How to automatically choose mongodb based on location?

This can be achieved using tag based sharding or tag aware sharding. Basically, it works like this:

1. You have a value in your documents which denotes wether this specific document belongs to your chinese or your japanese data set, eg. location: 'CHN' or location: 'JPN'
2. You set up a sharded cluster, where one shard is in a datacenter in China and the other resides in Japan.
3. You assign tags to each shard. In your case it would make sense to tag the Chinese shard with CHN and the Japanese shard with JPN

4. Now you can configure the shards that the Japanese shard should hold all documents with location: 'JPN' and the Chinese shard to hold all documents to hold all documents with the location: CHN with assigning those values to the accordingly tagged shards:

   sh.addTagRange("database.collcetion", { location: "CHN" }, { location: "CHN" }, "CHN") sh.addTagRange("database.collcetion", { location: "JPN" }, { location: "JPN" }, "JPN")

5. Use location as your shard key.

6. Now, you can use a single database and the data will be automatically geographically distributed and your application's queries will always be routed to the geographically matching data center.

Could you post the part of your code where you are sending the data to MongoDB, since you should have opened two connection.

Open two connection, one will be MongoChina, the other MongoJapan.

When a user connect, get his ip adress, and choose where which connection use based on this.

Pseudo code:

var MongoChina = //Connecting to the chinese mongo server,
    MongoJapan = //Connecting to the japanese mongo server;

function isJapanese(ip) {
    //Return true if IP is in japanese ip range
    //http://www.nirsoft.net/countryip/jp.html
}

function isChinese(ip) {
    //Return true if IP is in chinese ip range
    //http://www.nirsoft.net/countryip/cn.html
}

on.('user_connecting', function (req, res) {
    var ip = //Extract the user ip from the request;
    if (isJapanese(ip)) {
        getDataToSendToMongo(function (data) {
            MongoJapan.send(data);
        });
    } else if (isChinese(ip)) {
        getDataToSendToMongo(function (data) {
            MongoChina.send(data);
        });
    } else {
        //Do whatever you like
    }
});