Appengine之上的键值

Question

Although appengine already is schema-less, there still need to define the entities that needed to be stored into the Datastore through the Datanucleus persistence layer. So I am thinking of a way to get around this; by having a layer that will store Key-value at runtime, instead of compile-time Entities.

The way this is done with Redis is by creating a key like this:

private static final String USER_ID_FORMAT = "user:id:%s";
private static final String USER_NAME_FORMAT = "user:name:%s";

From the docs Redis types are: String , Linked-list , Set , Sorted set . I am not sure if there's more.

As for the GAE datastore is concerned a String "Key" and a "Value" have to be the entity that will be stored.

Like:

public class KeyValue {
 private String key;
 private Value value; // value can be a String, Linked-list, Set or Sorted set etc.
 // Code omitted
}

The justification of this scheme is rooted to the Restful access to the datastore (that is provided by Datanucleus-api-rest)

Using this rest api, to persist a object or entity:

POST http://datanucleus.appspot.com/dn/guestbook.Greeting
{"author":null,
  "class":"guestbook.Greeting",
  "content":"test insert",
  "date":1239213923232}

The problem with this approach is that in order to persist a Entity the actual class needs to be defined at compile time; unlike with the idea of having a key-value store mechanism we can simplify the method call:

POST http://datanucleus.appspot.com/dn/org.myframework.KeyValue
{ "class":"org.myframework.KeyValue"
  "key":"user:id:johnsmith;followers",
  "value":"the_list",
}

Passing a single string as "value" is fairly easy, I can use JSON array for list, set or sorted list. The real question would be how to actually persist different types of data passed into the interface. Should there be multiple KeyValue entities each representing the basic types it support: KeyValueString? KeyValueList? etc.

Answer 1

看起来您正在使用基于JSON的REST API，那么为什么不只将Value存储为JSON字符串呢？

Answer 2

You do not need to use the Datanucleus layer, or any of the other fine ORM layers (like Twig or Objectify). Those are optional, and are all based on the low-level API. If I interpret what you are saying properly, perhaps it already has the functionality that you want. See: https://developers.google.com/appengine/docs/java/datastore/entities

Answer 3

Datanucleus is a specific framework that runs on top of GAE. You can however access the database at a lower, less structured, more key/value-like level - the low-level API. That's the lowest level you can access directly. BTW, the low-level-"GAE datastore" internally runs on 6 global Google Megastore tables, which in turn are hosted on the Google Big Table database system. Saving JSON as a String works fine. But you will need ways to retrieve your objects other than by ID. That is, you need a way to index your data to support any kind of useful query on it.