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# Appendix A: Backup Callback Interface

## mnesia_backup Callback Behavior

```erlang
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% This module contains one implementation of callback functions
%% used by Mnesia at backup and restore. The user may however
%% write an own module the same interface as mnesia_backup and
%% configure Mnesia so the alternate module performs the actual
%% accesses to the backup media. This means that the user may put
%% the backup on media that Mnesia does not know about, possibly
%% on hosts where Erlang is not running.
%%
%% The OpaqueData argument is never interpreted by other parts of
%% Mnesia. It is the property of this module. Alternate implementations
%% of this module may have different interpretations of OpaqueData.
%% The OpaqueData argument given to open_write/1 and open_read/1
%% are forwarded directly from the user.
%%
%% All functions must return {ok, NewOpaqueData} or {error, Reason}.
%%
%% The NewOpaqueData arguments returned by backup callback functions will
%% be given as input when the next backup callback function is invoked.
%% If any return value does not match {ok, _} the backup will be aborted.
%%
%% The NewOpaqueData arguments returned by restore callback functions will
%% be given as input when the next restore callback function is invoked
%% If any return value does not match {ok, _} the restore will be aborted.
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

-module(mnesia_backup).

-include_lib("kernel/include/file.hrl").

-export([
         %% Write access
         open_write/1,
         write/2,
         commit_write/1,
         abort_write/1,

         %% Read access
         open_read/1,
         read/1,
         close_read/1
        ]).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Backup callback interface
-record(backup, {tmp_file, file, file_desc}).

%% Opens backup media for write
%%
%% Returns {ok, OpaqueData} or {error, Reason}
open_write(OpaqueData) ->
    File = OpaqueData,
    Tmp = lists:concat([File,".BUPTMP"]),
    file:delete(Tmp),
    case disk_log:open([{name, make_ref()},
                        {file, Tmp},
                        {repair, false},
                        {linkto, self()}]) of
        {ok, Fd} ->
            {ok, #backup{tmp_file = Tmp, file = File, file_desc = Fd}};
        {error, Reason} ->
            {error, Reason}
    end.

%% Writes BackupItems to the backup media
%%
%% Returns {ok, OpaqueData} or {error, Reason}
write(OpaqueData, BackupItems) ->
    B = OpaqueData,
    case disk_log:log_terms(B#backup.file_desc, BackupItems) of
        ok ->
            {ok, B};
        {error, Reason} ->
            abort_write(B),
            {error, Reason}
    end.

%% Closes the backup media after a successful backup
%%
%% Returns {ok, ReturnValueToUser} or {error, Reason}
commit_write(OpaqueData) ->
    B = OpaqueData,
    case disk_log:sync(B#backup.file_desc) of
        ok ->
            case disk_log:close(B#backup.file_desc) of
                ok ->
                    file:delete(B#backup.file),
                    case file:rename(B#backup.tmp_file, B#backup.file) of
                        ok ->
                            {ok, B#backup.file};
                        {error, Reason} ->
                            {error, Reason}
                    end;
                {error, Reason} ->
                    {error, Reason}
            end;
        {error, Reason} ->
            {error, Reason}
    end.

%% Closes the backup media after an interrupted backup
%%
%% Returns {ok, ReturnValueToUser} or {error, Reason}
abort_write(BackupRef) ->
    Res = disk_log:close(BackupRef#backup.file_desc),
    file:delete(BackupRef#backup.tmp_file),
    case Res of
        ok ->
            {ok, BackupRef#backup.file};
        {error, Reason} ->
            {error, Reason}
    end.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Restore callback interface

-record(restore, {file, file_desc, cont}).

%% Opens backup media for read
%%
%% Returns {ok, OpaqueData} or {error, Reason}
open_read(OpaqueData) ->
    File = OpaqueData,
    case file:read_file_info(File) of
        {error, Reason} ->
            {error, Reason};
        _FileInfo -> %% file exists
            case disk_log:open([{file, File},
                                {name, make_ref()},
                                {repair, false},
                                {mode, read_only},
                                {linkto, self()}]) of
                {ok, Fd} ->
                    {ok, #restore{file = File, file_desc = Fd, cont = start}};
                {repaired, Fd, _, {badbytes, 0}} ->
                    {ok, #restore{file = File, file_desc = Fd, cont = start}};
                {repaired, Fd, _, _} ->
                    {ok, #restore{file = File, file_desc = Fd, cont = start}};
                {error, Reason} ->
                    {error, Reason}
            end
    end.

%% Reads BackupItems from the backup media
%%
%% Returns {ok, OpaqueData, BackupItems} or {error, Reason}
%%
%% BackupItems == [] is interpreted as eof
read(OpaqueData) ->
    R = OpaqueData,
    Fd = R#restore.file_desc,
    case disk_log:chunk(Fd, R#restore.cont) of
        {error, Reason} ->
            {error, {"Possibly truncated", Reason}};
        eof ->
            {ok, R, []};
        {Cont, []} ->
            read(R#restore{cont = Cont});
        {Cont, BackupItems, _BadBytes} ->
            {ok, R#restore{cont = Cont}, BackupItems};
        {Cont, BackupItems} ->
            {ok, R#restore{cont = Cont}, BackupItems}
    end.

%% Closes the backup media after restore
%%
%% Returns {ok, ReturnValueToUser} or {error, Reason}
close_read(OpaqueData) ->
    R = OpaqueData,
    case disk_log:close(R#restore.file_desc) of
        ok -> {ok, R#restore.file};
        {error, Reason} -> {error, Reason}
    end.
```