# Introduction

Obnam2 is a backup system.

In 2004 I started a project to develop a backup program for myself,
which in 2006 I named Obnam. In 2017 I retired the project, because it
was no longer fun. The project had some long-standing, architectural
issues related to performance that had become entrenched and were hard
to fix, without breaking backwards compatibility.

In 2020, with Obnam2 I'm starting over from scratch. The new software
is not, and will not become, compatible with Obnam1 in any way. I aim
the new software to be more reliable and faster than Obnam1, without
sacrificing security or ease of use, while being maintainable in the
long run. I also intend to have fun while developing the new software.

Part of that maintainability is going to be achieved by using Rust as
the programming language (which has a strong, static type system)
rather than Python (which has a dynamic, comparatively weak type
system). Another part is more strongly aiming for simplicity and
elegance. Obnam1 used an elegant, but not very simple copy-on-write
B-tree structure; Obnam2 will use [SQLite][].

[SQLite]: https://sqlite.org/index.html

## Glossary

This document uses some specific terminology related to backups. Here
is a glossary of such terms.

* a **chunk** is a relatively small amount of live data or metadata
  about live data, as chosen by the client
* a **client** is the computer system where the live data lives, also
  the part of Obnam running on that computer
* a **generation** is a snapshot of live data, also known as **a
  backup**
* **live data** is the data that gets backed up
* a **repository** is where the backups get stored
* a **server** is the computer system where the repository resides,
  also the part of Obnam running on that computer


# Requirements

The following high-level requirements are not meant to be verifiable
in an automated way:

* _Done:_ **Easy to install:** available as a Debian package in an APT
  repository. Other installation packages will also be provided,
  hopefully.
* _Not done:_ **Easy to configure:** only need to configure things
  that are inherently specific to a client, when sensible defaults are
  impossible.
* _Not done:_ **Excellent documentation:** although software ideally
  does not need documentation, in practice is usually does, and Obnam
  should have documentation that is clear, correct, helpful,
  unambiguous, and well-liked.
* _Done_: **Easy to run:** making a backup is a single command line
  that's always the same.
* _Not done:_ **Detects corruption:** if a file in the repository is
  modified or deleted, the software notices it automatically.
* _Not done:_ **Repository is encrypted:** all data stored in the
  repository is encrypted with a key known only to the client.
* _Not done:_ **Fast backups and restores:** when a client and server
  both have sufficient CPU, RAM, and disk bandwidth, the software
  makes a backup or restores a backup over a gigabit Ethernet using at
  least 50% of the network bandwidth.
* _Done:_ **Snapshots:** Each backup is an independent snapshot: it
  can be deleted without affecting any other snapshot.
* _Done:_ **Deduplication:** Identical chunks of data are stored only
  once in the backup repository.
  - Note: The chunking is very simplistic, for now, but that can be
    improved later. The changes will only affect the backup part of
    the client.
* _Not done:_ **Compressed:** Data stored in the backup repository is
  compressed.
* _Not done:_ **Large numbers of live data files:** The system must
  handle at least ten million files of live data. (Preferably much
  more, but I want some concrete number to start with.)
* _Not done:_ **Live data in the terabyte range:** The system must
  handle a terabyte of live data. (Again, preferably more.)
* _Not done:_ **Many clients:** The system must handle a thousand
  total clients and one hundred clients using the server concurrently,
  on one physical server.
* _Not done:_ **Shared repository:** The system should allow people
  who don't trust each other to share a repository without fearing
  that their own data leaks, or even its existence leaks, to anyone.
* _Not done:_ **Shared backups:** People who do trust each other
  should be able to share backed up data in the repository.

The detailed, automatically verified acceptance criteria are
documented below, as _scenarios_ described for the [Subplot][] tool.
The scenarios describe specific sequences of events and the expected
outcomes.

[Subplot]: https://subplot.liw.fi/

# Threat model

This chapter discusses the various threats against backups. Or it
will. For now it's very much work in progress. This version of the
chapter is only meant to get threat modeling started by having the
simplest possible model that is in any way useful.

## Backed up data is readable by server operator

This threat is about the operator of the backup server being able to
read the data backed up by any user of the server. We have to assume
that the operator can read any file and can also eavesdrop all network
traffic. The operator can even read all physical and virtual memory on
the server.

The mitigation strategy is to encrypt the data before it is sent to
the server. If the server never receives cleartext data, the operator
can't read it.

Backups have four kinds of data:

* actual contents of live data files
* metadata about live data files, as stored on the client file system,
  such as the name, ownership, or size of each file
* metadata about the contents of live data, such as its cryptographic
  checksum
* metadata about the backup itself

For now, we are concerned about the first two kinds. The rest will be
addressed later.

The mitigation technique against this threat is to encrypt the live
data and its metadata before uploading it to the server.

# Software architecture

## Effects of requirements

The requirements stated above drive the software architecture of
Obnam. Some requirements don't affect the architecture at all: for
example, "excellent documentation". This section discusses the various
requirements and notes how they affect the architecture.

* **Easy to install:** Does not affect the architeture.
* **Easy to configure:** Does not affect the architeture.
* **Excellent documentation:** Does not affect the architeture.
* **Easy to run:** Obnam may not require that its user provide any
  information specific to a backup run. For example, it may not
  require a name or identifier to be provided. The software must
  invent any identifiers itself.
* **Detects corruption:** The client must provide a strong checksum of
  the data it uploads, and verify the checksum for data it downloads.
  Note that the server can't compute or verify the checksum, at least
  not for the cleartext data, which it never sees. Also, having the
  server compute a checksum is too late: corruption may have happened
  during the upload already.
* **Repository is encrypted:** Client must do the encryption and
  decryption. The server may only see encrypted data. Note that this
  must include metadata, such as the checksum of cleartext data. The
  client will encrypt the checksum for a chunk and the server must not
  interpret or use the checksum in any way.
* **Fast backups and restores:** The architecture needs to enable the
  implementation to use concurrency and protocols that can saturate
  fast network connections, and handle network problems well.
* **Snapshots:** We can't do deltas from one backup run to another. If
  Obnam does a tape-like full backup, and then an incremental one as a
  delta from the full one, it can't delete the full backup until all
  the incremental ones have been deleted. This complicated management
  of backup storage.
* **Deduplication:** The client sees the cleartext and can make more
  intelligent decisions about how to split live data into chunks.
  Further, the client has fast access to the live data, which the
  server does not. Ideally, we design the server in a way that does
  not care about how data is split into chunks.
* **Compressed:** Compression should be done prior to encryption: if
  encrypted data can be significantly compressed that leaks
  information about the nature of the cleartext data.
* **Large numbers of live data files:** Storing and accessing lists of
  and meta data about files needs to done using data structures that
  are efficient for that.
* **Live data in the terabyte range:** 
* **Many clients:** The architecture should enable flexibly managing
  clients.
* **Shared repository:** The server component needs identify and
  distinguish between clients and data in backups made by different
  clients. Access to backups to be strictly controlled so that each
  client can only ever access its own data, or even query about the
  presence of specific data.
* **Shared backups:** Clients should be able to specify, for each
  chunk of data separately, which other clients should be able to
  access that.


