Store BackendType as string instead of enum in the DB

contributing/BACKENDS.md

@@ -97,8 +97,6 @@ Add any dependencies required by your cloud provider to `setup.py`. Create a sep
 Add a new enumeration member for your provider to `BackendType` (`src/dstack/_internal/core/models/backends/base.py`).
 Use the name of the provider.
 
-Then create a database [migration](MIGRATIONS.md) to reflect the new enum member.
-
 ##### 2.4.2. Create the backend directory
 
 Create a new directory under `src/dstack/_internal/core/backends` with the name of the backend type.

src/dstack/_internal/server/migrations/versions/bc8ca4a505c6_store_backendtype_as_string.py

@@ -0,0 +1,171 @@
+"""Store BackendType as string
+
+Revision ID: bc8ca4a505c6
+Revises: 98d1b92988bc
+Create Date: 2025-03-10 14:49:06.837118
+
+"""
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision = "bc8ca4a505c6"
+down_revision = "98d1b92988bc"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("backends", schema=None) as batch_op:
+        batch_op.alter_column(
+            "type",
+            existing_type=postgresql.ENUM(
+                "AWS",
+                "AZURE",
+                "CUDO",
+                "DATACRUNCH",
+                "DSTACK",
+                "GCP",
+                "KUBERNETES",
+                "LAMBDA",
+                "LOCAL",
+                "REMOTE",
+                "NEBIUS",
+                "OCI",
+                "RUNPOD",
+                "TENSORDOCK",
+                "VASTAI",
+                "VULTR",
+                name="backendtype",
+            ),
+            type_=sa.String(length=100),
+            existing_nullable=False,
+        )
+
+    with op.batch_alter_table("instances", schema=None) as batch_op:
+        batch_op.alter_column(
+            "backend",
+            existing_type=postgresql.ENUM(
+                "AWS",
+                "AZURE",
+                "CUDO",
+                "DATACRUNCH",
+                "DSTACK",
+                "GCP",
+                "KUBERNETES",
+                "LAMBDA",
+                "LOCAL",
+                "REMOTE",
+                "NEBIUS",
+                "OCI",
+                "RUNPOD",
+                "TENSORDOCK",
+                "VASTAI",
+                "VULTR",
+                name="backendtype",
+            ),
+            type_=sa.String(length=100),
+            existing_nullable=True,
+        )
+
+    sa.Enum(
+        "AWS",
+        "AZURE",
+        "CUDO",
+        "DATACRUNCH",
+        "DSTACK",
+        "GCP",
+        "KUBERNETES",
+        "LAMBDA",
+        "LOCAL",
+        "REMOTE",
+        "NEBIUS",
+        "OCI",
+        "RUNPOD",
+        "TENSORDOCK",
+        "VASTAI",
+        "VULTR",
+        name="backendtype",
+    ).drop(op.get_bind())
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    sa.Enum(
+        "AWS",
+        "AZURE",
+        "CUDO",
+        "DATACRUNCH",
+        "DSTACK",
+        "GCP",
+        "KUBERNETES",
+        "LAMBDA",
+        "LOCAL",
+        "REMOTE",
+        "NEBIUS",
+        "OCI",
+        "RUNPOD",
+        "TENSORDOCK",
+        "VASTAI",
+        "VULTR",
+        name="backendtype",
+    ).create(op.get_bind())
+    with op.batch_alter_table("instances", schema=None) as batch_op:
+        batch_op.alter_column(
+            "backend",
+            existing_type=sa.String(length=100),
+            type_=postgresql.ENUM(
+                "AWS",
+                "AZURE",
+                "CUDO",
+                "DATACRUNCH",
+                "DSTACK",
+                "GCP",
+                "KUBERNETES",
+                "LAMBDA",
+                "LOCAL",
+                "REMOTE",
+                "NEBIUS",
+                "OCI",
+                "RUNPOD",
+                "TENSORDOCK",
+                "VASTAI",
+                "VULTR",
+                name="backendtype",
+            ),
+            existing_nullable=True,
+            postgresql_using="backend::VARCHAR::backendtype",
+        )
+
+    with op.batch_alter_table("backends", schema=None) as batch_op:
+        batch_op.alter_column(
+            "type",
+            existing_type=sa.String(length=100),
+            type_=postgresql.ENUM(
+                "AWS",
+                "AZURE",
+                "CUDO",
+                "DATACRUNCH",
+                "DSTACK",
+                "GCP",
+                "KUBERNETES",
+                "LAMBDA",
+                "LOCAL",
+                "REMOTE",
+                "NEBIUS",
+                "OCI",
+                "RUNPOD",
+                "TENSORDOCK",
+                "VASTAI",
+                "VULTR",
+                name="backendtype",
+            ),
+            existing_nullable=False,
+            postgresql_using="type::VARCHAR::backendtype",
+        )
+
+    # ### end Alembic commands ###

