From 0afd76e332ce4aa621e72bc3ce94929a685ef730 Mon Sep 17 00:00:00 2001
From: Patrick Brosset <patrick.brosset@microsoft.com>
Date: Mon, 4 Nov 2024 10:56:49 +0100
Subject: [PATCH] Add more IDB demo code

---
 idb-getallrecords/index.html | 154 +++++++++++++++++++++++------------
 1 file changed, 102 insertions(+), 52 deletions(-)

diff --git a/idb-getallrecords/index.html b/idb-getallrecords/index.html
index fff2a780..4559125a 100644
--- a/idb-getallrecords/index.html
+++ b/idb-getallrecords/index.html
@@ -5,9 +5,15 @@
 
 <h1>IndexedDB: getAllRecords()</h1>
 
+<button id="read" disabled>Read in batches</button>
+<button id="read-reverse" disabled>Read in reverse order</button>
+
 <script>
+  const readButton = document.querySelector("#read");
+  const readReverseButton = document.querySelector("#read-reverse");
+
   // Open the DB.
-  const openRequest = indexedDB.open("db", 3);
+  const openRequest = indexedDB.open("db", 4);
 
   // The first time, the onupgradeneeded event is triggered, and we use it
   // to create an object store (similar to a table in SQL databases).
@@ -17,14 +23,8 @@ <h1>IndexedDB: getAllRecords()</h1>
     const db = openRequest.result
 
     // Creating an object store.
-    // Providing keyPath means that objects stored here must have an "id" property.
     console.log("creating user store");
-    const store = db.createObjectStore("features", { keyPath: "id" });
-
-    // Indexes are used to query object stores by various properties.
-    // Creating one to search by the "name" property.
-    console.log("creating name index");
-    const index = store.createIndex("name", "name");
+    const store = db.createObjectStore("features");
   };
 
   function clearStore(db) {
@@ -37,15 +37,15 @@ <h1>IndexedDB: getAllRecords()</h1>
         console.error("Clear transaction error");
         reject(new Error("Clear transaction error", event));
       };
-      
+
       // The transaction gives us access to the store, but we need to name it.
       // Indeed, we could have opened the transaction for multiple stores at once.
       const clearTransactionStore = clearTransaction.objectStore("features");
-      
+
       // We can now clear the store.
       console.log("Clearing the store");
       const clearRequest = clearTransactionStore.clear();
-      
+
       clearRequest.onsuccess = event => {
         console.log("Store cleared");
         resolve();
@@ -68,15 +68,15 @@ <h1>IndexedDB: getAllRecords()</h1>
         console.error("Add transaction error");
         reject(new Error("Add transaction error", event));
       };
-      
+
       // The transaction gives us access to the store, but we need to name it.
       // Indeed, we could have opened the transaction for multiple stores at once.
       const addTransactionStore = addTransaction.objectStore("features");
-      
+
       // We can now add data to the store.
       console.log("Adding the data");
-      const addRequest = addTransactionStore.add({ id, name, description });
-      
+      const addRequest = addTransactionStore.add({ name, description }, id);
+
       addRequest.onsuccess = event => {
         console.log("Data added");
         resolve();
@@ -95,61 +95,111 @@ <h1>IndexedDB: getAllRecords()</h1>
     const data = await response.json();
 
     const featureAdditionPromises = Object.keys(data.features).map(id => {
-      return addOneFeatureToStore(db, id, data.features[id].name, data.features[id].description);
+      const { name, description } = data.features[id];
+      return addOneFeatureToStore(db, id, name, description);
     });
 
     return Promise.all(featureAdditionPromises);
   }
 
-  // Transactions can't run before the onupgradeneeded event has finished.
-  // We can wait by using the onsuccess event of the request.
-  openRequest.onsuccess = async event => {
-    console.log("onsuccess");
+  // Reading a DB in order can be done in mulitple ways.
+  // With a cursor, you do it one at a time, which means that if you're reading a
+  // lot of records, you will suffer through a lot of back and forth between the
+  // main thread and the IDB engine thread.
+  // You could also just read the entire DB at once, but if that DB is big, you
+  // will have memory problems.
+  // Below, we use getAll+getAllKeys to read the DB in batches, limiting the number
+  // of back-and-forth with the IDB engine thread.
+  // However this doesn't support reading in reverse order, and forces to use both
+  // getAll and getAllKeys together. Plus, finding the right batch size might be
+  // difficult. 
+  async function readInBatches(db) {
+    const batchSize = 10;
 