## Overall shape

It seems fairly clear that a simple shape of the software architecture
of Obnam2 is to have a client and server component, where one server
can handle any number of clients. They communicate over HTTPS, using
proven web technologies for authentication and authorization.

~~~pikchr
Live1: cylinder "live1" bold big big
move
Live2: cylinder "live2" bold big big
move
Live3: cylinder "live3" bold big big
move
Live4: cylinder "live4" bold big big
move
Live5: cylinder "live5" bold big big

down

arrow from Live1.s
C1: ellipse "client1" bold big big
arrow from Live2.s
C2: ellipse "client2" bold big big
arrow from Live3.s
C3: ellipse "client3" bold big big
arrow from Live4.s
C4: ellipse "client4" bold big big
arrow from Live5.s
C5: ellipse "client5" bold big big

S: ellipse "server" bold big big at 2*boxwid south of C3
arrow from C1.s to S.n "HTTPS" bold big big aligned below
arrow from C2.s to S.n
arrow from C3.s to S.n
arrow from C4.s to S.n
arrow from C5.s to S.n

arrow from S.s
cylinder "disk" bold big big
~~~

The responsibilities of the server are roughly:

* provide an HTTP API for managing chunks and their metadata: create,
  retrieve, search, delete; note that updating a chunk is not needed
* keep track of the client owning each chunk
* allow clients to manage sharing of specific chunks between clients

The responsibilities of the client are roughly:

* split live data into chunks, upload them to server
* store metadata of live data files in a generation file (an SQLite
  database), store that too as chunks on the server
* retrieve chunks from server when restoring
* let user manage sharing of backups with other clients

There are many details to add to both to the client and the server,
but that will come later.

It is possible that an identity provider needs to be added to the
architecture later, to provide strong authentication of clients.
However, that will not be necessary for the minimum viable product
version of Obnam. For the MVP, authentication will happen using
RSA-signed JSON Web Tokens. The server is configured to trust specific
public keys. The clients have the private keys and generate the tokens
themselves.

## Logical structure of backups

For each backup (generation) the client stores, on the server, exactly
one _generation chunk_. This is a chunk that is specially marked as a
generation, but is otherwise not special. The generation chunk content
is a list of identifiers for chunks that form an SQLite database.

The SQLite database lists all the files in the backup, as well as
their metadata. For each file, a list of chunk identifiers are listed,
for the content of the file. The chunks may be shared between files in
the same backup or different backups.

File content data chunks are just blobs of data with no structure.
They have no reference to other data chunks, or to files or backups.
This makes it easier to share them between files.

Let's look at an example. In the figure below there are three backups,
each using three chunks for file content data. One chunk, "data chunk
3", is shared between all three backups.

~~~pikchr
GEN1: ellipse "Backup 1" big big
move 200%
GEN2: ellipse "Backup 2" big big
move 200%
GEN3: ellipse "Backup 3" big big

arrow from GEN1.e right to GEN2.w
arrow from GEN2.e right to GEN3.w

arrow from GEN1.s down 100%
DB1: box "SQLite" big big

arrow from DB1.s left 20% then down 100%
C1: file "data" big big "chunk 1" big big

arrow from DB1.s right 0% then down 70% then right 100% then down 30%
C2: file "data" big big "chunk 2" big big

arrow from DB1.s right 20% then down 30% then right 200% then down 70%
C3: file "data" big big "chunk 3" big big



arrow from GEN2.s down 100%
DB2: box "SQLite" big big

arrow from DB2.s left 20% then down 100% then down 0.5*C3.height then to C3.e

arrow from DB2.s right 0% then down 70% then right 100% then down 30%
C4: file "data" big big "chunk 4" big big

arrow from DB2.s right 20% then down 30% then right 200% then down 70%
C5: file "data" big big "chunk 5" big big




arrow from GEN3.s down 100%
DB3: box "SQLite" big big

arrow from DB3.s left 50% then down 100% then down 1.5*C3.height \
  then left until even with C3.s then up to C3.s

arrow from DB3.s right 20% then down 100%
C6: file "data" big big "chunk 6" big big

arrow from DB3.s right 60% then down 70% then right 100% then down 30%
C7: file "data" big big "chunk 7" big big
~~~


## On SFTP versus HTTPS

Obnam1 supported using a standard SFTP server as a backup repository,
and this was a popular feature. This section argues against supporting
SFTP in Obnam2.

The performance requirement for network use means favoring protocols
such as HTTPS, or even QUIC, rather than SFTP.

SFTP works on top of SSH. SSH provides a TCP-like abstraction for
SFTP, and thus multiple SFTP connections can run over the same SSH
connection. However, SSH itself uses a single TCP connection. If that
TCP connection has a dropped packet, all traffic over the SSH
connections, including all SFTP connections, waits until TCP
re-transmits the lost packet and re-synchronizes itself.

With multiple HTTP connections, each on its own TCP connection, a
single dropped packet will not affect other HTTP transactions. Even
better, the new QUIC protocol doesn't use TCP.

The modern Internet is to a large degree designed for massive use of
the world wide web, which is all HTTP, and adopting QUIC. It seems
wise for Obnam to make use of technologies that have been designed
for, and proven to work well with concurrency and network problems.

