# Transaction logger

The transaction logger is a subsystem designed to write file
system updates to another device.

The changes are written to an internal queue. This queue is then
written to a device, tape, or network by a dequeueing process run
on a phantom line.

If the dequeueing process can keep up with the enqueueing of
items by the users, then up to the second crash protection can be
accomplished. Used with a file-save tape, all updates are recorded
for restoration. If used with another machine, it can provide a hot
backup.

startlog
on the primary machine and tlog-restore
on the secondary machine provides the basic functionality for hot
backups.

Transaction Log is a menu available to help manage the
transaction logger. It can start and stop the dequeueing process as
well as filter data going into the queue. All files can be logged
or only files with an l
in attribute one.

set-dptr
is a command that can change a D-pointer or all D-pointers in an
account to add or remove the l
from attribute one. If only dl type files are being logged then a
normal create-file
does not cause a create-file
on the backup machine. So, changing the D-pointer to dl later does
not cause a create-file
either. This is part of the design and should be noted.

A device must be attached to the port that is starting the
dequeueing process. The transaction logger steals the device and
begin dequeueing to it. The logger runs on a phantom process,
usually the last one, right before the scheduler.

When a stoplog
is entered, the current reel on the dequeueing device is terminated
with a file mark and a t-det
is executed by the dequeueing process. It remembers which tape
device it was using, but now this device is free. The next
startlog
entered re-attaches the tape and increments the reel number.

Each reel in a transaction log dequeue session is independent
and can be restored from TCL one after the other or separately.

A list of devices is available to choose from. To cause the
startlog
command to display this list, detach all the tape devices prior to
invoking startlog.

Between stoplog
and the next startlog,
a new reel must be inserted if not doing hot backups.

## Example(s)

This sequence allows all updates to the invoices file to be
enqueued for writing to the SCT drive.

```
set-dptr +l invoices (f
invoices : D -> DL
invoices : D -> DL
set-sct
Block size: 16384
[1709] Tape device is assigned to med density (150M) quarter inch tape (SCT).
startlog
Activate transaction logger (y/n)? y
[607] Transaction logger started.
```
This command writes a file mark on the tape and releases it from
the transaction logger, t-det.
Updates are enqueued automatically. This is reel one.

```
stoplog
[602] The transaction logger is disabled.
```
This restarts the logger to the SCT drive on behalf of reel
two.

```
startlog
[607] Transaction logger started.
```

---
Source: https://d3codex.com/definitions/transaction-logger/ - part of the D3Codex reference.