sharding JDBC讀寫分離原理

一、基礎信息

    maven依賴：

<groupId>org.apache.shardingsphere</groupId>
<artifactId>sharding-jdbc-spring-namespace</artifactId>
<version>4.0.0-RC3</version>

二、原理

    流程圖：

    在MasterSlaveRouter的route方法中實現的讀寫分離：

@RequiredArgsConstructor
public final class MasterSlaveRouter {
    
    private final MasterSlaveRule masterSlaveRule;
    
    private final SQLParseEngine parseEngine;
    
    private final boolean showSQL;

    /**
     * Route Master slave.
     *
     * @param sql SQL
     * @param useCache use cache or not
     * @return data source names
     */
    // TODO for multiple masters may return more than one data source
    public Collection<String> route(final String sql, final boolean useCache) {
        Collection<String> result = route(parseEngine.parse(sql, useCache));
        if (showSQL) {
            SQLLogger.logSQL(sql, result);
        }
        return result;
    }
    
    private Collection<String> route(final SQLStatement sqlStatement) {
        if (isMasterRoute(sqlStatement)) {
            MasterVisitedManager.setMasterVisited();
            return Collections.singletonList(masterSlaveRule.getMasterDataSourceName());
        }
        return Collections.singletonList(masterSlaveRule.getLoadBalanceAlgorithm().getDataSource(
                masterSlaveRule.getName(), masterSlaveRule.getMasterDataSourceName(), new ArrayList<>(masterSlaveRule.getSlaveDataSourceNames())));
    }

    private boolean isMasterRoute(final SQLStatement sqlStatement) {
        return containsLockSegment(sqlStatement) || !(sqlStatement instanceof SelectStatement) || MasterVisitedManager.isMasterVisited() || HintManager.isMasterRouteOnly();
    }

    private boolean containsLockSegment(final SQLStatement sqlStatement) {
        return sqlStatement instanceof SelectStatement && ((SelectStatement) sqlStatement).getLock().isPresent();
    }
}