Johnny Lin — software engineer in Taiwan. Projects, case studies and notes on e-commerce, payments and the systems I build.https://johnnyjclin.com/https://johnnyjclin.com/blog/aws-saa-01-exam-map/https://johnnyjclin.com/blog/aws-saa-01-exam-map/Turn the four exam domains into a repeatable way to read requirements, eliminate distractors and select an architecture.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSSAA-C03exam strategyWell-Architectedhttps://johnnyjclin.com/blog/aws-saa-02-identity-and-security/https://johnnyjclin.com/blog/aws-saa-02-identity-and-security/Connect IAM evaluation, Organizations, KMS, secrets, network protection and detection services into one security model.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSIAMKMSOrganizationssecurityhttps://johnnyjclin.com/blog/aws-saa-03-networking-and-edge/https://johnnyjclin.com/blog/aws-saa-03-networking-and-edge/Connect subnets, routing, security, private access, hybrid links, DNS and global delivery through packet paths.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSVPCRoute 53CloudFrontDirect Connecthttps://johnnyjclin.com/blog/aws-saa-04-compute-and-elasticity/https://johnnyjclin.com/blog/aws-saa-04-compute-and-elasticity/Choose among EC2, Auto Scaling, load balancers, containers and Lambda by control, workload shape and state.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSEC2Auto ScalingELBECSLambdahttps://johnnyjclin.com/blog/aws-saa-05-storage/https://johnnyjclin.com/blog/aws-saa-05-storage/Compare object, block and file storage before choosing classes, replication, lifecycle and hybrid transfer tools.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSS3EBSEFSFSxstoragehttps://johnnyjclin.com/blog/aws-saa-06-databases-and-cache/https://johnnyjclin.com/blog/aws-saa-06-databases-and-cache/Choose RDS, Aurora, DynamoDB, ElastiCache or a purpose-built database from consistency, query and scale requirements.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSRDSAuroraDynamoDBElastiCachehttps://johnnyjclin.com/blog/aws-saa-07-messaging-and-serverless/https://johnnyjclin.com/blog/aws-saa-07-messaging-and-serverless/Separate queues, pub/sub, streams and orchestration before connecting Lambda, API Gateway and event-driven workloads.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSSQSSNSKinesisLambdaEventBridgehttps://johnnyjclin.com/blog/aws-saa-08-observability-and-governance/https://johnnyjclin.com/blog/aws-saa-08-observability-and-governance/Stop mixing CloudWatch, EventBridge, CloudTrail and Config by asking what each service records and acts on.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSCloudWatchCloudTrailConfigEventBridgehttps://johnnyjclin.com/blog/aws-saa-09-resilience-dr-and-migration/https://johnnyjclin.com/blog/aws-saa-09-resilience-dr-and-migration/Separate high availability from disaster recovery, then map business recovery targets to replication and migration tools.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSdisaster recoveryRTORPOmigrationhttps://johnnyjclin.com/blog/aws-saa-10-data-and-analytics/https://johnnyjclin.com/blog/aws-saa-10-data-and-analytics/Map ad hoc SQL, warehouses, ETL, streams, search, dashboards and clusters to the service that owns each job.