add offical sample question answer

README.md

@@ -1,9 +1,168 @@
 # CDA_resource
 for AWS Certified Developer – Associate test prepare
+這裡是以筆記形式紀錄我在CDA考試所準備的東西
+如果需要更詳細的操作
+請至[我的網誌](http://g23988.blogspot.tw/)觀看
+
+
+## 重點節錄
+
+## 官方範例
+聽說必出，所以自己先拿來回答了
+有找到正確答案再修正
+`
+Which of the following statements about SQS is true?
+A. Messages will be delivered exactly once and messages will be delivered in First in, First out order
+B. Messages will be delivered exactly once and message delivery order is indeterminate
+C. Messages will be delivered one or more times and messages will be delivered in First in, First out order
+D. Messages will be delivered one or more times and message delivery order is indeterminate
+`
+> D
+
+SQS可以提供多次讀取，雖說目前已經有FIFO (First in, First out)，但是還不會反映在考試上
+
+`
+EC2 instances are launched from Amazon Machine Images (AMIs). A given public AMI:
+A. can be used to launch EC2 instances in any AWS region
+B. can only be used to launch EC2 instances in the same country as the AMI is stored
+C. can only be used to launch EC2 instances in the same AWS region as the AMI is stored
+D. can only be used to launch EC2 instances in the same AWS availability zone as the AMI is stored
+`
+> C
+
+AMI僅會儲存在當下的region，如果想要讓AMI能夠在別的region使用，可以使用 Copy AMI 功能複製到別的region
+
+`
+Company B provides an online image recognition service and utilizes SQS to decouple system
+components for scalability. The SQS consumers poll the imaging queue as often as possible to keep endto-end
+throughput as high as possible. However, Company B is realizing that polling in tight loops is
+burning CPU cycles and increasing costs with empty responses. How can Company B reduce the number
+of empty responses?
+A. Set the imaging queue VisibilityTimeout attribute to 20 seconds
+B. Set the imaging queue ReceiveMessageWaitTimeSeconds attribute to 20 seconds
+C. Set the imaging queue MessageRetentionPeriod attribute to 20 seconds
+D. Set the DelaySeconds parameter of a message to 20 seconds
+`
+> B
+
+因為向SQS要Queue，SQS會馬上回應。
+如果SQS是空的那就慘了，他會一直一直回應直到把你CPU吃滿
+所以AWS有設計一個long pool，最長可以設定到20秒
+那個選項在aws console 上的label是 Receive Message Wait Time
+
+`
+You attempt to store an object in the US-STANDARD region in Amazon S3, and receive a confirmation
+that it has been successfully stored. You then immediately make another API call and attempt to read
+this object. S3 tells you that the object does not exist. What could explain this behavior?
+A. US-STANDARD uses eventual consistency and it can take time for an object to be readable in a bucket.
+B. Objects in Amazon S3 do not become visible until they are replicated to a second region.
+C. US-STANDARD imposes a 1 second delay before new objects are readable
+D. You exceeded the bucket object limit, and once this limit is raised the object will be visible.
+`
+> A
+
+S3有設計最終一致性的功能(eventual consistency)，所以最新的PUT object並不會馬上反映在讀取上
+直到所有的準備都完成，才能取得object
+順帶一提，美國標準(US-STANDARD) 已經改名為 北維吉尼亞 (N.Virginia us-east-1)
+
+`
+You have reached your account limit for the number of CloudFormation stacks in a region. How do you
+increase your limit?
+A. Make an API call
+B. Contact AWS
+C. Use the console
+D. You cannot increase your limit
+`
+> B
+
+找飯店？trivago！
+
+`
+Which statements about DynamoDB are true? (Pick 2 correct answers)
+A. DynamoDB uses a pessimistic locking model
+B. DynamoDB uses optimistic concurrency control
+C. DynamoDB uses conditional writes for consistency
+D. DynamoDB restricts item access during reads
+E. DynamoDB restricts item access during writes
+`
+> BC
+
+這題答案我也不確定，不過這兩項看起來是DynamoDB能夠辦到的
+
+`
+What is one key difference between an Amazon EBS-backed and an instance-store backed instance?
