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DynamoDB Cheatsheet – Everything you need to know about Dynamo DB for the 2020 AWS Certified Developer Associate Certification

andrewbrown profile image Andrew Brown πŸ‡¨πŸ‡¦ Originally published at freecodecamp.org ・9 min read

The emergence of cloud services has changed the way we build web-applications. This in turn has changed the responsibilities of a Web Developer.

We used to build everything into a single web-application on a single server. This encompassed multiple responsibilities such as storage, databases, authentication, background jobs, caching, and more.

Cloud services allows us to reduce the complexity of our web-app and web-servers by pushing the responsibilities to these highly available, scalable, and durable cloud services.

A Web Developer who knows how to deploy and integrate cloud services with a web-application is what we call a Cloud Engineer.

If you want to fast-track your career as a Web Developer in 2020 then the AWS Developer Associate Certification can help you achieve that end goal.

The most important AWS service you need to study to pass that AWS Developer Associate exam is DynamoDB. So I have released to you for free what I call The Ultimate DynamoDB Cheatsheet. You can print this out on the day of your exam to increase your chances of passing.

It was Nader the AWS Developer Advocate for AWS Amplify who suggested I release my entire cheatsheet for free. You would not have this resource if it wasn't for him.

It was Kirk the AWS Senior Technologist specializing in DynamoDB who volunteered his time to ensure the accuracy of this Cheatsheet. This turned it from 5 pages to 8 pages long! πŸ“„πŸ“„πŸ“„πŸ“„πŸ“„πŸ“„πŸ“„πŸ“„

If you have Twitter, please do me the favour of thanking them by tweeting at @dabit3 and @NoSQLKnowHow with the #AWSCertified hashtag.

So lets move on to the Cheatsheet:

The Basics of DynamoDB

DynamoDB is a fully managed NoSQL key/value and document database.

DynamoDB is suited for workloads with any amount of data that require predictable read and write performance and automatic scaling from large to small and everywhere in between.

DynamoDB scales up and down to support whatever read and write capacity you specify per second in provisioned capacity mode. Or you can set it to On-Demand mode and there is little to no capacity planning.

  • DynamoDB stores 3 copies of data on SSD drives across 3 AZs in a region.
  • DynamoDB's most common datatypes are B (Binary), N (Number), and S (String)
  • Tables consist of Items (rows) and Items consist of Attributes (columns)

Reads and Writes Consistency

DynamoDB can be set to support Eventually Consistent Reads (default) and Strongly Consistent Reads on a per-call basis.

Eventually consistent reads data is returned immediately but data can be inconsistent. Copies of data will be generally consistent in 1 second.

Strongly Consistent Reads will always read from the leader partition since it always has an up-to-date copy. Data will never be inconsistent but latency may be higher. Copies of data will be consistent with a guarantee of 1 second.

Partitions
A Partition is when DynamoDB slices your table up into smaller chunks of data. This speeds up reads for very large tables.

DynamoDB automatically creates Partitions for:

Every 10 GB of Data or
When you exceed RCUs (3000) or WCUs (1000) limits for a single partition
When DynamoDB sees a pattern of a hot partition, it will split that partition in an attempt to fix the issue.
DynamoDB will try to evenly split the RCUs and WCUs across Partitions

Primary Key Design

Primary keys define where and how your data will be stored in partitions

The Key schema can be made up of two keys:

Partition Key (PK) is also known as **HASH*

  • The Sort Key (SK) is also known as RANGE

When using the AWS DynamoDB API eg. CLI, SDK they refer to the PK and SK by their alternative names due to legacy reasons.
Primary key comes in two types:

  • Simple Primary Key (Using only a Partition Key)
  • Composite Primary Key (Using both a Partition and Sort Key)
    Key Uniqueness is as follows:

  • When creating a Simple Primary Key the PK value may be unique

  • When creating a Composite Primary Key the combined PK and SK must be unique

When using a Sort key, records on the partition are logically grouped together in Ascending order.

Secondary Indexes

DynamoDB has two types of Indexes:

  • LSI - Local Secondary index
  • GSI - Global Secondary Index

LSI - Local Secondary index

  • Supports strongly or eventual consistency reads
  • Can only be created with initial table (cannot be modified or and cannot deleted unless also deleting the table)
  • Only Composite
  • 10GB or less per partition
  • Share capacity units with base table
  • Must share Partition Key (PK) with base table.

