All about JSON: How to develop frontends

Borg does not have a public API on the Python level. That does not keep you from writing import borg, but does mean that there are no release-to-release guarantees on what you might find in that package, not even for point releases (1.1.x), and there is no documentation beyond the code and the internals documents.

Borg does on the other hand provide an API on a command-line level. In other words, a frontend should to (for example) create a backup archive just invoke borg create, give commandline parameters/options as needed and parse JSON output from borg.

Important: JSON output is expected to be UTF-8, but currently borg depends on the locale being configured for that (must be a UTF-8 locale and not “C” or “ascii”), so that Python will choose to encode to UTF-8. The same applies to any inputs read by borg, they are expected to be UTF-8 encoded also.

We consider this a bug (see #2273) and might fix it later, so borg will use UTF-8 independent of the locale.

On POSIX systems, you can usually set environment vars to choose a UTF-8 locale:

export LANG=en_US.UTF-8
export LC_CTYPE=en_US.UTF-8

Logging

Especially for graphical frontends it is important to be able to convey and reformat progress information in meaningful ways. The --log-json option turns the stderr stream of Borg into a stream of JSON lines, where each line is a JSON object. The type key of the object determines its other contents.

Since JSON can only encode text, any string representing a file system path may miss non-text parts.

The following types are in use. Progress information is governed by the usual rules for progress information, it is not produced unless --progress is specified.

archive_progress

Output during operations creating archives (borg create and borg recreate). The following keys exist, each represents the current progress.

original_size
Original size of data processed so far (before compression and deduplication)
compressed_size
Compressed size
deduplicated_size
Deduplicated size
nfiles
Number of (regular) files processed so far
path
Current path
time
Unix timestamp (float)
progress_message

A message-based progress information with no concrete progress information, just a message saying what is currently being worked on.

operation
unique, opaque integer ID of the operation
msgid
Message ID of the operation (may be null)
finished
boolean indicating whether the operation has finished, only the last object for an operation can have this property set to true.
message
current progress message (may be empty/absent)
time
Unix timestamp (float)
progress_percent

Absolute progress information with defined end/total and current value.

operation
unique, opaque integer ID of the operation
msgid
Message ID of the operation (may be null)
finished
boolean indicating whether the operation has finished, only the last object for an operation can have this property set to true.
message
A formatted progress message, this will include the percentage and perhaps other information
current
Current value (always less-or-equal to total)
info
Array that describes the current item, may be null, contents depend on msgid
total
Total value
time
Unix timestamp (float)
file_status

This is only output by borg create and borg recreate if --list is specified. The usual rules for the file listing applies, including the --filter option.

status
Single-character status as for regular list output
path
Path of the file system object
log_message

Any regular log output invokes this type. Regular log options and filtering applies to these as well.

time
Unix timestamp (float)
levelname
Upper-case log level name (also called severity). Defined levels are: DEBUG, INFO, WARNING, ERROR, CRITICAL
name
Name of the emitting entity
message
Formatted log message
msgid
Message ID, may be null or absent

See Prompts for the types used by prompts.

Examples (reformatted, each object would be on exactly one line)

borg extract progress:

{"message": "100.0% Extracting: src/borgbackup.egg-info/entry_points.txt",
 "current": 13000228, "total": 13004993, "info": ["src/borgbackup.egg-info/entry_points.txt"],
 "operation": 1, "msgid": "extract", "type": "progress_percent", "finished": false}
{"message": "100.0% Extracting: src/borgbackup.egg-info/SOURCES.txt",
 "current": 13004993, "total": 13004993, "info": ["src/borgbackup.egg-info/SOURCES.txt"],
 "operation": 1, "msgid": "extract", "type": "progress_percent", "finished": false}
{"operation": 1, "msgid": "extract", "type": "progress_percent", "finished": true}

borg create file listing with progress:

{"original_size": 0, "compressed_size": 0, "deduplicated_size": 0, "nfiles": 0, "type": "archive_progress", "path": "src"}
{"type": "file_status", "status": "U", "path": "src/borgbackup.egg-info/entry_points.txt"}
{"type": "file_status", "status": "U", "path": "src/borgbackup.egg-info/SOURCES.txt"}
{"type": "file_status", "status": "d", "path": "src/borgbackup.egg-info"}
{"type": "file_status", "status": "d", "path": "src"}
{"original_size": 13176040, "compressed_size": 11386863, "deduplicated_size": 503, "nfiles": 277, "type": "archive_progress", "path": ""}

Internal transaction progress:

