changes, pull, push, scan – client–server replica management

replica/pull [ –nv ] [ –c name ]... [ –s name ]... name [ path ]
replica/push [ –nv ] name [ path ]
replica/changes name [ path ]
replica/scan name [ path ]

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 relative 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 system 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 Dir structure (see stat(5)). For deletion events, the metadata 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, simultaneous 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 specifies 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:
A function that mounts the server; run on both client and server.
A function that rescans the server for changes. Typically this command dials a CPU server known to be close to the file server and runs replica/scan on that well–connected machine.
The path to the root of the replicated file system on the server, as it will be in the name space after running servermount.
The path to the server's change log, after running servermount.
The path to the proto file describing the server's files, after running servermount. Only used by scan.
The path to the server's file database, after running servermount. Only used by scan.
A function to mount the client file system; run only on the client.
The path to the root of the replicated file system on the client, after running clientmount.
The path to the client's copy of the server log file. The client log is maintained by pull.
The path to the proto file describing the client's files. Only used by changes. Often just a copy of $serverproto.
The path to the client's file database, after running clientmount.
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='' }
fn clientmount { 9fs kfs }

(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.)

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.)


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