86
86
<p>In one specific kind of hook, this is easy to deal with. A relation-changed hook can always exit without error when the current remote unit is missing data, because the hook is guaranteed to be run again when that data changes -- and, assuming the remote unit is running a charm that agrees on how to implement the interface, the data <em>will</em> change and the hook <em>will</em> be run again.</p>
87
87
<p>In <em>all</em> other cases -- unit hooks, relation hooks for a different relation, relation hooks for a different remote unit in the same relation, and even relation hooks other than -changed for the <em>same</em> remote unit -- there is no such guarantee. These hooks all run on their own schedule, and there is no reason to expect them to be re-run on a predictable schedule, or in some cases ever again.</p>
88
88
<p>This means that all such hooks need to be able to handle missing relation data, and to complete successfully; they mustn't fail, because the user is powerless to resolve the situation, and they can't even wait for state to change, because they all see their own sandboxed composite snapshot of fairly-recent state, which never changes.</p>
89
<p>So, outside a vey narrow range of circumstances, relation-get should be treated with particular care. The corresponding advice for relation-set is very simple by comparison: relation-set should be called early and often. Because the unit agent serializes hook execution, there is never any danger of concurrent changes to the data, and so a null setting change can be safely ignored, and will not cause other units to react.</p>
89
<p>So, outside a very narrow range of circumstances, relation-get should be treated with particular care. The corresponding advice for relation-set is very simple by comparison: relation-set should be called early and often. Because the unit agent serializes hook execution, there is never any danger of concurrent changes to the data, and so a null setting change can be safely ignored, and will not cause other units to react.</p>
92
92
<section id="relations-in-depth">
added
removed
htmldocs/authors-relations-in-depth.html