Further, having used SFTP with Obnam1, it is not always an easy
protocol to use. Further, if there is a desire to have controlled
sharing of parts of one client's data with another, this would require
writing a custom SFTP service, which seems much harder to do than
writing a custom HTTP service. From experience, a custom HTTP service
is easy to do. A custom SFTP service would need to shoehorn the
abstractions it needs into something that looks more or less like a
Unix file system.

The benefit of using SFTP would be that a standard SFTP service could
be used, if partial data sharing between clients is not needed. This
would simplify deployment and operations for many. However, it doesn't
seem important enough to warrant the implementation effort.

Supporting both HTTP and SFTP would be possible, but also much more
work and against the desire to keep things simple.

## On "btrfs send" and similar constructs

The btrfs and ZFS file systems, and possibly others, have a way to
mark specific states of the file system and efficiently generate a
"delta file" of all the changes between the states. The delta can be
transferred elsewhere, and applied to a copy of the file system. This
can be quite efficient, but Obnam won't be built on top of such a
system.

On the one hand, it would force the use of specific file systems:
Obnam would no be able to back up data on, say, an ext4 file system,
which seems to be the most popular one by far.

Worse, it also for the data to be restored to the same type of file
system as where the live data was originally. This onerous for people
to do.


## On content addressable storage

[content-addressable storage]: https://en.wikipedia.org/wiki/Content-addressable_storage
[git version control system]: https://git-scm.com/

It would be possible to use the cryptographic checksum ("hash") of the
contents of a chunk as its identifier on the server side, also known
as [content-addressable storage][]. This would simplify de-duplication
of chunks. However, it also has some drawbacks:

* it becomes harder to handle checksum collisions
* changing the checksum algorithm becomes harder

In 2005, the author of [git version control system][] chose the
content addressable storage model, using the SHA1 checksum algorithm.
At the time, the git author considered SHA1 to be reasonably strong
from a cryptographic and security point of view, for git. In other
words, given the output of SHA1, it was difficult to deduce what the
input was, or to find another input that would give the same output,
known as a checksum collision. It is still difficult to deduce the
input, but manufacturing collisions is now feasible, with some
constraints. The git project has spent years changing the checksum
algorithm.

Collisions are problematic for security applications of checksum
algorithms in general. Checksums are used, for example, in storing and
verifying passwords: the cleartext password is never stored, and
instead a checksum of it is computed and stored. To verify a later
login attempt a new checksum is computed from the newly entered
password from the attempt. If the checksums match, the password is
accepted.[^passwords] This means that if an attacker can find _any_ input that
gives the same output for the checksum algorithm used for password
storage, they can log in as if they were a valid user, whether the
password they have is the same as the real one.

[^passwords]: In reality, storing passwords securely is much more
    complicated than described here.

For backups, and version control systems, collisions cause a different
problem: they can prevent the correct content from being stored. If
two files (or chunks) have the same checksum, only one will be stored.
If the files have different content, this is a problem. A backup
system should guard against this possibility.

As an extreme and rare, but real, case consider a researcher of
checksum algorithms. They've spent enormous effort to produce two
distinct files that have the same checksum. They should be able make a
backup of the files, and restore them, and not lose one. They should
not have to know that their backup system uses the same checksum
algorithm they are researching, and have to guard against the backup
system getting the files confused. (Backup systems should be boring
and just always work.)

Attacks on security-sensitive cryptographic algorithms only get
stronger by time. It is therefore necessary for Obnam to be able to
easily change the checksum algorithm it uses, without disruption for
user. To achieve this, Obnam does not use content-addressable storage.

Obnam will (eventually, as this hasn't been implemented yet) allow
storing multiple checksums for each chunk. It will use the strongest
checksum available for a chunk. Over time, the checksums for chunks
can be replaced with stronger ones. This will allow Obnam to migrate
to a stronger algorithm when attacks against the current one become
too scary.

## On pull versus push backups

Obnam only does push backups. This means the client runs on the host
where the live data is, and sends it to the server.

Backups could also be pulled, in that the server reaches out tot he
host where the live data is, retrieves the data, and stores it on the
server. Obnam does not do this, due to the hard requirement that live
data never leaves its host in cleartext.

The reason pull backups are of interest in many use cases is because
they allow central administration of backups, which can simplify
things a lot in a large organization. Central backup administration
can be achieved with Obnam in a more complicated way: the installation
and configuration of live data hosts is done in a central fashion
using configuration management, and if necessary, backups can be
triggered on each host by having the server reach out and run the
Obnam client.

# File metadata

Files in a file system contain data and have metadata: data about the
file itself. The most obvious metadata is the file name, but there is
much more. A backup system needs to back up, but also restore, all
relevant metadata. This chapter discusses all the metadata the Obnam
authors know about, and how they understand it, and how Obnam handles
it, and why it handles it that way.

The long term goal is for Obnam to handle everything, but it may take
a while to get there.

## On portability

Currently, Obnam is developed on Linux, and targets Linux only. Later,
it may be useful to add support for other systems, and Obnam should
handle file metadata in a portable way, when that makes sense and is
possible. This means that if a backup is made on one type of system,
but restored on another type, Obnam should do its best to make the
restored data as identical as possible to what the data would be if it
had been copied over directly, with minimal change in meaning.

This affects not only cases when the operating system changes, but
also when the file system changes. Backing up on Linux ext4 file
system and restoring to a vfat file system brings up the same class of
issues with file metadata.

There are many [type of file systems][] with varying capabilities and
behaviors. Obnam attempts to handle everything the Linux system it
runs on can handle.

[type of file systems]: https://en.wikipedia.org/wiki/Comparison_of_file_systems

## Filenames

On Unix, the filename is a sequence of bytes. Certain bytes have
special meaning:

* byte 0, ASCII NUL character: terminates filename
* byte 56, ASCII period character: used for . and .. directory entries
* byte 57, ASCII slash character: used to separate components in a pathname

On generic Unix, the operating system does not interpret other bytes.
It does not impose a character set. Binary filenames are OK, as long
as they use the above bytes only in the reserved manner. It is up to
the presentation layer (the user interface) to present the name in a
way suitable for humans.

