unregister for memory banks and remove update API (#458)

The semantics of an Update on resources is very tricky to reason about
especially for memory banks and models. The best way to go forward here
is for the user to unregister and register a new resource. We don't have
a compelling reason to support update APIs.


Tests:
pytest -v -s llama_stack/providers/tests/memory/test_memory.py -m
"chroma" --env CHROMA_HOST=localhost --env CHROMA_PORT=8000

pytest -v -s llama_stack/providers/tests/memory/test_memory.py -m
"pgvector" --env PGVECTOR_DB=postgres --env PGVECTOR_USER=postgres --env
PGVECTOR_PASSWORD=mysecretpassword --env PGVECTOR_HOST=0.0.0.0

$CONDA_PREFIX/bin/pytest -v -s -m "ollama"
llama_stack/providers/tests/inference/test_model_registration.py

---------

Co-authored-by: Dinesh Yeduguru <dineshyv@fb.com>

llama_stack/providers/inline/memory/faiss/faiss.py

@@ -4,6 +4,8 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import base64
+import json
 import logging
 
 from typing import Any, Dict, List, Optional
@@ -37,10 +39,52 @@ class FaissIndex(EmbeddingIndex):
     id_by_index: Dict[int, str]
     chunk_by_index: Dict[int, str]
 
-    def __init__(self, dimension: int):
+    def __init__(self, dimension: int, kvstore=None, bank_id: str = None):
         self.index = faiss.IndexFlatL2(dimension)
         self.id_by_index = {}
         self.chunk_by_index = {}
+        self.kvstore = kvstore
+        self.bank_id = bank_id
+        self.initialize()
+
+    async def initialize(self) -> None:
+        if not self.kvstore:
+            return
+
+        index_key = f"faiss_index:v1::{self.bank_id}"
+        stored_data = await self.kvstore.get(index_key)
+
+        if stored_data:
+            data = json.loads(stored_data)
+            self.id_by_index = {int(k): v for k, v in data["id_by_index"].items()}
+            self.chunk_by_index = {
+                int(k): Chunk.model_validate_json(v)
+                for k, v in data["chunk_by_index"].items()
+            }
+
+            index_bytes = base64.b64decode(data["faiss_index"])
+            self.index = faiss.deserialize_index(index_bytes)
+
+    async def _save_index(self):
+        if not self.kvstore or not self.bank_id:
+            return
+
+        index_bytes = faiss.serialize_index(self.index)
+
+        data = {
+            "id_by_index": self.id_by_index,
+            "chunk_by_index": {k: v.json() for k, v in self.chunk_by_index.items()},
+            "faiss_index": base64.b64encode(index_bytes).decode(),
+        }
+
+        index_key = f"faiss_index:v1::{self.bank_id}"
+        await self.kvstore.set(key=index_key, value=json.dumps(data))
+
+    async def delete(self):
+        if not self.kvstore or not self.bank_id:
+            return
+
+        await self.kvstore.delete(f"faiss_index:v1::{self.bank_id}")
 
     @tracing.span(name="add_chunks")
     async def add_chunks(self, chunks: List[Chunk], embeddings: NDArray):
@@ -51,6 +95,9 @@ class FaissIndex(EmbeddingIndex):
 
         self.index.add(np.array(embeddings).astype(np.float32))
 
+        # Save updated index
+        await self._save_index()
+
     async def query(
         self, embedding: NDArray, k: int, score_threshold: float
     ) -> QueryDocumentsResponse:
@@ -85,7 +132,7 @@ class FaissMemoryImpl(Memory, MemoryBanksProtocolPrivate):
         for bank_data in stored_banks:
             bank = VectorMemoryBank.model_validate_json(bank_data)
             index = BankWithIndex(
-                bank=bank, index=FaissIndex(ALL_MINILM_L6_V2_DIMENSION)
+                bank=bank, index=FaissIndex(ALL_MINILM_L6_V2_DIMENSION, self.kvstore)
             )
             self.cache[bank.identifier] = index
 
@@ -110,13 +157,19 @@ class FaissMemoryImpl(Memory, MemoryBanksProtocolPrivate):
 
         # Store in cache
         index = BankWithIndex(
-            bank=memory_bank, index=FaissIndex(ALL_MINILM_L6_V2_DIMENSION)
+            bank=memory_bank,
+            index=FaissIndex(ALL_MINILM_L6_V2_DIMENSION, self.kvstore),
         )
         self.cache[memory_bank.identifier] = index
 
     async def list_memory_banks(self) -> List[MemoryBank]:
         return [i.bank for i in self.cache.values()]
 
+    async def unregister_memory_bank(self, memory_bank_id: str) -> None:
+        await self.cache[memory_bank_id].index.delete()
+        del self.cache[memory_bank_id]
+        await self.kvstore.delete(f"{MEMORY_BANKS_PREFIX}{memory_bank_id}")
+
     async def insert_documents(
         self,
         bank_id: str,