[AWS - DA - Guru] DynamoDB Exam Tips

DynamoDB Consistency Models

Eventually Consistent Reads

Consistency across all copies of data is usually reached within a second

Best for read perforamnce

Strongly Consistent Reads

A strongly consistent read always reflects all successful writes. Writes are reflected across all 3 locations at once.

Best for read consistency

ACID Transactions

DynamoDB Transactions provide the ability to perform ACID Transactions (Atomic, Consistent, Isolated, Durable) Read or write multiple items across multiple tables as an all or nothing operations.

Goo for payment process.

 

Primary Keys

Partition Key

A unique attribute.

Composite Key

1. Partition Key + Sort Key

When partition key is not unique. 

For example: Forum posts, Users post multiple messages.

Combination of:

• User_ID
• Sort key (timestamp)

　　2. A Unique Combination

Items in the table may have the same partition key, but they must have a differnet sort key.

　　3. Storage

All items with the same partition key are stored together and then sorted according to the sort key value.

 

Access Control

1. IAM
2. IAM Permission: access and create DynamoDB tables
3. IAM Roles: Enabling temporary access to DynamoDB

Restricting User Access

You can also use a special IAM Condtion to restrict user access to only their own records.

IAM condition paramter dynamodb:LeadingKeys allow users to access only the itmes where partition key value matches their User_ID

 

 

Indexes Deepdive

Flexible Querying

Query based on an attribute that is not the primary key.

DynamoDB allows you to run a query on non-primary key attributes using global secondary indexes and local secodnary index.

Local Secondary Index

• Must be created when creating the table
• Same partition key and different sort key to your table

Gloabl Secondary Index

• Different partition key and different sort key to your table
• Can be created any time
• Creates copy of the data in a table (data is available via GSI after some delay)

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Creating indexes in DynamoDB

1. Create table with Partition key and sort key

 

2. Add Local secondary Indexes (LSI)

Recall that for LSI, the partition key is the same as Primary key's partiion key, when this can only be created during creating new table.

Remeber to check the checkbox for LSI

3. After creating table, we can create Global secondary index

Recll that GSI can be created any time, the partiion key can be different from Primary key's partition key.

 

4. GSI can ONLY be used by CLI / SDK, not possible yet with GUI.

Therefore when you do Query, you can only see the LSI and Primary key.

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Scan vs Query

Query

A query find items in a table based on the primary key attribute and a distinct value to search for.

 

 

Scan

A scan operation examines every item in the table. By default, it returns all data attributes.

Use the ProjectionExpression parameter to refine the scan to only return the attributes you want. 

 

Query vs Scan

1. Query is More efficient Than a Scan: Because A Scan dump the entire table and filters out the values to provide the desired result, removing the unwanted data.
2. Extra Step: Adds an extra step of remoing the data you don't want. As table grows, the scan operation takes longer.
3. Provisioned Thoughput: A scan operation on a large table can use up the provisioned throughput for a large table in just a single operation.

 

Improving Performance

• Set a smaller page size (e.g page size of 40 items)
• Running a larger number of smaller operations will allow other requests to succeed iwthout throttling.
• AVOID SCANS!
• Design tables in a way that you can use the Query, Get, or BatchGetItem APIs.

 

Improving Scan Performance 

• Isolate scan operations to specific tables and segregate them from your mission critical traffic. Even if that means writing data to 2 different tables
• By default, a scan operation processes data sequentially in returning 1MB increments before moving on to retrieve the next 1 MB of data. It can only scan one partition at a time.
• You can configure DynamoDB to use Parallel scans instead of logically dividing a table or index into segments and scanning each segment in parallel
• Best to AVOID parallel scans if your table or index is already incurring heavy read / write activity from other applications.

 

DynamoDB Provisioned Throuhhput

DynamoDB Provisioned Throughput is measured in Capacity Units.

When you create your table, you specify your requirements in terms of Read Capacity Units and Write Capacity Units.

Write Capacity Units

1 * 1KB write per second

Strongly Consistent Reads

1 * 4KB read per second

Eventually Consistent

2 * 4KB per second

 

Your application needs to read 80 items per second, each item is 3 KB in size. You need strongly consistent reads. How many read capacity units will you need?

1. Calculate the size of each item / 4 KB -> 3KB/4KB = 0.75, round to 1, so need 1 read capacity unit

2. Multiply by the number of read operations per second -> 80 * 1 = 80

 

How about eventually consistent read?

Stongly consistent read / 2 -> 80 / 2 = 40

 

You want to write 100 items per scond. Each item is 512 bytes in size, how many write capacity units do you think you will need?

512byte/1KB = 0.5

round to 1

100 * 1 = 100

 

DynamoDB Low-Level API

Doc: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Programming.LowLevelAPI.html

DynamoDB provide SDK for many programming language, but if there is no such SDK for your project using langauge, you can use low-level API to using DynamoDB

The Amazon DynamoDB low-level API is the protocol-level interface for DynamoDB. At this level, every HTTP(S) request must be correctly formatted and carry a valid digital signature.

