chore(aws/rds): upgrade engines to new versions

knowledge base/cloud computing/aws/README.md

@@ -817,6 +817,8 @@ Available discount options:
 
 Order of application: reserved instances -> Savings plans (EC2 instances -> Compute)
 
+Also check [Branko Petric's X posts] for further suggestions.
+
 ### Free Tier
 
 **New** AWS customers get **1 year** of free tier access to **selected** services only.
@@ -1248,6 +1250,7 @@ If one can, prefer just build the image from an EC2 instance.
 [aws config tutorial by stephane maarek]: https://www.youtube.com/watch?v=qHdFoYSrUvk
 [AWS Fundamentals Blog]: https://awsfundamentals.com/blog
 [aws savings plans vs. reserved instances: when to use each]: https://www.cloudzero.com/blog/savings-plans-vs-reserved-instances/
+[Branko Petric's X posts]: https://x.com/brankopetric00
 [date & time policy conditions at aws - 1-minute iam lesson]: https://www.youtube.com/watch?v=4wpKP1HLEXg
 [difference in boto3 between resource, client, and session?]: https://stackoverflow.com/questions/42809096/difference-in-boto3-between-resource-client-and-session
 [Gateway Endpoints vs Internet Routing for S3]: https://awsfundamentals.com/blog/gateway-endpoints-vs-internet-routing-s3
@@ -1255,7 +1258,7 @@ If one can, prefer just build the image from an EC2 instance.
 [Introduction to the AWS Virtual Private Cloud (VPC) - Part 2]: https://awsfundamentals.com/blog/introduction-to-the-aws-virtual-private-cloud-vpc-part-2
 [Introduction to the AWS Virtual Private Cloud (VPC) - Part 3]: https://awsfundamentals.com/blog/amazon-vpc-introduction-part-3
 [Learn AWS]: https://www.learnaws.org/
+[The $1,000 AWS mistake]: https://www.geocod.io/code-and-coordinates/2025-11-18-the-1000-aws-mistake/
 [using aws kms via the cli with a symmetric key]: https://nsmith.net/aws-kms-cli
 [VPC Endpoints: Secure and Direct Access to AWS Services]: https://awsfundamentals.com/blog/vpc-endpoints
 [What Is OIDC and Why Do We Need It?]: https://awsfundamentals.com/blog/oidc-introduction
-[The $1,000 AWS mistake]: https://www.geocod.io/code-and-coordinates/2025-11-18-the-1000-aws-mistake/

knowledge base/cloud computing/aws/rds.md

@@ -17,6 +17,9 @@
 1. [Multi-AZ instances](#multi-az-instances)
    1. [Converting instances between Multi-AZ and Single-AZ](#converting-instances-between-multi-az-and-single-az)
 1. [Operations](#operations)
+    1. [Upgrade the engine version](#upgrade-the-engine-version)
+       1. [Upgrade to a new major version](#upgrade-to-a-new-major-version)
+       1. [Upgrade to a new minor version](#upgrade-to-a-new-minor-version)
     1. [PostgreSQL: reduce allocated storage by migrating using transportable databases](#postgresql-reduce-allocated-storage-by-migrating-using-transportable-databases)
     1. [Stop instances](#stop-instances)
     1. [Cancel pending modifications](#cancel-pending-modifications)
@@ -151,6 +154,20 @@ aws rds cancel-export-task --export-task-identifier 'my_export'
 
