Hi Murtaza, The short answer is that it's not necessarily that there are pros or cons between having components know about each other in their "constructed" vs "started" forms- it's that immutability imposes limitations in the way that components can maintain local references to other components in the context of the composite system global state management pattern Stuart is describing.
If you want to use this pattern and have components that have circular dependencies on each other, those components better have mutable initialization operations. If they don't, the circular dependencies will need to be factored out. Jonah == Longer answer with an example- Stuart's original insight, documented here [1] and referenced in that post has to do with keeping all global system entities- like a database connection, a routing table, a cache, a configuration object, etc, anything that would be a singleton in a generic java application- in a single composite data structure (map, record, vector, etc) that itself can be a local variable, rather than storing each component individually as a global var in a namespace, as has been the informal convention in much open source Clojure application code. This single composite data structure pattern allows for a clean code reloads and application state reconstruction at the repl, easy testability, and a host of other benefits. However, the pattern is not without its complications. Once you make a commitment to working in accordance with this pattern, you run into the situation where some of those individual singleton components have lifecycle requirements, where after being created/constructed, they need further setup to be made ready for use. A database has to be connected to. A cache has to be started and given a backing store. A configuration object has to be loaded with data. In that situation you then run into the problem of managing the sequencing of and dependencies between those singleton lifecycle operations. Perhaps the configuration object has to be loaded with data and then given to the database which needs to be opened, and then the opened database connection needs to be given to the cache, which fronts the database. And whatever is done at initialization time to get all those components ready probably has to be undone in the reverse order when the system is shutting down. Finally, the last piece that adds complexity to the mix is immutability. >From an immutability perspective, the result of a lifecycle initialization operation on a component should be a new version of that component- even if in practice the initialization of the particular component under the covers is some mutable operation. But immutability leading to the expected creation of a new version creates issues when there are dependencies between the components. For instance, say that the configuration object has to know about the cache, because other objects expect to get the cache through configuration. Then say the cache has to know about the database, because it has to know about its backing store. But then the database may have to know about the configuration object, because that's where it gets credentials and connection URIs from. With immutability, this cyclical knowledge dependency is impossible to implement, as the new instance of each individual component as it is updated with the knowledge of a predecessor dependent and is initialized, or "started", obviates the reference that a successor dependent was initialized or "started" with. When individual component initialization is a mutable operation, the cyclical update problem goes away- but there is still a bit of code smell because the pattern of preserving the results of what looks to be an immutable operation isn't maintained. In any event, this is what Stuart is talking about when he makes the distinction between components knowing about each other in the "constructed" forms- as an unconnected database, a non-started cache, etc- and their "started" forms- their post-initialization state of being connected to database or as a cache that is ready for work. Hope that helps. 1. http://thinkrelevance.com/blog/2013/06/04/clojure-workflow-reloaded On Fri, Sep 20, 2013 at 1:21 PM, Murtaza Husain < murtaza.hus...@sevenolives.com> wrote: > Hi, > > I am going through the blog of Stuart Sierra regarding lifecycle > composition. http://stuartsierra.com/2013/09/15/lifecycle-composition > > In the end there is this para - > > "This technique is a form of dependency injection through constructors. > The choice of whether to make the individual components mutable, > stateful objects has an impact on how I can use them later on. In the > original version of this pattern using mutable objects, each component > gets stable references to other components it depends on. In the later > version using immutable data structures, each component gets > references to the *constructed* versions of other components it > depends on, but not the *started* versions of those components, > i.e. the values returned from start." > > I am unable to discern the pros and cons of both the versions > (approaches). Any concrete examples will be appreciated. > > Thanks, > Murtaza > > -- > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
