[IndexedDB] Update getAllRecords explainer to add direction to getAll and getAllKeys

IndexedDbGetAllEntries/explainer.md

@@ -7,23 +7,38 @@
 ## Participate
 
 - https://github.com/w3c/IndexedDB/issues/206
+- https://github.com/w3c/IndexedDB/issues/130
 
 ## Introduction
 
 [`IndexedDB`](https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API) is a transactional database for client-side storage. Each record in the database contains a key-value pair. [`getAll()`](https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore/getAll) enumerates database record values sorted by key in ascending order. [`getAllKeys()`](https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore/getAllKeys) enumerates database record primary keys sorted by key in ascending order.
 
 This explainer proposes a new operation, `getAllRecords()`, which combines [`getAllKeys()`](https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore/getAllKeys) with [`getAll()`](https://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore/getAll) to enumerate both primary keys and values at the same time. For an [`IDBIndex`](https://developer.mozilla.org/en-US/docs/Web/API/IDBIndex), `getAllRecords()` also provides the record's index key in addition to the primary key and value. Lastly, `getAllRecords()` offers a new direction option to enumerate records sorted by key in descending order.
 
+To add the direction option to the existing `getAll()` and `getAllKeys()` operations, this explainer proposes new function overloads that accept the same argument as `getAllRecords()`: the `IDBGetAllOptions` dictionary.
+
 ## Goals
 
 Decrease the latency of database read operations. By retrieving the primary key, value and index key for database records through a single operation, `getAllRecords()` reduces the number of JavaScript events required to read records. Each JavaScript event runs as a task on the main JavaScript thread. These tasks can introduce overhead when reading records requires a sequence of tasks that go back and forth between the main JavaScript thread and the IndexedDB I/O thread.
 
 For batched record iteration, for example, retrieving *N* records at a time, the primary and index keys provided by `getAllRecords()` can eliminate the need for an [`IDBCursor`](https://developer.mozilla.org/en-US/docs/Web/API/IDBCursor), which further reduces the number of JavaScript events required. To read the next *N* records, instead of advancing a cursor to determine the range of the next batch, getAllRecords() can use the primary key or the index key retrieved by the results from the previous batch.
 
+Update the existing operations `getAll()` and `getAllKeys()` to support the same query options as `getAllRecords()`, which adds direction.  For some scenarios, `getAll()` and `getAllKeys()` may suffice.  For example, developers may use `getAllKeys()` to defer loading values until needed.  For records with inline keys, `getAll()` already retrieves both key and value.
+
 ## `IDBObject::getAllRecords()` and `IDBIndex::getAllRecords()`
 
 This explainer proposes adding `getAllRecords()` to both [`IDBObjectStore`](https://www.w3.org/TR/IndexedDB/#idbobjectstore) and [`IDBIndex`](https://www.w3.org/TR/IndexedDB/#idbindex). `getAllRecords()` creates a new `IDBRequest` that queries its `IDBObjectStore` or `IDBIndex` owner. The `IDBRequest` completes with an array of `IDBRecord` results. Each `IDBRecord` contains the `key`, `primaryKey` and `value` attributes. For `IDBIndex`, `key` is the record's index key. For `IDBObjectStore`, both `key` and `primaryKey` return the same value. The pre-existing [`IDBCursorWithValue`](https://www.w3.org/TR/IndexedDB/#idbcursorwithvalue) interface contains the same attributes and values for both `IDBObjectStore` and `IDBIndex`. However, unlike `getAllRecords()`, a cursor may only read one record at a time.
 
+## Adding direction to `getAll()` and `getAllKeys()`
+
+This explainer proposes using `getAllRecords()` as feature detection for direction support in `getAllKeys()` and `getAll()`.  `getAllRecords()` introduces the `IDBGetAllOptions` dictionary, which developers may also use with `getAll()` and `getAllKeys()`.  Before using `IDBGetAllOptions`, developers must check for the existence of `getAllRecords()` in `IDBObjectStore` or `IDBIndex`.
+
+## Compatibility risk
+
+Overloading `getAll()` and `getAllKeys()` to accept the `IDBGetAllOptions` dictionary introduces compatibility risk.  Prior to this proposal, when passed a dictionary argument, both `getAll()` and `getAllKeys()` throw an exception after [failing to convert the dictionary to a key range](https://w3c.github.io/IndexedDB/#convert-a-value-to-a-key-range).  After the overload, `getAllKeys()` and `getAll()` will no longer throw for dictionary input.  When the `IDBGetAllOptions` dictionary initializes with its default values, it creates a query that retrieves all of the keys or values from the entire database.
+
+Since using a dictionary with `getAll()` and `getAllKeys()` is a programming error, we believe compat risk is low.
+
 ## Key scenarios
 
