Transaction condition expressions

Transaction condition expressions are available in requests of all four types of operations in TransactWriteItems, namely, PutItem, DeleteItem, UpdateItem, and ConditionCheck.

For PutItem, DeleteItem, and UpdateItem, the transaction condition expression is optional. For ConditionCheck, the transaction condition expression is required.

Example 1

The following transactional DeleteItem function request handler does not have a condition expression. As a result, it deletes the item in DynamoDB.

export const onPublish = {
  request(ctx) {
    const table = "events"
    return ddb.transactWrite({
      items: ctx.events.map(({ payload }) => ({
        deleteItem: { table, key: { id: payload.id } }
      }))
    })
  },
  response: (ctx) => ctx.events
}

Example 2

The following transactional DeleteItem function request handler does have a transaction condition expression that allows the operation succeed only if the author of that post equals a certain name.

export const onPublish = {
  request(ctx) {
    return ddb.remove({
      items: ctx.events.map(({ payload }) => ({
        deleteItem: { 
          table: 'events', 
          key: { id: payload.id }, 
          condition: { owner: { eq: payload.owner } }
        }
      }))
    })
  },
  response: (ctx) => ctx.events
}

If the condition check fails, it will cause TransactionCanceledException and the error detail will be returned in ctx.result.cancellationReasons.