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Improving Robustness In Distributed Systems
 

Improving Robustness In Distributed Systems

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    Improving Robustness In Distributed Systems Improving Robustness In Distributed Systems Presentation Transcript

    • Improving Robustness in Distributed Systems Per Bergqvist [email_address] Erlang User Conference 2001 (courtesy CellPoint Systems AB)
    • Design base
      • Cluster of cooperating hosts
      • Erlang and C
      • COTS hardware based
      • Unix based (i.e. Solaris or Linux)
      • 10/100/1000 base-T back plane (”system area network”)
    • Cluster
      • Shared, distributed, system configuration
      • Each host have ONE cluster controller
      • Dispatch and supervise worker tasks
      • Master cluster controller: holds configuration database (persistent replica)
      • Slave cluster controller: gets configuration from master cluster controllers
      • Cluster is DOWN when all master cluster controllers are inaccessible
    • Typical system Firewall Switch Traffic Control
    • Cluster Key Benefits
      • Single system view
      • Enforces decoupling of parts of O&M from actual traffic processing
    • Implementing a cluster
      • Cluster->Host->Node->NodeData
      • Cluster global parameters
      • Subscription mechanisms for conf. changes
      • Mnesia as configuration database on master cluster controllers
      • Homebrewn configuration distribution to slave controllers (NOT using mnesia)
      • (Worker) node supervision
    • Mnesia gotchas
      • First distributed node startup
        • Disallow writes when all replicas not accessible
        • Use timeout on table load and force load
    • ... BUT ...
      • TCP based distribution
      • Network partitioning
    • Network parameters
      • Align TCP retransmission intervals w/ Erlang heartbeats
      • Align TCP and IP rerouting parameters
    • Typical system II: Dual back plane Firewall Switch Traffic Control
    • Erlang multi-homing problem Host A Host B Host C
    • Multi-home Erlang w/ TCP
      • Add an alias interface to loop back i/f
      • Patch tcp distribution to bind to alias
      • Publish alias interface on (all wanted) via real hw i/f’s
        • Method 1: Static routes and gratuitous/proxy arp
        • Method 2: Use new (routing) protocol
    • ARP method
      • Implement a utility to: - broadcast unsolicited ARP responses - respond to ARP requests for the alias i/f address
      • Add static routes on all far end systems
      • NOTE: all real i/f needs to be on same IP subnet
    • New routing protocol
      • Broadcast (Ethernet frames) what you have, including interface priority
      • Let the far end select path based on what/when they receive
      • Far end dynamically sets up host routes
      • Use short retransmission intervals
    • Erlang multi-homing resolved ? Host A Host B Host C
    • Summing up
      • Erlang can support multihoming with some additional work
      • By using loop back alias i/f, link failure becomes a routing problem (peer-peer association is kept intact)
      • Solaris TCP/IP stack parameters are: - hard to find (only in out-of-date app. notes) - hard to set ”right” - host global
      • A distribution mechanism with built-in support for multi-homing preferred
    • Erlang Distribution over SCTP Per Bergqvist et al [email_address] Erlang User Conference 2002