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In some circumstances, it is very useful to trigger actions upon the detection of changes to an archive. For example, you might want to send an email notification whenever new revisions are checked in.
The command notify
provides this capability:
% larch notify NOTIFICATION-DIR
That command reads configuration files found in NOTIFICATION-DIR
.
Those configuration files explain what parts of which repositories to
monitor, and what actions to take upon changes. When changes are
detected, notify
invokes actions.
This chapter explains notify
in general. It documents the format of
a NOTIFICATION-DIR
and the configuration files. It explains what
notify
does. Later in the manual is documentation explaining how to
use notify
to achieve certain specific effects.
|=rules.archives| |=rules.categories| |=rules.branches| |=rules.versions|
notify
is controlled by four configuration files, found at the root
of a NOTIFICATION-DIR
. These are:
=rules.archives Specifies actions to take whenever new categories are added to specific archives.
=rules.categories Specifies actions to take whenever new branches are added to specific categories.
=rules.branches Specifies actions to take whenever new versions are added to specific branches.
=rules.versions Specifies actions to take whenever new revisions are added to specific versions.
Within these files, blank lines are permitted and comments begin with '#' and continue to the end of line.
The file =rules.versions
contains lines with the format:
VERSION ACTION
where VERSION
is a fully-qualified version name, and action a string
(with no whitespace) of the form:
COMMAND:THUNK
Whenever new revisions are detected in VERSION
, notify
invokes:
larch COMMAND THUNK RVN ...
where COMMAND
and THUNK
are from the configuration file, and the
trailing RVN
arguments are a list of fully qualified revision names
of newly detected revisions.
The file =rules.branches
is similar. It contains lines of the form:
BRANCH COMMAND:THUNK VERSION-ACTION
Whenever new versions are created in BRANCH
, notify
takes two
actions. First, it invokes:
larch COMMAND THUNK VSN ...
Second, it adds new rules to =rules.versions
of the form:
VSN VERSION-ACTION
The files =rules.categories
and =rules.archives
are similar. The
former contains lines of the form:
CATEGORY COMMAND:THUNK BRANCH-ACTION VERSION-ACTION
and the latter lines of the form:
ARCHIVE COMMAND:THUNK CATEGORY-ACT BRANCH-ACT VERSION-ACT
In addition to the configuration files, notify
maintains its private
state in NOTIFY-DIR
. In particular, it keeps track of what
notifications have already been sent, in order to try to avoid sending
redundant notifications.
Configuring triggers for every category, branch, and version in an
archive is quite simple: you only need to write the =rules.archives
file -- other configuration files will be automatically genrated from
that.
The initial =rules.archives
file should contain lines of the form:
ARCHIVE COMMAND:THUNK CATEGORY-ACT BRANCH-ACT VERSION-ACT
for ach set of actions you want to trigger. After creating the config
file, run notify
in that directory. The first run will detect all
categories in the repository, invoke COMMAND
for those, and create a
rules.categories
file with lines:
CATEGORY CATEGORY-ACT BRANCH-ACT VERSION-ACT
Then notify
will read that config file, detect all the branches in
the repository, invoke CATEGORY-ACT
, and create =rules.branches
.
Finally, =rules.versions
will be automatically created in a similar
way.
Whenever a new category, branch, or version is added to an archive,
the rules files will be automatically updated to reflect the addition
(when notify
is run, of course).
Unfortunately, some fundamental physical properties of the universe
make it impossible for notify
to guarantee that actions will be
invoked only once for each new category, branch, version, or revision.
A (presumably rare) well timed interrupt or system failure can cause
notify
to invoke actions more than once for a given change to the
archive.
Consequently, actions should be designed to be robust against that eventuality.
notify
is designed to be safe in the face of concurrent invocations:
if two processes invoke notify
at the same time, everything should
work wonderfully with no resultant redundant actions or NOTIFY-DIR
corruptions.
One strategy for using notify
is to run it from a cron
job.
These commands (and only these commands) modify arch repositories in
ways that are significant to notify
:
make-category make-branch make-version import commit
Corresponding to those commands, are four others:
% larch my-category-action COMMAND ARGUMENTS...
% larch my-branch-action COMMAND ARGUMENTS...
% larch my-version-action COMMAND ARGUMENTS...
% larch my-revision-action COMMAND ARGUMENTS...
which in those invocations, set an action to be taken upon successful completion of an archive transaction, and in these invocations:
% larch my-category-action
% larch my-branch-action
% larch my-version-action
% larch my-revision-action
report the actions to be taken.
The actions have a fairly obvious meaning. After creating a new
branch, for example, the command make-branch
will invoke:
% COMMAND ARGUMENTS... BRANCH
where BRANCH
is a fully qualified branch name.
A useful action to take is to invoke notify
directly. Another
useful action (though imposing a greater system administration burden)
is to invoke a script which sends mail to some address, informing the
recipient of the repository change. The recipient can be, for
example, an automated process which invokes notify
.
When the repository-modifying commands invoke one of these actions,
they run the action as a daemon
: an asynchronous process with no
controlling terminal, with standard input redirected from /dev/null
,
and with stdout
and stderr
saved in a temporary file. If the
command exits with non-0 status, the output from the command is sent
to the user in an email notice.
Once again, the damn constraints of physics, particularly the
prohibition against "action at a distance", impose a constraint: it
is impossible to guarantee the COMMAND
will be invoked and in
(hopefully rare) circumstances, it might not be. COMMANDs
should be
chosen with that constraint in mind.
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