Topics Covered

On this page

1. NOT EXECUTED
2. RDS (managed PostgreSQL/MySQL)
3. BigQuery
4. Cosmos DB
5. Managed against self-hosted

NOT EXECUTED

This repository has no cloud account, and none will be created. Creating one requires a payment card and accepts a billing relationship, which is not something a study repository should do on anyone's behalf.

So this file is what you type, with the traps marked. It has never been run here, and nothing in the notes claims an output for it.

The runnable half is 09_etl_warehouse.py, which runs the same extract and the same warehouse queries locally.

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RDS (managed PostgreSQL/MySQL)

aws rds create-db-instance \
  --db-instance-identifier retail-db --db-instance-class db.t3.micro \
  --engine postgres --allocated-storage 20 \
  --master-username admin --manage-master-user-password \
  --no-publicly-accessible --backup-retention-period 7

psql -h retail-db.xxxx.us-east-1.rds.amazonaws.com -U admin -d postgres

--no-publicly-accessible is the right default. Reach it from an EC2 instance in the same VPC, or through a bastion, or with SSM Session Manager. A publicly reachable database with a weak password is found by scanners in hours.

And the security group needs an inbound rule for port 5432 from your instance's security group — group-to-group, not a CIDR. That is the fix for almost every "connection timed out".

BigQuery

bq mk --dataset retail
bq load --autodetect --source_format=CSV retail.sales gs://bucket/sales.csv

bq query --use_legacy_sql=false --dry_run \
  'SELECT region, SUM(revenue) FROM retail.sales GROUP BY region'

--dry_run prints the bytes the query would scan without running it, and therefore what it would cost. Run it before every large query. It is the single most valuable BigQuery habit.

bq query --use_legacy_sql=false --maximum_bytes_billed=1000000000 '...'

--maximum_bytes_billed is a seatbelt: the query fails rather than billing more than you said. Set it in every automated job.

Cosmos DB

az cosmosdb create --name retail-cosmos --resource-group rg \
  --kind GlobalDocumentDB --default-consistency-level Session
az cosmosdb sql database create --account-name retail-cosmos \
  --resource-group rg --name retail

The consistency level is the interesting choice, and it is Course 10's CAP discussion made into a dropdown:

Level	Guarantee	Cost
Strong	linearizable	highest RU, single region write
Bounded staleness	lag bounded by time or versions	high
Session	your own writes are read back	the default, and usually right
Consistent prefix	order preserved, some lag	low
Eventual	none	lowest RU

Cosmos bills in Request Units, and stronger consistency costs more RUs per read. That is CAP with a price tag attached.

Managed against self-hosted

	Self-hosted on EC2	Managed (RDS)
Patching	you	automatic, in a window
Backups	you write them	continuous, point-in-time
Failover	you build it	Multi-AZ, ~60 s
Version upgrade	your weekend	a click, with a restart
Root/superuser	yes	no
Cost	instance only	instance + ~20-30%

"No superuser" is the row that surprises people. Some extensions, some ALTER SYSTEM settings and any filesystem access are unavailable. If your application needs those, managed is not an option — and knowing that before you migrate is the point.