DynamoDB Capacity Mode Switching: On-Demand to Provisioned Around Cliffs

# ✅ Estimate peak from Insights / billing metrics, not folklore
# PeakConsumedWriteCapacityUnits * safety (1.4–2.0) → target provisioned WCU
peak_wcu = 12_000 # from CloudWatch max over last 4 similar events
safety = 1.5
target_wcu = int(peak_wcu * safety) # 18_000
# On-demand bill ≈ peak * hours * price; provisioned = reserved + auto-scale burst

# ✅ Pre-warm: create provisioned table settings BEFORE traffic arrives
# 1) UpdateTable → BillingMode=PROVISIONED with RCU/WCU >= target
# 2) Wait TableStatus=ACTIVE and Consumed < Provisioned for 10+ min
# 3) Enable Application Auto Scaling (target 70% utilization)
# 4) Only then route launch traffic / open feature flag

# ❌ Flip to provisioned at T-0 with default 5 WCU — instant throttle storm

# ✅ boto3 cutover helper (run in change window, not in the request path)
import boto3

ddb = boto3.client("dynamodb")

def to_provisioned(table: str, rcu: int, wcu: int) -> None:
 ddb.update_table(
 TableName=table,
 BillingMode="PROVISIONED",
 ProvisionedThroughput={"ReadCapacityUnits": rcu, "WriteCapacityUnits": wcu},
 )
 waiter = ddb.get_waiter("table_exists")
 waiter.wait(TableName=table) # ACTIVE again

def rollback_on_demand(table: str) -> None:
 ddb.update_table(TableName=table, BillingMode="PAY_PER_REQUEST")

# ✅ Canary: synthetic checkout writer at 20% of cliff for 15 minutes post-switch
# Abort / rollback if ThrottledRequests > 0 sustained for 60s