src/dstack/_internal/server/models.py

@@ -1,3 +1,4 @@
+import enum
 import uuid
 from datetime import datetime
 from typing import Callable, List, Optional, Union
@@ -112,7 +113,11 @@ def set_encrypt_decrypt(
         cls._encrypt_func = encrypt_func
         cls._decrypt_func = decrypt_func
 
-    def process_bind_param(self, value: Union[DecryptedString, str], dialect):
+    def process_bind_param(
+        self, value: Optional[Union[DecryptedString, str]], dialect
+    ) -> Optional[str]:
+        if value is None:
+            return None
         if isinstance(value, str):
             # Passing string allows binding an encrypted value directly
             # e.g. for comparisons
@@ -130,6 +135,29 @@ def process_result_value(self, value: Optional[str], dialect) -> Optional[Decryp
             return DecryptedString(plaintext=None, decrypted=False, exc=e)
 
 
+class EnumAsString(TypeDecorator):
+    """
+    A custom type decorator that stores enums as strings in the DB.
+    """
+
+    impl = String
+    cache_ok = True
+
+    def __init__(self, enum_class: type[enum.Enum], *args, **kwargs):
+        self.enum_class = enum_class
+        super().__init__(*args, **kwargs)
+
+    def process_bind_param(self, value: Optional[enum.Enum], dialect) -> Optional[str]:
+        if value is None:
+            return None
+        return value.name
+
+    def process_result_value(self, value: Optional[str], dialect) -> Optional[enum.Enum]:
+        if value is None:
+            return None
+        return self.enum_class[value]
+
+
 constraint_naming_convention = {
     "ix": "ix_%(column_0_label)s",
     "uq": "uq_%(table_name)s_%(column_0_name)s",
@@ -222,7 +250,7 @@ class BackendModel(BaseModel):
     )
     project_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("projects.id", ondelete="CASCADE"))
     project: Mapped["ProjectModel"] = relationship()
-    type: Mapped[BackendType] = mapped_column(Enum(BackendType))
+    type: Mapped[BackendType] = mapped_column(EnumAsString(BackendType, 100))
 
     config: Mapped[str] = mapped_column(String(20000))
     auth: Mapped[DecryptedString] = mapped_column(EncryptedString(20000))
@@ -533,7 +561,7 @@ class InstanceModel(BaseModel):
     last_termination_retry_at: Mapped[Optional[datetime]] = mapped_column(NaiveDateTime)
 
     # backend
-    backend: Mapped[Optional[BackendType]] = mapped_column(Enum(BackendType))
+    backend: Mapped[Optional[BackendType]] = mapped_column(EnumAsString(BackendType, 100))
     backend_data: Mapped[Optional[str]] = mapped_column(Text)
 
     # offer

src/dstack/_internal/server/services/backends/__init__.py

@@ -102,7 +102,7 @@ async def validate_and_create_backend_model(
     )
     return BackendModel(
         project_id=project.id,
-        type=configurator.TYPE.value,
+        type=configurator.TYPE,
         config=backend_record.config,
         auth=DecryptedString(plaintext=backend_record.auth),
     )