What You Can Do By The End

• Define the three words in your own words
• Classify real services by machines and by control
• Name the three axes of decentralization
• Weigh trade-offs of each design
• Decide when a central server is the right choice

Two Questions Behind Every System

• Where do the computers run, and how do they connect?
• Who decides what the system does?
• Three words answer these in different ways

Baran'S Three Pictures

• Paul Baran, RAND Corporation, 1964
• Centralized: one hub, many spokes
• Decentralized: several hubs linked together
• Distributed: a net with no special center

Centralized     Decentralized      Distributed

   o o o          o-o   o-o         o-o-o-o
    \|/            \|     |/         |\|/|/|
  o--X--o        o--X---X--o        o-o-o-o
    /|\            /|     |\         |/|\|\|
   o o o          o-o   o-o         o-o-o-o

Centralized: One Place Decides

• One party holds the data and sets the rules
• Example: a school registrar, a social media company
• One source of truth for everyone
• Easy to build and to change

Where Centralized Designs Hurt

• The center fails, everyone loses service
• The owner changes the rules or closes accounts
• An attacker who enters the center reaches everything
• Users must trust one party completely

Decentralized: Control Is Shared

• Several independent parties each hold part of the control
• Shared rules (protocols) let them work together
• Example: email across many providers
• A matter of degree, not a yes or no

Distributed: Many Machines, One Job

• Computers in different places act as one system
• Describes machines, not owners
• A big video service is distributed but centrally owned
• Bitcoin is distributed and decentralized

Four Combinations

• One server, one owner: centralized
• Many data centers, one owner: distributed, centralized control
• Many owners, shared rules: decentralized
• Many owners, many machines: both

                  one owner        many owners
one machine       small web shop   (rare)
many machines     big tech cloud   Bitcoin, email

Three Axes Of Decentralization

• Architectural: how many machines can fail
• Political: how many owners control them
• Logical: one shared state or separate parts
• A system can score high on one and low on another

The Trade-Offs

• Centralized: fast, cheap, simple, one point of failure
• Decentralized: shared power, harder to switch off
• Distributed: scales and survives hardware loss
• The price is complexity and slower agreement

Why People Want Decentralization

• Censorship resistance
• Uptime when one operator fails
• Users keep their own data and keys
• Credible neutrality: rules nobody can quietly change

When A Central Server Wins

• Everyone already trusts one operator
• Speed and low cost matter most
• Rules must change often
• Legal accountability must sit with one party

Five Questions For Any System

• Who runs the machines?
• Who can change the rules?
• What happens if one machine fails?
• What happens if one operator turns hostile?
• Who can see the data?

Recap

• Distributed describes machines, decentralized describes control
• Decentralization has three axes and many degrees
• Each design has real strengths and costs
• Pick the design by the threat you face

Next Week

• Distributed systems fundamentals
• Networks, latency and failures
• The CAP theorem and eventual consistency

Questions And Exercise

• Classify five services you use
• Fill the table of machines, control and failure
• Score each on the three axes
• Bring the table to week 2