Reversible Engineering


Reverting Time for Fun and Profit

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Alex Fernández

Physicist by education

Software engineer by trade

@pinchito, alexfernandez, alejandrofer

Ground to Cover


Reversibility and Irreversibility


Software Development


Software Architecture


DevOps


Decision Making

First, What Not to Expect


Not reversible computing


Not reverse engineering


Not thermodynamics

nor information theory


Not hard-core science

Reversibility

and Irreversibility

Entropy is not...


heat

energy

chaos

disorder

irreversibility

uncertainty

degradation

complexity

Reversibility


Want to know what "entropy" is?


Who cares, just worry about reversibility!


A reversible system:

  • always has minimum entropy
  • has the most efficient process
  • minimizes complexity

The Reversible Process


Is the most efficient process


Can be reversed in time

using no extra energy


Can work in either direction


Takes the system back to its initial state

with no side effects

Example: a Perfect Pendulum


Always near the stable point

Problems of Thermodynamic Reversibility


The process must be infinitely slow


Cannot have side effects:

no way to know it has happened


Oh and by the way:

it is impossible in the real world

Some Ideas about Reversibility


Efficiency



Stability



Predictability



Slowness

Irreversibility

Is inefficient (but necessary)

The other Reversible Process


Can be reversed in time

(at least for our system)


Can work in either direction


Takes the system back to its initial state

with some side effects


Uses an efficient process

Example: Management Application


CRUD:

  • Create
  • Read
  • Update
  • Delete


Create ↔ Delete

Update

Read

Example: Testing a CRUD App


  • First create a random key
  • Create a record
  • Read the record
  • Then modify the record
  • Read the record again
  • Finally delete the record
  • Verify that the record is deleted


Self-contained test: leaves the system intact

But the tests have side effects

The World is Irreversible

Deal with it

Some Ideas about IrReversibility


Speed



Turbulence



Chaos



Complexity

Reversible

Development

The Reversible Machine


Works in cycles


Each cycle costs some effort


Each cycle can be reversed

at a small cost


Small cycles are better than big cycles

An Analogy



Example: Version Control


CVS stores changes in a log file


SVN stores revisions as diffs


Git stores history in deltas

Easy to go back to any previous HEAD

Easy to revert a previous commit

Easy to truncate history and reset

Example: Undo


GUI application with commands


Each command can be undone

... and redone


Much easier to try out changes

The Development Process


Small cycles of write-compile-link-run


Bigger cycles of write-test-debug-fix


Large cycles of write-integrate-deploy


The smaller the cycle, the most efficient the process

Reversible

Architecture

The Stateless API


Every request is self-contained


No context is stored in the server


The client maintains the state of the client


Can be parallelized

The Stateless Server


Does not hold any state in memory

(except for cached data)


Can be reset at will


All information is dumped on external systems

(and in log files)

A Small Detour

Pile your heat onto an exhaust!

Information is Entropy


Information is not “negative entropy”

Entropy is the same as information!


Pile your information onto an “exhaust”


Information exhausts:

  • databases
  • files

Two Tier Architecture


Information exhaust in database
(and in log files)

API Evolution


Avoid breaking existing APIs

Please don't


Keep backwards compatibility

... and forwards compatibility!


Warn about deprecated functions


OK to remove if there are no users

What is missing


More compatibility shims


Forwards-backwards compatibility


Example: Python 2.x → 3.x migration

and 3.x → 2.x migration!

Reversible

Operations

Another view of reversibility

The reversible process is always at equilibrium

Testing Equilibrium


System is stable when it passes the tests

Testing


Don't let your system stray from equilibrium

It is much harder to get it back on track


Do your development in small steps


Pass your test suites often

Integration Environment


After integration should go back to its initial state


Should be repeatable


Ideal environment: created ad hoc

and then destroyed

like Travis-CI

Snapshots


Disk snapshots


OS snapshots


Cloud: ubiquitous snapshots


Allow immediate rollbacks to previous states

What is missing


Differential snapshots

Roll back one feature while keeping another

E.g.: rollback OS, keep applications


Version control for configuration files

Continuous Deployment


Put your code in production


Deploy in small increments


Immediate rollbacks


Automatic rollbacks

What Is Missing


Make it easier to go to a previous time

and then to go forward again!


Automatic rollbacks should be easier

and ubiquitous

Dark Deployment


Cycle is made of two parts


First deploy inactive code


Then activate feature


Both parts are reversible

Reversible

Management

Heroic efforts

... are irreversible by nature


A repeatable process can approach reversibility

Typical Management View


Lots of irreversible steps

Closed tasks are not revisited

Timeboxed Releases


Features are released only when ready


Tasks can be reopened if needed

and it is not a failure!


Features can be perfected


Releases focus on improvements

Release Process


Small releases better than big changes


E.g.: Linux 2.4.x -> 2.6.x: big ugly change

Linux 2.6.x -> 3.x: continuous improvement


Avoid releases if at all possible

Rolling releases are the best!


Making Decisions


Thus the key to avoiding big decisions becomes to just avoiding to do things that can’t be undone.  Don’t get ushered into a corner from which you cannot escape. A cornered rat may be dangerous - a cornered manager is just pitiful.

Linus Torvalds

Thanks!


https://slid.es/alexfernandez/reversible-engineering


All pictures from Wikimedia Foundation

published under CC license



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Reversible Engineering

By Alex Fernández

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