
+
+Use extended library
+------------------------
+In the application source project, it will include the WAMR built-in API's header file and platform extension header files. Assuming the board vendor extends the library which added an API called customized(), the WASM application would be like this:
+``` C
+#include <stdio.h>
+#include "lib_export_dec.h" // provided by the platform vendor
+
+int main(int argc, char **argv)
+{
+    int I;
+    char *buf = “abcd”;
+    customized();                   // customized API provided by the platform vendor
+    return i;
+}
+```
+
+
+Communication programming models
+=========================
+WAMR supports two typical communication programming models, the microservice model and the pub/sub model. 
+
+
+Microservice model
+-------------------------
+The microservice model is also known as request and response model. One WASM application acts as the server which provides a specific service. Other WASM applications or host/cloud applications request that service and get the response.
+<img src="./pics/request.PNG" width="60%" height="60%">
+
+Below is the reference implementation of the server application. It provides room temperature measurement service.
+
+``` C
+void on_init()
+{
+    api_register_resource_handler("/room_temp", room_temp_handler);
+}
+
+void on_destroy() 
+{
+}
+
+void room_temp_handler(request_t *request)
+{
+    response_t response[1];
+    attr_container_t *payload;
+    payload = attr_container_create("room_temp payload");
+    if (payload == NULL)
+        return;
+
+    attr_container_set_string(&payload, "temp unit", "centigrade");
+    attr_container_set_int(&payload, "value", 26);
+
+    make_response_for_request(request, response);
+    set_response(response,
+                 CONTENT_2_05,
+                 FMT_ATTR_CONTAINER,
+                 payload,
+                 attr_container_get_serialize_length(payload));
+
+    api_response_send(response);
+    attr_container_destroy(payload);
+}
+```
+
+
+Pub/sub model
+-------------------------
+One WASM application acts as the event publisher. It publishes events to notify WASM applications or host/cloud applications which subscribe to the events.
+
+<img src="./pics/sub.PNG" width="60%" height="60%">
+
+Below is the reference implementation of the pub application. It utilizes a timer to repeatedly publish an overheat alert event to the subscriber applications. Then the subscriber applications receive the events immediately.
+
+``` C
+/* Timer callback */
+void timer_update(user_timer_t timer
+{
+    attr_container_t *event;
+
+    event = attr_container_create("event");
+    attr_container_set_string(&event,
+                              "warning",
+                              "temperature is over high");
+
+    api_publish_event("alert/overheat",
+                      FMT_ATTR_CONTAINER,
+                      event,
+                      attr_container_get_serialize_length(event));
+
+    attr_container_destroy(event);
+}
+
+void on_init()
+{
+    user_timer_t timer;
+    timer = api_timer_create(1000, true, true, timer_update);
+}
+
+void on_destroy()
+{
+}
+```
+
+Below is the reference implementation of the sub application.
+``` C
+void overheat_handler(request_t *event)
+{
+    printf("Event: %s\n", event->url);
+
+    if (event->payload != NULL && event->fmt == FMT_ATTR_CONTAINER)
+       attr_container_dump((attr_container_t *) event->payload);
+}
+
+void on_init(
+{
+    api_subscribe_event ("alert/overheat", overheat_handler);
+}
+
+void on_destroy()
+{
+}
+```
+**Note:** You can also subscribe this event from host side by using host tool. Please refer `samples/simple` project for deail usage.