8.2 Example: configuration parameters 示例：配置参数

Configuration parameters are an example of moving complexity upwards instead of down. Rather than determining a particular behavior internally, a class can export a few parameters that control its behavior, such as the size of a cache or the number of times to retry a request before giving up. Users of the class must then specify appropriate values for the parameters. Configuration parameters have become very popular in systems today; some systems have hundreds of them.

配置参数是提高复杂度而不是降低复杂度的一个示例。类可以在内部输出一些控制其行为的参数，而不是在内部确定特定的行为，例如高速缓存的大小或在放弃之前重试请求的次数。然后，该类的用户必须为参数指定适当的值。在当今的系统中，配置参数已变得非常流行。有些系统有数百个。

Advocates argue that configuration parameters are good because they allow users to tune the system for their particular requirements and workloads. In some situations it is hard for low-level infrastructure code to know the best policy to apply, whereas users are much more familiar with their domains. For instance, a user might know that some requests are more time-critical than others, so it makes sense for the user to specify a higher priority for those requests. In situations like this, configuration parameters can result in better performance across a broader variety of domains.

拥护者认为配置参数不错，因为它们允许用户根据他们的特定要求和工作负载来调整系统。在某些情况下，低级基础结构代码很难知道要应用的最佳策略，而用户则对其域更加熟悉。例如，用户可能知道某些请求比其他请求更紧迫，因此用户为这些请求指定更高的优先级是有意义的。在这种情况下，配置参数可以在更广泛的域中带来更好的性能。

However, configuration parameters also provide an easy excuse to avoid dealing with important issues and pass them on to someone else. In many cases, it’s difficult or impossible for users or administrators to determine the right values for the parameters. In other cases, the right values could have been determined automatically with a little extra work in the system implementation. Consider a network protocol that must deal with lost packets. If it sends a request but doesn’t receive a response within a certain time period, it resends the request. One way to determine the retry interval is to introduce a configuration parameter. However, the transport protocol could compute a reasonable value on its own by measuring the response time for requests that succeed and then using a multiple of this for the retry interval. This approach pulls complexity downward and saves users from having to figure out the right retry interval. It has the additional advantage of computing the retry interval dynamically, so it will adjust automatically if operating conditions change. In contrast, configuration parameters can easily become out of date.

但是，配置参数还提供了一个轻松的借口，可以避免处理重要问题并将其传递给其他人。在许多情况下，用户或管理员很难或无法确定参数的正确值。在其他情况下，可以通过在系统实现中进行一些额外的工作来自动确定正确的值。考虑必须处理丢失数据包的网络协议。如果它发送请求但在一定时间内未收到响应，则重新发送该请求。确定重试间隔的一种方法是引入配置参数。但是，传输协议可以通过测量成功请求的响应时间，然后将其倍数用于重试间隔，自己计算出一个合理的值。这种方法降低了复杂性，使用户不必找出正确的重试间隔。它具有动态计算重试间隔的其他优点，因此，如果操作条件发生变化，它将自动进行调整。相反，配置参数很容易过时。

Thus, you should avoid configuration parameters as much as possible. Before exporting a configuration parameter, ask yourself: “will users (or higher-level modules) be able to determine a better value than we can determine here?” When you do create configuration parameters, see if you can compute reasonable defaults automatically, so users will only need to provide values under exceptional conditions. Ideally, each module should solve a problem completely; configuration parameters result in an incomplete solution, which adds to system complexity.

因此，您应尽可能避免使用配置参数。在导出配置参数之前，请问自己：“用户（或更高级别的模块）是否能够确定比我们在此确定的更好的值？” 当您创建配置参数时，请查看是否可以自动计算合理的默认值，因此用户仅需在特殊情况下提供值即可。理想情况下，每个模块都应完全解决问题。配置参数导致解决方案不完整，从而增加了系统复杂性。