Add precondition check (#21)

* Add precondition check

* Better message

* Add precondition check for upsert

Author: javivelasco

src/handler/replace-document.ts

@@ -31,7 +31,19 @@ export default async (
 
   if (data.id !== body.id) {
     res.statusCode = 400;
-    return { Message: "replacing id is not allowed" };
+    return {
+      code: "BadRequest",
+      message: "replacing id is not allowed"
+    };
+  }
+
+  if (req.headers["if-match"] && req.headers["if-match"] !== data._etag) {
+    res.statusCode = 412;
+    return {
+      code: "PreconditionFailed",
+      message:
+        "Operation cannot be performed because one of the specified precondition is not met."
+    };
   }
 
   return collection.documents.replace(body);

src/handler/upsert-document.ts

@@ -26,5 +26,17 @@ export default async (
     return {};
   }
 
+  if (req.headers["if-match"]) {
+    const data = collection.document(body.id).read();
+    if (data && req.headers["if-match"] !== data._etag) {
+      res.statusCode = 412;
+      return {
+        code: "PreconditionFailed",
+        message:
+          "Operation cannot be performed because one of the specified precondition is not met."
+      };
+    }
+  }
+
   return collection.documents.upsert(body);
 };