AWS Get Closest DynamoDB Region

Dns.ts

// DNS Stack
export function Dns({ stack, app }: StackContext) {
  ...
  createLatencyRecords(stack, 'example.app', 'zone_id')
}
function createLatencyRecords(stack: Stack, domainName: string, hostedZoneId: string) {
   // Create TXT Records with region names under record name: lbr.example.app
   // This allows us to query `dns.resolveTxt('lbr.example.app')` to get the 
   // region w/ the lowest latency for DynamoDB to connect to.
   REGIONS.map(region => {
     const record = new CfnRecordSet(stack as any, `Latency ${region}`, {
       setIdentifier: `lbr_${region}`,
       name: `lbr.${domainName}`,
       type: 'TXT',
       hostedZoneId,
       region,
       ttl: '31540000', // 1 year in seconds
       resourceRecords: [`"${region}"`]
     })
     // This record is global across all stages, so don't delete when removing stack.
     record.applyRemovalPolicy(RemovalPolicy.RETAIN)
   })
 }

utils.ts

import dns from 'dns/promises'

export const DYNAMODB_REGIONS = ['us-east-1', 'us-west-2', 'eu-west-2'] as const
type REGION = typeof DYNAMODB_REGIONS[number]

/**
 * Gets the closest DynamoDB region based on server location.
 * The closest region is determined by a TXT record DNS lookup.
 * The TXT record contains the region info.
 * 
 * Currently, the DynamoDB regions are:
 * main: us-east-1
 * replicas: us-west-2, eu-west-2
 * 
 * If a user triggers an edge function in us-west-1, this will return the closest region
 * us-west-2.
 * 
 * 
 * @returns region
 */
export async function getClosestDynamoReplicaRegion(): Promise<string> {
  let region = process.env.AWS_REGION as REGION

  // The region is in one of the replica regions
  if (DYNAMODB_REGIONS.includes(region)) {
    return region
  }

  // Get the closest TXT value from DNS
  try {
    const r = await dns.resolveTxt('lbr.example.app')
    region = r[0][0] as REGION
  } catch (err) {
    console.error('Failed to resolve TXT record', {
      region, err
    })
    region = 'us-east-1'
  }
  return region
}