When you have to maintain a sprawling documentation set, the ability to reuse content can be a lifesaver. It can also be a disaster. The wall staving off disaster is strategy, consistency, and discipline. Without a good strategy, reuse becomes a rat's nest. If the writers are not consistent in applying the strategy, reuse creates snarls. If the writers are not disciplined, reuse exacerbates the problems.
The first thing that needs be done in forming a good reuse strategy is defining what reuse means. One definition of reuse is that content modules are used in multiple published documents. For example, a standard warning message is placed in a module and then that module is imported into any document that requires it. Another common definition of reuse that content is cloned (copied) to other modules where it may be useful. For example, the standard warning is simply pasted into all of the modules that require it.
In my mind the first definition of reuse is the one that knowledge set maintainers should aspire. It truly leads to reduction in workload and possibility for error. Writers only need to maintain a single copy of a module and when that module is updated, all importing modules take advantage of the update. The second definition, in contrast, saves writers some amount of work up front since they do not have to originate any content, but increases the maintenance load on the backend. A change in one of the cloned sections requires the writer to not only update the original, but to hunt down all of the copies, determine if the change is appropriate, and then make the update. It is more work and far more error prone.
The idea of cloning content is not without merit and does have a place in a solid reuse strategy, but by itself is not a solid reuse strategy. Cloning is useful when the content in a module is a close, but not exact, fit. For example, two products may use a common log-in module and have a similar log-in procedure. However, the username/password requirements may be very different or one of the products may require an additional step. In this case, it may make sense to maintain two copies of the log-in procedure.
Cloning is also routinely used to perform versioning. Each product library I work on has at least three versions. Each of these versions is a clone of the other versions. The entire collection of modules is cloned into a single version such that module A and module B will share the same version in an instance of a library instance. Trying to make an update to multiple version of a library will highlight the issues with cloning as a primary reuse strategy.
So, if cloning is not a useful primary reuse strategy what is? Reuse is complex and any strategy will require many tactics:
* constraining writing to make all of the content stylistically uniform
* wise use of variables
* wise use of conditional text
* sensible chunking rules
* open communication channels
* sensible cloning rules
* scoping rules
* clear versioning policies for shared content collections
Using variables and conditional text make it easier to share modules between products or in places where there are minor variations required in the content. They are useful for places where a product name changes or when two products use different version schemes. Conditional text can allow for slight variances in procedures. Variables and conditional text can have pitfalls as well. They can hinder translation and can get convoluted and hard to manage. When a module becomes too heavy with conditionals and variables, it might be a good idea to consider cloning.
One of the most important parts of a reuse strategy is the size of the modules allowed. They must be fine grained enough to maximize reuse. For example, a book is not a great a great module size since a books reusability is limited to one per library. The modules need to be large grained enough to maintain. For example, a phrase, or even a sentence, does not make a great content module because there would simply be too many of them to manage. I generally think that the DocBook section element is a good module delimiter. Sections are fine grained enough to be shared in multiple places in a library or set of libraries and rough grained enough to hold a useful amount of information. In case by case instances, tables, lists, admonitions, and examples also make good modules.
In situations where you are only dealing with one product library a strict versioning policy may not be critical. All of the modules will ostensibly share the same version for an entire library. However, if you are working in an environment where products share large components, it makes sense to have a strict and well understood policy in place about how component and product versions work. We currently have two products that share a number of common components and the products can at any one time be using different versions of the components. To handle this we version the documentation for each component independently of the products in which they are used. Each product imports the required version of the component sets and when a release is built tags are added to the component libraries to mark the product revision. This allows us to make on going updates to all of the content with reasonable assurance that we won't accidentally cross contaminate a product library. It does, however, add administrative overhead and require some extra caution on the part of the writers.
There is not a one-size-fits-all answer for how to implement these things. Every team has slightly different requirements, slightly different content, and slightly different capabilities. If your requirements put ease of translation over ease of reuse you will choose a different set or parameters. If your team is made up of newbies, you will choose a less strict set of parameters. Your tools will also alter the parameters you choose. The trick is to choose wisely and honestly.
Once you have chosen stick to the plan and look for ways to improve the process. Just know that once you have chosen, changing your mind becomes harder and harder. Reuse creates tangles and dependencies that are not easily unwound.