For now, Obnam stores fully qualified pathnames as strings of bytes as
above. Arguably, Obnam could split the pathname into components,
stored separately, to avoid having to give ASCII slash characters
special meaning. The `.` and `..` directory entries are not stored by
Obnam.

Different versions of Unix, and different file system types, put
limits on the length of a filename or components of a pathname. Obnam
does not.

On other operating systems, and on some file system types, filenames
are more restricted. For example, on MacOS, although nominally a Unix
variant, filenames must form valid UTF-8 strings normalized in a
particular way. While Obnam does not support MacOS at the time of
writing, if it ever will, that needn't affect the way filenames are
stored. They will be stored as strings of bytes, and if necessary,
upon restore, a filename can be morphed into a form required by MacOS
or the filename being written to. The part of Obnam that restores
files will have to learn how to do that.

The generic Unix approach does not allow for "drive letters", used by
Windows. Not sure if supporting that is needed.


## Unix inode metadata: `struct stat`

[stat(2)]: https://linux.die.net/man/2/stat
[lstat(2)]: https://linux.die.net/man/2/lstat
[inode]: https://en.wikipedia.org/wiki/Inode

The basic Unix system call for querying a file's metadata is
[stat(2)][]. However, since it follows symbolic links, Obnam needs to
use [lstat(2)][] instead. The metadata is stored in an [inode][]. Both
variants return a C `struct stat`. On Linux, it has the following
fields:

* `st_dev` &ndash; id of the block device containing file system where
  the file is; this encodes the major and minor device numbers
  - this field can't be restored as such, it is forced by the
    operating system for the file system to which files are restored
  - Obnam stores it so that hard links can be restored, see below
* `st_ino` &ndash; the inode number for the file
  - this field can't be restored as such, it is forced by the file
    system whan the restored file is created
  - Obnam stores it so that hard links can be restored, see below
* `st_nlink` &ndash; number of hard links referring to the inode
  - this field can't be restored as such, it is maintained by the
    operating system when hard links are created
  - Obnam stores it so that hard links can be restored, see below
* `st_mode` &ndash; file type and permissions
  - stored and restored
* `st_uid` &ndash; the numeric id of the user account owning the file
  - stored
  - restored if restore is running as root, otherwise not restored
* `st_gid` &ndash; the numeric id of the group owning the file
  - stored
  - restored if restore is running as root, otherwise not restored
* `st_rdev` &ndash; the device this inode represents
  - not stored?
* `st_size` &ndash; size or length of the file in bytes
  - stored
  - restored implicitly be re-creating the origtinal contents
* `st_blksize` &ndash; preferred block size for efficient I/O
  - not stored?
* `st_blocks` &ndash; how many blocks of 512 bytes are actually
    allocated to store this file's contents
  - see below for discussion about sparse files
  - not stored by Obnam
* `st_atime` &ndash; timestamp of latest access
  - stored and restored
  - On Linux, split into two integer fields
* `st_mtime` &ndash; timestamp of latest modification
  - stored and restored
  - On Linux, split into two integer fields
* `st_ctime` &ndash; timestamp of latest inode change
  - On Linux, split into two integer fields
  - stored
  - not restored

Obnam stores most these fields. Not all of them can be restored,
especially not explicitly. The `st_dev` and `st_ino` fields get set by
the file system when when a restored file is created. They're stored
so that Obnam can restore all hard links to the same inode.

## Hard links and symbolic links

In Unix, filenames are links to an inode. The inode contains all the
metadata, except the filename. Many names can link to the same inode.
These are called hard links.

On Linux, hard links can be created explicitly only for regular files,
not for directories. This avoids creating cycles in the directory
tree, which simplifies all software that traverses the file system.
However, hard links get created implicitly when creating
sub-directories: the `..` entry in the sub-directory is a hard link to
the inode of the parent directory.

Unix also supports symbolic links, which are tiny files that contain
the name of another file. The kernel will follow a symbolic link
automatically by reading the tiny file, and pretending the contents of
the file was used instead. Obnam stores the contents of a symbolic
link, the "target" of the link, and restores the original value
without modification.

## On access time stamps

The `st_atime` field is automatically updated when a file or directory
is "accessed". This means reading a file or listing the contents of a
directory. Accessing a file in a directory does count as accessing the
directory.

The `st_atime` update can be prevented by updating the file system as
read-only, or using a mount option `noatime`, `nodiratime`, or
`relatime`, or by opening the file or directory with the `O_NOATIME`
option (under certain conditions). This can be a useful for a system
administrator to do to avoid needless updates if nothing needs the
access timestamp. There are few uses for it.

Strictly speaking, a backup program can't assume the access timestamp
is not needed and should do its best to back it up and restore it.
However, this is trickier that one might think. A backup program can't
change mount options, or make the file system be read-only. It thus
needs to use the `NO_ATIME` flag to the [open(2)][] system call.

Obnam does not do this yet. In fact, it doesn't store or restore the
access time stamp yet.

[open(2)]: https://linux.die.net/man/2/open

## Time stamp representation

Originally, Unix (and Linux) stored file time stamps as whole seconds
since the beginning of 1970. Linux now stores timestamp with up to
nanosecond precision, depending on file system type. Obnam handles
this by storing and restoring nanosecond timestamps. If, when
restoring, the target file system doesn't support that precision, then
some accuracy is lost.

Different types of file system store timestamps at different
precision, and sometimes support a different precision for different
types of timestamp. The Linux [ext4][] file system supports nanosecond
precision for all timestamps. The [FAT][] file system supports a 2
seconds for last modified time, 10 ms for creation time, 1 day for
access date (if at all), 2 seconds for deletion time.

[ext4]: https://en.wikipedia.org/wiki/Ext4
[FAT]: https://en.wikipedia.org/wiki/File_Allocation_Table

Obnam uses the same Linux system calls for retrieve timestamps, and
those always return them at nanosecond precision (if not accuracy).
Likewise when restoring, Obnam attempts to set the timestamps in the
same way, and if the target file system supports less precision, the
result may be imperfect, but there isn't really anything Obnam can do
to improve that

## Sparse files

On Unix a [sparse file][] is one where some blocks of the file are not
stored explicitly, but the file still has a length. Instead, the file
system return zero bytes for the missing blocks. The blocks that
aren't explicitly stored form "holes" in the file.