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSAthenaRedshiftGlueKinesisanalyticshttps://johnnyjclin.com/blog/aws-saa-11-cost-and-final-review/https://johnnyjclin.com/blog/aws-saa-11-cost-and-final-review/Finish the series with cost levers, frequently confused pairs and a compact method for reviewing practice questions.Tue, 22 Sep 2026 00:00:00 GMTcertificationsAWSSAA-C03cost optimizationreviewhttps://johnnyjclin.com/blog/cs411-01-relational-model-and-sql/https://johnnyjclin.com/blog/cs411-01-relational-model-and-sql/Relations, schemas, keys, bags, NULL, and SQL's logical processing order—one mental model for reasoning about query results.Tue, 22 Sep 2026 00:00:00 GMTmastersrelational modelSQLNULLquery semanticshttps://johnnyjclin.com/blog/cs411-02-joins-nulls-and-set-operations/https://johnnyjclin.com/blog/cs411-02-joins-nulls-and-set-operations/Join cardinality, outer-join filters, NULL-safe anti-joins, and when UNION ALL is the honest operation.Tue, 22 Sep 2026 00:00:00 GMTmastersSQLJOINNULLEXISTSset operationshttps://johnnyjclin.com/blog/cs411-03-aggregation-views-and-updates/https://johnnyjclin.com/blog/cs411-03-aggregation-views-and-updates/GROUP BY semantics, ordinary and materialized views, atomic updates, and when a stored routine is worth the coupling.Tue, 22 Sep 2026 00:00:00 GMTmastersSQLGROUP BYviewsupdatesstored procedureshttps://johnnyjclin.com/blog/cs411-04-constraints-and-triggers/https://johnnyjclin.com/blog/cs411-04-constraints-and-triggers/A practical hierarchy for NOT NULL, CHECK, UNIQUE, foreign keys, and triggers—with failure behavior you can test.Tue, 22 Sep 2026 00:00:00 GMTmastersSQLconstraintstriggersintegrityhttps://johnnyjclin.com/blog/cs411-05-conceptual-to-logical-design/https://johnnyjclin.com/blog/cs411-05-conceptual-to-logical-design/A repeatable path from requirements to entities, relationships, keys, cardinalities, and relational tables.Tue, 22 Sep 2026 00:00:00 GMTmastersdata modelingERUMLschema designhttps://johnnyjclin.com/blog/cs411-06-functional-dependencies-and-normalization/https://johnnyjclin.com/blog/cs411-06-functional-dependencies-and-normalization/A worked reasoning chain for attribute closure, candidate keys, minimal covers, lossless decomposition, 3NF, and BCNF.Tue, 22 Sep 2026 00:00:00 GMTmastersfunctional dependenciesnormalization3NFBCNFhttps://johnnyjclin.com/blog/cs411-07-mongodb-document-model/https://johnnyjclin.com/blog/cs411-07-mongodb-document-model/Choose embedding or references from access patterns, growth bounds, update ownership, and measurable query plans.Tue, 22 Sep 2026 00:00:00 GMTmastersMongoDBNoSQLdocument modelindexinghttps://johnnyjclin.com/blog/cs411-08-graph-model-and-cypher/https://johnnyjclin.com/blog/cs411-08-graph-model-and-cypher/Model nodes, relationships, direction, and traversal cost in Neo4j without turning every dataset into a graph.Tue, 22 Sep 2026 00:00:00 GMTmastersgraph databasesNeo4jCypherdata modelinghttps://johnnyjclin.com/blog/cs411-09-bplus-tree-and-hash-index/https://johnnyjclin.com/blog/cs411-09-bplus-tree-and-hash-index/Tree structure, fanout math, update cost, runnable PostgreSQL, EXPLAIN interpretation, and a practical index decision table.Tue, 22 Sep 2026 00:00:00 GMTmastersB+ Treehash indexPostgreSQLEXPLAINpage I/Ohttps://johnnyjclin.com/blog/cs411-10-transactions-serializability-and-isolation/https://johnnyjclin.com/blog/cs411-10-transactions-serializability-and-isolation/From ACID and schedules to conflict graphs, anomalies, isolation levels, locks, and evidence from two database sessions.Tue, 22 Sep 2026 00:00:00 GMTmasterstransactionsserializabilityisolationconcurrencyhttps://johnnyjclin.com/blog/cs411-11-relational-algebra-and-physical-operators/https://johnnyjclin.com/blog/cs411-11-relational-algebra-and-physical-operators/Separate logical meaning from scan, sort, hash, and join algorithms, then estimate their page-I/O cost.Tue, 22 Sep 2026 00:00:00 GMTmastersrelational algebraquery executionjoinsI/O costhttps://johnnyjclin.com/blog/cs411-12-query-optimization/https://johnnyjclin.com/blog/cs411-12-query-optimization/Rule-based