Transcript XML in Corba Component Model (CCM)

The Time-Triggered Architecture
Krishnakumar B
[email protected]
Institute for Software Integrated Systems
Vanderbilt University, Nashville, TN
January 23rd, 2003
Krishnakumar B
The Time-Triggered Architecture
Outline of Talk
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Overview of TTA
Architecture Model
Design Principles
Communication
Fault Tolerance
Design Methodology
Questions ?
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Time-Triggered Architecture
• Treatment of physical time as a first-order quantity
• Provides fault-tolerant global time base
• Decomposes a large application into:
– Clusters
– Nodes
– Combination of both
• Use global time to specify interfaces between nodes
• Communication and agreement protocols
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Model of Time
• Time progresses along a dense timeline
• Duration – Interval delimited by two instants
• Event occurs at an instant
– E.g. Observation of state
• Time-stamping
– Assign state of node-local global time to event
• How to synchronize clocks ?
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Sparse Time Base
• Continuum of time is partitioned
• Infinite sequence of alternating durations of activity & silence
• Duration of the activity interval > precision of clock
synchronization
• All events that occur within an interval of activity considered
simultaneous
• External representation of time
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
RT Entities and RT Images
• TTA system
– Node, Communication Network Interface, Host
– Time domain and value domain
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
RT Entities and RT Images (Contd…)
• Real-Time Entities
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State variables used to model dynamics of system
Change their state as time progresses
Mix of both static and dynamic attributes
E.g Flow of a liquid in a pipe, Temperature of valve
• Observation
– State of RT Entity at a particular instant tobs
– Observation = <Name, Value, tobs>
• Real-Time Image
– Temporally accurate picture of RT entity at instant t
– Duration b/w time of observation and instant t < dacc
• Observation valid forever, not true of validity of image
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
State-Information vs Event-Information
• State attribute
– Property of a RT entity at a particular instant
• State Information
– (state variable, value, time of observation)
– Idempotent, atleast-once semantics
– Sender-side – Not consumed
– Receiver-side – Update-in-place, non-consuming read
• Event
– Sudden change of state of an RT Entity at an instant
• Event Information
– (state variable, value difference, time of event)
– Exactly-once semantics
– Sender-side – Consumed on sending
– Receiver-side – Queued and consumed on reading
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Structure of TTA
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Node
– Self-contained unit
Communication system
– Replicated channels
– Autonomous
– Executes periodically
– a priori TDMA schedule
Fetch Instant
– Reads state message from
CNI
Delivery instant
– Delivers it to CNI of all other
nodes of cluster
– Overwriting previous version
of state message
Fetch, delivery instants in
message scheduling table
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Interconnection topology
• TTA-bus
– Replicated passive buses
– Each node has 3
subsystems
• Node, 2 guardians
• Spatial proximity faults
• Fail-safe vs fail-operational
• TTA-star
– Independent guardians
– n+2 packages vs 3n
– Reshape physical signals
& resilient to Slightly-offspecification (SOS) faults
– Additional monitoring,
better EMI characteristics
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Design Principles of TTA
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Consistent Distributed Computing Base
Unification of Interfaces – Temporal Firewalls
Composability
Scalability
Transparent Fault Tolerance
Openness
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Consistent Distributed Computing Base
• Distributed algorithms dependent on consistent data
• TTA exploits short error detection latency of protocol
– Error-detection at protocol level
– Distributed agreement (membership) algorithm
• Checking membership of all nodes to ascertain correct
operation
• Detect faulty outgoing link
• Violation of fault-hypothesis
– Distributed agreement protocol unable to reach
conclusion
– Result: Clique avoidance algorithm is activated
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Unification of Interfaces – Temporal Firewalls
• Uni-directional data-flow interfaces
– Elementary – Uni-directional control flow
– Composite – Bi-directional control flow
• TTA CNI is an elementary interface
• Control-error propagation prevented by design
• Interface called temporal firewall
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Different Interfaces of a Node
• Real-Time Service (RS) Interface
– Provides timely real-time services to node environment
– Must satisfy temporal specification under all conditions
– Affects temporal composability
• Diagnostic & Maintenance (DM) Interface