The low-level DynamoDB API uses JavaScript Object Notation (JSON) as a wire protocol format. 

Requet Format:

POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date> 
X-Amz-Target: DynamoDB_20120810.GetItem

{
    "TableName": "Pets",
    "Key": {
        "AnimalType": {"S": "Dog"},
        "Name": {"S": "Fido"}
    }
}

• The Authorization header contains information required for DynamoDB to authenticate the request. Need to sign the request using AWS access keys and Signature Version 4
• The X-Amz-Target header contains the name of a DynamoDB operation: GetItem.
• The payload (body) of the request contains the parameters for the operation, in JSON format. For the GetItem operation, the parameters are TableName and Key.

 

DynamoDB API

It is important to know about what apis you can use for DynamoDB.

Such as GetItem, BatchGetItem, PutItem, UpdateItem, ConditionalWrites, ConditionalDelete 

Here we just memetion Conditional write/delete:

• Accept a write / update only if conditions are respected, otherwise reject
• Help with concurrent access to items
• No performacne impact

 

DynamoDB Indexes and Throttling 

GSI:

• if the writes are throttled on GSI, then the main table will be throttle
• Event if the WCU on the main tables are fine
• We need to choose GSI partition key carefully
• Make sure GSI has enough WCU as well

GSI creates a NEW Table, when creating the GSI, you need to define RCU / WCU as well.

 

DynamoDB Concurrency

• DynamoDB has a feature called "Conditional Update / Delete"
• That means that you can ensure an item hasn't changed before altering it
• That makes DynamoDB an Optimistic locking / concurrency database

DynamoDB On-Demand Capacity

 

DynamoDB Accelerator (DAX)

Solve the Hot Key Problem (too many reads)

TTL for 5 mins

DynamoDB Accelerator (or DAX) is a fully managed, clustered in-memory cache for DynamoDB.

Delivers up to 10x read performance improvement. Microsecond performance for millions of requests per second.

 

 

How does it work?

DAX is a write-through caching service. Data is written to the cache and the backend store at the same time.

This allows you to point your DynamoDB API calls at the DAX cluster first.

If the item you are query is in the cache (cache hit), DAX return the result to application.

If the item is not available (cache miss), then DAX performs an eventually consistent GetItem operation against DynamoBD and returns the result of the API call.

 

When is DAX Not suitable?

Caters for eventually consistent reads only.

• Not suitable for applications that require stronly consistent reads.
• Not suitable for applications which are mainly write-intensive
• Application that do not perform many read operations.
• Applications that do not require microsecond response times

 

DAX vs ElastiCache 

• DAX: caching individual object for Query or Scan for simple use cases
• ElastiCachce: more advance, store aggregation result, so next time you don't need to do the same query, just return the result.

DynamoDB Time To Live (TTL)

Defines an expiry time of your data. Expired itmes marked for deletion in next 48 hours.

• Greate for removing irrlevant or old data, (e.g, session data, event logs, and temporary data)
• Reduces the cost of your table by automatically removing data which is no longer relevant.

DynamoDB Streams

Good for trigger Lambda event.

 

Recoreds are not retroactively populated in a stream after enabling it. Only the future data after enabled stream. 

 

4 Different Stream types

• KEYS_ONLY: Only the key attributes of the modified item
• NEW_IMAGE: The entire item, as it appears after it was modfiied
• OLD_IMAGE: The entire item, as it apperared before it was modified
• NEW_AND_OLD_IMAES: Both the new and the old images of the item

 

DynamoDB Exponential Backoff Exam

1. ProvisionedThroughputExceeded: ProvisionedThroughputExceededException error means the number of requests is too high.
2. Better Flow Control: Exponential backoff improves flow by retrying requests using progressively longer waits.
3. Every AWS SDK: Exponential backoff is a feature of every AWS SDK and applies to many serices within AWS.

 

Atomic is mainly increase / descrese by the number

 

Using DynamoDB, max size is 400KB for each item.

If larger than 400KB, you need to save the item into S3.

Then save the metadata into DynamoDB table.

Then clinet will get object from S3.

 

If we save itme into S3, we lost the search functionality. Way to solve it is trigger a Lambda function, write metadata into DynamoDB then we can do the searching in DynamoDB table.

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Global Secondary indexes has a limit 20.

 

You need use filter, by default there is no filter.

 

https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html

Only `GetItem`, `UpdateItem`, `DeleteItem` for single item

And `BatchGetItem`, `BatchExecuteStatement`, `BatchWriteItem`.

 

(Item size / read size) * itemPreSecond / 2 (if eventually consistent)

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来源： https://www.cnblogs.com/Answer1215/p/14667218.html