REPLICA(1) REPLICA(1)
NAME
changes, pull, push, scan - client-server replica management
SYNOPSIS
replica/pull [ -nv ] [ -c name ]... [ -s name ]... name [
path ]
replica/push [ -nv ] name [ path ]
replica/changes name [ path ]
replica/scan name [ path ]
DESCRIPTION
These shell scripts provide a simple log-based client-server
replica management. The server keeps a log of changes made
to its file system, and clients synchronize by reading the
log and applying these changes locally.
These scripts are a polished interface to the low-level
tools described in replica(8). See replica(8) for details on
the inner workings of replica management. These tools were
written primarily as the fourth edition Plan 9 distribution
mechanism, but they have wider applicability. For example,
they could be used to synchronize one's home directory
between a laptop and a central file server.
Replicas are described by configuration files. The name in
all the replica commands is a configuration file. Paths
that do not begin with /, ./, or ../ are assumed to be rela-
tive to $home/lib/replica. Configuration files are
described below.
Replica/scan is the only one of these programs that does not
need to be run on the client. It scans the server file sys-
tem for changes and appends entries for those changes into
the server log. Typically it is run on a machine with a
fast network connection to the server file system.
Replica/pull copies changes from the server to the client,
while replica/push copies changes from the client to the
server. (Both run on the client.) If a list of paths is
given, only changes to those paths or their children are
copied. The -v flag causes pull or push to print a summary
of what it is doing. Each status line is of the form
verb path serverpath mode uid gid mtime length
Verb describes the event: addition of a file (a), deletion
of a file (d), a change to a file's contents (c), or a
change to a file's metadata (m). Path is the file path on
the client; serverpath is the file path on the server.
Mode, uid, gid, and mtime are the file's metadata as in the
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Dir structure (see stat(5)). For deletion events, the meta-
data is that of the deleted file. For other events, the
metadata is that after the event. The -n flag causes pull
or push to print the summary but not actually carry out the
actions.
Push and pull are careful to notice simultaneous changes to
a file or its metadata on both client and server. Such
simultaneous changes are called conflicts. Here, simultane-
ous does not mean at the same instant but merely that both
changes were carried out without knowledge of the other.
For example, if a client and server both make changes to a
file without an intervening push or pull, the next push or
pull will report an update/update conflict. If a conflict
is detected, both files are left the same. The -c flag to
pull specifies that conflicts for paths beginning with name
should be resolved using the client's copy, while -s speci-
fies the server's copy. The -c and -s options may be
repeated.
Replica/changes prints a list of local changes made on the
client that have not yet been pushed to the server. It is
like push with the -n flag, except that it does not check
for conflicts and thus does not require the server to be
available.
The replica configuration file is an rc(1) script that must
define the following functions and variables:
servermount
A function that mounts the server; run on both client
and server.
serverupdate
A function that rescans the server for changes. Typi-
cally this command dials a CPU server known to be close
to the file server and runs replica/scan on that well-
connected machine.
serverroot
The path to the root of the replicated file system on
the server, as it will be in the name space after run-
ning servermount.
serverlog
The path to the server's change log, after running
servermount.
serverproto
The path to the proto file describing the server's
files, after running servermount. Only used by scan.
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serverdb
The path to the server's file database, after running
servermount. Only used by scan.
clientmount
A function to mount the client file system; run only on
the client.
clientroot
The path to the root of the replicated file system on
the client, after running clientmount.
clientlog
The path to the client's copy of the server log file.
The client log is maintained by pull.
clientproto
The path to the proto file describing the client's
files. Only used by changes. Often just a copy of
$serverproto.
clientdb
The path to the client's file database, after running
clientmount.
clientexclude
A (potentially empty) list of paths to exclude from
synchronization. A typical use of this is to exclude
the client database and log files. These paths are
relative to the root of the replicated file system.
As an example, the Plan 9 distribution replica configuration
looks like:
fn servermount { 9fs sources; bind /n/sources/plan9 /n/dist }
fn serverupdate { status='' }
serverroot=/n/dist
s=/n/dist/dist/replica
serverlog=$s/plan9.log
serverproto=$s/plan9.proto
fn clientmount { 9fs kfs }
clientroot=/n/kfs
c=/n/kfs/dist/replica
clientlog=$c/client/plan9.log
clientproto=$c/plan9.proto
clientdb=$c/client/plan9.db
clientexclude=(dist/replica/client)
(Since the Plan 9 developers run scan manually to update the
log, the clients need not do anything to rescan the file
system. Thus serverupdate simply returns successfully.)
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The fourth edition Plan 9 distribution uses these tools to
synchronize installations with the central server at Bell
Labs. The replica configuration files and metadata are kept
in /dist/replica. To update your system, make sure you are
connected to the internet and run
replica/pull /dist/replica/network
If conflicts are reported (say you have made local changes
to /rc/bin/cpurc and /rc/bin/termrc, but only want to keep
the cpurc changes), use
replica/pull -c rc/bin/cpurc -s rc/bin/termrc /dist/replica/network
to instruct pull to ignore the server's change to cpurc.
The script /usr/glenda/bin/rc/pull runs pull with the -v
flag and with /dist/replica/network inserted at the right
point on the command line. Logged in as glenda, one can
repeat the above example with:
pull -c rc/bin/cpurc -s rc/bin/termrc
To see a list of changes made to the local file system since
installation, run
replica/changes /dist/replica/network
(Although the script is called network, since changes is a
local-only operation, the network need not be configured.)
SOURCE
/rc/bin/replica
SEE ALSO
replica(8)
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