[sparse file]: https://en.wikipedia.org/wiki/Sparse_file
[truncate(1)]: https://linux.die.net/man/1/truncate

As an example, one can create a very large file with the command line
[truncate(1)][] command:

~~~sh
$ truncate --size 1T sparse
$ ls -l sparse
-rw-rw-r-- 1 liw liw 1099511627776 Dec  8 11:18 sparse
$ du sparse
0	sparse
~~~

It's a one-terabyte long file that uses no space! If the file is read,
the file system serves one terabyte of zero bytes. If it's written,
the file system creates a new block at the location of the write, and
fills it new data, and fills the rest of the block with zeroes.

The metadata fields `st_size` and `st_blocks` make this visible. The
`ls` command shows the `st_size` field. The `du` command reports disk
usage based on the `st_blocks` field.

Sparse files are surprisingly useful. They can, for example, be used
to implement large virtual disks without using more space than is
actually stored on the file system on the virtual disk.

Sparse files are a challenge to backup systems: it is wasteful to
store very large amounts of zeroes. Upon restore, the hole should be
re-created rather then zeroes written out, or else the restored files
will use much more disk space than the original files.

Obnam will store sparse files explicitly. It will find the holes in a
file and store only the parts of a file that are not holes, and their
position. But this isn't implemented yet.


## Access control lists (Posix ACL)

FIXME

## Extended attributes

FIXME

## Extra Linux ext2/3/4 metadata

FIXME

## On implementation and abstractions

Obnam clearly needs to abstract metadata across target systems. There
are two basic appraches:

* every target gets its own, distinct metadata structure:
  LinuxMetadata, NetbsdMetadata, MacosMetadata, WindowsMetadata, and
  so on
* all targets share a common metadata structure that gets created in a
  target specific way

The first approach seems likely to cause an explosion of variants, and
thus lead to more complexity overall. Thus, Obnam uses the second
approach.

The Obnam source code has the `src/fsentry.rs` module, which is the
common metadata structure, `FsEntry`. It has a default value that is
adjusted using system specific functions, based on operating system
specific variants of the `std::fs::Metadata` structure in the Rust
standard library.

In addition to dealing with different `Metadata` on each system, the
`FsEntry` needs to be stored in an SQLite database and retrieved from
there. Initially, this will be done by serializing it into JSON and
back. This is done at early development time, to simplify the process
in which new metadata fields are added. It will be changed later, if
there is need to.

# Implementation

The minimum viable product will not support sharing of data between
clients.

## Chunks

Chunks consist of arbitrary binary data, a small amount of metadata,
and an identifier chosen by the server. The chunk metadata is a JSON
object, consisting of the following fields:

* `sha256` &ndash; the SHA256 checksum of the chunk contents as
  determined by the client
  - this MUST be set for every chunk, including generation chunks
  - the server allows for searching based on this field
  - note that the server doesn't verify this in any way, to pave way
    for future client-side encryption of the chunk data
* `generation` &ndash; set to `true` if the chunk represents a
  generation
  - may also be set to `false` or `null` or be missing entirely
  - the server allows for listing chunks where this field is set to
    `true`
* `ended` &ndash; the timestamp of when the backup generation ended
  - note that the server doesn't process this in any way, the contents
    is entirely up to the client
  - may be set to the empty string, `null`, or be missing entirely
  - this can't be used in searches

When creating or retrieving a chunk, its metadata is carried in a
`Chunk-Meta` header as a JSON object, serialized into a textual form
that can be put into HTTP headers.


## Server

The server has the following API for managing chunks:

* `POST /chunks` &ndash; store a new chunk (and its metadata) on the
  server, return its randomly chosen identifier
* `GET /chunks/<ID>` &ndash; retrieve a chunk (and its metadata) from
  the server, given a chunk identifier
* `GET /chunks?sha256=xyzzy` &ndash; find chunks on the server whose
  metadata indicates their contents has a given SHA256 checksum
* `GET /chunks?generation=true` &ndash; find generation chunks
* `GET /chunks?data=True` &ndash; find chunks with file data
  - this is meant for testing only
  - it excludes generation chunks, and chunks used to store the
    generation's SQLite file

HTTP status codes are used to indicate if a request succeeded or not,
using the customary meanings.

When creating a chunk, chunk's metadata is sent in the `Chunk-Meta`
header, and the contents in the request body. The new chunk gets a
randomly assigned identifier, and if the request is successful, the
response body is a JSON object with the identifier:

~~~json
{
    "chunk_id": "fe20734b-edb3-432f-83c3-d35fe15969dd"
}
~~~

The identifier is a [UUID4][], but the client should not assume that
and should treat it as an opaque value.

[UUID4]: https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)

When a chunk is retrieved, the chunk metadata is returned in the
`Chunk-Meta` header, and the contents in the response body.

It is not possible to update a chunk or its metadata.

When searching for chunks, any matching chunk's identifiers and
metadata are returned in a JSON object:

~~~json
{
  "fe20734b-edb3-432f-83c3-d35fe15969dd": {
     "sha256": "09ca7e4eaa6e8ae9c7d261167129184883644d07dfba7cbfbc4c8a2e08360d5b",
     "generation": null,
	 "ended: null,
  }
}
~~~

There can be any number of chunks in the search response.



## Client

The client scans live data for files, reads each file, splits it into
chunks, and searches the server for chunks with the same checksum. If
none are found, the client uploads the chunk. For each backup run, the
client creates an [SQLite][] database in its own file, into which it
inserts each file, its metadata, and list of chunk ids for its
content. At the end of the backup, it uploads the SQLite file as
chunks, and finally creates a generation chunk, which has as its
contents the list of chunk identifiers for the SQLite file.

[SQLite]: https://sqlite.org/

For an incremental backup, the client first retrieves the SQLite file
for the previous generation, and compares each file's metadata with
that of the previous generation. If a live data file does not seem to
have changed, the client copies its metadata to the new SQLite file.

