Distributed systems use multiple computers that interact over a network to achieve common goals like scalability and high availability. They work to handle increasing loads by either scaling up individual nodes or scaling out by adding more nodes. However, distributed systems face challenges in maintaining consistency, availability, and partition tolerance as defined by the CAP theorem. Techniques like caching, queues, logging, and understanding failure modes can help address these challenges.

1. Distributed Systems
scalability and high availability
Renato Lucindo - lucindo.github.com - @rlucindo

2. Renato Lucindo
Call me Lucindo (or Linus)
2002 - Bachelor Computer Science
2007 - M.Sc. Computer Science (Combinatorial
Optimization)
7+ year developing Distributed Systems
My default answer: "I don't know."

3. Agenda
Scalability
High Availability
Problems
Tips and Tricks
Learning More

4. Distributed Systems
Multiple computers that interact with each other over a
network to achieve a common goal
Purpose
Scalability
High availability
source: http://www.cnds.jhu.edu/

5. Scalability
System ability to handle gracefully a growing amount of
work
Scale up (vertical)
Add resources to a single node
Improve existing code to handle more work
Scale out (horizontal)
Add more nodes to a system
Linear (or better) scalability

6. Scalability - Vertical
Add: CPU, Memory, Disks (bigger box)
Handling more simultaneous:
Connections
Operations
Users
Choose a good I/O and concurrency model
Non-blocking I/O
Asynchronous I/O
Threads (single, pool, per-connection)
Event handling patterns (Reactor, Proactor, ...)
Memory model?
STM

7. Scalability - Vertical
Careful with numbers
Requests per second
# of Connections
Simultaneous operations
Event handling
Think front-end
Slow connections/clients
It's slower than other options
In doubt, go async
Back-end
Thread pool (thread per-connection)
No events
Process per-core

8. Scalability - Horizontal
Add nodes to handle more work
Front-end
Straightforward
Stateless
Back-end
Master/Slave(s)
Partitioning
DHT
Volatile Index

9. Scalability - Horizontal
Master/Slave
Write on single Master
Read on Slaves (one or more)
Scales reads

10. Scalability - Horizontal
Partitioning (Sharding)
Distribute dada across nodes
Generally involves data de-normalization
Where is some specific data?
Master Index
Hash (DTH, Consistent Hashing)
Volatile Index
Joins done in application level
NoSQL friendly

11. Scalability - Horizontal
Volatile Index: build and maintain data index as cached
information (all clients)

12. High Availability
"Processes, as well as people, die"
Handle hardware and software failures
Eliminate single point of failure
Redundancy
Failover
Replicas

13. High Availability - Failover/Redundancy

14. High Availability - Replicas
Two or more copies of same data
Replica granularity
From node replica to "row" replica
Load balancing
Write concurrency
Replica updates
Key for high availability and root of several problems

15. Problems

16. Problems - CAP Theorem

17. Problems - CAP Theorem
Consistency: all operations (reads/writes) yield a global
consistent state
Availability: all requests (on non-failed servers) must have
a response
Partition Tolerance: nodes may not be able
to communicate with each other.
Pick Two

18. Problems - CAP Theorem
C + A: network problems might stop the system
Examples:
Oracle RAC, IBM DB2 Parallel
RDBMS (Master/Slave)
Google File System
HDFS (Hadoop)

19. Problems - CAP Theorem
C + P: clients can't always perform operations
Examples:
Distributed lock-systems: Chubby, ZooKeeper
Paxos protocol (consensus)
BigTable, Hbase
Hypertable
MongoDB

20. Problems - CAP Theorem
A + P: clients may read inconsistent (old or undone) data
Examples:
Amazon Dynamo
Cassandra
Voldemort
CouchDB
Riak
Caches

21. Problem with CAP Theorem
In practice, C + A and C + P systems are the same.
C + A: not tolerant of network partitions
C + P: not available when a network partition occurs
Big problem: network partition
Not so big (how often does it happens?)
Pick two
Availability
Consistency
The forgotten: Latency
Or, how long the system waits before considering a
partitioned network?

22. Problems - Real World
Every component may fail:
Network failure
Hardware failure
Electricity
Natural disasters
Code failure

23. Tips & Tricks

24. Tips & Tricks - Pyramid
Capacity (connections, operations, ...) Pyramid

25. Tips & Tricks - Reply Fast
FAIL Fast
Break complex requests into smaller ones
Use timeouts
No transactions
Be aware that a single slow operation or component can
generate contention
Self-denial attack

26. Tips & Tricks - Cache
Cache: component location, data, dns lookups, previous
requests, etc
Use negative cache for failed requests (low expiration)
Don't rely on cache
Your system must work with no cache

27. Tips & Tricks - Queues
Easy way to add asynchronous processing an decouple
your system.

28. Tips & Tricks - DNS

29. Tips & Tricks - Logs
Log everything
Use several log levels
On every log message
User
Request host
Component involved
Version
Filename and line
If log level not enabled do not process log message
Avoid lookup calls (gettimeofday)

30. Tips & Tricks - Domino Effect
Make sure your load balancer won't overload components
User smart algorithms
Load Balance
Resource Allocation

31. Tips & Tricks - (Zero) Configuration
No configuration files
Use good defaults
Auto-discovery (multicast, gossip, ...)
Make everything configurable
Administrative command
No need to stop for changes
Automatic self adjusts when possible

32. Tips & Tricks - STOP Test
With your system under load: kill -STOP <component>

33. Tips & Tricks - Know your tools
load average (uptime)
stats tools
vmstat
iostat
mpstat
tcpstat, tcprstat, etc
tcpdump, nc, netstat
tunning
/proc/net/*
ulimit
sysctl
oprofile
debuging tools (gdb, valgrind)
...

34. Tips & Tricks - Count
Count everything
Connections
Operations
Failures
Successes
Request times (granularity)
Total, average, standard deviation
Monitor counters

35. Tips & Tricks - Stability Patterns
Use Timeouts
Circuit Breaker
Bulkheads
Steady State
Fail Fast
Handshaking
Test Harness
Decoupling Middleware

36. Tips & Tricks - Don't Panic!

37. Learning More - Books
TCP/IP Illustrated, Vol. 1: The Protocols

38. Learning More - Books
Unix Network Programming, Vol. 1: The Sockets Networking

39. Learning More - Books
Pattern Oriented Software Architecture, Vol. 2

40. Learning More - Books
Release It!

41. Learning More - Papers
The Google File System
Bigtable: A Distributed Storage System for Structured Data
Dynamo: Amazon's Highly Available Key-Value Store
PNUTS: Yahoo!’s Hosted Data Serving Platform
MapReduce: Simplified Data Processing on Large Clusters
Towards robust distributed systems
Brewer's conjecture and the feasibility of consistent,
available, partition-tolerant web services
BASE: An Acid Alternative
Looking up data in P2P systems

42. Thanks!!! Questions?
lucindo.github.com - @rlucindo