sh.startBalancer(timeout, interval) 参数的含义？

Question

在 mongo db 命令中 sh.startBalancer(timeout, interval) 根据文档： timeout integer 要等待的毫秒数。 interval integer 等待每个周期休眠的毫秒数。 有人可以详细解释一下参数的含义。运行 sh.startBalancer() 时相同的默认值是什么？

Answer 1

sh.startBalancer(timeout, interval) 会做两件事：

1. 更新 config.settings {_id: 'balancer'} 文档，以允许平衡器运行。
2. 每隔'interval'毫秒检查是否已获取分布式锁，表明平衡器正在运行。它将一直执行到指定的“超时”毫秒，此时此方法将断言。

即使您达到了超时，config.settings 更新仍将保持在启用平衡器的位置。

如果您有兴趣查看代码，可以运行不带括号和参数的 sh.startBalancer，对其调用的函数执行相同操作。以下是针对 MongoDB 3.2.6 的：

> sh.startBalancer
function (timeout, interval) {
    sh.setBalancerState(true);
    sh.waitForBalancer(true, timeout, interval);
}

> sh.setBalancerState
function (onOrNot) {
    sh._getConfigDB().settings.update(
    {_id: "balancer"}, {$set: {stopped: onOrNot ? false : true}}, true);
}

> sh.waitForBalancer
function (onOrNot, timeout, interval) {

    // If we're waiting for the balancer to turn on or switch state or
    // go to a particular state
    if (onOrNot) {
        // Just wait for the balancer lock to change, can't ensure we'll ever see it
        // actually locked
        sh.waitForDLock("balancer", undefined, timeout, interval);
    } else {
        // Otherwise we need to wait until we're sure balancing stops
        sh.waitForBalancerOff(timeout, interval);
    }

}

> sh.waitForDLock
function (lockId, onOrNot, timeout, interval) {
    // Wait for balancer to be on or off
    // Can also wait for particular balancer state
    var state = onOrNot;
    var configDB = sh._getConfigDB();

    var beginTS = undefined;
    if (state == undefined) {
        var currLock = configDB.locks.findOne({_id: lockId});
        if (currLock != null)
            beginTS = currLock.ts;
    }

    var lockStateOk = function() {
        var lock = configDB.locks.findOne({_id: lockId});

        if (state == false)
            return !lock || lock.state == 0;
        if (state == true)
            return lock && lock.state == 2;
        if (state == undefined)
            return (beginTS == undefined && lock) ||
                (beginTS != undefined && (!lock || lock.ts + "" != beginTS + ""));
        else
            return lock && lock.state == state;
    };

    assert.soon(
        lockStateOk,
        "Waited too long for lock " + lockId + " to " +
            (state == true ? "lock" : (state == false ? "unlock" : "change to state " + state)),
        timeout,
        interval);
}