（10）ElasticSearch 版本控制

　　ElasticSearch是多用户操作的 ，需要保证数据的一致性，它用乐观锁保证数据的一致性，用户每次操作一次文档，它的版本号就会自动加1，用户对文档操作时，不需要对文档加锁、解锁，只需要指定要操作的版本。当版本号一致时，ElasticSearch会允许该操作顺利执行，而当版本号存在冲突时，ElasticSearch会提示冲突并抛出异常。ElasticSearch的版本号的取值范围是1到2^63-1。

　　1、内部版本控制

　　使用的是_version，下面是个简单例子：

POST /lib/books/_bulk
{"index":{"_id":1}}
{"title":"Html5","price":45}

　　显示版本号是3，"_version": 3，结果如下：

{
  "took": 19,
  "errors": false,
  "items": [
    {
      "index": {
        "_index": "lib",
        "_type": "books",
        "_id": "1",
        "_version": 3,
        "result": "updated",
        "_shards": {
          "total": 2,
          "successful": 1,
          "failed": 0
        },
        "_seq_no": 7,
        "_primary_term": 1,
        "status": 200
      }
    }
  ]
}

　　现在将价格修改为50，执行下面的语句，从结果中看到"_version"：4，版本号增加了1，

PUT /lib/books/1?version=3
{
  "title":"Html5",
  "price":50
}

{
  "_index": "lib",
  "_type": "books",
  "_id": "1",
  "_version": 4,
  "result": "updated",
  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0
  },
  "_seq_no": 8,
  "_primary_term": 1
}

　　现在版本号是4，如果执行版本号为5的操作，会报冲突，如下所示：

PUT /lib/books/1?version=5
{
  "title":"Html5",
  "price":50
}

{
  "error": {
    "root_cause": [
      {
        "type": "version_conflict_engine_exception",
        "reason": "[books][1]: version conflict, current version [4] is different than the one provided [5]",
        "index_uuid": "yL8koqQqQVqf3uoyvTcBHA",
        "shard": "3",
        "index": "lib"
      }
    ],
    "type": "version_conflict_engine_exception",
    "reason": "[books][1]: version conflict, current version [4] is different than the one provided [5]",
    "index_uuid": "yL8koqQqQVqf3uoyvTcBHA",
    "shard": "3",
    "index": "lib"
  },
  "status": 409
}

　　2、外部版本控制

　　所谓外部版本号指的是ElasticSearch与关系型数据库一块使用时，关系型数据实现锁机制中的版本号。ElasticSearch在处理外部版本号时与对内部版本号的处理有些不同。它不再检查_version是否与请求中指定的数值相同，而是检查当前的_version是否比指定的数值小。如果请求成功，那么外部的版本号就会被存储到文档中的_version中。为了保持_version与外部版本控制的数据一致，使用version_type=external。

　　参考下面的例子，现在版本号是4，执行为版本号为5的操作，可以成功，版本号变为5

PUT /lib/books/1?version=5&version_type=external
{
  "title":"Html5",
  "price":50
}

{
  "_index": "lib",
  "_type": "books",
  "_id": "1",
  "_version": 5,
  "result": "updated",
  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0
  },
  "_seq_no": 9,
  "_primary_term": 1
}

　　现在版本号是5，执行版本号是5的操作会报冲突，执行的只能大于当前的。如下：

PUT /lib/books/1?version=5&version_type=external
{
  "title":"Html5",
  "price":50
}

{
  "error": {
    "root_cause": [
      {
        "type": "version_conflict_engine_exception",
        "reason": "[books][1]: version conflict, current version [5] is higher or equal to the one provided [5]",
        "index_uuid": "yL8koqQqQVqf3uoyvTcBHA",
        "shard": "3",
        "index": "lib"
      }
    ],
    "type": "version_conflict_engine_exception",
    "reason": "[books][1]: version conflict, current version [5] is higher or equal to the one provided [5]",
    "index_uuid": "yL8koqQqQVqf3uoyvTcBHA",
    "shard": "3",
    "index": "lib"
  },
  "status": 409
}

　　现在版本号是5，执行一次版本号是12的，执行完后，版本号变为了12，如下：

PUT /lib/books/1?version=12&version_type=external
{
  "title":"Html5",
  "price":50
}

{
  "_index": "lib",
  "_type": "books",
  "_id": "1",
  "_version": 12,
  "result": "updated",
  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0
  },
  "_seq_no": 10,
  "_primary_term": 1
}