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docs/distributed-system/gray-environment.md

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+#### 准备一个数据库和一个表（也可以用Apollo配置中心、Redis、ZooKeeper，其实都可以），放一个灰度发布启用表
+
+```
+id service_id path enable_gray_release
+
+CREATE TABLE `gray_release_config` (
+   `id` int(11) NOT NULL AUTO_INCREMENT,
+   `service_id` varchar(255) DEFAULT NULL,
+   `path` varchar(255) DEFAULT NULL,
+   `enable_gray_release` int(11) DEFAULT NULL,
+   PRIMARY KEY (`id`)
+ ) ENGINE=InnoDB DEFAULT CHARSET=utf8
+ 
+
+
+在zuul里面加入下面的filter，可以在zuul的filter里定制ribbon的负载均衡策略
+
+<dependency>
+			<groupId>io.jmnarloch</groupId>
+			<artifactId>ribbon-discovery-filter-spring-cloud-starter</artifactId>
+			<version>2.1.0</version>
+		</dependency>
+
+写一个zuul的filter，对每个请求，zuul都会调用这个filter
+
+@Configuration
+public class GrayReleaseFilter extends ZuulFilter {
+
+@Autowired
+private JdbcTemplate jdbcTemplate;
+
+    @Override
+    public int filterOrder() {
+        return PRE_DECORATION_FILTER_ORDER - 1;
+    }
+ 
+    @Override
+    public String filterType() {
+        return PRE_TYPE;
+    }
+ 
+    @Override
+    public boolean shouldFilter() {
+    	
+    }
+ 
+    @Override
+    public Object run() {
+        RequestContext ctx = RequestContext.getCurrentContext();
+        HttpServletRequest request = ctx.getRequest();
+
+Random random = new Random();
+int seed = random.getInt() * 100;
+
+        if (seed = 50) {
+            // put the serviceId in `RequestContext`
+            RibbonFilterContextHolder.getCurrentContext()
+                    .add("version", "new");
+        }  else {
+            RibbonFilterContextHolder.getCurrentContext()
+                    .add("version", "old");
+        }
+        
+        return null;
+    }
+}
+ ```
+
+eureka:
+instance:
+metadata-map:
+version: new
+
+
+