今天阅读到阮一峰前辈的“软件架构入门”一文，其中谈到“微核架构”有良好的功能延伸性（extensibility），和较差的扩展性（scalability）。
extensibility 和 scalability 在谷歌翻译里的注释都是“可扩展性”，但阮一峰前辈在文中对这两个词分别用延伸性和扩展性表述，这引起了我对这两个词的词义的好奇，于是写下这篇博文做一次探究和记录。
extensibility
通过 Collins English Dictionary 查阅到，extensibility 是 extensible 的派生形式，而 extensible 的释义是 capable of being extended 。于是继续检索 extend 的含义
1. 动词
If you say that something, usually something large, extends for a particular distance or extends from one place to another, you are indicating its size or position. # 延伸
The caves extend for some 18 kilometres. [VERB + for]
The main stem will extend to around 12ft, if left to develop naturally. [VERB + to]
Our personal space extends about 12 to 18 inches around us. [VERB amount]
Diyala extends from the suburbs of Baghdad to the Iranian border. [VERB from noun to noun]
The new territory would extend over one-fifth of Canada’s land mass. [VERB + over]
同义词： spread out, reach, stretch, continue extend 的更多同义词
2. 动词
If an object extends from a surface or place, it sticks out from it.
A shelf of land extended from the escarpment. [VERB + from]
3. 动词
If an event or activity extends over a period of time, it continues for that time.
…a playing career in first-class cricket that extended from 1894 to 1920. [VERB from noun to noun]
The courses are based on a weekly two-hour class, extending over a period of 25 weeks. [VERB + over]
同义词： last, continue, go on, stretch extend 的更多同义词
4. 动词
If something extends to a group of people, things, or activities, it includes or affects them.
The service also extends to wrapping and delivering gifts. [VERB + to]
The talks will extend to the church, human rights groups and other social organizations. [Vto n/-ing]
His influence extends beyond the TV viewing audience. [V + beyond]
5. 动词
If you extend something, you make it longer or bigger.
This year they have introduced three new products to extend their range. [VERB noun]
The building was extended in 1500. [be VERB-ed]
…an extended exhaust pipe. [VERB-ed]
同义词： widen, increase, develop, expand extend 的更多同义词
6. 动词
If a piece of equipment or furniture extends, its length can be increased.
… a table which extends to accommodate extra guests. [VERB]
The table extends to 220cm. [VERB + to]
7. 动词
If you extend something, you make it last longer than before or end at a later date.
They have extended the deadline by twenty-four hours. [VERB noun]
…an extended contract. [VERB-ed]
同义词： make longer, prolong, lengthen, draw out extend 的更多同义词
8. 动词
If you extend something to other people or things, you make it include or affect more people or things.
It might be possible to extend the technique to other crop plants. [VERB noun + to]
9. 动词
If someone extends their hand, they stretch out their arm and hand to shake hands with someone.
The man extended his hand: ‘I’m Chuck’. [VERB noun]
从释义里看出，extend 其实更接近中文意思里的延伸。
从什么地方延伸到什么地方，将什么东西延伸到什么状态。进而发展成主动地拉扯使物体伸长变大，拉扯时间使截止日期延后等，其他还有事物对人群或其他事物的影响，伸出手臂和别人握手。
于是到这里似乎可以判断 capable of being extended 的意思就是“延展伸长的能力”，那么 extensibility 自然就是“延展性”。
scalability
用同样的方法查到 scalability 的释义是 the ability of something, esp a computer system, to adapt to increased demands —— 某种东西（尤其是计算机系统）适应不断增长的需求的能力
scalable 的释义
1.
capable of being scaled or climbed
2. computing
(of a network) able to be expanded to cope with increased use
第 2 条用与计算机时的释义为 (of a network) able to be expanded to cope with increased use —— （网络）可以扩展以应对增加的使用需求
scale 的释义
1. 单数名词
If you refer to the scale of something, you are referring to its size or extent, especially when it is very big. # 指物体的规模或事物的程度，特别是当改事物已经非常大的时候
However, he underestimates the scale of the problem. [+ of]
You may feel dwarfed by the sheer scale of the place.
The break-down of law and order could result in killing on a massive scale.
The British aid programme is small in scale.
同义词： degree, size, range, spread scale 的更多同义词
2. See also full-scale, large-scale, small-scale # 全尺寸，大尺寸，小尺寸
3. 可数名词
A scale is a set of levels or numbers which are used in a particular system of measuring things or are used when comparing things. # 量级，单位，层次
…an earthquake measuring five-point-five on the Richter scale.
The patient rates the therapies on a scale of zero to ten.
The higher up the social scale they are, the more the men have to lose.
同义词： system of measurement, register, measuring system, graduated system scale 的更多同义词
4. See also sliding scale, timescale
5. 可数名词
A pay scale or scale of fees is a list that shows how much someone should be paid, depending, for example, on their age or what work they do.
[British]
…those on the high end of the pay scale.
A Registered Osteopath will be pleased to tell you his scale of fees before you decide on a consultation.
6. 可数名词
The scale of a map, plan, or model is the relationship between the size of something in the map, plan, or model and its size in the real world. # 比例，比例尺
The map, on a scale of 1:10,000, shows over 5,000 individual paths. [+ of]
同义词： ratio, proportion, relative size scale 的更多同义词
7. See also full-scale, large-scale
8. 形容词 [ADJECTIVE noun]
A scale model or scale replica of a building or object is a model of it which is smaller than the real thing but has all the same parts and features. # 等比例模型
Franklin made his mother an intricately detailed scale model of the house.
9. 可数名词
In music, a scale is a fixed sequence of musical notes, each one higher than the next, which begins at a particular note. # 调
…the scale of C major. [+ of]
10. 可数名词 [usually plural]
The scales of a fish or reptile are the small, flat pieces of hard skin that cover its body.
11. 复数名词 [oft a pair of NOUN]
Scales are a piece of equipment used for weighing things, for example for weighing amounts of food that you need in order to make a particular meal.
…a pair of kitchen scales.
…bathroom scales.
I step on the scales practically every morning.
同义词： weighing machine, balance, scale, weigh beam scale 的更多同义词
12. 动词
If you scale something such as a mountain or a wall, you climb up it or over it.
[written]
…Rebecca Stephens, the first British woman to scale Everest. [VERB noun]
The men scaled a wall and climbed down scaffolding on the other side. [VERB noun]
同义词： climb up, mount, go up, ascend scale 的更多同义词
13.
See out of scale
14.
See to scale
在 scale 的释义里可以看出，scale 主要强调事物的大小、规模。
于是 scalability 的释义似乎就是“可以按需扩展的能力”。
二者的对比
从上面的探究里，我的感性认识是，scalability 强调根据需求的增加，对原有事物的规模进行扩展；而 extensibility 则没有强调要根据需求的变化来使自身本体的规模扩大。
再结合实际工作中接触到的场景，extensibility 更多的出现在功能的增加，提升多样性的场景；extensibility 则更多出现在规模的动态扩展场景里，其本身的功能没有改变。
所以 extensibility 描述功能时，是添加不同功能的能力；scalability 则是规模的动态扩展能力，体现在文中即是分布式扩展能力。