GSI - Global Secondary Index

  • Only eventual consistency reads (cannot provide strong consistency)
  • Can create, modify, or delete at anytime
  • Simple and Composite
  • Can have whatever attributes as Primary Key (PK) or Secondary Key (SK)
  • No size restriction per partition
  • Has its own capacity settings (does not share with base table)

Scan

Your table(s) should be designed in such a way that your workload primary access patterns do not use Scans. Overall, scans should be needed sparingly, for example for an infrequent report.

  • Scans through all items in a table and then returns one or more items through filters
  • By default returns all attributes for every item (use ProjectExpression to limit)
  • Scans are sequential, and you can speed up a scan through parallel scans using Segments and Total Segments
  • Scans can be slow, especially with very large tables and can easily consume your provisioned throughput.
  • Scans are one of the most expensive ways to access data in DynamoDB.

Query

  • Find items based on primary key values
  • Table must have a composite key in order to be able to query
  • By default queries are Eventually Consistent (use ConsistentRead True to change Strongly Consistent)
  • By default returns all attributes for each item found by a query (use ProjectExpression to limit)
  • By default is sorted ascending (use ScanIndexForward to False to reverse order to descending)

Capacity Modes

DynamoDB has two capacity modes, Provisioned and On-Demand. You can switch between these modes once every 24 hours.

Provisioned

Provisioned Throughput Capacity is the maximum amount of capacity your application is allowed to read or write per second from a table or index

  • Provisioned is suited for predictable or steady state workloads
  • RCUs is Read Capacity Unit
  • WCUs is Write Capacity Unit

You should enable Auto Scaling with Provisioned capacity mode. In this mode, you set a floor and ceiling for the capacity you wish the table to support. DynamoDB will automatically add and remove capacity to between these values on your behalf and throttle calls that go above the ceiling for too long.

If you go beyond your provisioned capacity, you’ll get an Exception: ProvisionedThroughputExceededException (throttling)

Throttling is when requests are blocked due to read or write frequency higher than set thresholds. E.g. exceeding set provisioned capacity, partitions splitting, table/index capacity mismatch.

On-Demand

On-Demand Capacity is pay per request. So you pay only for what you use.

  • On-Demand is suited for new or unpredictable workloads
  • The throughput is only limited by the default upper limits for a table (40K RCUs and 40K WCUs)
  • Throttling can occur if you exceed double your previous peak capacity (high water mark) within 30 minutes. For example, if you previously peaked to a maximum of 30,000 ops/sec, you could not peak immediately to 90,000 ops/sec, but you could to 60,000 ops/sec.
  • Since there is no hard limit, On-Demand could become very expensive based on emerging scenarios

Calculating Reads and Writes

Calculating Reads (RCU)

A read capacity unit represents:

  • one strongly consistent read per second,
  • or two eventually consistent reads per second,
  • for an item up to 4 KB in size.

How to calculate RCUs for strong

  1. Round data up to nearest 4.
  2. Divide data by 4
  3. Times by number of reads

Here's an example:

  • 50 reads at 40KB per item. (40/4) x 50 = 500 RCUs
  • 10 reads at 6KB per item. (8/4) x 10 = 20 RCUs
  • 33 reads at 17KB per item. (20/4) x 33 = 132 RCUs

How to calculate RCUs for eventual

  1. Round data up to nearest 4.
  2. Divide data by 4
  3. Times by number of reads
  4. Divide final number by 2
  5. Round up to the nearest whole number

Here's an example:

  • 50 reads at 40KB per item. (40/4) x 50 / 2 = 250 RCUs
  • 11 reads at 9KB per item. (12/4) x 11 / 2 = 17 RCUs
  • 14 reads at 24KB per item. (24/4) x 14 / 2 = 35 RCUs

Calculating Writes (Writes)

A write capacity unit represents:

  • one write per second,
  • for an item up to 1 KB

How to calculate Writes

  1. Round data up to nearest 1.
  2. Times by number of writes

Here's an example:

  • 50 writes at 40KB per item. 40 x 50 = 2000 WCUs
  • 11 writes at 1KB per item. 1 x 11 = 11 WCUs
  • 18 writes at 500 BYTES per item. 1 x 18 = 18 WCUs

