HiTraceChain is the module that provides APIs to implement call chain tracing throughout a service process. With HiTraceChain, you can quickly obtain the run log for the call chain of a specified service process and locate faults in inter-device, inter-process, or inter-thread communications.
## Available APIs
The APIs for distributed call chain tracing are provided by the **hiTraceChain** module. For details, see [API Reference](../reference/apis/js-apis-hitracechain.md).
In this example, distributed call chain tracing begins when the application startup execution page is loaded and stops when the service usage is completed.
1. Create a JS application project. In the displayed **Project** window, choose **entry** > **src** > **main** > **js** > **default** > **pages** > **index**, and double-click **index.js**. Add the code to implement call chain tracing upon page loading. The sample code is as follows:
```
import hiTraceChain from '@ohos.hiTraceChain'
export default {
data: {
title: ""
},
onInit() {
this.title = this.$t('strings.world');
// 1. Enable distributed call chain tracing.
let asyncTraceId = hiTraceChain.begin("business", hiTraceChain.HiTraceFlag.INCLUDE_ASYNC | hiTraceChain.HiTraceFlag.DONOT_CREATE_SPAN);
// 2. Start the service process.
console.log(`business start`);
// 3. Add a trace point.
hiTraceChain.tracepoint(hiTraceChain.HiTraceCommunicationMode.THREAD, hiTraceChain.HiTraceTracepointType.SS, asyncTraceId, "Just an example");
// 4. Keep the service process running.
console.log(`business running`);
// 5. End the service process.
console.log(`business end`);
// 6. Stop call chain tracing.
hiTraceChain.end(asyncTraceId);
}
}
```
2. Click the run button on the application page. Then, you'll obtain the log information for service analysis.
hiTraceChain is a lightweight implementation of the cloud-based distributed call chain tracing. It allows applications to trace cross-thread, cross-process, and cross-device service calls.
## Basic Concepts
-**chainId**
Distributed call chain tracing ID, which is a part of **HiTraceId** and is used to identify the service process being traced.
## Working Principles
The hiTraceChain module generates a unique **chainId** for a service process and passes it to various information (including application events, system time, and logs) specific to the service process. During debugging and fault locating, you can use the unique **chainId** to quickly correlate various information related to the service process.
## Constraints
All APIs provided by the hiTraceChain module work in synchronous mode.
HiTraceMeter provides APIs for system performance tracing. You can call the APIs provided by HiTraceMeter module in your own service logic to effectively track service processes and check the system performance.
## Available APIs
The performance tracing APIs are provided by the **hiTraceMeter** module. For details, see [API Reference](../reference/apis/js-apis-hitracemeter.md).
| hiTraceMeter.startTrace(name: string, taskId: number, expectedTime?: number) | void | Starts a trace task. If multiple trace tasks with the same name need to be performed at the same time or a trace task needs to be performed multiple times concurrently, different task IDs must be specified in **startTrace**. If the trace tasks with the same name are not performed at the same time, the same task ID can be used.|
| hiTraceMeter.finishTrace(name: string, taskId: number) | void | Stops a trace task. The values of **name** and **taskId** must be the same as those of **hiTraceMeter.startTrace**.|
| hiTraceMeter.traceByValue(name: string, value: number) | void | Traces the value changes of a variable.|
## How to Develop
In this example, distributed call chain tracing begins when the application startup execution page is loaded and stops when the service usage is completed.
1. Create a JS application project. In the displayed **Project** window, choose **entry** > **src** > **main** > **js** > **default** > **pages** > **index**, and double-click **index.js**. Add the code to implement performance tracing upon page loading. The sample code is as follows:
```js
importhiTraceMeterfrom'@ohos.hiTraceMeter'
exportdefault{
data:{
title:""
},
onInit(){
this.title=this.$t('strings.world');
// The expected duration of the trace task is 5 ms.
hiTraceMeter.startTrace("business",1);
hiTraceMeter.startTrace("business",1,5);
// Start track tasks with the same name concurrently.
hiTraceMeter.startTrace("business",1);
// Keep the service process running.
console.log(`business running`);
hiTraceMeter.startTrace("business",2);// Start the second trace task while the first task is still running. The first and second tasks have the same name but different task IDs.
// Keep the service process running.
console.log(`business running`);
hiTraceMeter.finishTrace("business",1);
// Keep the service process running.
console.log(`business running`);
hiTraceMeter.finishTrace("business",2);
// Start track tasks with the same name at different times.
hiTraceMeter.startTrace("business",1);
// Keep the service process running.
console.log(`business running`);
hiTraceMeter.finishTrace("business",1);// End the first trace task.
// Keep the service process running.
console.log(`business running`);
hiTraceMeter.startTrace("business",1);// Start the second trace task after the first trace task ends. The two tasks have the same name and task ID.
hiTraceMeter is a tool for you to trace service processes and monitor system performance. Through encapsulating and extending the ftrace inside the kernel, hiTraceMeter supports performance tracing for code execution in the user space. You can use hiTraceMeter APIs to implement performance tracing and use the hiTraceMeter CLI tool to collect traced data.
## Basic Concepts
-**hiTraceMeter Tag**
Tag used for tracing data categorization. It is also known as **hiTraceMeter Category**. Generally, one subsystem maps to a tag. The tag is passed as the **Tag** parameter in performance tracing APIs. When you use the hiTraceMeter CLI tool to collect tracing data, only the tracing data specified by the **Tag** parameter is collected.
## Working Principles
- The application calls hiTraceMeter APIs to perform performance tracing. The APIs output the tracing data to the kernel's ftrace data buffer through the kernel's sysfs file interface.
- The hiTraceMeter CLI tool reads the tracing data in the ftrace data buffer and saves the trace data as a text file on the device.
## Constraints
- Due to the asynchronous I/O feature of JS, the hiTraceMeter module provides only asynchronous APIs.