-    const db = openRequest.result
+    console.log("starting read transaction");
+    const readTransaction = db.transaction("features", "readonly");
 
-    // Start by clearing the entire store
-    await clearStore(db);
+    readTransaction.oncomplete = event => console.log("read transaction complete");
+    readTransaction.onerror = event => console.log("read transaction error");
 
-    // Add some data to the store.
-    await addAllFeatures(db);
+    const readTransactionStore = readTransaction.objectStore("features");
+
+    function getAllKeys(range) {
+      return new Promise(resolve => {
+        readTransactionStore.getAllKeys(range, batchSize).onsuccess = event => {
+          resolve(event.target.result);
+        };
+      });
+    }
+
+    function getAllValues(range) {
+      return new Promise(resolve => {
+        readTransactionStore.getAll(range, batchSize).onsuccess = event => {
+          resolve(event.target.result);
+        };
+      });
+    }
+
+    let range = null;
+
+    while (true) {
+      const [keys, values] = await Promise.all([getAllKeys(range), getAllValues(range)]);
+      if (keys && values && values.length === batchSize) {
+        // There could be more records, set a starting range for next iteration.
+        range = IDBKeyRange.lowerBound(keys.at(-1), true);
+
+        console.log(`Read ${batchSize} records`, keys, values);
+      } else {
+        break;
+      }
+    }
+  }
+
+  // Reading in reserve order requires the use of a cursor.
+  // This means that it has to be done one item at a time.
+  async function readReverse(db) {
+    const batchSize = 10;
 
-    // To read data from the database, we need another transaction.
     console.log("starting read transaction");
     const readTransaction = db.transaction("features", "readonly");
 
-    // We can also log the transaction's events.
     readTransaction.oncomplete = event => console.log("read transaction complete");
     readTransaction.onerror = event => console.log("read transaction error");
 
-    // Get the store from the transaction.
     const readTransactionStore = readTransaction.objectStore("features");
 
-    // We can now read data from the store.
-    console.log("reading data - all records");
-    const getAllRequest = readTransactionStore.getAll();
-    getAllRequest.onsuccess = event => console.log("all records", getAllRequest.result);
+    readTransactionStore.openCursor(null, "prev").onsuccess = event => {
+      const cursor = event.target.result;
+      if (cursor) {
+        console.log(`Read one item backward`, cursor.key, cursor.value);
+        cursor.continue();
+      } else {
+        console.log("Reverse read done");
+      }
+    };
+  }
 
-    console.log("reading data - all keys");
-    const getAllKeysRequest = readTransactionStore.getAllKeys();
-    getAllKeysRequest.onsuccess = event => console.log("all keys", getAllKeysRequest.result);
-  };
+  // Transactions can't run before the onupgradeneeded event has finished.
+  // We can wait by using the onsuccess event of the request.
+  openRequest.onsuccess = async event => {
+    console.log("onsuccess");
+
+    const db = openRequest.result;
 
-  /*
-  
-  EXISTING
-  
-  api.IDBIndex.getAll(query, count) // both optional
-  api.IDBIndex.getAllKeys(query, count) // both optional
-  
-  api.IDBObjectStore.getAll(query, count) // both optional
-  api.IDBObjectStore.getAllKeys(query, count) // both optional
-  
-  PROPOSED
-  
-  api.IDBIndex.getAllRecords({ count, direction })
-  api.IDBObjectStore.getAllRecords({ count, direction })
-  
-  */
+    // Start by clearing the entire store
+    await clearStore(db);
 
+    // Add some data to the store.
+    await addAllFeatures(db);
+
+    readButton.removeAttribute("disabled");
+    readButton.addEventListener("click", () => {
+      readInBatches(db);
+    });
+
+    readReverseButton.removeAttribute("disabled");
+    readReverseButton.addEventListener("click", () => {
+      readReverse(db);
+    });
+  };
 </script>
\ No newline at end of file