Patterns for the People

@KevlinHenney kevlin@curbralan.com

Habitability is the characteristic of

source code that enables programmers,

coders, bug-fixers, and people coming

to the code later in its life to

understand its construction and

intentions and to change it comfortably

and confidently.

Habitability makes a place livable, like

home. And this is what we want in

software — that developers feel at

home, can place their hands on any

item without having to think deeply

about where it is.

pattern

a regular form or sequence discernible in the way in which something happens or is done.

an example for others to follow.

a particular recurring design problem that arises in specific design contexts and presents a well-proven solution for the problem. The solution is specified by describing the roles of its constituent participants, their responsibilities and relationships, and the ways in which they collaborate.

Concise Oxford English Dictionary

Pattern-Oriented Software Architecture, Volume 5: On Patterns and Pattern Languages

I don't make stupid mistakes.

Only very, very clever ones.

John Peel

http://xkcd.com/612/

Failure is a far better teacher

than success.

Philip Delves Broughton

http://www.ft.com/cms/s/0/f33f5508-f010-11e0-bc9d-00144feab49a.html

If you want to learn how to build a

house, build a house. Don't ask

anybody, just build a house.

Christopher Walken

Programming is difficult

business. It should never

be undertaken in ignorance.

Douglas Crockford JavaScript: The Good Parts

Mark Pagel at the University of Reading, UK, doubts that hominins before Homo sapiens had what it takes to innovate and exchange ideas, even if they wanted to. He draws a comparison with chimps, which can make crude stone tools but lack technological progress. They mostly learn by trial and error, he says, whereas we learn by watching each other, and we know when something is worth copying.

http://www.newscientist.com/article/mg21328571.400- puzzles-of-evolution-why-was-technological-development-so-slow.html

Anti-patterns don't provide a resolution of forces as

patterns do, and they are dangerous as teaching

tools: good pedagogy builds on positive examples

that students can remember, rather than negative

examples. Anti-patterns might be good diagnostic

tools to understand system problems.

James Coplien, Software Patterns

Wise men profit more from fools than

fools from wise men; for the wise men

shun the mistakes of fools, but fools do

not imitate the successes of the wise. Cato the Elder

One of the hallmarks

of architectural design

is the use of idiomatic

patterns of system

organization. Many of

these patterns — or

architectural styles —

have been developed

over the years as

system designers

recognized the value

of specific

organizational

principles and

structures for certain

classes of software.

We know that every pattern is an instruction of the general form:

context conflicting forces configuration

So we say that a pattern is good, whenever we can show that it meets the following two empirical conditions:

1. The problem is real. This means that we can express the problem as a conflict among forces which really do occur within the stated context, and cannot normally be resolved within that context. This is an empirical question.

2. The configuration solves the problem. This means that when the stated arrangement of parts is present in the stated context, the conflict can be resolved, without any side effects. This is an empirical question.

Style is the art of getting yourself out of the way, not putting yourself in it.

David Hare

Some people, when confronted with a problem, think, "I know, I'll use threads," and then two they hav erpoblesms.

Ned Batchelder https://twitter.com/#!/nedbat/status/194873829825327104

Concurrency

Concurrency

Threads

Concurrency

Threads

Locks

Immutable Value

References to value objects are commonly distributed and

stored in fields. However, state changes to a value caused

by one object can have unexpected and unwanted side-

effects for any other object sharing the same value

instance. Copying the value can reduce the

synchronization overhead, but can also incur object

creation overhead.

Therefore:

Define a value object type whose instances are immutable.

The internal state of a value object is set at construction

and no subsequent modifications are allowed.

public class Date implements ... { ... public int getYear() ... public int getMonth() ... public int getDayInMonth() ... public void setYear(int newYear) ... public void setMonth(int newMonth) ... public void setDayInMonth(int newDayInMonth) ... ... }

public class Date implements ... { ... public int getYear() ... public int getMonth() ... public int getWeekInYear() ... public int getDayInYear() ... public int getDayInMonth() ... public int getDayInWeek() ... public void setYear(int newYear) ... public void setMonth(int newMonth) ... public void setWeekInYear(int newWeek) ... public void setDayInYear(int newDayInYear) ... public void setDayInMonth(int newDayInMonth) ... public void setDayInWeek(int newDayInWeek) ... ... }

