db.bind('tag', {
    inc: function(name) {
        console.log('function inc');
        this.update({name: name}, {'$inc': {count: 1}}, {upsert: true}, function() {
            console.log('inc callback');
        });
    },
    
    dec: function(name) {
        console.log('function dec');
        var self = this;
        self.update({name: name}, {'$inc': {count: -1}}, {upsert: true}, function() {
            console.log('function dec callback');
            self.findOne({name: name}, function(error, item){
                if (item && item.count <= 0) {
                    self.remove(item);
                }
            })
        });
    },
    
    all: function(callback) {
        console.log('function all');
        this.find().toArray(callback);
    }
});

for (var i = 0; i < 1000; i++) {
    db.tag.inc('电影');
}

for (var i = 0; i < 500; i++) {
    db.tag.dec('电影');
}

db.tag.all(function(err, data) {
    console.log('function all callback');
    for (var index in data) {
        console.dir(data);
    }
})

多运行几次，看输出。会发现所有的操作是串行的。不知道是因为mongo native的驱动本身就是串行的还是因为mongoskin在处理的时候串行的。

为了测试是不是因为mongo native驱动的问题,然后用mongoose写了一个另外类似功能的代码，如下：

var mongoose = require('mongoose');
var Schema   = mongoose.Schema;
    
var TagSchema = new Schema({
    name  : { type: String, index: true },
    count : { type: Number },
});

TagSchema.statics.inc = function(name) {
    console.log('function inc');
    this.update({name: name}, {'$inc': {count: 1}}, {upsert: true}, function() {
        console.log('function inc callback!');
    });
}

TagSchema.statics.dec = function(name) {
    console.log('function dec');
    var self = this;
    self.update({name: name}, {'$inc': {count: -1}}, {upsert: true}, function(error) {
        console.log('function dec callback');
        if (error) {
            return;
        }
        
        self.findOne({name: name}, function(error, item) {
            if (error) {
                return;
            }
            
            if (item && item.count <= 0) {
                item.remove();
            } 
        });
    });
}

TagSchema.statics.all = function(callback) {
    console.log('function all');
    this.find().desc('count').exec(callback);
}


var Tag = mongoose.model('Tag', TagSchema);
mongoose.connect('mongodb://localhost/testdb2');

for (var i = 0; i < 1000; i++) {
    Tag.inc('电影');
}

for (var i = 0; i < 500; i++) {
    Tag.dec('电影');
}

Tag.all(function(err, data) {
    console.log('function all callback');
    for (var index in data) {
        console.dir(data);
    }
})

任然是运行看结果。你会发现，mongoose不是串行的。