Case Study: Distributed OS

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Case Study Distributed OS

• Distributed Systems, Lecture 17

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Amoeba OS

• What if we could afford hundreds of computers per
  user?
• Use a shared pool of servers
• These days this is called blade computing
• Grid Computing?
• Location independent
• Amake

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Architecture
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Micro-kernel

• Process and thread (kernel/user space) management
• IPC
• Low level I/O management
• Low level Memory management

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Client-Server Model

• Clients are written by users
• Servers are written by system programmers
• Essentially isolating faults, and reliability
  guarantees

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Objects

• Servers manage objects
• Clients can instantiate and access objects
• Objects protected by cryptographically protected
  capability
• Files
• Directories
• Memory Segments
• Screen Windows
• Processors
• Disks

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RPC Communication

• Primitive for location independence
• Stub acts as a proxy in the client address space
• Resolve name
• Marshal parameters
• Block for reply

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Bullet Soap Server

• Modular Architecture
• Naming
• File Manipulation
• Bullet File server
• Read-only file server
• Files can only created and deleted
• Not modified!
• Soap Directory Server
• Maps file names to capabilties

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Other Servers

• Object replication server
• Process maintenance
• Health-monitoring server
• Inter-operability servers

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Objects and Capabilities

• Objects make the system modular
• Secure
• Fault-tolerant
• More available

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Capabilities

• Naming as well as protection
• Managed in user space
• Rights management
• Can downgrade rights of a capability
• Takes original bits
• XOR with old rights
• Take a one-way hash

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Process Management

• Process is also an object
• Accessed by process descriptor capability
• Three-levels of interfaces
• RPC with process servers
• Library Calls
• Do-all dispatcher Run-server

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Memory Management

• Not paged or swapped
• Contiguous in memory
• Segments may be created, destroyed, read and
  written
• Simple and fast in an RPC model
• Overly restrictive

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Message passing

• Servers declare ports
• Get_port
• Identifies a service
• Clients put-ports in their messages
• At-most once semantics
• As opposed to atleast once

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Group Communication

• Closed groups
• Reliable Broadcast
• Sequencer model
• Lazy-ack
• Client broadcast vs Sequencer broadcast

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Layered Communication Model

• LAN
• IP
• FLIP
• Fast local internet protocol
• FLIP addresses are persistent and
  location-independent
• From the put-port to the FLIP address
• From the FLIP to the network address

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