IDBTransaction - Web APIs 编辑

The IDBTransaction interface of the IndexedDB API provides a static, asynchronous transaction on a database using event handler attributes. All reading and writing of data is done within transactions. You use IDBDatabase to start transactions, IDBTransaction to set the mode of the transaction (e.g. is it readonly or readwrite), and you access an IDBObjectStore to make a request. You can also use an IDBTransaction object to abort transactions.

Note:

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Transactions are started when the transaction is created, not when the first request is placed; for example consider this:

var trans1 = db.transaction("foo", "readwrite");
var trans2 = db.transaction("foo", "readwrite");
var objectStore2 = trans2.objectStore("foo")
var objectStore1 = trans1.objectStore("foo")
objectStore2.put("2", "key");
objectStore1.put("1", "key");

After the code is executed the object store should contain the value "2", since trans2 should run after trans1.

Transaction failures

Transactions can fail for a fixed number of reasons, all of which (except the user agent crash) will trigger an abort callback:

• Abort due to bad requests, e.g. trying to add() the same key twice, or put() with the same index key with a uniqueness constraint. This causes an error on the request, which can bubble up to an error on the transaction, which aborts the transaction. This can be prevented by using preventDefault() on the error event on the request.
• An explicit abort() call from script.
• An uncaught exception in the request's success/error handler.
• An I/O error (e.g. an actual failure to write to disk, or other OS/hardware failure).
• Quota exceeded.
• A user agent crash.

Firefox durability guarantees

Note that as of Firefox 40, IndexedDB transactions have relaxed durability guarantees to increase performance (see bug 1112702.) Previously in a readwrite transaction IDBTransaction.oncomplete was fired only when all data was guaranteed to have been flushed to disk. In Firefox 40+ the complete event is fired after the OS has been told to write the data but potentially before that data has actually been flushed to disk. The complete event may thus be delivered quicker than before, however, there exists a small chance that the entire transaction will be lost if the OS crashes or there is a loss of system power before the data is flushed to disk. Since such catastrophic events are rare, most consumers should not need to concern themselves further.

If you must ensure durability for some reason (e.g. you're storing critical data that cannot be recomputed later) you can force a transaction to flush to disk before delivering the complete event by creating a transaction using the experimental (non-standard) readwriteflush mode (see IDBDatabase.transaction.

Properties

IDBTransaction.db Read only

The database connection with which this transaction is associated.

IDBTransaction.error Read only

Returns a DOMException indicating the type of error that occurred when there is an unsuccessful transaction. This property is null if the transaction is not finished, is finished and successfully committed, or was aborted with theIDBTransaction.abort() function.

IDBTransaction.mode Read only

The mode for isolating access to data in the object stores that are in the scope of the transaction. The default value is readonly.

IDBTransaction.objectStoreNames Read only

Returns a DOMStringList of the names of IDBObjectStore objects associated with the transaction.

Methods

Inherits from: EventTarget

IDBTransaction.abort()

Rolls back all the changes to objects in the database associated with this transaction. If this transaction has been aborted or completed, this method fires an error event.

IDBTransaction.objectStore()

Returns an IDBObjectStore object representing an object store that is part of the scope of this transaction.

IDBTransaction.commit()

For an active transaction, commits the transaction. Note that this doesn't normally have to be called — a transaction will automatically commit when all outstanding requests have been satisfied and no new requests have been made. commit() can be used to start the commit process without waiting for events from outstanding requests to be dispatched.

Events

Listen to these events using addEventListener() or by assigning an event listener to the oneventname property of this interface.

abort

Fired when an IndexedDB transaction is aborted.
Also available via the onabort property.

complete

Fired when a transaction successfully completes.
Also available via the oncomplete property.

error

Fired when a request returns an error and the event bubbles up to the transaction object.
Also available via the onerror property.

Mode constants

Deprecated since Gecko 13 (Firefox 13 / Thunderbird 13 / SeaMonkey 2.10)

This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

These constants are no longer available — they were removed in Gecko 25. You should use the string constants directly instead. (bug 888598)

Transactions can have one of three modes:

Even if these constants are now deprecated, you can still use them to provide backward compatibility if required (in Chrome the change was made in version 21). You should code defensively in case the object is not available anymore:

var myIDBTransaction = window.IDBTransaction || window.webkitIDBTransaction || { READ_WRITE: "readwrite" };

Examples

In the following code snippet, we open a read/write transaction on our database and add some data to an object store. Note also the functions attached to transaction event handlers to report on the outcome of the transaction opening in the event of success or failure. For a full working example, see our To-do Notifications app (view example live.)

// Let us open our database
var DBOpenRequest = window.indexedDB.open("toDoList", 4);

DBOpenRequest.onsuccess = function(event) {
  note.innerHTML += '<li>Database initialised.</li>';

  // store the result of opening the database in the db
  // variable. This is used a lot below
  db = DBOpenRequest.result;

  // Add the data to the database
  addData();
};

function addData() {
  // Create a new object to insert into the IDB
  var newItem = [ { taskTitle: "Walk dog", hours: 19, minutes: 30, day: 24, month: "December", year: 2013, notified: "no" } ];

  // open a read/write db transaction, ready to add data
  var transaction = db.transaction(["toDoList"], "readwrite");

  // report on the success of opening the transaction
  transaction.oncomplete = function(event) {
    note.innerHTML += '<li>Transaction completed: database modification finished.</li>';
  };

  transaction.onerror = function(event) {
  note.innerHTML += '<li>Transaction not opened due to error. Duplicate items not allowed.</li>';
  };

  // create an object store on the transaction
  var objectStore = transaction.objectStore("toDoList");

  // add our newItem object to the object store
  var objectStoreRequest = objectStore.add(newItem[0]);

  objectStoreRequest.onsuccess = function(event) {
    // report the success of the request (this does not mean the item
    // has been stored successfully in the DB - for that you need transaction.oncomplete)
    note.innerHTML += '<li>Request successful.</li>';
  };
};

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Using IndexedDB
• Starting transactions: IDBDatabase
• Using transactions: IDBTransaction
• Setting a range of keys: IDBKeyRange
• Retrieving and making changes to your data: IDBObjectStore
• Using cursors: IDBCursor
• Reference example: To-do Notifications (view example live.)