amazon-dynamodb - DynamoDB放置项目ConditionalCheckFailedException

Question

给定下面定义的表模式(create-table.json)，我先使用put-item和add-event1.json调用add-event2.json后，收到以下错误:

A client error (ConditionalCheckFailedException) occurred when calling the PutItem operation: The conditional request failed

为什么ConditionExpression不允许我同时写两条记录？ (我预计第二次操作后会有2条记录)

我怀疑这是因为使用了非关键条件，但是在docs中我没有看到任何表示缺乏对非关键条件的支持的信息。

创建表

$ aws dynamodb create-table --cli-input-json file://create-table.json

create-table.json

{
    "TableName": "EVENTS_TEST",
    "KeySchema": [
      { "AttributeName": "aggregateId", "KeyType": "HASH" },
      { "AttributeName": "streamRevision", "KeyType": "RANGE" }
    ],
    "AttributeDefinitions": [
      { "AttributeName": "aggregateId", "AttributeType": "S" },
      { "AttributeName": "streamRevision", "AttributeType": "N" }
    ],
    "ProvisionedThroughput": {
      "ReadCapacityUnits": 10,
      "WriteCapacityUnits": 10
    }
 }

添加第一条记录

$ aws dynamodb put-item --cli-input-json file://add-event1.json

add-event1.json

{
  "TableName": "EVENTS_TEST",
  "Item": {
    "aggregateId": { "S": "id" },
    "id": { "S": "119" },
    "context": { "S": "*" },
    "aggregate": { "S": "*" },
    "streamRevision": { "N": "0" },
    "commitId": { "S": "1119" },
    "commitSequence": { "N": "0" },
    "commitStamp": { "N": "1470185631511" },
    "dispatched": { "BOOL": false },
    "payload": { "S": "{ \"event\": \"bla\" }" }
  },
  "ExpressionAttributeNames": { "#name": "aggregate" },
  "ConditionExpression": "attribute_not_exists(aggregateId) and attribute_not_exists(streamRevision) and #name <> :name and context <> :ctx",
  "ExpressionAttributeValues": {
    ":name": { "S": "*" },
    ":ctx": { "S": "*" }
  }
}

添加第二条记录

$ aws dynamodb put-item --cli-input-json file://add-event2.json

add-event2.json

{
  "TableName": "EVENTS_TEST",
  "Item": {
    "aggregateId": { "S": "id" },
    "id": { "S": "123" },
    "context": { "S": "myCtx" },
    "aggregate": { "S": "myAgg" },
    "streamRevision": { "N": "0" },
    "commitId": { "S": "1123" },
    "commitSequence": { "N": "0" },
    "commitStamp": { "N": "1470185631551" },
    "dispatched": { "BOOL": false },
    "payload": { "S": "{ \"event\": \"bla2\" }" }
  },
  "ExpressionAttributeNames": { "#name": "aggregate" },
  "ConditionExpression": "aggregateId <> :id and streamRevision <> :rev and #name <> :name and context <> :ctx",
  "ExpressionAttributeValues": {
     ":id": { "S": "id" },
     ":rev": { "N": "0" },
     ":name": { "S": "myAgg" },
     ":ctx": { "S": "myCtx" }
  }
}

Answer 1

您的目标是保存两个记录。这里有2个问题

通过选择哈希和范围键，无法保存两个记录

哈希和范围键的组合使记录变得唯一。
事件1和事件2的哈希键和范围键具有相同的值。
因此，第二个放置项将简单地覆盖第一个记录。

您的ConditionExpression禁止将记录1替换为记录2

在放置记录之前对ConditionExpression进行求值。您的表达式失败是因为，当您要插入事件2时，DynamoDB会发现已经存在具有aggregateId“id1”的记录。该条件在“attribute_not_exists(aggregateId)”上失败，并且您收到ConditionalCheckFailedException。此表达式防止记录2覆盖记录1。

如果要保存两个记录，则必须提出另一种选择，即更好地表示数据项的唯一性的哈希键和/或范围键。您无法使用ConditionExpression解决此问题。