Application Data Stores

• Pick the data store from the access pattern, not the data shape
• Keep large objects in S3 and store only the key in the database
• A cache is never the source of truth
• DynamoDB primary key is partition-only or partition + sort
• Pick a high-cardinality partition key to avoid hot partitions
• GSI: different key, addable any time, eventual reads only
• LSI: same partition key, create-time only, strong reads allowed
• On-demand absorbs spiky traffic; provisioned is cheaper for steady load
• Read costs 1 RCU strong / 2 eventual per 4 KB; write 1 WCU per 1 KB
• DynamoDB reads are eventually consistent by default; ask for strong only when needed
• DynamoDB transactions are ACID but cost 2x the capacity
• Use a conditional write with a version attribute for optimistic locking
• Query targets one partition key; Scan reads the whole table
• Every Query/Scan is capped at 1 MB and paginates via LastEvaluatedKey
• Batch APIs cut round trips but are not atomic
• TTL auto-deletes expired items for free, but on a best-effort delay
• DynamoDB Streams give an ordered 24-hour change log for event-driven code
• DAX is a zero-code, write-through cache for DynamoDB only
• DAX only helps read-heavy DynamoDB workloads, and strong reads bypass it
• Lazy loading caches on miss; write-through caches on write
• Redis for replication and data structures; Memcached for simple key/value
• Choose RDS/Aurora over DynamoDB when you need joins or multi-column filters
• An AbortIncompleteMultipartUpload lifecycle rule cleans up failed multipart upload parts
• CompleteMultipartUpload requires the upload ID plus every part's number and ETag
• Every multipart part except the last must be at least 5 MB
• A presigned URL dies when its signing credentials expire, regardless of ExpiresIn
• Control versioned-bucket storage with NoncurrentVersionExpiration and NewerNoncurrentVersions
• Lifecycle transitions tier objects down for cost; transition to Standard-IA needs a 30-day minimum
• Deleting in a versioned bucket adds a delete marker; clean expired markers with ExpiredObjectDeleteMarker
• Enable S3 Bucket Keys to slash SSE-KMS request costs at scale
• RDS point-in-time recovery restores to a NEW instance within the backup retention window
• Pick io2 Block Express for max IOPS/durability; gp3 to provision IOPS independently of size
• Memory-intensive RDS workloads need a memory-optimized instance class
• A Multi-AZ DB instance takes backups from the standby, avoiding primary I/O suspension
• RDS storage autoscaling has a 6-hour cooldown and a minimum max-threshold
• ACUUtilization near 100% means an Aurora Serverless v2 cluster is capped at its max ACUs
• Set Aurora Serverless v2 minimum capacity to 0 ACUs to auto-pause when idle
• Use RDS Proxy to stop Lambda concurrency from exhausting database connections

Unlock with Premium — includes all practice exams and the complete study guide.