Journal Export Formats

Journal Export Format

Note that this document describes the binary serialization format of journals only, as used for transfer across the network. For interfacing with web technologies there’s the Journal JSON Format, described below. The binary format on disk is documented as the Journal File Format.

Before reading on, please make sure you are aware of the basic properties of journal entries, in particular realize that they may include binary non-text data (though usually don’t), and the same field might have multiple values assigned within the same entry (though usually hasn’t).

When exporting journal data for other uses or transferring it via the network/local IPC the journal export format is used. It’s a simple serialization of journal entries, that is easy to read without any special tools, but still binary safe where necessary. The format is like this:

Two journal entries that follow each other are separated by a double newline.
Journal fields consisting only of valid non-control UTF-8 codepoints are serialized as they are (i.e. the field name, followed by ‘=’, followed by field data), followed by a newline as separator to the next field. Note that fields containing newlines cannot be formatted like this. Non-control UTF-8 codepoints are the codepoints with value at or above 32 (‘ ‘), or equal to 9 (TAB).
Other journal fields are serialized in a special binary safe way: field name, followed by newline, followed by a binary 64-bit little endian size value, followed by the binary field data, followed by a newline as separator to the next field.
Entry metadata that is not actually a field is serialized like it was a field, but beginning with two underscores. More specifically, __CURSOR=, __REALTIME_TIMESTAMP=, __MONOTONIC_TIMESTAMP=, __SEQNUM=, __SEQNUM_ID are introduced this way. Note that these meta-fields are only generated when actual journal files are serialized. They are omitted for entries that do not originate from a journal file (for example because they are transferred for the first time to be stored in one). Or in other words: if you are generating this format you shouldn’t care about these special double-underscore fields. But you might find them usable when you deserialize the format generated by us. Additional fields prefixed with two underscores might be added later on, your parser should skip over the fields it does not know.
The order in which fields appear in an entry is undefined and might be different for each entry that is serialized. And that’s already it.

This format can be generated via journalctl -o export.

Here’s an example for two serialized entries which consist only of text data:

__CURSOR=s=739ad463348b4ceca5a9e69c95a3c93f;i=4ece7;b=6c7c6013a26343b29e964691ff25d04c;m=4fc72436e;t=4c508a72423d9;x=d3e5610681098c10;p=system.journal
__REALTIME_TIMESTAMP=1342540861416409
__MONOTONIC_TIMESTAMP=21415215982
_BOOT_ID=6c7c6013a26343b29e964691ff25d04c
_TRANSPORT=syslog
PRIORITY=4
SYSLOG_FACILITY=3
SYSLOG_IDENTIFIER=gdm-password]
SYSLOG_PID=587
MESSAGE=AccountsService-DEBUG(+): ActUserManager: ignoring unspecified session '8' since it's not graphical: Success
_PID=587
_UID=0
_GID=500
_COMM=gdm-session-wor
_EXE=/usr/libexec/gdm-session-worker
_CMDLINE=gdm-session-worker [pam/gdm-password]
_AUDIT_SESSION=2
_AUDIT_LOGINUID=500
_SYSTEMD_CGROUP=/user/lennart/2
_SYSTEMD_SESSION=2
_SELINUX_CONTEXT=system_u:system_r:xdm_t:s0-s0:c0.c1023
_SOURCE_REALTIME_TIMESTAMP=1342540861413961
_MACHINE_ID=a91663387a90b89f185d4e860000001a
_HOSTNAME=epsilon

__CURSOR=s=739ad463348b4ceca5a9e69c95a3c93f;i=4ece8;b=6c7c6013a26343b29e964691ff25d04c;m=4fc72572f;t=4c508a7243799;x=68597058a89b7246;p=system.journal
__REALTIME_TIMESTAMP=1342540861421465
__MONOTONIC_TIMESTAMP=21415221039
_BOOT_ID=6c7c6013a26343b29e964691ff25d04c
_TRANSPORT=syslog
PRIORITY=6
SYSLOG_FACILITY=9
SYSLOG_IDENTIFIER=/USR/SBIN/CROND
SYSLOG_PID=8278
MESSAGE=(root) CMD (run-parts /etc/cron.hourly)
_PID=8278
_UID=0
_GID=0
_COMM=run-parts
_EXE=/usr/bin/bash
_CMDLINE=/bin/bash /bin/run-parts /etc/cron.hourly
_AUDIT_SESSION=8
_AUDIT_LOGINUID=0
_SYSTEMD_CGROUP=/user/root/8
_SYSTEMD_SESSION=8
_SELINUX_CONTEXT=system_u:system_r:crond_t:s0-s0:c0.c1023
_SOURCE_REALTIME_TIMESTAMP=1342540861416351
_MACHINE_ID=a91663387a90b89f185d4e860000001a
_HOSTNAME=epsilon