When restoring, the user provides the chunk id of the generation to be
restored. The client retrieves the generation chunk, gets the list of
chunk ids for the corresponding SQLite file, retrieves those, and then
restores all the files in the SQLite database.



## Encryption and authenticity of chunks

*This is a plan that will be implemented soon. When it has been, this
section needs to be updated to to use present tense.*

Obnam encrypts data it stores on the server, and checks that the data
it retrieves from the server is what it stored. This is all done in
the client: the server should never see any data isn't encrypted, and
the client can't trust the server to validate anything.

Obnam will be using _Authenticated Encryption with Associated Data_ or
[AEAD][]. AEAD both encrypts data, and validates it before decrypting.
AEAD uses two encryption keys, one algorithm for symmetric encryption,
and one algorithm for a message authentication codes or [MAC][]. AEAD
encrypts the plaintext with a symmetric encryption algorithm using the
first key, giving a ciphertext. It then computes a MAC of the
ciphertext using the second key. Both the ciphertext and MAC are
stored on the server.

For decryption, a MAC is computed against the retrieved
ciphertext, and compared to the retrieved MAC. If the MACs differ,
that's an error and no decryption is done. If they do match, the
ciphertext is decrypted.

Obnam will require the user to provide a passphrase, and will derive
the two keys from the single passphrase, using [PBKDF2][], rather than
having the user provide two passphrases. The derived keys will be
stored in a file that only the owner can read. (This is simple, and good
enough for now, but needs to improved later.)

When this is all implemented, there will be a setup step before the
first backup:

~~~sh
$ obnam init
Passphrase for encryption:
Re-enter to make sure: 
$ obnam backup
~~~

The `init` step asks for a passphrase, uses PBKDF2 (with the [pbkdf2
crate][]) to derive the two keys, and writes a JSON file with the keys
into `~/.config/obnam/keys.json`, making that file be readable only by
the user running Obnam. Other operations get the keys from that file.
For now, we will use the default parameters of the pbkdf2 crate, to
keep things simple. (This will need to be made more flexible later: if
nothing else, Obnam should not be vulnerable to the defaults
changing.)

The `init` step will not be optional. There will only be encrypted
backups.

Obnam will use the [aes-gcm crate][] for AEAD, since it has been
audited. If that choice turns out to be less than optimal, it can be
reconsidered later. The `encrypt` function doesn't return the MAC and
ciphertext separately, so we don't store them separately. However,
each chunk needs its own [nonce][], which we will generate. We'll use
a 96-bit (or 12-byte) nonce. We'll use the [rand crate][] to generate
random bytes.

The chunk sent to the server will be encoded as follows:

* chunk format: a 32-bit unsigned integer, 0x0001, store in
  little-endian form.
* a 12-byte nonce unique to the chunk
* the ciphertext

The format version prefix dictates the content and structure of the
chunk. This document defines version 1 of the format. The Obnam client
will refuse to operate on backup generations which use chunk formats
it cannot understand.


[AEAD]: https://en.wikipedia.org/wiki/Authenticated_encryption#Authenticated_encryption_with_associated_data_(AEAD)
[MAC]: https://en.wikipedia.org/wiki/Message_authentication_code
[aes-gcm crate]: https://crates.io/crates/aes-gcm
[PBKDF2]: https://en.wikipedia.org/wiki/PBKDF2
[pbkdf2 crate]: https://crates.io/crates/pbkdf2
[nonce]: https://en.wikipedia.org/wiki/Cryptographic_nonce
[rand crate]: https://crates.io/crates/rand


# Acceptance criteria for the chunk server

These scenarios verify that the chunk server works on its own. The
scenarios start a fresh, empty chunk server, and do some operations on
it, and verify the results, and finally terminate the server.

## Chunk management happy path

We must be able to create a new chunk, retrieve it, find it via a
search, and delete it. This is needed so the client can manage the
storage of backed up data.

~~~scenario
given an installed obnam
and a running chunk server
and a file data.dat containing some random data
when I POST data.dat to /chunks, with chunk-meta: {"sha256":"abc"}
then HTTP status code is 201
and content-type is application/json
and the JSON body has a field chunk_id, henceforth ID
and server has 1 file chunks
~~~

We must be able to retrieve it.

~~~scenario
when I GET /chunks/<ID>
then HTTP status code is 200
and content-type is application/octet-stream
and chunk-meta is {"sha256":"abc","generation":null,"ended":null}
and the body matches file data.dat
~~~

We must also be able to find it based on metadata.

~~~scenario
when I GET /chunks?sha256=abc
then HTTP status code is 200
and content-type is application/json
and the JSON body matches {"<ID>":{"sha256":"abc","generation":null,"ended":null}}
~~~

Finally, we must be able to delete it. After that, we must not be able
to retrieve it, or find it using metadata.

~~~scenario
when I DELETE /chunks/<ID>
then HTTP status code is 200

when I GET /chunks/<ID>
then HTTP status code is 404

when I GET /chunks?sha256=abc
then HTTP status code is 200
and content-type is application/json
and the JSON body matches {}
~~~

## Retrieve a chunk that does not exist

We must get the right error if we try to retrieve a chunk that does
not exist.

~~~scenario
given an installed obnam
and a running chunk server
when I try to GET /chunks/any.random.string
then HTTP status code is 404
~~~

## Search without matches

We must get an empty result if searching for chunks that don't exist.

~~~scenario
given an installed obnam
and a running chunk server
when I GET /chunks?sha256=abc
then HTTP status code is 200
and content-type is application/json
and the JSON body matches {}
~~~

## Delete chunk that does not exist

We must get the right error when deleting a chunk that doesn't exist.

~~~scenario
given an installed obnam
and a running chunk server
when I try to DELETE /chunks/any.random.string
then HTTP status code is 404
~~~


## Persistent across restarts

Chunk storage, and the index of chunk metadata for searches, needs to
be persistent across restarts. This scenario verifies it is so.

