Refactoring work to support multi-tenancy and multiple storage, database backends (#598)

* Backend infrastructure
* New backend configuration work
* Factor out KV prefixing
* Don't put assets in the manifest cache
* Removed fancy authentication stuff
* Documentation updates

server/db_backend.ts

@@ -0,0 +1,44 @@
+import { DenoKvPrimitives } from "../plugos/lib/deno_kv_primitives.ts";
+import { KvPrimitives } from "../plugos/lib/kv_primitives.ts";
+import { path } from "./deps.ts";
+
+/**
+ * Environment variables:
+ * - SB_DB_BACKEND: "denokv" or "off" (default: denokv)
+ * - SB_KV_DB (denokv only): path to the database file (default .silverbullet.db) or ":cloud:" for cloud storage
+ */
+
+export async function determineDatabaseBackend(
+  singleTenantFolder?: string,
+): Promise<
+  KvPrimitives | undefined
+> {
+  const backendConfig = Deno.env.get("SB_DB_BACKEND") || "denokv";
+  switch (backendConfig) {
+    case "denokv": {
+      let dbFile: string | undefined = Deno.env.get("SB_KV_DB") ||
+        ".silverbullet.db";
+
+      if (singleTenantFolder) {
+        // If we're running in single tenant mode, we may as well use the tenant's space folder to keep the database
+        dbFile = path.resolve(singleTenantFolder, dbFile);
+      }
+
+      if (dbFile === ":cloud:") {
+        dbFile = undefined; // Deno Deploy will use the default KV store
+      }
+      const denoDb = await Deno.openKv(dbFile);
+      console.info(
+        `Using DenoKV as a database backend (${
+          dbFile || "cloud"
+        }), running in server-processing mode.`,
+      );
+      return new DenoKvPrimitives(denoDb);
+    }
+    default:
+      console.info(
+        "Running in databaseless mode: no server-side indexing and state keeping (beyond space files) will happen.",
+      );
+      return;
+  }
+}