 ### Read multiple database records through a single request
@@ -182,6 +197,27 @@ if (!results.done) {
 }
 ```
 
+### Use direction with `getAllKeys()` after feature detection
+
+`getAllRecords()` introduces the `IDBGetAllOptions` dictionary, which developers may also use with `getAll()` and `getAllKeys()`.  Before using `IDBGetAllOptions`, developers must check for the existence of `getAllRecords()` in `IDBObjectStore` or `IDBIndex`.
+
+```js
+const read_transaction = database.transaction('my_object_store', "readonly");
+const object_store = read_transaction.objectStore('my_object_store');
+
+// Use feature detection to determine if this browser supports `getAllRecords()`.
+if ('getAllRecords' in object_store) {
+  // Request the last 5 primary keys in `object_store`.
+  const get_all_options = {
+    direction = 'prev',
+    count: 5    
+  };
+  const request = object_store.getAllKeys(get_all_options);
+} else {
+  // Fallback to a cursor with direction: 'prev' for this query.
+}
+```
+
 ## Considered alternatives
 
 ### `getAllEntries()`
@@ -194,14 +230,10 @@ Similar to `getAllRecords()` but [provides results as an array of entries](https
 
 Developers may directly use the entry results to construct a `Map` or `Object` since the entry results are inspired by ECMAScript's [Map.prototype.entries()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/entries).  However, `getAllEntries()` has unusual ergonomics, requiring indices like `0` and `1` to access the record properties like `key` and `value`.  Also, IndexedDB database records do not map cleanly to ECMAScript entries.  For `IDBIndex`, the results contain a third element for index key.  For an alternate form, `[[ indexKey1, [ primaryKey1, value1]], [ indexKey2, [ primaryKey2, value2]], ... ]`, the index key cannot always serve as the entry's key since the index key may not be unique across all records.
 
-### Adding direction to `getAll()` and `getAllKeys()`
-
-This will be pursued separately.  Join the discussion at https://github.com/w3c/IndexedDB/issues/130.  Providing the direction option on `getAllKeys()` might be useful for reverse iteration scenarios that don't need to load every value enumerated.
-
 ## WebIDL
 
 ```js
-dictionary IDBGetAllRecordsOptions {
+dictionary IDBGetAllOptions {
   // A key or an `IDBKeyRange` identifying the records to retrieve.
   any query = null;
 
@@ -228,6 +260,20 @@ partial interface IDBObjectStore {
   // `[[primaryKey1, value1], [primaryKey2, value2], ... ]`
   [NewObject, RaisesException]
   IDBRequest getAllRecords(optional IDBGetAllRecordsOptions options = {});
+
+  // For `getAll()` and `getAllKeys()`, add support for the direction option
+  // through a new overload, which accepts a `IDBGetAllOptions` dictionary as
+  // the first and only argument.
+  // 
+  // IDBRequest getAll(optional IDBGetAllOptions options);
+  // IDBRequest getAllKeys(optional IDBGetAllOptions options);
+  // 
+  [NewObject, RaisesException]
+  IDBRequest getAll(optional any query_or_options = null,
+                    optional [EnforceRange] unsigned long count);  
+  [NewObject, RaisesException]
+  IDBRequest getAllKeys(optional any query_or_options = null,
+                        optional [EnforceRange] unsigned long count);                    
 }
 
 [Exposed=(Window,Worker)]
@@ -237,6 +283,19 @@ partial interface IDBIndex {
   // `[[primaryKey1, value1, indexKey1], [primaryKey2, value2, indexKey2], ... ]`
   [NewObject, RaisesException]
   IDBRequest getAllRecords(optional IDBGetAllRecordsOptions options = {});
+
+  // Like `IDBObjectStore` above, IDBIndex overloads `getAll()` and `getAllKeys()`
+  // to support direction:
+  //
+  // IDBRequest getAll(optional IDBGetAllOptions options);
+  // IDBRequest getAllKeys(optional IDBGetAllOptions options);
+  // 
+  [NewObject, RaisesException]
+  IDBRequest getAll(optional any query_or_options = null,
+                    optional [EnforceRange] unsigned long count);  
+  [NewObject, RaisesException]
+  IDBRequest getAllKeys(optional any query_or_options = null,
+                        optional [EnforceRange] unsigned long count);     
 }
 ```