about:navigating_principle_languages
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about:navigating_principle_languages [2013-09-08 17:18] – Traversing a PL christian | about:navigating_principle_languages [2013-09-16 17:27] (current) – decision christian | ||
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When making a design decision based on principles, it is necessary to find those principles which fit to the given design problem. This means the [[glossary: | When making a design decision based on principles, it is necessary to find those principles which fit to the given design problem. This means the [[glossary: | ||
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- | ===== Characterizing Sets ===== | ||
- | |||
- | A principle language cannot free the designer from actually taking a decision. There is no algorithm which replaces a sound judgment. Rather principle languages point to the relevant aspects to consider. For a given design problem, the result is a characterizing set of principles which describes the dimensions of the design space, i.e. the advantages and disadvantages of possible solutions. A typical characterizing set has around five principles. | ||
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- | There are two typical questions which can be answered using a characterizing set: | ||
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- | - Given a design problem and a solution, is the solution good? | ||
- | - Given a design problem and several solutions which one is the best, i.e. the appropriate one? | ||
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- | In the first case the solution can be rated according to the principles in the characterizing set. If the benefits justify the liabilities, | ||
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- | For the second question all the solutions are rated. The solutions which are not pareto-optimal (see [[conflicting principles]]) are clearly bad solutions. The others are all " | ||
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- | ===== Weighting Principles ===== | ||
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- | Note that it's not the number of principles which is essential. Weighting the principles and taking the decision is still the task of the designer. Sometimes the weights may be derived from the requirements. Where this is not possible the weighting is an expression of the personal style of the designer, the team or the project. | ||
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- | The [[principles: | ||
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- | Certainly some people will rather favor KISS and others will prefer DRY. Some will say one duplication is tolerable and you should only refactor when there are three similar pieces of code (see [[principles: | ||
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===== Example ===== | ===== Example ===== | ||
+ | ==== Context ==== | ||
+ | |||
+ | The following---rather sophisticated---example shows the usage of the OOD Principle Language. It details the assessment of a solution found in the CoCoME system((http:// | ||
+ | |||
+ | In CoCoME there is a mechanism for getting access to other components. In a nutshell it works like this: | ||
+ | * For the data layer there is a data component, a class '' | ||
+ | |||
+ | <code java> | ||
+ | public class DataImpl implements DataIf | ||
+ | { | ||
+ | public EnterpriseQueryIf getEnterpriseQueryIf() | ||
+ | { | ||
+ | return new EnterpriseQueryImpl(); | ||
+ | } | ||
+ | |||
+ | public PersistenceIf getPersistenceManager() | ||
+ | { | ||
+ | return new PersistenceImpl(); | ||
+ | } | ||
+ | |||
+ | public StoreQueryIf getStoreQueryIf() | ||
+ | { | ||
+ | return new StoreQueryImpl(); | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | * There are " | ||
+ | * The " | ||
+ | |||
+ | <code java> | ||
+ | public class DataIfFactory | ||
+ | { | ||
+ | private static DataIf dataaccess = null; | ||
+ | |||
+ | private DataIfFactory() {} | ||
+ | |||
+ | public static DataIf getInstance() | ||
+ | { | ||
+ | if (dataaccess == null) | ||
+ | { | ||
+ | dataaccess = new DataImpl(); | ||
+ | } | ||
+ | return dataaccess; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | Essentially '' | ||
+ | [[factory: | ||
+ | |||
+ | ==== Question ==== | ||
+ | |||
+ | //Is this a good solution?// | ||
+ | |||
+ | ==== Finding a Characterizing Set ==== | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | We will examine this question using the OOD principle language. First we have to find suitable starting principles. This is one of the rather sophisticated cases where finding a starting principle is at least not completely obvious. If we don't have a clue where to start, we'll have a look at the different categories of principles in the language. Essentially the " | ||
+ | |||
+ | Now we'll have a look at the relationships. LC lists [[principles: | ||
+ | |||
+ | * KISS | ||
+ | * The mechanism may be considered simple or complex, so KISS qualifies | ||
+ | * We'll insert it into the characterizing set: {LC, KISS} | ||
+ | * HC | ||
+ | * The mechanism does not have any influence on cohesion; HC does not qualify | ||
+ | * RoE | ||
+ | * The access to the data modules is gained implicitly, so RoE fits | ||
+ | * New characterizing set: {LC, KISS, RoE} | ||
+ | * TdA/IE | ||
+ | * Getting hold of a data module requires chained gets: '' | ||
+ | * Although we first did not think about TdA/IE, it qualifies; We'll add it: {LC, KISS, RoE, TdA/IE} | ||
+ | * MP | ||
+ | * There is no obvious model involved | ||
+ | * IH/E | ||
+ | * Does not quite fit. We might think about the data component (not) hiding some information but we can regard that a bit far-fetched. | ||
+ | * DIP | ||
+ | * Does not qualify let alone replace LC. | ||
+ | |||
+ | |||