First, create a chunk.

~~~scenario
given an installed obnam
and a running chunk server
and a file data.dat containing some random data
when I POST data.dat to /chunks, with chunk-meta: {"sha256":"abc"}
then HTTP status code is 201
and content-type is application/json
and the JSON body has a field chunk_id, henceforth ID
~~~

Then, restart the server.

~~~scenario
when the chunk server is stopped
given a running chunk server
~~~

Can we still find it by its metadata?

~~~scenario
when I GET /chunks?sha256=abc
then HTTP status code is 200
and content-type is application/json
and the JSON body matches {"<ID>":{"sha256":"abc","generation":null,"ended":null}}
~~~

Can we still retrieve it by its identifier?

~~~scenario
when I GET /chunks/<ID>
then HTTP status code is 200
and content-type is application/octet-stream
and chunk-meta is {"sha256":"abc","generation":null,"ended":null}
and the body matches file data.dat
~~~

# Acceptance criteria for the Obnam client

The scenarios in chapter verify that the Obnam client works as it
should, when it is used independently of an Obnam chunk server.

## Client shows its configuration

This scenario verifies that the client can show its current
configuration, with the `obnam config` command. The configuration is
stored as YAML, but the command outputs JSON, to make sure it doesn't
just copy the configuration file to the output.

~~~scenario
given an installed obnam
and file config.yaml
and JSON file config.json converted from YAML file config.yaml
when I run obnam --config config.yaml config
then stdout, as JSON, matches file config.json
~~~

~~~{#config.yaml .file .yaml .numberLines}
roots: [live]
server_url: https://backup.example.com
verify_tls_cert: true
~~~


## Client requires https

This scenario verifies that the client rejects a configuration with a
server URL using `http:` instead of `https:`.


~~~scenario
given an installed obnam
and file http.yaml
when I try to run obnam --config http.yaml config
then command fails
then stderr contains "https:"
~~~

~~~{#http.yaml .file .yaml .numberLines}
roots: [live]
server_url: http://backup.example.com
verify_tls_cert: true
~~~

## Client refuses a self-signed certificate

This scenario verifies that the client refuses to connect to a server
if the server's TLS certificate is self-signed. The test server set up
by the scenario uses self-signed certificates.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on ca-required.yaml
and a file live/data.dat containing some random data
when I try to run obnam --config ca-required.yaml backup
then command fails
then stderr contains "self signed certificate"
~~~

~~~{#ca-required.yaml .file .yaml .numberLines}
verify_tls_cert: true
roots: [live]
~~~


# Acceptance criteria for Obnam as a whole

The scenarios in this chapter apply to Obnam as a whole: the client
and server working together.

## Smoke test for backup and restore

This scenario verifies that a small amount of data in simple files in
one directory can be backed up and restored, and the restored files
and their metadata are identical to the original. This is the simplest
possible useful use case for a backup system.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a manifest of the directory live in live.yaml
when I run obnam --config smoke.yaml backup
then backup generation is GEN
when I run obnam --config  smoke.yaml list
then generation list contains <GEN>
when I invoke obnam --config smoke.yaml restore <GEN> rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

~~~{#smoke.yaml .file .yaml .numberLines}
verify_tls_cert: false
roots: [live]
~~~


## Back up regular file

The scenarios in this section back up a single regular file each, and
verify that is metadata is restored correctly. There is a separate
scenario for each bit of metadata so that it's clear what fails, if
anything.

All these scenarios use the following configuration file.

~~~{#metadata.yaml .file .yaml .numberLines}
verify_tls_cert: false
roots: [live]
~~~

### Modification time

This scenario verifies that the modification time is restored correctly.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on metadata.yaml
and a file live/data.dat containing some random data
and a manifest of the directory live in live.yaml
when I run obnam --config metadata.yaml backup
then backup generation is GEN
when I invoke obnam --config metadata.yaml restore <GEN> rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

### Mode bits

This scenario verifies that the mode ("permission") bits are restored
correctly.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on metadata.yaml
and a file live/data.dat containing some random data
and file live/data.dat has mode 464
and a manifest of the directory live in live.yaml
when I run obnam --config metadata.yaml backup
then backup generation is GEN
when I invoke obnam --config metadata.yaml restore <GEN> rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

### Symbolic links

This scenario verifies that symbolic links are restored correctly.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on metadata.yaml
and a file live/data.dat containing some random data
and symbolink link live/link that points at data.dat
and symbolink link live/broken that points at does-not-exist
and a manifest of the directory live in live.yaml
when I run obnam --config metadata.yaml backup
then backup generation is GEN
when I invoke obnam --config metadata.yaml restore <GEN> rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

## Set chunk size

This scenario verifies that the user can set the chunk size in the
configuration file. The chunk size only affects the chunks of live
data.

~~~scenario
given an installed obnam
given a running chunk server
given a client config based on tiny-chunk-size.yaml
given a file live/data.dat containing "abc"
when I run obnam --config tiny-chunk-size.yaml backup
then server has 3 file chunks
~~~

~~~{#tiny-chunk-size.yaml .file .yaml .numberLines}
verify_tls_cert: false
roots: [live]
chunk_size: 1
~~~


## Backup or not for the right reason

The decision of whether to back up a file or keep the version in the
previous backup is crucial. This scenario verifies that Obnam makes
the right decisions.

### First backup backs up all files because they're new

This scenario verifies that in the first backup all files are backed
up because they were new.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a manifest of the directory live in live.yaml
when I run obnam --config smoke.yaml backup
when I run obnam --config  smoke.yaml list-files
then file live/data.dat was backed up because it was new
~~~

### All files in second backup are unchanged

This scenario verifies that if a file hasn't been changed, it's not
backed up.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a manifest of the directory live in live.yaml
when I run obnam --config smoke.yaml backup
when I run obnam --config smoke.yaml backup
when I run obnam --config  smoke.yaml list-files
then file live/data.dat was not backed up because it was unchanged
~~~

### Second backup back up changed file

This scenario verifies that if a file has indeed been changed, it's
backed up.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a manifest of the directory live in live.yaml
when I run obnam --config smoke.yaml backup
given a file live/data.dat containing some random data
when I run obnam --config smoke.yaml backup
when I run obnam --config  smoke.yaml list-files
then file live/data.dat was backed up because it was changed
~~~

