[HiCache][HA 1/N] Support HiCache storage runtime attach/detach (#15892)

2026-01-26 19:33:19 -08:00
parent 1b56a886bb
commit fd3b179ffd
10 changed files with 1488 additions and 124 deletions
@@ -6,3 +6,4 @@ Hierarchical KV Caching (HiCache)

   hicache_best_practices.md
   hicache_design.md
+   hicache_storage_runtime_attach_detach.md
@@ -19,6 +19,10 @@ SGLang HiCache extends the traditional RadixAttention with a three-tier hierarch
 --hicache-storage-backend             # Optional storage backend (e.g., hf3fs, mooncake, etc.)
 ```

+Notes:
+
+- Besides configuring `--hicache-storage-backend` at startup, SGLang also supports **runtime attach/detach** of the HiCache storage backend (no restart required) via HTTP admin endpoints. See [Runtime Attach/Detach HiCache Storage Backend](hicache_storage_runtime_attach_detach.md).
+
 ## Key Configurations with Storage Backends Enabled

 ### Memory Layout Optimization
@@ -0,0 +1,132 @@
+# Runtime Attach/Detach HiCache Storage Backend (No Restart)
+
+This document explains how to **dynamically attach/detach the HiCache L3 storage backend at runtime** (e.g., `mooncake` / `hf3fs` / `nixl` / `file` / `aibrix` / `eic`) while **SGLang is already running and serving traffic**, without restarting the process.
+
+For safety and consistency, the current implementation **strictly requires** these operations to happen only when the service is **idle**:
+
+- **No running requests**
+- **No waiting/queued requests**
+
+If the idle condition is not met, the API will fail fast (HTTP 400) and **will not modify** the current service state.
+
+---
+
+## 1. Background and implementation overview
+
+### 1.1 Architecture / control path
+
+The control path is:
+
+1. **HTTP Server** (`python/sglang/srt/entrypoints/http_server.py`)
+   - Exposes `PUT /hicache/storage-backend`, `DELETE /hicache/storage-backend`, `GET /hicache/storage-backend`
+2. **TokenizerManager** (`python/sglang/srt/managers/tokenizer_communicator_mixin.py`)
+   - Sends the request to the Scheduler via `_Communicator`
+3. **Scheduler** (`python/sglang/srt/managers/scheduler.py`)
+   - Performs a **strict idle check**
+   - Calls `tree_cache.attach_storage_backend(...)` / `detach_storage_backend(...)`
+4. **HiRadixCache** (`python/sglang/srt/mem_cache/hiradix_cache.py`)
+   - Parses `hicache_storage_backend_extra_config_json` (supports both backend config and prefetch knobs)
+   - Calls `cache_controller.attach_storage_backend(...)` / `detach_storage_backend(...)`
+5. **HiCacheController** (`python/sglang/srt/managers/cache_controller.py`)
+   - Creates/destroys the storage backend instance (via `StorageBackendFactory`)
+   - Starts/stops backend background threads at runtime (prefetch/backup)
+
+---
+
+## 2. Idle-state requirement (strict)
+
+The Scheduler uses a stricter `_is_idle_for_hicache_storage_op()`:
+
+- `_is_no_request()` is true (covers running/overlap/pp/disagg and other active states)
+- `waiting_queue` is empty
+- `grammar_queue` is empty (if the grammar backend is enabled)
+
+If the condition is not met, attach/detach returns an error like:
+
+- `Reject attach: scheduler is not idle. #queue-req=... #running-req=...`
+
+> Tip: before switching, drain upstream traffic and wait for the server to become idle, then call attach/detach.
+
+### 2.1 DP (data parallel) semantics
+
+When `dp_size > 1`, the tokenizer dispatches the request to **all DP scheduler instances** and aggregates their responses:
+
+- The final `success` is **true only if all DP ranks return success**
+- The final `message` concatenates messages from all DP ranks
+
+This is intended to prevent “silent partial success”, but it also means you may see:
+
+- Overall **failure** even though **some ranks already succeeded**
+
+Currently there is **no automatic partial rollback** across DP ranks (see TODO in code). Operationally:
+
+- Prefer to keep backend config identical across ranks
+- If attach fails, immediately call detach (best-effort/idempotent), fix config, then retry attach
+
+---
+
+## 3. How to use (HTTP Admin API)
+
+The examples below assume your SGLang HTTP server is at `http://127.0.0.1:30000`.
+
+### 3.1 Query current storage backend status
+
+```bash
+curl -s http://127.0.0.1:30000/hicache/storage-backend
+```
+
+Example response:
+
+```json
+{
+  "hicache_storage_backend": "mooncake",
+  "hicache_storage_backend_extra_config": "{\"master_server_address\":\"127.0.0.1:50051\", ...}"
+}
+```
+
+### 3.2 Attach (enable) a storage backend
+```bash
+curl -s -X PUT http://127.0.0.1:30000/hicache/storage-backend \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "hicache_storage_backend": "mooncake"
+  }'
+```
+
+```bash
+curl -s -X PUT http://127.0.0.1:30000/hicache/storage-backend \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "hicache_storage_backend": "mooncake",
+    "hicache_storage_backend_extra_config_json": "{\"master_server_address\":\"127.0.0.1:50051\",\"protocol\":\"tcp\",\"global_segment_size\":\"4gb\",\"prefetch_threshold\":256}",
+    "hicache_storage_prefetch_policy": "timeout"
+  }'
+```
+
+Notes:
+
+- `hicache_storage_backend_extra_config_json` can include both:
+  - **Backend configuration** (e.g., Mooncake master/metadata/protocol, etc.)
+  - **Prefetch configuration** (`prefetch_threshold`, `prefetch_timeout_base`, `prefetch_timeout_per_ki_token`, `hicache_storage_pass_prefix_keys`)
+
+### 3.3 Detach (disable) the storage backend
+
+```bash
+curl -s -X DELETE http://127.0.0.1:30000/hicache/storage-backend
+```
+
+Notes:
+
+- Detach only makes SGLang **stop using** the L3 storage backend and stops prefetch/backup threads
+- It **does not automatically delete** data stored in Mooncake/HF3FS (or other remote backends)
+
+---
+
+## 4. Behavior and caveats
+
+- **No restart required**: attach/detach switches in-process at runtime
+- **Must be idle**: otherwise the request is rejected to avoid consistency issues
+- **Host KV layout constraints still apply**: for example, Mooncake still requires layouts like `page_first/page_first_direct/page_head`; if the server's HiCache host-memory layout does not satisfy the backend requirements, attach will fail with an error
+- **Observability**:
+  - After attach, `server_args.hicache_storage_backend*` is updated on both the tokenizer and scheduler sides
+  - If metrics are enabled, attach will create a storage metrics collector in `HiRadixCache` on demand