[IOS] How to get data from DynamoDB

Introduction

I think that I may develop an application that works with DynamoDB with swift, but since there were few articles, I tried to summarize the method of acquiring data.

Development environment

• Swift 5
• Xcode 12
• aws-amplify/aws-sdk-ios
• cocoapods

1. AWS authentication settings

Please refer to it as it is summarized here. [IOS] How to get table name from AWS DynamoDB

2. Add AWS DyanamoDB to pod

It is assumed that the pod is already installed in the project. If it is not installed, please install it.

Add AWD DyanmoDB to your Podfile.

Podfile

target 'aws-sdk' do
  # Comment the next line if you don't want to use dynamic frameworks
  use_frameworks!

  pod 'AWSDynamoDB'
end

Install pod and reopen Xcode.

Terminal

$ pod install --repo-update
$ xed .

3. Create table and insert data in AWS DynamoDB

Press Create Table to create the table. This time, I changed the table name to user.

User table

Once the table is created, select the tab for items. Then press Create Item to create the data. I created the following data.

4. Create model

Create a model to get DynamoDB data. Please change to the class name and property as appropriate.

Data cannot be obtained without @objc. I'm addicted to this, so be careful not to forget it. .. ..

User.swift

import AWSDynamoDB

class User: AWSDynamoDBObjectModel, AWSDynamoDBModeling {
    
    @objc private var id: NSNumber = 0
    @objc private var email: String = ""
    @objc private var password: String = ""

    class func dynamoDBTableName() -> String {
        return "user"  //Describe the table name created in DyanmoDB
    }

    class func hashKeyAttribute() -> String {
        return "id"
    }
}

5. Create a method to get the data

This time, I'm using scan because I want to get all the data.

ViewController.swift

import UIKit
import AWSDynamoDB

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        fetchDynamoDBData()
    }
    
    private func fetchDynamoDBData() {
        let dynamoDBObjectMapper = AWSDynamoDBObjectMapper.default()
        let scanExpression = AWSDynamoDBScanExpression()

        //User part is the class name of the created Model
        dynamoDBObjectMapper.scan(User.self, expression: scanExpression).continueWith() { (task:AWSTask<AWSDynamoDBPaginatedOutput>!) -> Void in
            
            guard let items = task.result?.items as? [User] else { return }
            if let error = task.error as NSError? {
                print("The request failed. Error: \(error)")
                return
            }
            print(items)
        }
    }
}

Now when you build, the data will be displayed in the console.

If DyanmoDB has a lot of data

If there is a lot of data, you may not be able to get all the data at once even if you scan. (I didn't notice it and was really into it ...) You can only get up to 1MB with a single scan.

(Using Scan with DynamoDB)

If not all items have been acquired, a task.result? .LastEvaluatedKey will appear. In other words, unless this becomes nil, you will not be able to get all the records. (Similar to the current page of pagenation)

If you have a lot of data but want to get all of them, you need to update the pagenation. As shown below, if task.result? .LastEvaluatedKey exists, update the starting page to task.result? .LastEvaluatedKey and fetch it again.

ViewController.swift

func fetchDynamoDBData() {
    dynamoDBObjectMapper.scan(User.self, expression: scanExpression).continueWith() { (task:AWSTask<AWSDynamoDBPaginatedOutput>!) -> Void in

        guard let key = task.result?.lastEvaluatedKey else {
            print("end of scan")
            return
        }

        if key != nil {
            // MEMO:Update the page to start
            scanExpression.exclusiveStartKey = key
            fetchDynamoDBData()
        }
    }
}

Repository

For your reference. https://github.com/shungo0525/aws-sdk

Reference article

https://stackoverflow.com/questions/26958637/best-way-to-make-amazon-aws-dynamodb-queries-using-swift https://stackoverflow.com/questions/34049054/aws-dynamodb-swift-query-with-multiple-hashkeyvalues