+A. Instance-store backed instances can be stopped and restarted
+B. Auto scaling requires using Amazon EBS-backed instances
+C. Amazon EBS-backed instances can be stopped and restarted
+D. Virtual Private Cloud requires EBS backed instances
+`
+> C
+
+Instance-store 是設計在各台虛擬主機上的暫存空間，而EBS則是在AWS Storage設備上的iSCSI空間
+所以Instance-store 會隨著stop，或是terminate而消失
+但是兩者reboot都是沒問題的
+
+`
+A corporate web application is deployed within an Amazon VPC, and is connected to the corporate data
+center via IPSec VPN. The application must authenticate against the on-premise LDAP server. Once
+authenticated, logged-in users can only access an S3 keyspace specific to the user.
+Which two approaches can satisfy the objectives?
+A. The application authenticates against LDAP. The application then calls the IAM Security Service to login
+to IAM using the LDAP credentials. The application can use the IAM temporary credentials to access the
+appropriate S3 bucket.
+B. The application authenticates against LDAP, and retrieves the name of an IAM role associated with the
+user. The application then calls the IAM Security Token Service to assume that IAM Role. The application
+can use the temporary credentials to access the appropriate S3 bucket.
+C. The application authenticates against IAM Security Token Service using the LDAP credentials. The
+application uses those temporary AWS security credentials to access the appropriate S3 bucket.
+D. Develop an identity broker which authenticates against LDAP, and then calls IAM Security Token Service
+to get IAM federated user credentials. The application calls the identity broker to get IAM federated user
+credentials with access to the appropriate S3 bucket.
+E. Develop an identity broker which authenticates against IAM Security Token Service to assume an IAM
+Role to get temporary AWS security credentials. The application calls the identity broker to get AWS
+temporary security credentials with access to the appropriate S3 bucket.
+`
+> BD
+
+在LDAP做auth 使用情景下
+應用程式拿role，再用role給予的temporary credentials 做存取 (預設一小時後過期 3600s)
+使用者則拿federated user credential 做存取
+
+`
+You run an ad-supported photo sharing website using S3 to serve photos to visitors of your site. At some
+point you find out that other sites have been linking to the photos on your site, causing loss to your
+business. What is an effective method to mitigate this?
+A. Use CloudFront distributions for static content.
+B. Remove public read access and use signed URLs with expiry dates.
+C. Block the IPs of the offending websites in Security Groups.
+D. Store photos on an EBS volume of the web server.
+`
+> B
+
+先把暴露在外面的URL先拿掉，避免盜連
+然後使用signed urls去設定連結的過期時間
+
+`
+Your application is trying to upload a 6 GB file to Simple Storage Service and receive a "Your proposed
+upload exceeds the maximum allowed object size." error message. What is a possible solution for this?
+A. None, Simple Storage Service objects are limited to 5 GB
+B. Use the multi-part upload API for this object
+C. Use the large object upload API for this object
+D. Contact support to increase your object size limit
+E. Upload to a different region
+`
+> B
+
+AWS建議超過100MB就使用 Multi-part upload 了
+當然不做也可以，但是會被限制在 5 GB
+單個object最大可以存 5 TB
+而bucket能存放的object則是無限。
+
 
 ## 問題集
  - [範例問題](https://github.com/g23988/ACD_resource/tree/master/Sample_question/sample_question_200)
- - [官方範例問題解答](https://github.com/g23988/ACD_resource/blob/master/Sample_question/Pages%20from%20Amazon_ACDA.pdf)
+ - [範例問題解答](https://github.com/g23988/ACD_resource/blob/master/Sample_question/Pages%20from%20Amazon_ACDA.pdf)
+ - [網路搜尋範例問題](https://blog.cloudthat.com/sample-questions-for-amazon-web-services-certified-developer-associate-certification/)
 
 
 ## Resource