 # Change the storage type.
 aws rds modify-db-instance --db-instance-identifier 'instance-name' --storage-type 'gp3' --apply-immediately
+
+
+# Show available upgrade target versions for a given DB engine version.
+aws rds describe-db-engine-versions --engine 'postgres' --engine-version '13' \
+  --query 'DBEngineVersions[*].ValidUpgradeTarget[*]'
+aws rds describe-db-engine-versions --engine 'postgres' --engine-version '13.12' \
+  --query 'DBEngineVersions[*].ValidUpgradeTarget[*].{AutoUpgrade:AutoUpgrade,EngineVersion:EngineVersion}[?AutoUpgrade==`true`][]'
+
+# Start upgrading.
+# Requires downtime.
+aws rds modify-db-instance --db-instance-identifier 'my-db-instance' \
+  --engine-version '14.15' --allow-major-version-upgrade --no-apply-immediately
+aws rds modify-db-instance --db-instance-identifier 'my-db-instance' \
+  --engine-version '14.20' --apply-immediately
 ```
 
 </details>
@@ -600,6 +617,72 @@ deletes only the secondary instance and volumes. The change does **not** typical
 
 ## Operations
 
+### Upgrade the engine version
+
+> [!caution]
+> Database engine upgrades require downtime.<br/>
+> Minimize the downtime by using a blue/green deployment.
+
+In general, **major** engine version upgrades can introduce breaking changes.<br/>
+**Minor** version upgrades usually only include changes that are backward-compatible with existing applications.
+
+At the time of writing, multi-AZ DB clusters only support major version upgrades of PostgreSQL.<br/>
+Minor version upgrades are supported for **all** engines.
+
+> [!important]
+> One **cannot** modify a DB instance when it is being upgraded.
+>
+> During an engine upgrade, the DB instance status changes to `upgrading`.<br/>
+> This prevents further changes to the instance while the process is being carried out.
+
+#### Upgrade to a new major version
+
+Manually modify a DB engine version through the Console, CLI, or RDS API:
+
+```sh
+# Show available upgrade target versions for a given DB engine version.
+aws rds describe-db-engine-versions \
+        --engine 'postgres' --engine-version '13' \
+        --query 'DBEngineVersions[*].ValidUpgradeTarget[*]'
+
+# Start upgrading.
+aws rds modify-db-instance --db-instance-identifier 'my-db-instance' \
+  --engine-version '14.15' --allow-major-version-upgrade --no-apply-immediately
+aws rds modify-db-instance … --apply-immediately
+```
+
+#### Upgrade to a new minor version
+
+Either:
+
+- Manually modify a DB engine version through the Console, CLI, or RDS API:
+
+  ```sh
+  # Show available automatic minor upgrade target versions for a given DB engine version.
+  aws rds describe-db-engine-versions \
+    --engine 'postgres' --engine-version '13' \
+    --query 'DBEngineVersions[*].ValidUpgradeTarget[*].{AutoUpgrade:AutoUpgrade,EngineVersion:EngineVersion}[?AutoUpgrade==`true`][]'
+
+  # Start upgrading.
+  aws rds modify-db-instance --db-instance-identifier 'my-db-instance' --engine-version '14.20' --no-apply-immediately
+  aws rds modify-db-instance … --apply-immediately
+  ```
+
+- Enable automatic updates for the instance.
+
+Unless changes are applied immediately, RDS schedules the upgrade to run automatically in the preferred maintenance
+window.
+
+Automatic upgrades incur downtime too.<br/>
+The length of the downtime depends on various factors, including the DB engine type and the size of the database.
+
+During the upgrade, RDS:
+
+1. Runs a system pre-check to make sure the database can be upgraded.
+1. Upgrades the DB engine to the target minor engine version.
+1. Runs post-upgrade checks.
+1. Marks the database upgrade as complete.
+
 ### PostgreSQL: reduce allocated storage by migrating using transportable databases
 
 Refer [Migrating databases using RDS PostgreSQL Transportable Databases],

snippets/aws/other commands.fish

@@ -378,6 +378,18 @@ aws rds start-export-task \
 # Change the storage type
 aws rds modify-db-instance --db-instance-identifier 'instance-name' --storage-type 'gp3' --apply-immediately
 
+# Show available upgrade target versions for a given DB engine version.
+aws rds describe-db-engine-versions --engine 'postgres' --engine-version '13' \
+	--query 'DBEngineVersions[*].ValidUpgradeTarget[*]'
+aws rds describe-db-engine-versions --engine 'postgres' --engine-version '13.12' \
+	--query 'DBEngineVersions[*].ValidUpgradeTarget[*].{AutoUpgrade:AutoUpgrade,EngineVersion:EngineVersion}[?AutoUpgrade==`true`][]'
+
+# Start upgrading.
+# Requires downtime.
+aws rds modify-db-instance --db-instance-identifier 'my-db-instance' --engine-version '14.20' --apply-immediately
+aws rds modify-db-instance --db-instance-identifier 'my-db-instance' \
+	--engine-version '14.15' --allow-major-version-upgrade --no-apply-immediately
+
 # Max 5 running at any given time, RDS cannot queue
 echo {1..5} | xargs -p -n '1' -I '{}' aws rds start-export-task …