Make connection initialization more robust

proto/brad.proto

@@ -30,7 +30,14 @@ message StartSessionRequest {
 }
 
 message StartSessionResponse {
-  SessionId id = 1;
+  oneof result {
+    SessionId id = 1;
+    StartSessionError error = 2;
+  }
+}
+
+message StartSessionError {
+  string error_msg = 1;
 }
 
 message RunQueryRequest {

src/brad/connection/connection.py

@@ -40,5 +40,6 @@ def is_connected(self) -> bool:
 
 class ConnectionFailed(Exception):
     """
-    Used when
+    Used when an existing connection fails for any reason, or we failed to
+    establish a connection to an underlying engine.
     """

src/brad/front_end/front_end.py

@@ -18,6 +18,7 @@
 from brad.blueprint.manager import BlueprintManager
 from brad.config.engine import Engine
 from brad.config.file import ConfigFile
+from brad.connection.connection import ConnectionFailed
 from brad.daemon.monitor import Monitor
 from brad.daemon.messages import (
     ShutdownFrontEnd,
@@ -262,8 +263,24 @@ async def _run_teardown(self):
             self._estimator = None
 
     async def start_session(self) -> SessionId:
-        session_id, _ = await self._sessions.create_new_session()
-        return session_id
+        rand_backoff = None
+        while True:
+            try:
+                session_id, _ = await self._sessions.create_new_session()
+                return session_id
+            except ConnectionFailed:
+                if rand_backoff is None:
+                    rand_backoff = RandomizedExponentialBackoff(
+                        max_retries=10, base_delay_s=0.5, max_delay_s=10.0
+                    )
+                time_to_wait = rand_backoff.wait_time_s()
+                if time_to_wait is None:
+                    logger.exception(
+                        "Failed to start a new session due to a repeated "
+                        "connection failure (10 retries)."
+                    )
+                    raise
+                await asyncio.sleep(time_to_wait)
 
     async def end_session(self, session_id: SessionId) -> None:
         await self._sessions.end_session(session_id)

src/brad/front_end/grpc.py

@@ -4,6 +4,7 @@
 import brad.proto_gen.brad_pb2_grpc as rpc
 from brad.config.engine import Engine
 from brad.config.session import SessionId
+from brad.connection.connection import ConnectionFailed
 from brad.front_end.brad_interface import BradInterface
 from brad.front_end.errors import QueryError
 
@@ -24,8 +25,13 @@ def __init__(self, brad: BradInterface):
     async def StartSession(
         self, _request: b.StartSessionRequest, _context
     ) -> b.StartSessionResponse:
-        new_session_id = await self._brad.start_session()
-        return b.StartSessionResponse(id=b.SessionId(id_value=new_session_id.value()))
+        try:
+            new_session_id = await self._brad.start_session()
+            return b.StartSessionResponse(
+                id=b.SessionId(id_value=new_session_id.value())
+            )
+        except ConnectionFailed as ex:
+            return b.StartSessionResponse(error=b.StartSessionError(error_msg=repr(ex)))
 
     async def RunQuery(
         self, request: b.RunQueryRequest, _context

src/brad/grpc_client.py

@@ -122,7 +122,19 @@ def close(self) -> None:
     def start_session(self) -> SessionId:
         assert self._stub is not None
         result = self._stub.StartSession(b.StartSessionRequest())
-        return SessionId(result.id.id_value)
+        msg_kind = result.WhichOneof("result")
+        if msg_kind is None:
+            raise BradClientError(
+                message="BRAD RPC error: Unspecified start session result."
+            )
+        elif msg_kind == "id":
+            return SessionId(result.id.id_value)
+        elif msg_kind == "error":
+            raise BradClientError(message=result.error.error_msg)
+        else:
+            raise BradClientError(
+                message="BRAD RPC error: Unknown start session result."
+            )
 
     def end_session(self, session_id: SessionId) -> None:
         assert self._stub is not None

src/brad/proto_gen/blueprint_pb2.pyi

@@ -4,68 +4,55 @@ from google.protobuf import descriptor as _descriptor
 from google.protobuf import message as _message
 from typing import ClassVar as _ClassVar, Iterable as _Iterable, Mapping as _Mapping, Optional as _Optional, Union as _Union
 
-ATHENA: Engine
-AURORA: Engine
 DESCRIPTOR: _descriptor.FileDescriptor
-REDSHIFT: Engine
+
+class Engine(int, metaclass=_enum_type_wrapper.EnumTypeWrapper):
+    __slots__ = []  # type: ignore
+    UNKNOWN: _ClassVar[Engine]
+    AURORA: _ClassVar[Engine]
+    REDSHIFT: _ClassVar[Engine]
+    ATHENA: _ClassVar[Engine]
 UNKNOWN: Engine
+AURORA: Engine
+REDSHIFT: Engine
+ATHENA: Engine
 