{"message": "Saving files cache", "operation": 2, "msgid": "cache.commit", "type": "progress_message", "finished": false}
{"message": "Saving cache config", "operation": 2, "msgid": "cache.commit", "type": "progress_message", "finished": false}
{"message": "Saving chunks cache", "operation": 2, "msgid": "cache.commit", "type": "progress_message", "finished": false}
{"operation": 2, "msgid": "cache.commit", "type": "progress_message", "finished": true}

A debug log message:

{"message": "35 self tests completed in 0.08 seconds",
 "type": "log_message", "created": 1488278449.5575905, "levelname": "DEBUG", "name": "borg.archiver"}

Prompts

Prompts assume a JSON form as well when the --log-json option is specified. Responses are still read verbatim from stdin, while prompts are JSON messages printed to stderr, just like log messages.

Prompts use the question_prompt and question_prompt_retry types for the prompt itself, and question_invalid_answer, question_accepted_default, question_accepted_true, question_accepted_false and question_env_answer types for information about prompt processing.

The message property contains the same string displayed regularly in the same situation, while the msgid property may contain a msgid, typically the name of the environment variable that can be used to override the prompt. It is the same for all JSON messages pertaining to the same prompt.

Examples (reformatted, each object would be on exactly one line)

Providing an invalid answer:

{"type": "question_prompt", "msgid": "BORG_CHECK_I_KNOW_WHAT_I_AM_DOING",
 "message": "... Type 'YES' if you understand this and want to continue: "}
incorrect answer  # input on stdin
{"type": "question_invalid_answer", "msgid": "BORG_CHECK_I_KNOW_WHAT_I_AM_DOING", "is_prompt": false,
 "message": "Invalid answer, aborting."}

Providing a false (negative) answer:

{"type": "question_prompt", "msgid": "BORG_CHECK_I_KNOW_WHAT_I_AM_DOING",
 "message": "... Type 'YES' if you understand this and want to continue: "}
NO  # input on stdin
{"type": "question_accepted_false", "msgid": "BORG_CHECK_I_KNOW_WHAT_I_AM_DOING",
 "message": "Aborting.", "is_prompt": false}

Providing a true (affirmative) answer:

{"type": "question_prompt", "msgid": "BORG_CHECK_I_KNOW_WHAT_I_AM_DOING",
 "message": "... Type 'YES' if you understand this and want to continue: "}
YES  # input on stdin
# no further output, just like the prompt without --log-json

Passphrase prompts

Passphrase prompts should be handled differently. Use the environment variables BORG_PASSPHRASE and BORG_NEW_PASSPHRASE (see Environment Variables for reference) to pass passphrases to Borg, don’t use the interactive passphrase prompts.

When setting a new passphrase (borg init, borg key change-passphrase) normally Borg prompts whether it should display the passphrase. This can be suppressed by setting the environment variable BORG_DISPLAY_PASSPHRASE to no.

When “confronted” with an unknown repository, where the application does not know whether the repository is encrypted, the following algorithm can be followed to detect encryption:

  1. Set BORG_PASSPHRASE to gibberish (for example a freshly generated UUID4, which cannot possibly be the passphrase)

  2. Invoke borg list repository ...

  3. If this fails, due the repository being encrypted and the passphrase obviously being wrong, you’ll get an error with the PassphraseWrong msgid.

    The repository is encrypted, for further access the application will need the passphrase.

  4. If this does not fail, then the repository is not encrypted.

Standard output

stdout is different and more command-dependent than logging. Commands like borg info, borg create and borg list implement a --json option which turns their regular output into a single JSON object.

Dates are formatted according to ISO 8601 in local time. No explicit time zone is specified at this time (subject to change). The equivalent strftime format string is ‘%Y-%m-%dT%H:%M:%S.%f’, e.g. 2017-08-07T12:27:20.123456.

The root object at least contains a repository key with an object containing:

id
The ID of the repository, normally 64 hex characters
location
Canonicalized repository path, thus this may be different from what is specified on the command line
last_modified
Date when the repository was last modified by the Borg client

The encryption key, if present, contains:

mode
Textual encryption mode name (same as borg init --encryption names)
keyfile
Path to the local key file used for access. Depending on mode this key may be absent.

