Access shared datasets

Starting in Android 11 (API level 30), the system caches large datasets that multiple apps might access for use cases like machine learning and media playback. This functionality helps reduce data redundancy, both over the network and on disk.

When your app needs access to a shared large dataset, it can first look for these cached datasets, called shared data blobs, before determining whether to download a new copy. Apps can access these shared datasets functionality using the APIs in BlobStoreManager.

The system maintains the shared data blobs and controls which apps can access them. When your app contributes data blobs, you can indicate which other apps should have access by calling one of the following methods:

  • To grant access to a specific set of apps on a device, pass the package names of these apps into allowPackageAccess().
  • To allow only apps whose certificates are signed using the same key as the one used for your app—such as an app suite that you manage—call allowSameSignatureAccess().
  • To grant access to all apps on a device, call allowPublicAccess().

Access shared data blobs

The system represents each shared data blob using a BlobHandle object. Each instance of BlobHandle contains a cryptographically-secure hash and some identifying details for the dataset.

To access shared data blobs, download identifying details from the server. Using these details, check whether the dataset is already available on the system.

The next step depends on whether data is available.

Dataset available

If the dataset is already available on the device, then access it from the system, as shown in the following code snippet:

Kotlin

val blobStoreManager =
        getSystemService(Context.BLOB_STORE_SERVICE) as BlobStoreManager
// The label "Sample photos" is visible to the user.
val blobHandle = BlobHandle.createWithSha256(sha256DigestBytes,
        "Sample photos",
        System.currentTimeMillis() + TimeUnit.DAYS.toMillis(1),
        "photoTrainingDataset")
try {
    val input = ParcelFileDescriptor.AutoCloseInputStream(
            blobStoreManager.openBlob(blobHandle))
    useDataset(input)
}

Java

BlobStoreManager blobStoreManager =
        ((BlobStoreManager) getSystemService(Context.BLOB_STORE_SERVICE));
if (blobStoreManager != null) {
    // The label "Sample photos" is visible to the user.
    BlobHandle blobHandle = BlobHandle.createWithSha256(
            sha256DigestBytes,
            "Sample photos",
            System.currentTimeMillis() + TimeUnit.DAYS.toMillis(1),
            "photoTrainingDataset");
    try (InputStream input = new ParcelFileDescriptor.AutoCloseInputStream(
            blobStoreManager.openBlob(blobHandle))) {
        useDataset(input);
    }
}

Dataset unavailable

If the dataset isn't available, then download it from the server and contribute it to the system, as shown in the following code snippet:

Kotlin

val sessionId = blobStoreManager.createSession(blobHandle)
try {
    val session = blobStoreManager.openSession(sessionId)
    try {
        // For this example, write 200 MiB at the beginning of the file.
        val output = ParcelFileDescriptor.AutoCloseOutputStream(
                session.openWrite(0, 1024 * 1024 * 200))
        writeDataset(output)

        session.apply {
            allowSameSignatureAccess()
            allowPackageAccess(your-app-package,
                    app-certificate)
            allowPackageAccess(some-other-app-package,
                    app-certificate)
            commit(mainExecutor, callback)
        }
    }
}

Java

long sessionId = blobStoreManager.createSession(blobHandle);
try (BlobStoreManager.Session session =
        blobStoreManager.openSession(sessionId)) {
    // For this example, write 200 MiB at the beginning of the file.
    try (OutputStream output = new ParcelFileDescriptor.AutoCloseOutputStream(
            session.openWrite(0, 1024 * 1024 * 200)))
        writeDataset(output);
        session.allowSameSignatureAccess();
        session.allowPackageAccess(your-app-package,
                    app-certificate);
        session.allowPackageAccess(some-other-app-package,
                    app-certificate);
        session.commit(getMainExecutor(), callback);
    }
}