从源码看ShardingSphere设计-路由引擎篇

无论是分库分表、还是读写分离，一个SQL在DB上执行前都需要经过特定规则运算获得运行的目标库表信息。路由引擎的职责定位就是计算SQL应该在哪个数据库、哪个表上执行。前者结果会传给后续执行引擎，然后根据其数据库标识获取对应的数据库连接；后者结果则会传给改写引擎在SQL执行前进行表名的改写，即替换为正确的物理表名。

计算哪个数据库依据的算法是要用户配置的库路由规则，计算哪个表依据的算法是用户配置的表路由规则。目前在ShardingSphere中需要进行路由的功能模块有两个：分库分表sharding与读写分离master-slave。

代码调用分析

再回到BasePrepareEngine类中，在进行路由操作前先进行了路由装饰器的注册
org.apache.shardingsphere.underlying.pluggble.prepare.BasePrepareEngine#registerRouteDecorator

private void registerRouteDecorator() {
        for (Class each : OrderedRegistry.getRegisteredClasses(RouteDecorator.class)) {
            RouteDecorator routeDecorator = createRouteDecorator(each);
            Class ruleClass = (Class) routeDecorator.getType();
            // FIXME rule.getClass().getSuperclass() == ruleClass for orchestration, should decouple extend between orchestration rule and sharding rule
            rules.stream().filter(rule -> rule.getClass() == ruleClass || rule.getClass().getSuperclass() == ruleClass).collect(Collectors.toList())
                    .forEach(rule -> router.registerDecorator(rule, routeDecorator));
        }
    }

之后开始路由的真正流程，路由入口类是DataNodeRouter

org.apache.shardingsphere.underlying.route.DataNodeRouter

/**
 * Data node router.
 */
@RequiredArgsConstructor
    public final class DataNodeRouter {
    
    private final ShardingSphereMetaData metaData;
    
    private final ConfigurationProperties properties;
    
    private final SQLParserEngine parserEngine;
    
    private final Map decorators = new LinkedHashMap<>();
    
    private SPIRoutingHook routingHook = new SPIRoutingHook();
    
    /**
     * Register route decorator.
     *
     * @param rule rule
     * @param decorator route decorator
     */
    public void registerDecorator(final BaseRule rule, final RouteDecorator decorator) {
        decorators.put(rule, decorator);
    }
    
    /**
     * Route SQL.
     *
     * @param sql SQL
     * @param parameters SQL parameters
     * @param useCache whether cache SQL parse result
     * @return route context
     */
    public RouteContext route(final String sql, final List