rewrites, selectivity estimates, join order, physical-plan cost, and a disciplined way to read estimation errors.Tue, 22 Sep 2026 00:00:00 GMTmastersquery optimizationcardinalityEXPLAINjoin orderhttps://johnnyjclin.com/blog/cs411-13-choosing-a-data-model/https://johnnyjclin.com/blog/cs411-13-choosing-a-data-model/Compare data models by invariants, access paths, update ownership, relationship depth, latency, and operational cost.Tue, 22 Sep 2026 00:00:00 GMTmastersdata modelingrelationalMongoDBgraphNoDBhttps://johnnyjclin.com/blog/cs411-14-recovery-wal-and-checkpoints/https://johnnyjclin.com/blog/cs411-14-recovery-wal-and-checkpoints/Understand what survives a crash, why write-ahead logging works, and how buffer policies determine undo and redo work.Tue, 22 Sep 2026 00:00:00 GMTmastersrecoveryWALcheckpointtransactionstoragehttps://johnnyjclin.com/blog/cs411-15-parallel-and-distributed-databases/https://johnnyjclin.com/blog/cs411-15-parallel-and-distributed-databases/Connect partitioning, replication, parallel joins, skew, distributed commit, and consensus without collapsing them into one topic.Tue, 22 Sep 2026 00:00:00 GMTmastersdistributed databaseparallel querypartitioningreplication2PChttps://johnnyjclin.com/blog/cs411-16-information-integration/https://johnnyjclin.com/blog/cs411-16-information-integration/A practical guide to schema matching, entity resolution, provenance, merge policy, ETL, and federated queries.Tue, 22 Sep 2026 00:00:00 GMTmastersdata integrationETLentity resolutionprovenancedata qualityhttps://johnnyjclin.com/blog/cs411-17-research-nexus-design-review/https://johnnyjclin.com/blog/cs411-17-research-nexus-design-review/A design review of an 11-table research-discovery database built from DBLP-style citation data and Scholar enrichment.Tue, 22 Sep 2026 00:00:00 GMTmastersdatabase projectMySQLdata modelingETLresearch datahttps://johnnyjclin.com/blog/cs411-18-nodb-paper-review/https://johnnyjclin.com/blog/cs411-18-nodb-paper-review/A practical critique of NoDB's data-to-query argument, selective parsing, positional maps, caching, and break-even point.Tue, 22 Sep 2026 00:00:00 GMTmastersNoDBraw dataquery executionadaptive indexingpaper reviewhttps://johnnyjclin.com/blog/cs411-19-cost-aware-learned-caching/https://johnnyjclin.com/blog/cs411-19-cost-aware-learned-caching/Why miss count is the wrong objective for heterogeneous storage, how LBSC learns eviction, and how to scope a credible follow-up experiment.Tue, 22 Sep 2026 00:00:00 GMTmasterscachinglearned systemsLBSCcost modelpaper reviewhttps://johnnyjclin.com/blog/cs425-01-clouds-and-distributed-systems/https://johnnyjclin.com/blog/cs425-01-clouds-and-distributed-systems/Start from partial failure, concurrency, elasticity, virtualization, and the economic reason cloud systems exist.Tue, 22 Sep 2026 00:00:00 GMTmastersdistributed systemscloud computingpartial failureelasticityhttps://johnnyjclin.com/blog/cs425-02-p2p-overlays-and-dhts/https://johnnyjclin.com/blog/cs425-02-p2p-overlays-and-dhts/Compare centralized indexes, flooding, supernodes, BitTorrent swarms, and DHT routing through state, messages, and failure behavior.Tue, 22 Sep 2026 00:00:00 GMTmasterspeer to peerChordDHTBitTorrentconsistent hashinghttps://johnnyjclin.com/blog/cs425-03-gossip-membership-and-failure-detection/https://johnnyjclin.com/blog/cs425-03-gossip-membership-and-failure-detection/Connect epidemic dissemination, membership views, heartbeats, suspicion, false positives, and scalable failure detection.Tue, 22 Sep 2026 00:00:00 GMTmastersgossipmembershipfailure detectordistributed