diff --git a/da/api/__init__.py b/da/api/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/da/api/common.py b/da/api/common.py
new file mode 100644
index 0000000..ecf5845
--- /dev/null
+++ b/da/api/common.py
@@ -0,0 +1,32 @@
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import Optional, List
+
+from da.verifier import DABlob
+
+@dataclass
+class Metadata:
+    index: int
+    app_id: int 
+
+class BlobStore(ABC):
+    @abstractmethod
+    def add(certificate, metadata):
+        pass
+
+    @abstractmethod
+    def get_multiple(app_id, indexes) -> List[DABlob]:
+        pass
+
+class Api:
+    def __init__(self, bs: BlobStore):
+        self.store = bs
+
+    def write(self, certificate, metadata):
+        # TODO: Certificate indexing can fail.
+        self.store.add(certificate, metadata)
+        return
+
+    def read(self, app_id, indexes) -> Optional[List[Optional[DABlob]]]:
+        # Gather requested indexes for the app_id.
+        return self.store.get_multiple(app_id, indexes)
diff --git a/da/api/test_flow.py b/da/api/test_flow.py
new file mode 100644
index 0000000..477f410
--- /dev/null
+++ b/da/api/test_flow.py
@@ -0,0 +1,69 @@
+from unittest import TestCase
+from collections import defaultdict
+
+from da.api.common import *
+
+@dataclass
+class MockCertificate:
+    cert_id: int
+
+class MockStore(BlobStore):
+    def __init__(self):
+        self.blob_store = {}
+        self.app_id_store = defaultdict(dict)
+
+    def populate(self, blob, cert_id: bytes):
+        self.blob_store[cert_id] = blob
+
+    # cert, app_id, idx
+    def add(self, cert_id: bytes, metadata: Metadata):
+        if metadata.index in self.app_id_store[metadata.app_id]:
+            raise ValueError("index already written")
+
+        blob = self.blob_store.pop(cert_id)
+        self.app_id_store[metadata.app_id][metadata.index] = blob 
+
+    def get_multiple(self, app_id, indexes) -> List[Optional[DABlob]]:
+        return [
+                self.app_id_store[app_id].get(i) for i in indexes
+        ]
+
+
+class TestFlow(TestCase):
+    def test_api_write_read(self):
+        expected_blob = "hello"
+        cert_id = b"11"*32
+        app_id = 1
+        idx = 1
+        mock_meta = Metadata(1, 1)
+
+        mock_store = MockStore()
+        mock_store.populate(expected_blob, cert_id)
+
+        api = Api(mock_store)
+
+        api.write(cert_id, mock_meta)
+        blob = api.read(app_id, [idx])
+
+        self.assertEqual([expected_blob], blob)
+
+    def test_same_index(self):
+        expected_blob = "hello"
+        cert_id = b"11"*32
+        app_id = 1
+        idx = 1
+        mock_meta = Metadata(1, 1)
+
+        mock_store = MockStore()
+        mock_store.populate(expected_blob, cert_id)
+
+        api = Api(mock_store)
+
+        api.write(cert_id, mock_meta)
+        with self.assertRaises(ValueError):
+            api.write(cert_id, mock_meta)
+
+        blob = api.read(app_id, [idx])
+
+        self.assertEqual([expected_blob], blob)
+