+ | So up until now the characterizing set is {LC, KISS, RoE, TdA/IE}. Now let's examine the relationships of the newly added principles. KISS lists [[principles: | ||
+ | |||
+ | * GP | ||
+ | * Does not qualify. | ||
+ | * ML | ||
+ | * Certainly it is relevant to think about what can go wrong, so ML qualifies. | ||
+ | * {LC, KISS, RoE, TdA/IE, ML} | ||
+ | * MP | ||
+ | * Already examined above. | ||
+ | * MIMC | ||
+ | * KISS fits better | ||
+ | |||
+ | Characterizing set up until now: {LC, KISS, RoE, TdA/IE, ML}. ML was newly added. Maybe on this point we might decide to abort the process because we already have a good idea of the aspects. But for the sake of the example, we'll continue with the relationships of ML. The wiki page lists KISS as a contrary principle and [[principles: | ||
+ | |||
+ | * KISS | ||
+ | * Already examined above. | ||
+ | * DRY | ||
+ | * There is no duplication. DRY does not qualify. | ||
+ | * EUHM | ||
+ | * ML fits better. | ||
+ | * UP | ||
+ | * Whether UP qualifies or not, depends on the rest of the system, the team, other projects, etc. For the example, we'll assume that there are no other projects and parts of the system we want to be consistent with. | ||
+ | * IAP | ||
+ | * Does not fit better and adds no valuable aspect. | ||
+ | |||
+ | As a result we get {LC, KISS, RoE, TdA/IE, ML} as the characterizing set. | ||
+ | |||
+ | Note that although in this example the principles are examined in a certain order, the method does not prescribe any. | ||
+ | |||
+ | |||
+ | ==== Using the Characterizing Set ==== | ||
+ | |||
+ | In order to answer the above question, we have to informally rate the solution based on the principles of the characterizing set: | ||
+ | |||
+ | * LC | ||
+ | * The solution creates a relatively strong coupling to the concrete implementations of the components. If a class uses the " | ||
+ | * KISS | ||
+ | * The solution is pretty easy to implement. Furthermore it is easy to get access to an arbitrary component. So according to KISS this is a good solution. | ||
+ | * RoE | ||
+ | * Getting access to a component is implicit. There is no need to explicitly pass a reference around. There is not even the necessity to explicitly define an attribute for the dependent class. RoE tells, that the solution is bad. | ||
+ | * TdA/IE | ||
+ | * Getting access to a the '' | ||
+ | * ML | ||
+ | * There are no particular pitfalls with this solution. So ML has nothing against it. | ||
+ | |||
+ | So LC, RoE and TdA/IE are against the solution, KISS thinks it's good and ML has nothing against it. As it is not the number of principles which is important, the designer still has to make a sound judgment based on these results. What is more important: Coupling, testability, | ||
+ | |||
+ | ==== Deciding between Alternatives ==== | ||
+ | |||
+ | In the next step we would think about better alternatives and might come up with [[patterns: | ||
+ | |||
+ | We already constructed a characterizing set. So the only thing to do is to rate the ideas according to the principles: | ||
+ | |||
+ | The current " | ||
+ | |||
+ | * LC | ||
+ | * DI > SL > F | ||
+ | * Note that in the SL approach there is an additional coupling to the service locator | ||
+ | * KISS | ||
+ | * F > DI = SL | ||
+ | * All three solutions are rather simple but in DI there is complexity for passing around the references and in the SL approach there is complexity in maintaining the registry | ||
+ | * RoE | ||
+ | * The rating of RoE depends on the concrete variant of the pattern. In the DI approach the dependencies are explicitly visible on the interface, which is not the case in the two other approaches. In solution F the dependency is not visible from the interface at all. Same with SL if the service locator is globally accessible. Even if a reference to the service locator is explicitly passed around, it is still not visible which services provided by the locator are used. On the other hand getting a reference is explicit with F and SL. In the DI approach it is only explicit when it is done manually. Typical DI frameworks wire the instances implicitly. | ||
+ | * TdA/IE | ||
+ | * DI > SL = F | ||
+ | * For SL and F one first has to ask for an instance for calling a method on it. In DI the instance is already known, i.e. set from the outside. | ||
+ | * ML | ||
+ | * F > DI > SL | ||
+ | * In the DI solution a possible fault would be to have different modules reference different instances of the same class where they should rather reference the same instance. In SL solution there is even a more problematic fault which could be introduced. Eventually somebody might get the idea to change the registered instances in the locator at runtime. This would then be the source for some hard to find defects: Some modules will cache the instance they got from the service locator in an attribute and some won't. In such a case the latter will receive the new instance while the former won't. | ||
+ | |||
+ | As you can see all three possibilities have their advantages and disadvantages. The designer now has to weight the aspects in order to get to a decision. In this case we might state the following: | ||
+ | |||
+ | * F is ruled out because it is not testable. The other two approaches have lower couplings (LC) which make them better testable. The advantages wrt. KISS and ML do not justify that liability. | ||
+ | * The solutions DI and SL are not very far apart but DI is slightly better wrt. LC, TdA/IE and ML. | ||
+ | * TdA/IE can be regarded less important because it is a heuristic which is normally applied in other situations. | ||
+ | * For RoE we also have to decide whether to use a framework or not. In CoCoME we would rather want to avoid a framework because the rest of the system is implemented in that way ([[principles: | ||
+ | |||
+ | Based on this weighting, we decide to use DI. |
about/navigating_principle_languages.1378653493.txt.gz · Last modified: 2013-09-08 17:18 by christian