systemshttps://johnnyjclin.com/blog/cs425-04-time-causality-and-multicast-order/https://johnnyjclin.com/blog/cs425-04-time-causality-and-multicast-order/Move from clock synchronization to Lamport timestamps, vector clocks, FIFO, causal, and total-order multicast.Tue, 22 Sep 2026 00:00:00 GMTmastersLamport clocksvector clocksmulticastcausalityhttps://johnnyjclin.com/blog/cs425-05-consistent-snapshots/https://johnnyjclin.com/blog/cs425-05-consistent-snapshots/Understand consistent cuts, in-transit messages, and the Chandy-Lamport marker algorithm.Tue, 22 Sep 2026 00:00:00 GMTmastersglobal snapshotChandy Lamportconsistent cutdistributed statehttps://johnnyjclin.com/blog/cs425-06-consensus-and-paxos/https://johnnyjclin.com/blog/cs425-06-consensus-and-paxos/Separate consensus safety from progress, then follow proposals, acceptors, ballots, and quorum intersection in Paxos.Tue, 22 Sep 2026 00:00:00 GMTmastersconsensusPaxosquorumfault tolerancehttps://johnnyjclin.com/blog/cs425-07-mapreduce-at-scale/https://johnnyjclin.com/blog/cs425-07-mapreduce-at-scale/Understand map, shuffle, reduce, partitioning, stragglers, speculative execution, and lineage-based fault tolerance.Tue, 22 Sep 2026 00:00:00 GMTmastersMapReduceHadoopdistributed computingfault tolerancehttps://johnnyjclin.com/blog/cs425-08-cap-consistency-and-nosql-storage/https://johnnyjclin.com/blog/cs425-08-cap-consistency-and-nosql-storage/Connect partition behavior, consistency models, quorums, LSM-style writes, Cassandra, and HBase without reducing design to CAP slogans.Tue, 22 Sep 2026 00:00:00 GMTmastersCAP theoremconsistencyCassandraHBaseNoSQLhttps://johnnyjclin.com/blog/cs513-01-data-quality-fitness-for-use/https://johnnyjclin.com/blog/cs513-01-data-quality-fitness-for-use/Use fitness for use, quality dimensions, and cost to decide what must be cleaned and what is already good enough.Tue, 22 Sep 2026 00:00:00 GMTmastersdata qualityfitness for usedata cleaningquality dimensionshttps://johnnyjclin.com/blog/cs513-02-data-profiling-before-cleaning/https://johnnyjclin.com/blog/cs513-02-data-profiling-before-cleaning/Build a data profile that separates syntax, schema, semantic, duplicate, missingness, and distribution problems before changing values.Tue, 22 Sep 2026 00:00:00 GMTmastersdata profilingdata qualitySQLvalidationhttps://johnnyjclin.com/blog/cs513-03-regex-for-data-cleaning/https://johnnyjclin.com/blog/cs513-03-regex-for-data-cleaning/Build testable regexes for validation, extraction, and transformation while keeping parsing and semantic checks separate.Tue, 22 Sep 2026 00:00:00 GMTmastersregular expressionsregexdata transformationvalidationhttps://johnnyjclin.com/blog/cs513-04-openrefine-reproducible-cleaning/https://johnnyjclin.com/blog/cs513-04-openrefine-reproducible-cleaning/Use facets, clustering, GREL, reconciliation, and operation history without turning interactive cleaning into an unrepeatable black box.Tue, 22 Sep 2026 00:00:00 GMTmastersOpenRefinedata cleaningclusteringGRELreproducibilityhttps://johnnyjclin.com/blog/cs513-05-integrity-constraints-with-sql/https://johnnyjclin.com/blog/cs513-05-integrity-constraints-with-sql/Translate keys, references, domains, and cross-column business rules into violation queries and enforceable constraints.Tue, 22 Sep 2026 00:00:00 GMTmastersintegrity constraintsSQLdata qualityvalidationhttps://johnnyjclin.com/blog/cs513-06-datalog-and-recursive-queries/https://johnnyjclin.com/blog/cs513-06-datalog-and-recursive-queries/Understand facts, rules, joins, recursion, fixed points, and how the same reasoning appears in recursive SQL and data-quality checks.Tue, 22 Sep 2026 00:00:00 GMTmastersDatalogrecursive SQLlogic programmingdata