A message with a binary field produced by

python3 -c 'from systemd import journal; journal.send("foo\nbar")'
journalctl -n1 -o export

__CURSOR=s=bcce4fb8ffcb40e9a6e05eee8b7831bf;i=5ef603;b=ec25d6795f0645619ddac9afdef453ee;m=545242e7049;t=50f1202
__REALTIME_TIMESTAMP=1423944916375353
__MONOTONIC_TIMESTAMP=5794517905481
_BOOT_ID=ec25d6795f0645619ddac9afdef453ee
_TRANSPORT=journal
_UID=1001
_GID=1001
_CAP_EFFECTIVE=0
_SYSTEMD_OWNER_UID=1001
_SYSTEMD_SLICE=user-1001.slice
_MACHINE_ID=5833158886a8445e801d437313d25eff
_HOSTNAME=bupkis
_AUDIT_LOGINUID=1001
_SELINUX_CONTEXT=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
CODE_LINE=1
CODE_FUNC=<module>
SYSLOG_IDENTIFIER=python3
_COMM=python3
_EXE=/usr/bin/python3.4
_AUDIT_SESSION=35898
_SYSTEMD_CGROUP=/user.slice/user-1001.slice/session-35898.scope
_SYSTEMD_SESSION=35898
_SYSTEMD_UNIT=session-35898.scope
MESSAGE
^G^@^@^@^@^@^@^@foo
bar
CODE_FILE=<string>
_PID=16853
_CMDLINE=python3 -c from systemd import journal; journal.send("foo\nbar")
_SOURCE_REALTIME_TIMESTAMP=1423944916372858

Journal JSON Format

Note that this section describes the JSON serialization format of the journal only, as used for interfacing with web technologies. For binary transfer of journal data across the network there’s the Journal Export Format described above. The binary format on disk is documented as Journal File Format.

In most cases the Journal JSON serialization is the obvious mapping of the entry field names (as JSON strings) to the entry field values (also as JSON strings) encapsulated in one JSON object. However, there are a few special cases to handle:

A field that contains non-printable or non-UTF8 is serialized as a number array instead. This is necessary to handle binary data in a safe way without losing data, since JSON cannot embed binary data natively. Each byte of the binary field will be mapped to its numeric value in the range 0…255.
The JSON serializer can optionally skip huge (as in larger than a specific threshold) data fields from the JSON object. If that is enabled and a data field is too large, the field name is still included in the JSON object but assigned null.
Within the same entry, Journal fields may have multiple values assigned. This is not allowed in JSON. The serializer will hence create a single JSON field only for these cases, and assign it an array of values (which the can be strings, null or number arrays, see above).
If the JSON data originates from a journal file it may include the special addressing fields __CURSOR, __REALTIME_TIMESTAMP, __MONOTONIC_TIMESTAMP, __SEQNUM, __SEQNUM_ID, which contain the cursor string of this entry as string, the realtime/monotonic timestamps of this entry as formatted numeric string of usec since the respective epoch, and the sequence number and associated sequence number ID, both formatted as strings.

Here’s an example, illustrating all cases mentioned above. Consider this entry:

MESSAGE=Hello World
_UDEV_DEVNODE=/dev/waldo
_UDEV_DEVLINK=/dev/alias1
_UDEV_DEVLINK=/dev/alias2
BINARY=this is a binary value \a
LARGE=this is a super large value (let's pretend at least, for the sake of this example)

This translates into the following JSON Object:

{
  "MESSAGE" : "Hello World",
  "_UDEV_DEVNODE" : "/dev/waldo",
  "_UDEV_DEVLINK" : [ "/dev/alias1", "/dev/alias2" ],
  "BINARY" : [ 116, 104, 105, 115, 32, 105, 115, 32, 97, 32, 98, 105, 110, 97, 114, 121, 32, 118, 97, 108, 117, 101, 32, 7 ],
  "LARGE" : null
}