docs(obs): inventory ecstore dependency boundary (#4023)

docs/architecture/crate-boundaries.md

@@ -243,6 +243,9 @@ boundary files after the external runtime, test, and fuzz consumers have been
 narrowed. IAM, heal, scanner, notify, observability, Swift, S3 Select, e2e, and
 fuzz code must not reintroduce direct `rustfs_ecstore::api::...` references
 outside those boundary files.
+The observability ECStore dependency baseline is tracked in
+[`obs-ecstore-dependency-inventory.md`](obs-ecstore-dependency-inventory.md);
+future observability extraction PRs must update that inventory with the guard.
 
 ECStore ClusterControlPlane read models must stay owned by the crate-private
 `cluster` module. Public access goes through `rustfs_ecstore::api::cluster` so

docs/architecture/obs-ecstore-dependency-inventory.md

@@ -0,0 +1,64 @@
+# Observability ECStore Dependency Inventory
+
+This inventory closes the first `rustfs/backlog#735` step: make every
+observability dependency on ECStore visible before introducing traits or moving
+dependency direction.
+
+No behavior or crate movement is planned in inventory PRs. The current boundary
+is `crates/obs/src/metrics/storage_api.rs`; all direct `rustfs_ecstore` and
+`rustfs_storage_api` source references in `rustfs-obs` must stay in that file
+until the contracts below are extracted.
+
+## Dependency Inventory
+
+| Current symbol in `crates/obs/src/metrics/storage_api.rs` | Consumed by | Classification | Purpose |
+|---|---|---|---|
+| `rustfs_ecstore::api::storage::ECStore` as `ObsStore` | `stats_collector.rs` | Type dependency | Concrete object-store handle used to call storage admin methods and data-usage loaders. |
+| `rustfs_storage_api::{BucketOperations, BucketOptions, StorageAdminApi}` | `stats_collector.rs` | Type and trait dependency | Method-resolution and associated type contracts for bucket listing, backend info, and storage info. |
+| `rustfs_ecstore::api::runtime::object_store_handle` | `stats_collector.rs` | Runtime dependency | Resolves the currently published object-store handle for metric collection. |
+| `rustfs_ecstore::api::data_usage::load_data_usage_from_backend` | `stats_collector.rs` | Behavior dependency | Loads bucket/object usage and is projected into obs-local DTOs before collectors consume it. |
+| `rustfs_ecstore::api::capacity::{get_total_usable_capacity, get_total_usable_capacity_free}` | `stats_collector.rs` | Behavior dependency | Computes usable and free capacity from ECStore storage info. |
+| `rustfs_ecstore::api::bucket::metadata_sys::get_quota_config` | `stats_collector.rs` | Behavior dependency | Reads per-bucket quota limits used in bucket usage metrics. |
+| `rustfs_ecstore::api::bucket::bandwidth::monitor::Monitor` | `runtime_sources.rs`, `stats_collector.rs` | Type and runtime dependency | Reads replication bandwidth reports from the global bucket monitor handle. |
+| `rustfs_ecstore::api::runtime::bucket_monitor` | `runtime_sources.rs` | Runtime dependency | Resolves the global bucket bandwidth monitor for metric collection. |
+| `rustfs_ecstore::api::bucket::replication::{GLOBAL_REPLICATION_STATS, ReplicationStats}` | `runtime_sources.rs`, `stats_collector.rs` | Type and runtime dependency | Reads replication status, transfer, failure, and site-replication stats. |
+| `rustfs_ecstore::api::bucket::lifecycle::bucket_lifecycle_ops::{GLOBAL_ExpiryState, GLOBAL_TransitionState}` | `runtime_sources.rs`, `stats_collector.rs` | Runtime dependency | Reads lifecycle expiry and transition queue counters. |
+| `rustfs_ecstore::api::error::Result` as `ObsEcstoreResult` | `stats_collector.rs` | Type dependency | Preserves ECStore error propagation while data-usage behavior remains ECStore-owned. |
+
+## Classification
+
+The remaining coupling is not just a dependency declaration problem:
+
+- type coupling: `ObsStore`, `ObsEcstoreResult`, `ObsReplicationStats`,
+  `ObsBucketBandwidthMonitor`, `StorageAdminApi`, `BucketOperations`, and
+  `BucketOptions`;
+- runtime handle coupling: object-store handle, bucket monitor, replication
+  stats, expiry state, and transition state;
+- behavior coupling: data-usage loading, quota lookup, and capacity math.
+
+Removing `rustfs-ecstore` from `crates/obs/Cargo.toml` is unsafe until those
+three categories have replacement contracts and compile coverage.
+
+## Extraction Plan
+
+1. Keep all direct ECStore and storage-api imports centralized in
+   `crates/obs/src/metrics/storage_api.rs`.
+2. Keep projecting ECStore data-usage output into obs-local DTOs before
+   collectors consume it.
+3. Introduce obs-owned provider traits for storage info, bucket info, quota,
+   data usage, replication, bandwidth, and lifecycle queue snapshots.
+4. Implement those traits in ECStore or an ECStore-owned adapter crate after the
+   trait shapes are covered by focused tests.
+5. Remove the `rustfs-ecstore` dependency from `rustfs-obs` only after metrics
+   behavior is unchanged through the provider traits.
+
+## Guardrails
+
+The architecture guard enforces this inventory boundary:
+
+- `crates/obs/src/metrics/storage_api.rs` is the only `rustfs-obs` source file
+  allowed to reference `rustfs_ecstore` or `rustfs_storage_api`;
+- `rustfs-obs` must not add `storage_compat.rs` or `ecstore_compat.rs`
+  passthrough bridges;
+- future extraction PRs must update this inventory and the guard in the same
+  reviewed change when a dependency category is removed.

docs/architecture/overview.md

@@ -38,6 +38,9 @@ hot-path behavior must not drift during this migration.
 - [`global-state-crate-split-plan.md`](global-state-crate-split-plan.md): late
   global-state cleanup, runtime-source boundaries, fallback removal rules, and
   crate-split evaluation criteria.
+- [`obs-ecstore-dependency-inventory.md`](obs-ecstore-dependency-inventory.md):
+  observability-to-ECStore dependency inventory, classification, and extraction
+  guardrails.
 - [`ecstore-config-consumer-inventory.md`](ecstore-config-consumer-inventory.md):
   current `ecstore::config::{Config, KV, KVS}` definitions, consumers,
   migration risks, and do-not-change contract.