## Checksum verification

Each chunk has metadata with the checksum of the chunk contents. This
scenario verifies that the client checks the contents hasn't been
modified.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
when I run obnam --config smoke.yaml backup
then backup generation is GEN
when I invoke obnam --config smoke.yaml get-chunk <GEN>
then exit code is 0
when chunk <GEN> on chunk server is replaced by an empty file
when I invoke obnam --config smoke.yaml get-chunk <GEN>
then command fails
~~~

## Irregular files

This scenario verifies that Obnam backs up and restores files that
aren't regular files, directories, or symbolic links. Specifically,
Unix domain sockets and named pipes (FIFOs). However, block and
character device nodes are not tested, as that would require running
the test suite with `root` permissions and that would be awkward.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a Unix socket live/socket
and a named pipe live/pipe
and a manifest of the directory live in live.yaml
when I run obnam --config smoke.yaml backup
when I invoke obnam --config smoke.yaml restore latest rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

## Tricky filenames

Obnam needs to handle all filenames the underlying operating and file
system can handle. This scenario verifies it can handle a filename
that consists on a single byte with its top bit set. This is not
ASCII, and it's not UTF-8.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on metadata.yaml
and a file in live with a non-UTF8 filename
and a manifest of the directory live in live.yaml
when I run obnam --config metadata.yaml backup
then backup generation is GEN
when I invoke obnam --config metadata.yaml restore <GEN> rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests live.yaml and rest.yaml match
~~~

## Unreadable file

This scenario verifies that Obnam will back up all files of live data,
even if one of them is unreadable. By inference, we assume this means
other errors on individual files also won't end the backup
prematurely.


~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/data.dat containing some random data
and a file live/bad.dat containing some random data
and file live/bad.dat has mode 000
when I run obnam --config smoke.yaml backup
then backup generation is GEN
when I invoke obnam --config smoke.yaml restore <GEN> rest
then file live/data.dat is restored to rest
then file live/bad.dat is not restored to rest
~~~

## Unreadable directory

This scenario verifies that Obnam will skip a file in a directory it
can't read. Obnam should warn about that, but not give an error.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/unreadable/data.dat containing some random data
and file live/unreadable has mode 000
when I run obnam --config smoke.yaml backup
then stdout contains "live/unreadable"
then backup generation is GEN
when I invoke obnam --config smoke.yaml restore <GEN> rest
then file live/unreadable is restored to rest
then file live/unreadable/data.dat is not restored to rest
~~~

## Unexecutable directory

This scenario verifies that Obnam will skip a file in a directory it
can't read. Obnam should warn about that, but not give an error.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on smoke.yaml
and a file live/dir/data.dat containing some random data
and file live/dir has mode 600
when I run obnam --config smoke.yaml backup
then stdout contains "live/dir"
then backup generation is GEN
when I invoke obnam --config smoke.yaml restore <GEN> rest
then file live/dir is restored to rest
then file live/dir/data.dat is not restored to rest
~~~

## Restore latest generation

This scenario verifies that the latest backup generation can be
specified with literal string "latest". It makes two backups, which
are different.

~~~scenario
given an installed obnam
and a running chunk server
and a client config based on metadata.yaml

given a file live/data.dat containing some random data
when I run obnam --config metadata.yaml backup

given a file live/more.dat containing some random data
and a manifest of the directory live in second.yaml
when I run obnam --config metadata.yaml backup

when I invoke obnam --config metadata.yaml restore latest rest
given a manifest of the directory live restored in rest in rest.yaml
then manifests second.yaml and rest.yaml match
~~~

## Back up multiple directories

This scenario verifies that Obnam can back up more than one directory
at a time.


~~~scenario
given an installed obnam
and a running chunk server
and a client config based on roots.yaml
and a file live/one/data.dat containing some random data
and a file live/two/data.dat containing some random data
and a manifest of the directory live/one in one.yaml
and a manifest of the directory live/two in two.yaml
when I run obnam --config roots.yaml backup
then backup generation is GEN
when I invoke obnam --config roots.yaml restore <GEN> rest
given a manifest of the directory live/one restored in rest in rest-one.yaml
given a manifest of the directory live/two restored in rest in rest-two.yaml
then manifests one.yaml and rest-one.yaml match
then manifests two.yaml and rest-two.yaml match
~~~

~~~{#roots.yaml .file .yaml .numberLines}
roots:
- live/one
- live/two
~~~


# Acceptance criteria for backup encryption

This chapter outlines scenarios, to be implemented later, for
verifying that Obnam properly encrypts the backups. These scenarios
verify only encryption aspects of Obnam.

## Backup without passphrase fails

Verify that trying to backup without having set a passphrase fails
with an error message that clearly identifies the lack of a
passphrase.

## A passphrase can be set

Set a passphrase. Verify that it's stored in a file that is only
readable by it owner. Verify that a backup can be made.

## A passphrase stored insecurely is rejected

Verify that a backup fails if the file where the passphrase is stored
is readable by anyone but its owner. Verify that the error message
explains that the backup failed due to the passphrase file insecurity.

## The passphrase can be changed

Verify that the passphrase can be changed and that backups made before
the change can no longer be restored. (Later, this requirement will be
re-evaluated, but this is simple and gets us started.)

## The passphrase is not on server in cleartext

Verify that after the passphrase has been set, and a backup has been
made, the passphrase is not stored in cleartext on the server.

## A backup is encrypted

Verify that the backup repository does not contain the backed up data
in cleartext.


<!-- -------------------------------------------------------------------- -->

# Colophon

This manual is edited in Markdown and typeset using [Subplot][].

---
title: "Obnam2&mdash;a backup system"
author: Lars Wirzenius
documentclass: report
bindings:
  - subplot/server.yaml
  - subplot/client.yaml
  - subplot/data.yaml
  - subplot/vendored/files.yaml
  - subplot/vendored/runcmd.yaml
template: python
functions:
  - subplot/server.py
  - subplot/client.py
  - subplot/data.py
  - subplot/vendored/daemon.py
  - subplot/vendored/files.py
  - subplot/vendored/runcmd.py
classes:
  - json
abstract: |
  Obnam is a backup system, consisting of a not very smart server for
  storing chunks of backup data, and a client that splits the user's
  data into chunks. They communicate via HTTP.
  
  This document describes the architecture and acceptance criteria for
  Obnam, as well as how the acceptance criteria are verified.
...