DynamoDB Accelerator

DynamoDB Accelerator (DAX) is a fully managed in-memory write-through cache for DynamoDB that runs in a cluster

  • Reads are eventually consistent
  • Incoming requests are evenly distributed across all of the nodes in the cluster.
  • DAX can reduce read response times to microseconds

DAX is ideal for:

  • fastest response times possible
  • apps that read a small number of items more frequently
  • apps that are read intensive

DAX is not ideal for:

  • Apps that require strongly consistent reads
  • Apps that do not require microsecond read response times
  • Apps that are write intensive, or that do not perform much read activity
  • If you don’t need DAX consider using ElastiCache

DynamoDB Transactions

DynamoDB supports transactions via the TransactWriteItems and TransactGetItems API calls.

Transactions let you query multiple tables at once and are an all-or-nothing approach (all API calls must succeed).

Global tables

DynamoDB Global tables provide a fully managed solution for deploying multi-region, multi-master databases.

Streams

DynamoDB Streams allows you to setup a Lambda function triggered every time data is modified in a table to react to changes. Streams do not consume RCUs.

DynamoDB API

DynamoDB API's most notable commands via CLI: aws dynamodb

aws dynamodb get-item returns a set of attributes for the item with the given primary key. If no matching item, then it does not return any data and there will be no Item element in the response.

aws dynamodb put-item Creates a new item, or replaces an old item with a new item. If an item that has the same primary key as the new item already exists in the specified table, the new item completely replaces the existing item.

aws dynamodb update-item Edits an existing item's attributes, or adds a new item to the table if it does not already exist.

aws dynamodb batch-get-item returns the attributes of one or more items from one or more tables. You identify requested items by primary key. A single operation can retrieve up to 16 MB of data, which can contain as many as 100 items.

aws dynamodb batch-write-item puts or deletes multiple items in one or more tables. Can write up to 16 MB of data, which can comprise as many as 25 put or delete requests. Individual items to be written can be as large as 400 KB.

aws dynamodb create-table adds a new table to your account. Table names must be unique within each Region.

aws dynamodb update-table Modifies the provisioned throughput settings, global secondary indexes, or DynamoDB Streams settings for a given table.

aws dynamodb delete-table operation deletes a table and all of its items.

aws dynamodb transact-get-items is a synchronous operation that atomically retrieves multiple items from one or more tables (but not from indexes) in a single account and Region. Call can contain up to 25 objects. The aggregate size of the items in the transaction cannot exceed 4 MB.

aws dynamodb transact-write-items a synchronous write operation that groups up to 25 action requests. These actions can target items in different tables, but not in different AWS accounts or Regions, and no two actions can target the same item.

aws dynamodb query finds items based on primary key values. You can query table or secondary index that has a composite primary key.

aws dynamodb scan returns one or more items and item attributes by accessing every item in a table or a secondary index.

πŸš€ #rocketsToMars

I want to help you enter the web and cloud industry. That is why I am releasing my free AWS Developer Associate Certification 2020 course on the freeCodeCamp YouTube channel with 10+ hours of additional content I have never released previously.

This free course will be released in a few days as I apply the final touches.

I believe in making tech education accessible to the world because in-turn the more we upskill, the sooner we can lift people out of poverty, the sooner we can engineer sustainable solutions to keep our planet green and healthy, and the sooner we can launch rockets to Mars.

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Discussion

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Editor guide
 

For RCU:

for an item up to 4 KB in size

It’s not for β€œan item”, it’s for a read request of item(s) up to 4KB in size. For a collection of 400B items, a query for 10 items consumes 1 RCU.

The AWS docs are less than clear about this, but a query is a single read.

For reads, DynamoDB charges one read request unit for each strongly consistent read (up to 4 KB)

 

What's the cheapest setup when starting out? I am looking for a hosted mongodb solution to store data simply.

Amazon pricing is confusing and I am looking for 5-10 USD/month solutions for the db.

 

AWS has a service called DocumentDB however at its lowest price point it is currently 200 USD per month. If you want to use MongoDB instead go with MongoDB Atlas. That can deploy to AWS as an EC2 instance and it will make use of free tier or I believe 2 GB of ram will be around ~8 USD.

Whenever you see an hourly cost just times it by 730 to get the monthly cost. eg.

0.012/hr * 730 = 8.76