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I'm pasting the contents of
maven-components/maven-core/src/site/apt/offline-mode.apt in here again,
to update everyone on some of the details I've added.
Brett: I've changed requiredConnectivity to requiresOnline, per your
suggestion. I thought I had a use case for the requires-offline mode,
but cannot reconstruct my thoughts on this. So, simpler wins here,
unless/until we need more.
All: Thanks for again taking the time to deal with such a large piece of
list-spam. :)
Updated APT source follows:
- ------------------------------------------------------------------
---
Offline Mode Design
---
John Casey
---
08-April-2005
---
Offline Mode Design
* Assumptions: What is Offline?
For the purposes of determining the areas sensitive to offline status,
it is definitely useful to define what the offline state really means.
[[1]] This is obvious, but the network/internet is unavailable.
[[2]] Localhost (127.0.0.1) may also be unavailable if the whole
network stack is offline.
[[3]] "Remote" repositories referenced using the file:// protocol may
be available. However, if that file:// url references a
file-share, as in the case of an NFS or SMB mount, that will
be unavailable.
So, offline mode has several implications, some of which may not be
altogether obvious:
* Localhost may be unavailable. Therefore, even locally installed
server processes which work by conversing over a port may fail.
* Not all "remote" repositories will fail. Specifically, if the remote
repo uses the file:// protocol, and it doesn't refer to a shared
filesystem, it will continue to be available.
The question remaining is: Which level of offline mode will we support? It
seems reasonable to assume that users will be able to tell when
localhost is
not active (in most cases, localhost should be available, even if the
rest of
the network is not). Therefore, let's concentrate on the state where no
network <beyond localhost> exists, and leave the more extreme state to
users
to diagnose and correct as necessary.
* Why is Offline Mode Important?
Offline mode is essential for breaking the requirement that m2 operate
in a
network-connected environment. It means legitimizing a development
environment
in which there is no network connection, and providing a robust m2 service
offering in such circumstances. Introduction of offline mode allows m2 to
anticipate the inevitable network failures that accompany being physically
disconnected from the network, and adjust it's behavior accordingly.
It is more than simply understanding that m2 cannot go and check for the
latest version of some snapshot artifact. If m2 is offline, SCM
operations
cannot succeed; no artifact downloads can take place, regardless of
whether
they are snapshot versions; artifact deployment cannot take place;
certain
types of tests cannot be setup, since the container used to run them
cannot be
reached or started.
All of these operations will produce their own unique errors in the
absence of
a coordinated offline strategy. In addition, efforts to unite these
failing
behaviors behind a consistent user interface is much, much more
difficult if
the system can't tell whether it has access to the network required by
these
operations.
Offline mode really means anticipating a lack of network connectivity,
and as
a result turning off certain services provided by m2 and providing a
coherent
way of predicting and reporting when network-related failures will
take place.
It means warning users that since the network is missing, certain
features and
operations will be unavailable, rather than simply waiting for those
operations to fail, then trying to help users decipher the error
messages they
get as a result.
* Implications for Resolution
** Dependency Resolution
This one is obvious...we only have access to the repositories using
the file:// protocol and living on a truly local filesystem when
offline.
** Plugin Resolution
This is similar to dependency resolution. Plugin repositories not
using file:// or not residing on a local (not shared) filesystem will
be unavailable.
* Implications for Mojo Execution
** Deployment mojos
The concept of deployment is dependent on the availability of a some
remote repository. Just as above, if that repository is not using
file:// (which is highly likely to be the case), or the repository is
not on a local filesystem, deployment will fail when offline.
** Testing mojos
This can be a problem if the tests are more than simple unit tests;
that is, if they require configuration of a server process, and
subsequent testing in-container.
Since we're only going to concern ourselves with states where localhost is
still active, we only need to worry about this case when the server
container
is <<not>> installed on localhost. This allows the popular pattern of
starting
a server container in-JVM, running tests against it, and shutting it down.
** SCM mojos
See below for discussion on SCM-related operations. Any mojo which
carries out some analysis or other interaction with a SCM system
will likely be unavailable when in offline mode.
* Implications for Subsystems
** Maven-Wagon
Parts of Wagon will continue to function normally. These include:
* The file wagon, provided the referenced location is on a local
filesystem.
It is not possible to determine whether a file-based location will
be available except on a case-by-case basis (or a root-url by
root-url basis).
* If not otherwise specified, all other wagons are assumed to be
remote-only, and are therefore sensitive to offline mode.
** Maven-Artifact
This is wholly dependent on Maven-Wagon, above.
** Maven-SCM
In all but trivial examples, SCM operations cannot complete without
having access to the versioning server. Therefore, it is assumed that
any SCM-related activity will be unavailable when m2 is in offline
mode.
** Maven-Core
We'll examine the different parts of maven-core on a case-by-case
basis, below:
*** DefaultLifecycleExecutor
When binding goals to the project's configured lifecycle, each mojo
descriptor should declare whether it requires online/offline status.
This value should be a java.lang.Boolean, so it can implement 3VL
(three value logic: yes, no, don't-care). The requiresOnline
field in the mojo descriptor has the following semantics:
[true] Online status is required for this mojo to function
correctly.
[false] <<(Default)>> Either status is acceptable for the mojo to
execute. It doesn't care.
The majority of mojos will leave the requiresOnline == false,
since online/offline status will be irrelevant, provided they have
access to their required artifacts and other classpath elements. In
the case of required artifacts and other classpath elemtents, this is
assumed by the mojo API to be in a correct state, and will be handled
by the Wagon modifications.
* Implementation Notes
** Accessibility of offline status
Offline status should be indicated in the MavenSettings instance,
since it can conceivably be set from either the settings.xml or the
command-line.
In the event the '-o' switch is the impetus for setting offline mode,
this should result in modification of the active profile in the
MavenSettings instance, just as definition of the active profile from
the command-line should result in similar modification. This object is
not meant to be static within the build process, but rather to be
setup as an aggregation of all settings-related information passed
into the system.
** Control over downloads
Find the control point for m2 using maven-wagon. At this point, inject
a offline status parameter which is used when retrieving the specific
Wagon.
If <<<offline == true>>>:
* If the wagon is not bound to "file://", then ignore the request and
print a debug message.
* If the wagon is bound to "file://" then:
Retrieve the file or base-url file to be "downloaded".
* If the file (or more usefully, the base-url file) exists, proceed.
* If the file (or base-url file) doesn't exist, assume that this
location is part of a file-share. Ignore the request and print a
debug message as above.
** Control over mojos in the lifecycle
When binding a mojo to the project's lifecycle instance, check the
mojo descriptor's requiredConnectivity field.
* If <<<(offline == true) && (requiresOnline != true)>>>, bind
the mojo to the lifecycle.
In this case, the client is <<offline>>, and the mojo does not
require online status.
* If <<<(offline == false) && (requiresOnline == true)>>>, bind
the mojo to the lifecycle.
In this case, the client is <<online>>, and the mojo either requires
<<online>> status, or else doesn't care.
* Otherwise, don't bind the mojo. Log a debug message to indicate that
it is sensitive the the online state of the application, and that
this state is currently wrong for execution.
<<NOTE:>> Do we want to fail when we cannot bind a mojo to the
lifecycle because of offline/online status? That would probably
indicate that the user was trying to do something they cannot
succeed at for now...so we probably should throw an exception in
this case.
- ------------------------------------------------------------------
Regards,
john
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