qualityhttps://johnnyjclin.com/blog/cs513-07-database-repair-and-consistent-answers/https://johnnyjclin.com/blog/cs513-07-database-repair-and-consistent-answers/Reason about minimal repairs, repair policies, stable conclusions, and consistent query answers without hiding ambiguity.Tue, 22 Sep 2026 00:00:00 GMTmastersdatabase repairconsistent query answeringintegrity constraintsdata qualityhttps://johnnyjclin.com/blog/cs513-08-workflows-for-reproducible-cleaning/https://johnnyjclin.com/blog/cs513-08-workflows-for-reproducible-cleaning/Design cleaning pipelines around explicit inputs, outputs, parameters, dependencies, validation gates, and rerunnable evidence.Tue, 22 Sep 2026 00:00:00 GMTmastersworkflowreproducibilitydata pipelineautomationhttps://johnnyjclin.com/blog/cs513-09-data-provenance-and-yesworkflow/https://johnnyjclin.com/blog/cs513-09-data-provenance-and-yesworkflow/Separate prospective and retrospective provenance, model lineage at useful granularity, and annotate scripts with YesWorkflow-style dependencies.Tue, 22 Sep 2026 00:00:00 GMTmastersdata provenancelineageYesWorkflowreproducibilityhttps://johnnyjclin.com/blog/cs513-10-taxonomy-alignment/https://johnnyjclin.com/blog/cs513-10-taxonomy-alignment/Align hierarchical vocabularies with explicit semantic relations, logical constraints, possible worlds, and human review.Tue, 22 Sep 2026 00:00:00 GMTmasterstaxonomy alignmentcontrolled vocabularyontologydata integrationhttps://johnnyjclin.com/blog/cs513-11-nypl-menu-cleaning-case-study/https://johnnyjclin.com/blog/cs513-11-nypl-menu-cleaning-case-study/A project retrospective on profiling, canonical cities and meals, conservative inference, constraints, provenance, and honest validation.Tue, 22 Sep 2026 00:00:00 GMTmasterscase studyNYPL menusdata cleaningprovenancePythonSQLitehttps://johnnyjclin.com/blog/msbd5001-01-minhash-and-lsh/https://johnnyjclin.com/blog/msbd5001-01-minhash-and-lsh/Turn documents into shingles, compress Jaccard similarity into signatures, then use LSH to generate a small candidate set.Tue, 22 Sep 2026 00:00:00 GMTmastersMinHashLSHJaccard similaritynear duplicateshinglinghttps://johnnyjclin.com/blog/msbd5002-01-association-rules/https://johnnyjclin.com/blog/msbd5002-01-association-rules/Understand support, confidence and lift, then see why Apriori and FP-Growth spend work differently.Tue, 22 Sep 2026 00:00:00 GMTmastersassociation rulesAprioriFP-growthsupportlifthttps://johnnyjclin.com/blog/msbd5002-02-clustering-and-classification/https://johnnyjclin.com/blog/msbd5002-02-clustering-and-classification/Connect k-means, decision trees and SVMs through objectives, assumptions, boundaries and evaluation.Tue, 22 Sep 2026 00:00:00 GMTmastersclusteringk-meansdecision treeSVMevaluationhttps://johnnyjclin.com/blog/msbd5003-01-spark-rdd-execution/https://johnnyjclin.com/blog/msbd5003-01-spark-rdd-execution/Read Spark performance from the execution model instead of treating transformations as ordinary collection methods.Tue, 22 Sep 2026 00:00:00 GMTmastersSparkRDDlineagepartitionshufflehttps://johnnyjclin.com/blog/msbd5003-02-dataframes-and-spark-sql/https://johnnyjclin.com/blog/msbd5003-02-dataframes-and-spark-sql/Understand why schema, logical plans, Catalyst and columnar execution often beat opaque RDD code.Tue, 22 Sep 2026 00:00:00 GMTmastersSpark SQLDataFrameCatalystquery planjoinshttps://johnnyjclin.com/blog/msbd5003-03-streaming-graphs-and-ml/https://johnnyjclin.com/blog/msbd5003-03-streaming-graphs-and-ml/Use one execution foundation for incremental streams, property graphs and repeatable feature-to-model workflows.Tue, 22 Sep 2026 00:00:00 GMTmastersSpark StreamingGraphFramesMLlibcheckpointpipeline