Atomicity and Transactions原子性和事务

Example示例

Consider a collection with this document:考虑一个包含此文档的集合：

db.games.insertOne( { _id: 1, score: 80 } )

These update operations occur concurrently:这些更新操作同时发生：

// Update A
db.games.updateOne(
   { score: 80 },
   {
      $set: { score: 90 }
   }
)

// Update B
db.games.updateOne(
   { score: 80 },
   {
      $set: { score: 100 }
   }
)

One update operation sets the document's score field to either 90 or 100. After this update completes, the second update operation no longer matches the query predicate { score: 80 }, and is not performed.一个更新操作将文档的score字段设置为90或100。此更新完成后，第二次更新操作不再与查询谓词{ score: 80 }匹配，因此不会执行。

// Update A
db.games.updateOne(
   { _id: 1 },
   {
      $set: { score: 90 }
   }
)

// Update B
db.games.updateOne(
   { _id: 1 },
   {
      $set: { score: 100 }
   }
)

To prevent conflicting write operations when your update filter is on a different field than the one being updated, use the $inc operator.当更新筛选器位于与正在更新的字段不同的字段时，若要防止发生冲突的写入操作，请使用$inc运算符。

For example, consider if these update operations occur concurrently:例如，考虑这些更新操作是否同时发生：

// Update A
db.games.updateOne(
   { _id: 1 },
   {
      $inc: { score: 10 }
   }
)

// Update B
db.games.updateOne(
   { _id: 1 },
   {
      $inc: { score: 20 }
   }
)

After one update operation completes, the remaining operation still matches the query predicate { _id: 1 }. However, because the operations modify the current value of score, they don't overwrite each other. Both updates are reflected and the resulting score is 110.在一个更新操作完成后，剩余的操作仍然与查询谓词{ _id: 1 }匹配。但是，由于这些操作修改了score的当前值，因此它们不会相互覆盖。这两个更新都得到了反映，结果score为110。