 class Blueprint(_message.Message):
-    __slots__ = ["aurora", "policy", "redshift", "schema_name", "tables"]
-    AURORA_FIELD_NUMBER: _ClassVar[int]
-    POLICY_FIELD_NUMBER: _ClassVar[int]
-    REDSHIFT_FIELD_NUMBER: _ClassVar[int]
+    __slots__ = ["schema_name", "tables", "aurora", "redshift", "policy"]
     SCHEMA_NAME_FIELD_NUMBER: _ClassVar[int]
     TABLES_FIELD_NUMBER: _ClassVar[int]
-    aurora: Provisioning
-    policy: RoutingPolicy
-    redshift: Provisioning
+    AURORA_FIELD_NUMBER: _ClassVar[int]
+    REDSHIFT_FIELD_NUMBER: _ClassVar[int]
+    POLICY_FIELD_NUMBER: _ClassVar[int]
     schema_name: str
     tables: _containers.RepeatedCompositeFieldContainer[Table]
+    aurora: Provisioning
+    redshift: Provisioning
+    policy: RoutingPolicy
     def __init__(self, schema_name: _Optional[str] = ..., tables: _Optional[_Iterable[_Union[Table, _Mapping]]] = ..., aurora: _Optional[_Union[Provisioning, _Mapping]] = ..., redshift: _Optional[_Union[Provisioning, _Mapping]] = ..., policy: _Optional[_Union[RoutingPolicy, _Mapping]] = ...) -> None: ...
 
-class Index(_message.Message):
-    __slots__ = ["column_name"]
-    COLUMN_NAME_FIELD_NUMBER: _ClassVar[int]
-    column_name: _containers.RepeatedScalarFieldContainer[str]
-    def __init__(self, column_name: _Optional[_Iterable[str]] = ...) -> None: ...
-
-class Provisioning(_message.Message):
-    __slots__ = ["instance_type", "num_nodes"]
-    INSTANCE_TYPE_FIELD_NUMBER: _ClassVar[int]
-    NUM_NODES_FIELD_NUMBER: _ClassVar[int]
-    instance_type: str
-    num_nodes: int
-    def __init__(self, instance_type: _Optional[str] = ..., num_nodes: _Optional[int] = ...) -> None: ...
-
-class RoutingPolicy(_message.Message):
-    __slots__ = ["policy"]
-    POLICY_FIELD_NUMBER: _ClassVar[int]
-    policy: bytes
-    def __init__(self, policy: _Optional[bytes] = ...) -> None: ...
-
 class Table(_message.Message):
-    __slots__ = ["columns", "dependencies", "indexes", "locations", "table_name"]
+    __slots__ = ["table_name", "columns", "locations", "dependencies", "indexes"]
+    TABLE_NAME_FIELD_NUMBER: _ClassVar[int]
     COLUMNS_FIELD_NUMBER: _ClassVar[int]
+    LOCATIONS_FIELD_NUMBER: _ClassVar[int]
     DEPENDENCIES_FIELD_NUMBER: _ClassVar[int]
     INDEXES_FIELD_NUMBER: _ClassVar[int]
-    LOCATIONS_FIELD_NUMBER: _ClassVar[int]
-    TABLE_NAME_FIELD_NUMBER: _ClassVar[int]
+    table_name: str
     columns: _containers.RepeatedCompositeFieldContainer[TableColumn]
+    locations: _containers.RepeatedScalarFieldContainer[Engine]
     dependencies: TableDependency
     indexes: _containers.RepeatedCompositeFieldContainer[Index]
-    locations: _containers.RepeatedScalarFieldContainer[Engine]
-    table_name: str
     def __init__(self, table_name: _Optional[str] = ..., columns: _Optional[_Iterable[_Union[TableColumn, _Mapping]]] = ..., locations: _Optional[_Iterable[_Union[Engine, str]]] = ..., dependencies: _Optional[_Union[TableDependency, _Mapping]] = ..., indexes: _Optional[_Iterable[_Union[Index, _Mapping]]] = ...) -> None: ...
 
 class TableColumn(_message.Message):
-    __slots__ = ["data_type", "is_primary", "name"]
+    __slots__ = ["name", "data_type", "is_primary"]
+    NAME_FIELD_NUMBER: _ClassVar[int]
     DATA_TYPE_FIELD_NUMBER: _ClassVar[int]
     IS_PRIMARY_FIELD_NUMBER: _ClassVar[int]
-    NAME_FIELD_NUMBER: _ClassVar[int]
+    name: str
     data_type: str
     is_primary: bool
-    name: str
     def __init__(self, name: _Optional[str] = ..., data_type: _Optional[str] = ..., is_primary: bool = ...) -> None: ...
 
 class TableDependency(_message.Message):
@@ -76,5 +63,22 @@ class TableDependency(_message.Message):
     transform: str
     def __init__(self, source_table_names: _Optional[_Iterable[str]] = ..., transform: _Optional[str] = ...) -> None: ...
 
-class Engine(int, metaclass=_enum_type_wrapper.EnumTypeWrapper):  # type: ignore
-    __slots__ = []  # type: ignore
+class Provisioning(_message.Message):
+    __slots__ = ["instance_type", "num_nodes"]
+    INSTANCE_TYPE_FIELD_NUMBER: _ClassVar[int]
+    NUM_NODES_FIELD_NUMBER: _ClassVar[int]
+    instance_type: str
+    num_nodes: int
+    def __init__(self, instance_type: _Optional[str] = ..., num_nodes: _Optional[int] = ...) -> None: ...
+
+class RoutingPolicy(_message.Message):
+    __slots__ = ["policy"]
+    POLICY_FIELD_NUMBER: _ClassVar[int]
+    policy: bytes
+    def __init__(self, policy: _Optional[bytes] = ...) -> None: ...
+
+class Index(_message.Message):
+    __slots__ = ["column_name"]
+    COLUMN_NAME_FIELD_NUMBER: _ClassVar[int]
+    column_name: _containers.RepeatedScalarFieldContainer[str]
+    def __init__(self, column_name: _Optional[_Iterable[str]] = ...) -> None: ...