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# Sentinel: Sentinel of Your Application

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## What Does It Do?

As distributed systems become increasingly popular, the stability between services is becoming more important than ever before. Sentinel takes "flow" as breakthrough point, and works on multiple fields including **flow control**, **concurrency**, **circuit breaking** and **load protection**, to protect service stability.

Sentinel has the following features:

* **Rich applicable scenarios**:
Sentinel has been wildly used in Alibaba, and has covered almost all the core-scenarios in Double-11 Shopping Festivals in the past 10 years, such as “Second Kill” which needs to limit burst flow traffic to meet the system capacity, message peak clipping and valley fills, degrading unreliable downstream applications, etc.

* **Integrated monitor module**:
Sentinel also provides real-time monitoring function. You can see the runtime information of a single machine in real-time, and the summary runtime info of a cluster with less than 500 nodes.

* **Easy extension point**:
Sentinel provides easy-to-use extension points that allow you to quickly customize your logic, for example, custom rule management, adapting data sources, and so on.

## Documentation

See the [中文文档](https://github.com/alibaba/Sentinel/wiki/%E4%BB%8B%E7%BB%8D) for Chinese readme.

See the [Wiki](https://github.com/alibaba/Sentinel/wiki) for full documentation, examples, operational details and other information.

See the [Javadoc](https://github.com/alibaba/Sentinel/tree/master/doc) for the API.

**If you are using Sentinel, please [leave a comment here](https://github.com/alibaba/Sentinel/issues/18) to tell us your use scenario to make Sentinel better :-)**

## Quick Start

Below is a simple demo that guides new users to use Sentinel in just 3 steps. It also shows how to monitor this demo using the dashboard.

### 1.Download Library

**Note:** Sentinel requires Java 6 or later.

If your application is build in maven, just add the following code in pom.xml.

```xml
<dependency>
    <groupId>com.alibaba.csp</groupId>
    <artifactId>sentinel-core</artifactId>
    <version>x.y.z</version>
</dependency>
```

If not, you can download JAR in [Maven Center Repository](https://mvnrepository.com/artifact/com.alibaba.csp/sentinel-core).


### 2.Define Resource

Wrap code snippet via Sentinel API: `SphU.entry("RESOURCENAME")` and `entry.exit()`. In below example, it is `System.out.println("hello world");`:

```java
Entry entry = null;

try {   
  entry = SphU.entry("HelloWorld");
  
  // BIZ logic being protected
  System.out.println("hello world");
} catch (BlockException e) {
  // handle block logic
} finally {
  // make sure that the exit() logic is called
  if (entry != null) {
    entry.exit();
  }
}
```

So far the code modification is done.  

### 3.Define Rules

If we want to limit the access times of the resource, we can define rules. The following code defines a rule that limits access to the reource to 20 times per second at the maximum. 

```java
List<FlowRule> rules = new ArrayList<FlowRule>();
FlowRule rule = new FlowRule();
rule.setResource("HelloWorld");
// set limit qps to 20
rule.setCount(20);
rule.setGrade(RuleConstant.FLOW_GRADE_QPS);
rules.add(rule);
FlowRuleManager.loadRules(rules);
```

### 4. Check the Result

After running the demo for a while, you can see the following records in `~/logs/csp/${appName}-metrics.log.xxx`.

```
|--timestamp-|------date time----|--resource-|p |block|s |e|rt
1529998904000|2018-06-26 15:41:44|hello world|20|0    |20|0|0
1529998905000|2018-06-26 15:41:45|hello world|20|5579 |20|0|728
1529998906000|2018-06-26 15:41:46|hello world|20|15698|20|0|0
1529998907000|2018-06-26 15:41:47|hello world|20|19262|20|0|0
1529998908000|2018-06-26 15:41:48|hello world|20|19502|20|0|0
1529998909000|2018-06-26 15:41:49|hello world|20|18386|20|0|0

p stands for incoming request, block for intercepted by rules, success for success handled, e for exception, rt for average response time (ms)
```
This shows that the demo can print "hello world" 20 times per second.

More examples and information can be found in the [How To Use](https://github.com/alibaba/Sentinel/wiki/How-to-Use) section.

The working principles of Sentinel can be found in [How it works](https://github.com/alibaba/Sentinel/wiki/How-it-works) section.

Samples can be found in the [sentinel-demo](https://github.com/alibaba/Sentinel/tree/master/sentinel-demo) module.

### 5.Start Dashboard

Sentinel also provides a simple dashboard application, on which you can monitor the clients and configure the rules in real time.

For details please refer to [Dashboard](https://github.com/alibaba/Sentinel/wiki/Dashboard).

## Trouble Shooting and Logs

Sentinel will generate logs for troubleshooting. All the information can be found in [logs](https://github.com/alibaba/Sentinel/wiki/Logs).

## Bugs and Feedback

For bug report, questions and discussions please submit [GitHub Issues](https://github.com/alibaba/sentinel/issues).

Contact us: sentinel@linux.alibaba.com

## Contributing

Contributions are always welcomed! Please see [CONTRIBUTING](./CONTRIBUTING.md) for detailed guidelines.

