MongoDB维护

Question

我正在为 MongoDB 编写一个维护脚本，它将按计划压缩副本集中的集合。到目前为止，我的脚本看起来不错，可以针对主节点和辅助节点完成预期的工作。但是有一个问题:

使用 MongoDB 副本集的系统是一个高可用性网络队列，它不断地被写入和读取。因此，即使调用 rs.StepDown() 时出现几秒钟的停机时间也是绝对不能接受的。有没有办法安全地降级主节点，而不会出现来自数百个客户端的 MongoException？

谢谢!

附注这是脚本的实际版本，应该每月在低负载时间通过 cron-job 运行一次

// assuming we are a replica set ;-)
if(rs.isMaster().setName){
    try {
        //check if the script is run against a master
        if(rs.isMaster().ismaster){ //if so, step down as master        
            print("Connected to a PRIMARY node")
            print("Will try to step down as primary");
            rs.stepDown();

            // after stepdown connections are dropped. do an operation to cause reconnect:
            rs.isMaster();            

            // now ready to go.      
            // wait for another node to become primary -- it may need data from us for the last 
            // small sliver of time, and if we are already compacting it cannot get it while the 
            // compaction is running.            
            var counter = 1;
            while( 1 ) { 
                var m = rs.isMaster();
                if( m.ismaster ) {
                    print("ERROR: no one took over during our stepDown duration. we are primary again!");
                    assert(false);
                }

                if( m.primary ){ // someone else is, great
                    print("new master host is: "+m.primary);
                    break; 
                }
                print("waiting "+counter+" seconds");
                sleep(1000);
                counter++;
            }
        }else{
            print("Connected to a SECONDARY node");
            print("Going into Recovery Mode and Compacting");            
        }

        // someone else is primary, so we are ready to proceed with a compaction
        print(" ");
        print("Compacting...");
        print("- queue");
        printjson( db.runCommand({compact:"queue"}) );
        print(" ");

    } catch(e) { 
        print(" ");
        print("ACHTUNG! Exception:" + e);
        print(" ");
    }
}else{
    print('This script works with replica sets only');
}

Answer 1

Is there a way to safely demote a Primary node without MongoExceptions from the hundreds of clients?

否。 MongoDB 没有任何形式的“预期转换” 或“维护转换”。

执行此压缩循环是正确的做法。但是您将不得不等待维护窗口来压缩主数据库。

... is a high-availability web-queue.

你在使用 Capped Collections为了这？上限集合不需要压缩。上限集合中的对象不能调整大小，但这通常不是队列对象的问题。

虽然不是理想的解决方案，但它可能会解决您的问题。