Oliver Heger created CONFIGURATION-520:
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Summary: Rework reloading mechanisms
Key: CONFIGURATION-520
URL: https://issues.apache.org/jira/browse/CONFIGURATION-520
Project: Commons Configuration
Issue Type: Improvement
Components: File reloading
Affects Versions: 1.9
Reporter: Oliver Heger
Fix For: 2.0
The current implementation of reloading features is tightly coupled to
file-based configurations: On each property access a reloading strategy is
queried; if it indicates that a reload is required, the content of the
configuration is replaced by the file on hard disc.
The advantage of this approach is that it is very transparent for client code;
no specific actions have to be taken to enable reloading. However, there are
also many problems:
* There is no control over reloading. It can happen at any time. This could
cause problems if a configuration contains multiple related properties, and a
reload happens while an application reads them: properties read before the
reload may not be compatible with values read afterwards.
* There is no sound error handling. A failed reload operation corrupts the
configuration (see CONFIGURATION-136).
* There is no support for other reloading triggers (e.g. periodic checks) or
event notifications when the change of a configuration source is detected.
* The implementation of reloading features is spread over a bunch of methods in
{{AbstractFileConfiguration}}; each affected method contains a reload check.
* It is very hard (or even impossible) to provide an efficient thread-safe
implementation of configurations; there has to be synchronization with
reloading operations.
* The mechanisms available are specific to file-based configurations, there is
no generic approach.
The disadvantages listed above can be addressed by moving reloading
functionality from specific {{Configuration}} implementations to builder
objects responsible for the creation of configurations (see CONFIGURATION-519).
This would require client code to deal with reloading in a slightly different
(and probably slightly less transparent) way, but it would simplify the current
implementation and enable advanced reloading features.
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