The cache key, if present, contains:

path
Path to the local repository cache
stats

Object containing cache stats:

total_chunks
Number of chunks
total_unique_chunks
Number of unique chunks
total_size
Total uncompressed size of all chunks multiplied with their reference counts
total_csize
Total compressed and encrypted size of all chunks multiplied with their reference counts
unique_size
Uncompressed size of all chunks
unique_csize
Compressed and encrypted size of all chunks

Example borg info output:

{
    "cache": {
        "path": "/home/user/.cache/borg/0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
        "stats": {
            "total_chunks": 511533,
            "total_csize": 17948017540,
            "total_size": 22635749792,
            "total_unique_chunks": 54892,
            "unique_csize": 1920405405,
            "unique_size": 2449675468
        }
    },
    "encryption": {
        "mode": "repokey"
    },
    "repository": {
        "id": "0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
        "last_modified": "2017-08-07T12:27:20.789123",
        "location": "/home/user/testrepo"
    },
    "security_dir": "/home/user/.config/borg/security/0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
    "archives": []
}

Archive formats

borg info uses an extended format for archives, which is more expensive to retrieve, while borg list uses a simpler format that is faster to retrieve. Either return archives in an array under the archives key, while borg create returns a single archive object under the archive key.

Both formats contain a name key with the archive name, the id key with the hexadecimal archive ID, and the start key with the start timestamp.

borg info and borg create further have:

end
End timestamp
duration
Duration in seconds between start and end in seconds (float)
stats

Archive statistics (freshly calculated, this is what makes “info” more expensive)

original_size
Size of files and metadata before compression
compressed_size
Size after compression
deduplicated_size
Deduplicated size (against the current repository, not when the archive was created)
nfiles
Number of regular files in the archive
limits

Object describing the utilization of Borg limits

max_archive_size
Float between 0 and 1 describing how large this archive is relative to the maximum size allowed by Borg
command_line

Array of strings of the command line that created the archive

The note about paths from above applies here as well.

chunker_params
The chunker parameters the archive has been created with.

borg info further has:

hostname
Hostname of the creating host
username
Name of the creating user
comment
Archive comment, if any

Some keys/values are more expensive to compute than others (e.g. because it requires opening the archive, not just the manifest). To optimize for speed, borg list repo does not determine these values except when they are requested. The –format option is used for that (for normal mode as well as for –json mode), so, to have the comment included in the json output, you will need:

borg list repo --format "{name}{comment}" --json`

Example of a simple archive listing (borg list --last 1 --json):

{
    "archives": [
        {
            "id": "80cd07219ad725b3c5f665c1dcf119435c4dee1647a560ecac30f8d40221a46a",
            "name": "host-system-backup-2017-02-27",
            "start": "2017-08-07T12:27:20.789123"
        }
    ],
    "encryption": {
        "mode": "repokey"
    },
    "repository": {
        "id": "0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
        "last_modified": "2017-08-07T12:27:20.789123",
        "location": "/home/user/repository"
    }
}

The same archive with more information (borg info --last 1 --json):

{
    "archives": [
        {
            "chunker_params": [
                "buzhash",
                13,
                23,
                16,
                4095
            ],
            "command_line": [
                "/home/user/.local/bin/borg",
                "create",
                "/home/user/repository",
                "..."
            ],
            "comment": "",
            "duration": 5.641542,
            "end": "2017-02-27T12:27:20.789123",
            "hostname": "host",
            "id": "80cd07219ad725b3c5f665c1dcf119435c4dee1647a560ecac30f8d40221a46a",
            "limits": {
                "max_archive_size": 0.0001330855110409714
            },
            "name": "host-system-backup-2017-02-27",
            "start": "2017-02-27T12:27:20.789123",
            "stats": {
                "compressed_size": 1880961894,
                "deduplicated_size": 2791,
                "nfiles": 53669,
                "original_size": 2400471280
            },
            "username": "user"
        }
    ],
    "cache": {
        "path": "/home/user/.cache/borg/0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
        "stats": {
            "total_chunks": 511533,
            "total_csize": 17948017540,
            "total_size": 22635749792,
            "total_unique_chunks": 54892,
            "unique_csize": 1920405405,
            "unique_size": 2449675468
        }
    },
    "encryption": {
        "mode": "repokey"
    },
    "repository": {
        "id": "0cbe6166b46627fd26b97f8831e2ca97584280a46714ef84d2b668daf8271a23",
        "last_modified": "2017-08-07T12:27:20.789123",
        "location": "/home/user/repository"
    }
}

File listings

Listing the contents of an archive can produce a lot of JSON. Since many JSON implementations don’t support a streaming mode of operation, which is pretty much required to deal with this amount of JSON, output is generated in the JSON lines format, which is simply a number of JSON objects separated by new lines.

Each item (file, directory, …) is described by one object in the borg list output. Refer to the borg list documentation for the available keys and their meaning.

