Table of Contents Topics Performance Fault Tolerance Consistency Distributed Communication (Silberschatz Ch.15) Sockets RPC RPC semantics

• Distribution can hurt: network B/W and latency bottlenecks
  • lots of tricks: e.g. caching, threaded serves
• Distribution can help: parallelism, pick server near client
• Idea: scalable design
  • Nx servers -> Nx total performance
• Need a way to divide the load by N
  • device the state by N
    • split by user
    • split by file name
    • "sharding" or "partitioning"
• Rarely perfect -> only scales so far
  • Global operations: e.g. search
  • Load imbalance
    • One very active user
    • One very popular file
      • one server 100%, added servers mostly idle
      • Nx servers -> 1X performance

• Dropbox: ~10,000 servers: some fail
• Can I use my files if there's a failure?
  • Some part of the network, some set of servers
• Maybe: replicate the data on multiple servers
  • Perhaps client sends every operation to both
  • Maybe only needs to wait for one reply
• Opportunity: operate from two "replicas" independenty if partitioned
• Opportunity: can 2 servers yield 2x availability AND 2x performance?

• "contract with apps/users" about meaning of operations
  • e.g. "read yields most recently written value"
  • hard due to partial failure, replication/caching, concurrency
• Problem: keep replicas identical
  • If one is down, it will miss operations
    • Must be brought up to date after reboot
  • If net is broken, *both* replicas maybe live, and see different ops
    • Delete file, still visible via other replica
    • "split brain" -- usually bad
• Problem: clients may see updates in different orders
  • Due to caching or replication
  • I make grades.txt unreadable, then TA writes grades to it
  • What if the operations run in different order on different replicas?
• Consistency often hurts performance (communication, blocking)
  • Many systems cut corners -- "relaxed consistency"
  • Shifts burden to applications

• socket: endpoint for communication
  • a pair of (how about group?) processes communicating over a network employs a pair of sockets
  • socket: IP address + port

• SETUP
  • WLOG, there are only two machines: M0 and M1
  • one-caller, one-callee
    • M0 calls FOO on M1
  • one-caller, two-callee
    • M0 calls FOO and BAR on M1
  • two-caller, one-callee
    • M0 calls FOO twice (from different thread)
• at-least-once semantics
• at-most-once semantics
• exactly-once semantics
  • at-most-once semantics + unbounded retries + fault-tolerant service