REPL(8) REPL(8)
NAME
prune, scandb, syncdb - peer to peer replica management
SYNOPSIS
repl/prune repl...
repl/scandb [ -tv ] [ -p nb ] [ -n id ] [ -r replid ] [ -i
excldir ] dir db
repl/syncdb [ -12lnrv ] dir1 dir2 db1 db2 [ dir... ]
DESCRIPTION
These programs implement a peer-to-peer replica management
system for file trees. Each peer is called a replica and
holds both the replicated file tree and a data base. The
data base contains a textual representation of the file tree
together with the history of updates for each file as
described below. Any number of replicas can be defined and
any pair can be synchronized at any time. There is no cen-
tralized server nor there is a synchronization order
required to detect conflicts.
Replication is done using causal histories as defined by
Lamport. Each file entry in the data base holds a history
of change events. When a replica creates, changes, or
deletes a file, a change event is added to the file's his-
tory. A change event is a single character that identifies
the replica that did the change. The character is lower case
for creation, data changes, and metadata changes. The char-
acter is upper case for deletion events.
To synchronize a file between two replicas, their histories
for the file are compared. The file is considered up to date
if the histories are equal. If one of them is a prefix of
the other, it is considered out of date with respect to the
other. In this case the file and its history are updated in
the other replica. If the histories differ, but no one is a
prefix of the other, there was a concurrent update and a
conflict is reported.
Scandb is used to scan the file tree at dir for changes. It
builds a new data base that corresponds to the tree in disk
and compares it with db. The program prunes from the scan
process any directory not modified since the last scan per-
formed. Flag -t disables such prunning and is intended for
use in file systems that do not propagate modification times
from the changed file up to the root directory of the file
system.
The history for a file in the new data base is that found in
db perhaps followed by a change event (when the file has
changed).
REPL(8) REPL(8)
Option -n can be used when there is no db file. This happens
when defining a new replica. The option argument is the
name for the replica and must be a lower case letter. The
change histories in the new data base will contain a single
event (a change in the replica being defined).
Option -r can be used to define a new replica with name
replid while creating an initial data base for name id. This
is useful to build a data base for an already updated
replica that has been copied by other means. See the example
below.
Flag -v makes the program verbose. It will report any change
event noticed.
To ignore a directory give its name to option -i using an
relative path name starting at the replica directory. The
directory contents will be ignored. The option can be used
multiple times to ignore multiple directories. Be careful to
give the same ignore arguments to all the replicas.
To prune the history, you can use -p and pass the identifier
(a number) for the prune event. Such identifier must be used
at all the replicas. See the discussion below.
Syncdb is used to synchronize replicas. Its arguments are
the two directories and data bases for the replicas being
synchronized. Syncdb operates as said above to detect
updates and conflicts. It copies, creates, and deletes files
to bring both replicas to a synchronized state. If any dir
is supplied, relative to the top-dir of a replica, synchro-
nization applies only to it.
Flag -v makes the program report any change made. Flag -n
performs a dry run to report changes between both replicas
without synchronizing them. The flag -l can be used to syn-
crhonize just the left replica (in command line order) with
respect to the other. Flag -r does the same for the right
replica.
Flags -1 and -2 can be used to resolve conflicts in favor of
the first replica (the left one) and in favor of the second
one (the right one). When there are different conflicts to
be gained by different replicas, one replica can be updated
by hand, and then used as the prefered to settle down all
the conflicts.
To prune file histories follow this procedure.
1 Be sure that all the peers are synchronized.
2 Use prune to prune all the peers. You can pass the
REPL(8) REPL(8)
names for all the repl configuration files.
This program calls scandb for each replica using the -p
flag. The effect is to remove all but the last event in the
histories for files within the database. Any other proce-
dure would be very costly or complex, because it would
require finding the maximum common prefix for each history
and removing it from all the replicas.
Database files are exclusive-open text files that contain
the replica identifier in the first line, followed by one
line per file entry. Each entry contains the file name, the
update history, the user and group identifers, file permis-
sions (8 hex digits), modification time (8 hex digits), file
length (decimal), and Qid version. The file name starts with
'/' and is relative to the top directory of the replicated
file tree. All fields are separated by space. Entries for
deleted files have their history terminated on uppercase.
EXAMPLE
This defines for the system file tree, excluding /usr and
/dist, a replica called a. The same is done for /usr/nemo
excluding the directory /usr/nemo/dist and /usr/nemo/tmp.
; repl/scandb -n a -i usr -i dist /root /dist/sys.db
; repl/scandb -n a -i tmp -i dist $home $home/dist/home.db
Now we can replicate both the system and /usr/nemo into a
new laptop by defining two replicas, named b, and synchro-
nizing them.
# at the laptop, booted from the network.
; mount -c /dev/sdC0/fossil /n/fossil
; repl/scandb -n b /n/fossil /dist/nautilus.db
; replica/syncdb /root /fossil /dist/sys.db /dist/nautilus.db
; mkdir /fossil/$home /fossil/$home/dist
; repl/scandb -n b -i tmp -i dist /fossil/$home $home/dist/laptop.db
; replica/syncdb (/ /fossil)^$home $home/dist/^(home.db laptop.db)
SOURCE
/sys/src/cmd/repl
SEE ALSO
repl(1), replica(1).
BUGS
Append only files are not synchronized to prevent duplica-
tion of file contents.