Example (excerpt) of borg list --json-lines:

{"type": "d", "mode": "drwxr-xr-x", "user": "user", "group": "user", "uid": 1000, "gid": 1000, "path": "linux", "healthy": true, "source": "", "linktarget": "", "flags": null, "mtime": "2017-02-27T12:27:20.023407", "size": 0}
{"type": "d", "mode": "drwxr-xr-x", "user": "user", "group": "user", "uid": 1000, "gid": 1000, "path": "linux/baz", "healthy": true, "source": "", "linktarget": "", "flags": null, "mtime": "2017-02-27T12:27:20.585407", "size": 0}

Message IDs

Message IDs are strings that essentially give a log message or operation a name, without actually using the full text, since texts change more frequently. Message IDs are unambiguous and reduce the need to parse log messages.

Assigned message IDs are:

Errors
Archive.AlreadyExists
Archive {} already exists
Archive.DoesNotExist
Archive {} does not exist
Archive.IncompatibleFilesystemEncodingError
Failed to encode filename “{}” into file system encoding “{}”. Consider configuring the LANG environment variable.
Cache.CacheInitAbortedError
Cache initialization aborted
Cache.EncryptionMethodMismatch
Repository encryption method changed since last access, refusing to continue
Cache.RepositoryAccessAborted
Repository access aborted
Cache.RepositoryIDNotUnique
Cache is newer than repository - do you have multiple, independently updated repos with same ID?
Cache.RepositoryReplay
Cache is newer than repository - this is either an attack or unsafe (multiple repos with same ID)
Buffer.MemoryLimitExceeded
Requested buffer size {} is above the limit of {}.
ExtensionModuleError
The Borg binary extension modules do not seem to be properly installed
IntegrityError
Data integrity error: {}
NoManifestError
Repository has no manifest.
PlaceholderError
Formatting Error: “{}”.format({}): {}({})
KeyfileInvalidError
Invalid key file for repository {} found in {}.
KeyfileMismatchError
Mismatch between repository {} and key file {}.
KeyfileNotFoundError
No key file for repository {} found in {}.
PassphraseWrong
passphrase supplied in BORG_PASSPHRASE is incorrect
PasswordRetriesExceeded
exceeded the maximum password retries
RepoKeyNotFoundError
No key entry found in the config of repository {}.
UnsupportedManifestError
Unsupported manifest envelope. A newer version is required to access this repository.
UnsupportedPayloadError
Unsupported payload type {}. A newer version is required to access this repository.
NotABorgKeyFile
This file is not a borg key backup, aborting.
RepoIdMismatch
This key backup seems to be for a different backup repository, aborting.
UnencryptedRepo
Keymanagement not available for unencrypted repositories.
UnknownKeyType
Keytype {0} is unknown.
LockError
Failed to acquire the lock {}.
LockErrorT
Failed to acquire the lock {}.
ConnectionClosed
Connection closed by remote host
InvalidRPCMethod
RPC method {} is not valid
PathNotAllowed
Repository path not allowed
RemoteRepository.RPCServerOutdated
Borg server is too old for {}. Required version {}
UnexpectedRPCDataFormatFromClient
Borg {}: Got unexpected RPC data format from client.
UnexpectedRPCDataFormatFromServer
Got unexpected RPC data format from server: {}
Repository.AlreadyExists
Repository {} already exists.
Repository.CheckNeeded
Inconsistency detected. Please run “borg check {}”.
Repository.DoesNotExist
Repository {} does not exist.
Repository.InsufficientFreeSpaceError
Insufficient free space to complete transaction (required: {}, available: {}).
Repository.InvalidRepository
{} is not a valid repository. Check repo config.
Repository.AtticRepository
Attic repository detected. Please run “borg upgrade {}”.
Repository.ObjectNotFound
Object with key {} not found in repository {}.
Operations
  • cache.begin_transaction

  • cache.download_chunks, appears with borg create --no-cache-sync

  • cache.commit

  • cache.sync

    info is one string element, the name of the archive currently synced.

  • repository.compact_segments

  • repository.replay_segments

  • repository.check_segments

  • check.verify_data

  • extract

    info is one string element, the name of the path currently extracted.

  • extract.permissions

  • archive.delete

Prompts
BORG_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK
For “Warning: Attempting to access a previously unknown unencrypted repository”
BORG_RELOCATED_REPO_ACCESS_IS_OK
For “Warning: The repository at location … was previously located at …”
BORG_CHECK_I_KNOW_WHAT_I_AM_DOING
For “Warning: ‘check –repair’ is an experimental feature that might result in data loss.”
BORG_DELETE_I_KNOW_WHAT_I_AM_DOING
For “You requested to completely DELETE the repository including all archives it contains:”
BORG_RECREATE_I_KNOW_WHAT_I_AM_DOING
For “recreate is an experimental feature.”