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+# Proposal: Data Layer
+
+## Summary
+
+Introduce an **Async Data Layer** — a background observation pipeline that runs
+**outside the critical request path**. It collects runtime events fired by the producer
+(currently `server.go`), buffers them off the hot path, and dispatches them to registered
+`Extractor`s that compute aggregates and write them to the DataStore for the Model Selector.
+
+## Goal
+
+Track runtime information about inference requests to help make better routing decisions —
+for example, which model has the least in-flight requests or the lowest average latency.
+This information is read by the Model Selector (Filter / Score / Pick) when choosing
+where to route each request.
+
+## Requirements
+
+- **Non-blocking on the critical path** - data collection must add zero latency to request handling.
+- **Multiple independent extractors** - different metrics must be computable independently without coupling to each other.
+- **Extensible** - adding a new metric or event type must not require changes to existing extractors or the producer.
+- **Off the plugin pipeline** - the data layer is a background concern; it must not participate in the per-request plugin chain.
+
+## Proposal
+
+### Architecture
+
+```mermaid
+flowchart TD
+    P["Producer\n(server.go)"]
+    PP["Plugin Pipeline\nFilter → Score → Pick"]
+    NS["NotificationSource\nchan Event  (buffered)"]
+    EL["event loop"]
+    ExtA["InflightRequestsExtractor"]
+    ExtB["LatencyExtractor\n(future)"]
+    DS[("Datastore\npkg/datastore")]
+    MS["Model Selector\n(InflightRequestsScorer)"]
+
+    P -->|"Notify(event)  ~ns"| NS
+    P --> PP
+    NS --> EL
+    EL -->|"one event at a time"| ExtA
+    EL -->|"one event at a time"| ExtB
+    ExtA --> DS
+    ExtB --> DS
+    DS --> MS
+    PP -->|reads| MS
+```
+
+The **producer** (currently `server.go`) fires an `Event` on each request and response —
+a non-blocking channel write (~ns). The `NotificationSource` buffers it. A background
+event loop reads each event from the channel as it arrives and fans it out to all registered
+`Extractor`s. Each extractor switches on `Event.Type` and handles what it understands,
+ignoring the rest.
+
+### Types (`pkg/framework/datalayer/`)
+
+```go
+type DataSource interface {
+    framework.Plugin                  // TypedName() TypedName
+    Start(ctx context.Context) error
+    // Stop signals the component to shut down and blocks until it has fully stopped.
+    Stop()
+}
+
+// Event is the uniform carrier of all data layer events.
+// Type identifies what happened; Payload holds the event-specific data.
+type Event struct {
+    Type    EventType
+    Payload any
+}
+
+// EventNotifier is the narrow interface the producer uses to fire events.
+// Keeping it separate lets the server depend only on Notify, not on lifecycle
+// or extractor registration.
+type EventNotifier interface {
+    Notify(e Event)
+}
+
+// NotificationSource buffers events off the hot path and dispatches
+// each event to registered Extractors as it arrives.
+type NotificationSource interface {
+    DataSource
+    EventNotifier
+    // RegisterExtractor adds an extractor after construction.
+    // Extractors known at construction time can be passed to New directly.
+    RegisterExtractor(e Extractor)
+}
+
+// Extractor processes a batch of Events. It does not manage its own goroutines.
+type Extractor interface {
+    framework.Plugin
+    Extract(ctx context.Context, events []Event) error
+}
+```
+
+See [Appendix](#appendix) for payload struct definitions and a full extractor example.
+
+### DataStore injection
+
+The `DataStore` is passed directly to each extractor's constructor. This keeps the
+`NotificationSource` a pure event dispatcher with no knowledge of storage, and avoids routing the store through `framework.Handle`.
+
+The concrete implementation lives in `pkg/datastore/inmemory`, separate from the
+`pkg/datastore.Datastore` interface, to make room for future backends (e.g. Redis):
+
+```go
+ds := inmemory.NewDatastore()
+extractor := inflightrequests.NewInflightRequestsExtractor(ds)
+```
+
+### Registration (`runner.go`)
+
+```go
+ds := inmemory.NewDatastore()
+src, err := notificationsource.New("default", inflightrequests.NewInflightRequestsExtractor(ds))
+if err != nil { ... }
+if err := src.Start(ctx); err != nil { ... }
+// TODO: pass src to the producer so it can call src.Notify(...)
+```
+
+**Next:** define a configuration story for data layer plugins (NotificationSource, extractors)
+consistent with how model-selector plugins are configured via CLI flags.
+
+## Future
+
+- **LatencyExtractor** - handles `ResponseEventType`; per-model avg latency; owns `"pool-latency"` attribute
+- **PollingDataSource** - polls inference pool `/metrics` on a ticker; same `Extractor` interface
+
+## Implementation Steps
+
+1. Add `DataSource`, `DataStore`, `EventNotifier`, `Event`, `NotificationSource`, `Extractor`, payload types to `pkg/framework/datalayer/`
+2. Implement `NotificationSource` (buffered channel + event loop) in `pkg/plugins/datalayer/notificationsource/`
+3. Implement `InflightRequestsExtractor` in `pkg/plugins/datalayer/inflightrequests/`
+4. Implement `InflightRequestsScorer` in `pkg/framework/modelselector/scorer/inflightrequests/`
+5. Wire DataStore + extractor + NotificationSource in `runner.go`
+6. Add `src.Notify(...)` calls to the producer alongside existing pipeline dispatch
+7. Config-driven registration of data layer plugins
+
+---
+
+## Appendix
+
+### Payload types
+
+```go
+// package datalayer (pkg/framework/datalayer/)
+
+// RequestPayload is the Payload for RequestEventType.
+// Carries the already-parsed request — no re-parsing needed.
+type RequestPayload struct {
+    Request *framework.InferenceRequest
+}
+
+// ResponsePayload is the Payload for ResponseEventType.
+// Duration is computed by the producer and passed directly.
+// All response body fields are accessible via Response.Body.
+type ResponsePayload struct {
+    Request  *framework.InferenceRequest
+    Response *framework.InferenceResponse
+    Duration time.Duration
+}
+```
+
+### Extractor definitions
+
+#### `InflightRequestsExtractor` — owns `"inflight-requests"` (`pkg/plugins/datalayer/inflightrequests`)
+
+| | |
+|---|---|
+| Handles | `RequestEventType` (increment), `ResponseEventType` (decrement) |
+| Reads | `model`, `max_tokens` from `Request.Body` |
+| State | `map[string]InflightRequestsCount` — in-flight counters per model |
+| Writes | `InflightRequestsCount{Requests int64, Tokens int64}` per model |
+
+```go
+type InflightRequestsCount struct {
+    Requests int64
+    Tokens   int64 // sum of max_tokens across in-flight requests
+}
+```