public class Date implements ... { ... public int getYear() ... public int getMonth() ... public int getWeekInYear() ... public int getDayInYear() ... public int getDayInMonth() ... public int getDayInWeek() ... public void setYear(int newYear) ... public void setMonth(int newMonth) ... public void setWeekInYear(int newWeek) ... public void setDayInYear(int newDayInYear) ... public void setDayInMonth(int newDayInMonth) ... public void setDayInWeek(int newDayInWeek) ... ... private int year, month, dayInMonth; }

public class Date implements ... { ... public int getYear() ... public int getMonth() ... public int getWeekInYear() ... public int getDayInYear() ... public int getDayInMonth() ... public int getDayInWeek() ... public void setYear(int newYear) ... public void setMonth(int newMonth) ... public void setWeekInYear(int newWeek) ... public void setDayInYear(int newDayInYear) ... public void setDayInMonth(int newDayInMonth) ... public void setDayInWeek(int newDayInWeek) ... ... private int daysSinceEpoch; }

public final class Date implements ... { ... public int getYear() ... public int getMonth() ... public int getWeekInYear() ... public int getDayInYear() ... public int getDayInMonth() ... public int getDayInWeek() ... ... }

public final class Date implements ... { ... public int year() ... public int month() ... public int weekInYear() ... public int dayInYear() ... public int dayInMonth() ... public int dayInWeek() ... ... }

Copied Value

Value objects are commonly distributed and stored in

fields. If value objects are shared between threads,

however, state changes caused by one object to a value

can have unexpected and unwanted side effects for any

other object sharing the same value instance. In a multi-

threaded environment shared state must be synchronized

between threads, but this introduces costly overhead for

frequent access.

Therefore:

Define a value object type whose instances are copyable.

When a value is used in communication with another

thread, ensure that the value is copied.

class date { public: date(int year, int month, int day_in_month); date(const date &); date & operator=(const date &); ... int year() const; int month() const; int day_in_month() const; ... void year(int); void month(int); void day_in_month(int); ... };

class date { public: date(int year, int month, int day_in_month); date(const date &); date & operator=(const date &); ... int year() const; int month() const; int day_in_month() const; ... void set(int year, int month, int day_in_month); ... };

today.set(2014, 6, 4);

class date { public: date(int year, int month, int day_in_month); date(const date &); date & operator=(const date &); ... int year() const; int month() const; int day_in_month() const; ... };

today = date(2014, 6, 4);

Mutable

Immutable

Unshared Shared

Unshared mutable data needs no synchronisation

Unshared immutable data needs no synchronisation

Shared mutable data needs synchronisation

Shared immutable data needs no synchronisation

Shared memory is like a canvas where threads collaborate in painting images, except that they stand on the opposite sides of the canvas and use guns rather than brushes. The only way they can avoid killing each other is if they shout "duck!" before opening fire.

Bartosz Milewski "Functional Data Structures and Concurrency in C++"

http://bartoszmilewski.com/2013/12/10/functional-data-structures-and-concurrency-in-c/

Instead of using threads and shared memory

as our programming model, we can use

processes and message passing. Process here

just means a protected independent state

with executing code, not necessarily an

operating system process.

Languages such as Erlang (and occam before

it) have shown that processes are a very

successful mechanism for programming

concurrent and parallel systems. Such

systems do not have all the synchronization

stresses that shared-memory, multithreaded

systems have.

Russel Winder "Message Passing Leads to Better Scalability in Parallel Systems"

Multithreading is just one damn thing after, before, or simultaneous with another.

Andrei Alexandrescu

Actor-based concurrency is just one damn message after another.

Sender Receiver A

Message 1 Message 3

Receiver B

In response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message received.

http://en.wikipedia.org/wiki/Actor_model

A pattern’s audience

is ultimately always

human. Although a

developer may

support application

of a software pattern

solution through

libraries and

generators, it is the

developer and not the

technology that is

aware of the pattern.

History rarely happens in the right order or at the right time, but the job of a historian is to make it appear as if it did.

James Burke

http://upload.wikimedia.org/wikipedia/en/f/f0/Cave_of_time.jpg

James Siddle

"Choose Your Own Architecture" – Interactive Pattern Storytelling