第八节 处理并发读写操作

1、并发控制的必要性

• 两个 Web 程序同时更新某个⽂档，如果缺乏有效的并发，会导致更改的数据丢失
• 悲观并发控制
  • 假定有变更冲突的可能。会对资源加锁，防止冲突。例如数据库⾏锁
• 乐观并发控制
  • 假定冲突是不会发⽣的，不会阻塞正在尝试的操作。如果数据在读写中被修改，更新将会失败。 应⽤程序决定如何解决冲突，例如重试更新，使⽤新的数据，或者将错误报告给⽤户
  • ES 采⽤的是乐观并发控制

2、ES 的乐观并发控制

• ES 中的⽂档是不可变更的。如果你更新⼀个⽂档，会将就文档标记为删除，同时增加一个全新的⽂档。同时⽂档 的 version 字段加 1
• 内部版本控制
  • If_seq_no + If_primary_term
• 使⽤外部版本(使⽤其他数据库作为主要数据存储)
  • version + version_type=external

DELETE products
PUT products

Output

{
  "acknowledged" : true,
  "shards_acknowledged" : true,
  "index" : "products"
}

GET products/_doc/1

{
  "_index" : "products",
  "_type" : "_doc",
  "_id" : "1",
  "_version" : 1,
  "_seq_no" : 0,
  "_primary_term" : 1,
  "found" : true,
  "_source" : {
    "title" : "iphone",
    "count" : 100
  }
}

2-1 内部版本控制: If_seq_no + If_primary_term

PUT products/_doc/1?if_seq_no=0&if_primary_term=1
{
  "title":"iphone",
  "count":100
}

Output:

{
  "_index" : "products",
  "_type" : "_doc",
  "_id" : "1",
  "_version" : 2,
  "result" : "updated",
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "failed" : 0
  },
  "_seq_no" : 1,
  "_primary_term" : 1
}

• if_seq_no=0&if_primary_term=1
• 改变 count 到 99

PUT products/_doc/1?if_seq_no=0&if_primary_term=1
{
  "title":"iphone",
  "count":99
}

Output: 409 - Conflict

{
  "error" : {
    "root_cause" : [
      {
        "type" : "version_conflict_engine_exception",
        "reason" : "[1]: version conflict, required seqNo [0], primary term [1]. current document has seqNo [1] and primary term [1]",
        "index_uuid" : "YrNla_cjTweku9BxzjH2Ig",
        "shard" : "0",
        "index" : "products"
      }
    ],
    "type" : "version_conflict_engine_exception",
    "reason" : "[1]: version conflict, required seqNo [0], primary term [1]. current document has seqNo [1] and primary term [1]",
    "index_uuid" : "YrNla_cjTweku9BxzjH2Ig",
    "shard" : "0",
    "index" : "products"
  },
  "status" : 409
}

2-2 使⽤外部版本： version + version_type=external

PUT products/_doc/1?version=30000&version_type=external
{
  "title":"iphone",
  "count":100
}

Output

{
  "_index" : "products",
  "_type" : "_doc",
  "_id" : "1",
  "_version" : 30000,
  "result" : "updated",
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "failed" : 0
  },
  "_seq_no" : 2,
  "_primary_term" : 1
}

• "_version" : 30000
• 改变 count 到 99

PUT products/_doc/1?version=30000&version_type=external
{
  "title":"iphone",
  "count":99
}

Output: 409 - Conflict

{
  "error" : {
    "root_cause" : [
      {
        "type" : "version_conflict_engine_exception",
        "reason" : "[1]: version conflict, current version [30000] is higher or equal to the one provided [30000]",
        "index_uuid" : "YrNla_cjTweku9BxzjH2Ig",
        "shard" : "0",
        "index" : "products"
      }
    ],
    "type" : "version_conflict_engine_exception",
    "reason" : "[1]: version conflict, current version [30000] is higher or equal to the one provided [30000]",
    "index_uuid" : "YrNla_cjTweku9BxzjH2Ig",
    "shard" : "0",
    "index" : "products"
  },
  "status" : 409
}