### caching

```{eval-rst}
.. plantuml::

   @startuml
   title Kafka + Fast Cache + Tiered Storage + Lakehouse — Reference Architecture
   skinparam backgroundColor #FFFFFF
   skinparam defaultTextAlignment center
   skinparam packageStyle rectangle
   skinparam rectangle {
     BorderColor black
     RoundCorner 12
   }
   skinparam linetype ortho
   skinparam Arrow {
     Color black
     Thickness 1
     Padding 2  ' small arrowheads
   }
   ' left to right direction
   
   
   ' ===== Nodes =====
   ' Top row
   rectangle "Producers\n(Services, CDC, IoT)" as Producers #cce5ff
   rectangle "Ingress\n(Load Balancer)" as Ingress #cce5ff
   rectangle "Kafka Brokers\n(Cluster)" as Kafka #ffedcc
   rectangle "Policy Engine\nLRU / LFU / Hybrid\nWarm-up & Promotion" as Policy #e6ccff
   
   
   ' Middle row
   rectangle "Fast Cache (NVMe/SSD)\nSegment/Key Cache" as FastCache #ffd9b3
   
   
   ' Bottom row left (Consumers include ML/RAG)
   rectangle "Consumers\n(Apps, Stream proc, ML/RAG, BI, Replays)" as Consumers #cce5ff
   rectangle "AI Pipelines\n(Chunk/Embed, Feature Compute)" as AIPipe #cce5ff
   
   
   ' Bottom row right: Cold Tier containing Lakehouse tables + object store
   rectangle "Cold Tier\n(Storage: S3 / GCS / HDFS)" as ColdTier #d6f5d6 {
     rectangle "Lakehouse Tables\n(Iceberg / Delta / Hudi)" as Lakehouse #d6f5d6 {
       rectangle "AI Tables\n(Features, Embeddings, Labels)" as AITables #d6f5d6
     }
     rectangle "Object Store\n(Parquet/ORC files)" as ObjectStore #d6f5d6
   }
   
   
   rectangle "Control Plane & Admin UI\nTopics, Policies, SLAs, Quotas" as Control #f2f2f2
   rectangle "Observability\nMetrics, Traces, Cache Hit%, Backfill Latency, Cost" as Observability #f2f2f2
   
   
   ' ----- Soft positioning (hidden links to guide layout) -----
   Producers -[hidden]- Ingress
   Ingress -[hidden]- Kafka
   Kafka -[hidden]- Policy
   FastCache -[hidden]- ColdTier
   Consumers -[hidden]- AIPipe
   AIPipe -[hidden]- Lakehouse
   ColdTier -[hidden]- Control
   Control -[hidden]- Observability
   
   
   ' ===== Flows (dashed with small arrowheads) =====
   Producers -[#black,dashed]-> Ingress : produce
   Ingress -[#black,dashed]-> Kafka : ingest
   Kafka <-[#black,dashed]-> FastCache : spill / fetch
   FastCache <-[#black,dashed]-> ColdTier : tier / hydrate
   
   
   ' Operational consumption
   Kafka -[#black,dashed]-> Consumers : real-time events
   
   
   ' Control & policy
   Policy -[#black,dashed]-> FastCache : evict / promote policy
   Control -[#black,dashed]-> Kafka : configure policies
   
   
   ' Observability (components emit telemetry)
   Kafka -[#black,dashed]-> Observability : broker metrics / traces
   FastCache -[#black,dashed]-> Observability : cache hit% / latency
   ColdTier -[#black,dashed]-> Observability : storage metrics
   Policy -[#black,dashed]-> Observability : policy decisions / actions
   Consumers -[#black,dashed]-> Observability : app/ML latency & drift
   
   
   ' ===== Lakehouse (realized through Kafka) =====
   Kafka -[#black,dashed]-> Lakehouse : sink to tables\n(Stream→Batch materialization)
   Kafka -[#black,dashed]-> AIPipe : events/docs for features & RAG
   AIPipe <-[#black,dashed]- AITables : write features & embeddings
   Consumers <-[#black,dashed]- Lakehouse : BI / Analytics / RAG batch
   
   
   ' ===== Notes =====
   note bottom of Lakehouse
     Lakehouse is a logical layer on the Cold Tier,
     realized via Kafka sinks that write to Iceberg/Delta/Hudi tables.
     AI Pipelines produce unified AI Tables; Consumers (incl. ML/RAG) query them.
   end note
   
   
   ' ===== Legend =====
   legend right
   |= Color |= Component / Role |
   |<#cce5ff> Blue| Producers, Consumers & AI pipelines |
   |<#ffedcc> Orange (light)| Kafka Brokers |
   |<#ffd9b3> Orange (dark)| Fast Cache (NVMe/SSD) |
   |<#d6f5d6> Green| Cold Tier + Lakehouse tables (on object store) |
   |<#e6ccff> Purple| Policy Engine |
   |<#f2f2f2> Gray| Control Plane & Observability |
   endlegend
   @enduml