SDK: new 'bmgr' docs in the Dev Guide

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  <h2>bmgr quickview</h2>

<p><code>bmgr</code> lets you inspect and control the backup/restore system on an Android device.

  <h2>In this document</h2>

  <ol>

<li><a href="#transports">About backup transports</a></li>

<li><a href="#restoresets">About restore sets</a></li>

<li><a href="#backup">Forcing a backup operation</a></li>

<li><a href="#restore">Forcing a restore operation</a></li>

<li><a href="#other">Other commands</a></li>

  </ol>

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<p><code>bmgr</code> is a shell tool that developers can use to interact with the Backup Manager

on Android devices supporting API version 8 or later.  It provides commands for inducing backup

and restore operations on demand so that you do not need to repeatedly wipe data or take similar

intrusive steps to test the operation of an application's backup agent.  These commands are

accessed via the <a href="{@docRoot}guide/developing/tools/adb.html">adb</a> shell.

<p>There are a couple of basic concepts used by the Backup Manager that are built into the way

that <code>bmgr</code> operates.  These are <a href="#transports">backup transports</a> and

<a href="#restoresets">restore sets</a>.

<a name="transports"></a>

<h2>About backup transports</h2>

<p>A <em>backup transport</em> is the code module responsible for moving backup and restore data

to and from some storage location.  A device can have multipe transports installed, though only

one is active at any given time.  Transports are identified by name.  You can see what

transports are available on your device or emulator by running the

<code>bmgr list transports</code> command:

    <pre>adb shell bmgr list transports</pre>

<p>The output of this command is a list of the transports available on the device.  The currently

active transport is flagged with a <code>*</code> character.  Transport names may look like

component names -- for example, <code>android/com.android.internal.backup.LocalTransport</code> --

but they need not be, and the strings are never used as direct class references.  The use of

a component-like naming scheme is simply for purposes of preventing name collisions.

<p>You can change which transport is currently active from the command line as well:

    <pre>adb shell bmgr transport NAME</pre>

<p>where <code>NAME</code> is one of the names as printed by the <code>bmgr list transports</code>

command.  From this point forward, backup and restore operations will be directed through the

newly-selected transport.  Backup state tracking is managed separately for each transport, so

switching back and forth between them will not corrupt the saved state.

<a name="restoresets"></a>

<h2>About restore sets</h2>

<p>All of the application data that a device has written to a given backup transport is tracked

together, and is collectively sometimes called a <em>restore set,</em> because the typical use

of the data set is during restore operations.  Each time a device is initially provisioned, a

new data set is established.  The user can get a listing of all the data sets that can be

restored through the current transport by running this shell command:

    <pre>adb shell bmgr list sets</pre>

<p>The output is listing of available restore sets, one per line.  The first item on each line is

a <em>token,</em> a number in hexadecimal that identifies the restore set to the transport.  After

that is a string that may provide some brief identifying information about the restore set.  Only

the token is actually used within the backup and restore mechanism.

<a name="backup"></a>

<h2>Forcing a backup operation</h2>

<p>Normally, applications notify the backup manager directly that their data has changed, in

response to which the backup manager will make sure to invoke that application's agent when the

next backup operation is run.  You can simulate this manually from the command line if you wish,

by running this shell command:

    <pre>adb shell bmgr backup PACKAGE</pre>

<p><code>PACKAGE</code> is the formal package name of the application you wish to schedule for

backup.  At this point you know that the application's agent will be invoked for backup at some

point in the future, though there is no hard guarantee of when that will occur.  You can force

all pending backup operations to run immediately by using the following command:

    <pre>adb shell bmgr run</pre>

<p>This causes a backup pass to execute immediately, invoking the agents of all applications that

had called <code>BackupManager.dataChanged()</code> since the time of the last backup operation,

plus any applications which had been manually scheduled for backup via

<code>bmgr backup PACKAGE</code>.

<a name="restore"></a>

<h2>Forcing a restore operation</h2>

<p>Unlike backup operations, which are batched together and run on an occasional basis, restore

operations execute immediately.  The backup manager currently provides two kinds of restore

operations.  The first restores an entire device with the data from a given restore set.  This

is typically only performed when a device is first provisioned, to replicate settings and other

such saved state from the user's previous device.  The second kind of restore operation restores

a single application from the <em>active</em> data set; that is, from the data set currently

being written to by backup operations.  This second form is available as part of the public API.

It allows applications to abandon their current data and revert to the last-known-good data as

represented in their current backup image.

<p>A full-system restore operation can be initiated with this shell command:

    <pre>adb shell bmgr restore TOKEN</pre>

<p>where <code>TOKEN</code> is the desired restore set's token as printed out by the <code>bmgr

list sets</code> command.  <strong>Warning!</strong>  This operation will <em>replace</em> the

data of all backup-enabled applications with the contents of the given restore set.  Be careful,

and be aware of the potential consequences.

<p>A single-application restore operation does not reference a restore set token; it always uses

the data from the currently active data set.  You can induce such an operation from the command

line like this:

    <pre>adb shell bmgr restore PACKAGE</pre>

<p><code>PACKAGE</code> is the formal package name of an application that is participating in the

backup/restore mechanism.  The backup manager will immediately instantiate the application's

agent and invoke it for restore.

<a name="other"></a>

<h2>Other commands</h2>

<p>The data for a single application can be erased from the active data set on demand.  This is

very useful during development of backup agents, in case bugs lead you to write corrupt data

or saved state information.  The shell command for wiping an application's data is this:

    <pre>adb shell bmgr wipe PACKAGE</pre>

<p><code>PACKAGE</code> is the formal package name of the application whose data you wish to

erase.  The next backup operation that the application's agent processes will look as

though the application had never backed anything up before.

<p>You can see whether the backup manager is operational at all by running this command:

    <pre>adb shell bmgr enabled</pre>

<p>This might be useful if your application's agent is never being invoked for backup, to verify

whether the operating system thinks it should be performing such operations at all.  You can also

directly disable or enable the backup manager with this command:

    <pre>adb shell bmgr enable BOOLEAN</pre>

<p>where <code>BOOLEAN</code> is either <code>true</code> or <code>false</code>.  This is

equivalent to disabling or enabling backup in the device's main Settings UI.

<strong>Warning!</strong>  When backup is disabled, the current transport will explicitly wipe

the entire active data set from its backend storage.  This is so that when a user says that no,

they do not want their data backed up, the backup manager respects that wish.  No further data

will be saved from the device, and no restore operations will be possible, unless the backup

manager is re-enabled (either through Settings or through the above <code>bmgr</code> command).

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