Mongodb：用Java（和JavaScript）调用db.printShardingStatus（）/ sh.status（） [英] Mongodb: db.printShardingStatus() / sh.status() call in Java (and JavaScript)
问题描述
我需要在Java代码内部进行分片后得到一个块列表。我的代码很简单，看起来像这样：
I need to get a list of chunks after sharding inside my Java code. My code is simple and looks like this:
Mongo m = new Mongo( "localhost" , 27017 );
DB db = m.getDB( "admin" );
Object cr = db.eval("db.printShardingStatus()", 1);
调用eval（）会返回错误：
A call of eval() returns an error:
Exception in thread "main" com.mongodb.CommandResult$CommandFailure: command failed [$eval]: { "serverUsed" : "localhost/127.0.0.1:27017" , "errno" : -3.0 , "errmsg" : "invoke failed: JS Error: ReferenceError: printShardingStatus is not defined src/mongo/shell/db.js:891" , "ok" : 0.0}
at com.mongodb.CommandResult.getException(CommandResult.java:88)
at com.mongodb.CommandResult.throwOnError(CommandResult.java:134)
at com.mongodb.DB.eval(DB.java:340)
at org.sm.mongodb.MongoTest.main(MongoTest.java:35)
实际上，如果我们查看db.js的代码，在第891行中调用了一个未在文件中定义的方法printShardingStatus（）。在utils_sh.js文件中的sh.status（）方法内部，甚至还有一条注释：
And, really, if we look into the code of db.js, in line 891 there is a call to a method printShardingStatus() that is not defined inside a file. Inside of sh.status() method in utils_sh.js file, there is even a comment:
// TODO：移动实际的commadn这里
// TODO: move the actual commadn here
重要的是，当我在mongo命令行中运行这些命令时，一切正常！
Important to mention, when I run these commands in mongo command line, everything works properly!
我的问题是：
- 是否还有其他可能在Java代码中获得完整的分片状态？ （例如，使用DB.command（）方法）
- 如果没有，还有其他建议如何避免我的问题吗？
推荐答案
许多shell的辅助函数不能用于服务器端代码执行。在 printShardingStatus（）的情况下，它是有意义的，因为没有用于打印输出的控制台，你宁愿返回一个字符串。值得庆幸的是，您应该能够提取shell函数的源代码并在应用程序中重新实现它（例如，连接返回的字符串而不是直接打印）。
Many of the shell's helper functions are not available for server-side code execution. In the case of printShardingStatus(), it makes sense because there isn't a console to use for printing output and you'd rather have a string returned. Thankfully, you should be able to pull up the source of the shell function and reimplement it in your application (e.g. concatenating a returned string instead of printing directly).
$ mongo
MongoDB shell version: 2.2.0
connecting to: test
> db.printShardingStatus
function (verbose) {
printShardingStatus(this.getSiblingDB("config"), verbose);
}
那么，让我们看一下 printShardingStatus（） function ...
So, let's look at the printShardingStatus() function...
> printShardingStatus
function (configDB, verbose) {
if (configDB === undefined) {
configDB = db.getSisterDB("config");
}
var version = configDB.getCollection("version").findOne();
// ...
}
在全部转将输出语句转换为字符串连接，您需要确保其他DB方法都可供您使用。在性能方面，我认为最好的选择是将此函数的内部移植到Java并完全避免服务器端的JS评估。如果你深入研究 printShardingStatus（）函数，你会发现它只是在配置上发出 find（）数据库以及一些 group（）查询。
Before turning all of the output statements into string concatenation, you'd want to make sure the other DB methods are all available to you. Performance-wise, I think the best option is to port the innards of this function to Java and avoid server-side JS evaluation altogether. If you dive deeper into the printShardingStatus() function, you'll see it's just issuing find() on the config database along with some group() queries.
如果你确实想坚持评估JS而不愿意将此代码保存在Java应用程序中，您还可以查看存储服务器端的JS功能。
If you do want to stick with evaluating JS and would rather not keep this code within your Java application, you can also look into storing JS functions server-side.
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