– Opens channel to internals of a node
– Useful in configuring node parameters
– Retrieve node parameters for fault diagnosis
– Doesn’t affect temporal composability
• Configuration Planning (CP) Interface
– Connect node to other nodes of a system
– Used during integration phase to generate “glue”
– Not time critical
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Composability
• Independent development of nodes
– Differentiate between node and architecture design
– Precise specification of all node services => independent
design of nodes
• Stability of Prior services
– Validated service of a node should be unaffected by
integration of node into a system
• Constructive Integration
– n nodes already integrated => addition of n+1 doesn’t affect
previous n nodes
• Replica determinism
– All members have same externally visibile state
– Produce same output messages atmost d time units apart
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Scalability
• Complexity of system should not increase with growth of system
• In TTA, CNIs provides abstraction
– Encapsulate properties of environment
– Only essential properties available to nodes
• Example - Gateway nodes
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Transparent Fault-Tolerance
• Active redundancy by replication and voting
• Active replication is complex
– Shouldn’t be done at application level
• TTA provides dedicated Fault-Tolerance layer
– Fault-tolerant CNI (FTU-CNI)
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Openness
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Standardize interfaces
TTA interfaces submitted for standardization by OMG
Inter-operation with CORBA clients
RS, DM and CP interfaces available at the ORB level
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Communication
• Deliver information between CNIs
– Within interval delimited by fetch and delivery instants
• TTP/C Protocol
– Autonomous, fault-tolerant, TDMA based transport
– Fault-tolerant clock synchronization
– Membership service
• Inform every node about “health” of every other node
• Doubles as multicast acknowledgment
• Used in implementing fault-tolerant clock synchronization
– Clique avoidance to detect and eliminate the formation
of cliques when fault-hypothesis is violated
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Communication (contd…)
• TTP/A protocol
– Time-triggered field-bus protocol of TTA
– Connects low-cost smart transducers to a node of TTA
– Two types of rounds – Master/Slave (MS) & Multi-partner
(MP)
• MS – Read/write records from IFS to implement DM and CP
• MP – Periodic, implements the RS service
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Event Message Channels & Performance
• Event message channels
– Created by allocating portion of TT communication
– Push-pull model for events
– Filter service & Garbage collection service
• Performance of TTA
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Time distribution needs inter-frame gap of 5 μs
80% bandwidth utilization => 20 μs for send-phase
40,000 messages / second
10 clients => 250 μs sampling period => 4kHz loop
Amount of data
• 5 Mbps => 12 bytes / 20 μs
• 1 Gbps => 2400 bytes / 20 μs
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Fault Tolerance
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Fault Hypothesis
– States types and number of faults that the system should tolerate
TTA-star cluster
– Can tolerate an arbitrary failure of a single node
– Single faulty unit detected by membership protocol
– Isolated within two rounds (for single fault)
Fault-tolerant Units – Triple Modular redundancy
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Fault Tolerance (contd…)
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Till now assumed that environment complies with fault-hypothesis
If environment violates fault hypothesis
– TTA activates never-give-up strategy
– Initiated by TTP/C protocol in combination with application
– Only when necessary resources are unavailable to provide minimum required service
Redundant transducers
– Requires two independent TTP/A field buses
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Design Methodology
• Architecture Design
– Decompose into clusters and nodes
– Can use top-down or bottom-up
– Specify CNIs of nodes in both the temporal & value
domains
• Node design
– Delivery and fetch instants
• Used as pre-condition and post-condition by applications
• Validation
– Formal methods for consistent distributed computing
base algorithms
– Reproducable, observed without probe effect, DM
interface
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Concluding Remarks
• Autonomous clusters and nodes
• Global time used to specify interfaces among nodes
• Two-phased design
– Architecture and Component (Node) design
• Take advantage of global time
• Currently occupies a niche position
– Time considered a nuisance in mainstream computing
• Real-Time is an integral part of real-world
– Cannot be abstracted away
ISIS, Vanderbilt University
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Krishnakumar B
The Time-Triggered Architecture
Questions ?
ISIS, Vanderbilt University
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