The SOCK SAGA

1
The SOCK SAGA
Ivan Lanese Computer Science Department University
of Bologna Italy
Joint work with Gianluigi Zavattaro
2
The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

3

• Quest in progress...

4
The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

5
Service oriented computing and faults

• A world where different services interact
• Dynamically found and composed
• Interaction based on one-way and request-response
  invocations
• Safe composition of services requires to deal
  with faults
• No guarentee on components behaviour because of
  loose coupling
• Disconnections, message losses,
• Approaches based on long running transactions and
  compensations

6
Error handling

• A fault is an abnormal situation that forbids the
  continuation of an activity
• Faults should be managed so that the whole system
  reaches a consistent state
• Different mechanisms are commonly used
• Fault handlers specify how to recover from a
  fault
• Termination handlers specify how to terminate an
  ongoing activity when reached by a fault from a
  parallel activity
• Compensation handlers specify how to compensate
  a successfully terminated activity if requested
  for fault recovery

7
Formal models

• Different formal models have been proposed to
  analyze error handling in SOC
• Interaction based compensations, extending name
  passing calculi with operators for error handling
  
• Compensable flow composition, analyzing how
  compensations of simple activities are composed
• We compare two models, one for each approach
• SOCK for interaction based compensations
• SAGAs for compensable flow composition models

8
The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

9
SOCK (Service Oriented Computing Kernel)

• A calculus for modelling service oriented systems
• Strongly inspired by current technologies
• WSDL, WS-BPEL
• Implemented by Jolie
• but featuring a formal LTS semantics
• SOCK has three layers behaviour, engine and
  system
• Error handling is managed at behaviour layer

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Behaviour primitives

• and assignment

Output Input
One-way
Request-response
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Behaviour composition operators

• From sequential languages
• From concurrent calculi

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Error handling in SOCK

• Error handling exploits fault/termination/compensa
  tion handlers
• Handlers can be installed and updated dynamically
• At runtime the scope will also contain the active
  handlers PHq

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The scope hierarchy
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Throwing a fault
A fault f is raised by Throw(f)
(f,Q)
q2
(q2,T2)
q1
Throw (f)
(q1,T1)
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Throwing a fault
It propagates upward and kills the
traversed activities
(f,Q)
f
q2
(q2,T2)
q1
(q1,T1)
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Throwing a fault
Termination handlers of parallel activities are
executed
(f,Q)
f
q2
T2
q1
T1
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Throwing a fault
The fault handler for f is executed
Q
f
q2
T2
q1
T1
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Dynamic installation of handlers

• New handlers update the old ones
• Allowed for fault and termination handlers
• Allows to keep the handler up-to-date as far as
  the activity progresses
• Available handlers are installed before any fault
  is managed
• Always the most updated handler is used

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Installing a fault/termination handler
Inst (f,Q)
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Installing a fault/termination handler
(f,Q)
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Compensation handlers

• Allow to undo the effect of a successfully
  terminated activity
• Are the last available termination handlers
• Should be activated explicitly by comp(q)
• Only other handlers can do it

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Installing compensation handlers
q
q
Inst (q,Q)
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Installing compensation handlers
q
Q terminates
q
(q,Q)
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Installing compensation handlers
q
(q,Q)
Handlers in q can compensate q using comp(q)
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Faults and request-responses

• If a server is reached by a fault when processing
  a request-response, the fault is notified to the
  client
• A request-response always sends a response,
  either normal or faulty
• Allows recovery from remote faults
• A client always waits for the reply, even if
  reached by a local fault
• or(y,x,H) installs handlers in H only if a
  successful answer is received

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The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

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SAGAs

• A language for modelling compensable transactions
• The basic building blocks are compensable actions
  AB
• That can be composed in sequence and parallel
• and grouped into transactions

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SAGAs semantics

• SAGAs are equipped with a big-step semantics
• Observations contain the names of activities
  completed with success
• The final result of a SAGA can be
• success of the SAGA
• failure of the SAGA but success of the
  compensation
• crash, i.e. failure of both the SAGA and its
  compensation

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Atomic compensable activity AB

• Succeeds with observation A if A succeeds
• Compensation B is installed
• Fails with no observation if A fails
• No compensation is installed

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Sequential composition PP

• Succeeds if P and P succeed
• The observation is the sequential composition of
  observations
• Fails if P or P fail
• If P succeeds and P fails P should be
  compensated
• If the compensation succeeds the composition
  fails
• If the compensation fails the composition crashes
• Different sequential activities are compensated
  in reverse order

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Parallel composition PP

• Succeeds if P and P succeed
• The observation is the parallel composition of
  the observations
• Fails if any activity fails
• The other is stopped
• The activities executed so far are compensated
• If the compensation succeeds the composition
  fails
• If the compensation fails the composition crashes
• Different parallel activities are compensated in
  parallel

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Nested SAGA P

• Succeeds if P succeeds or fails
• If P succeeds the compensation is installed
• Crashes if P crashes

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The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

34
Idea of the mapping

• Activities are implemented by remote services
• If the activity succeeds, the service sends back
  a normal answer
• If the activity fails, the service sends back a
  fault
• We use fault f for failure of activities, c for
  crashes
• SAGAs are anonymous while SOCK scopes have unique
  names
• We give unique names to SAGAs
• From a SAGA we extract two components
• The structure of the process
• The structure of the compensations

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Extracting the structure of compensations
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Mapping basic activities

• An activity is a scope containing a call to the
  corresponding service
• If successful the compensation has to be
  installed
• But a fault in the compensation should be treated
  as a crash

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Composing basic activities

• The mapping is homomorphic

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Mapping SAGAs

• A SAGA is a scope
• In case of external fault the activities executed
  so far have to be compensated
• In case of internal fault the activities executed
  so far have to be compensated, and no
  compensation should be installed

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Behavioural correspondance

• From the SOCK lts we extract an abstract LTS
  tracking only the successful answers from
  request-responses
• A SAGA S has a big-step with observation O iff
  its translation can perform all the abstract LTSs
  compatible with O
• The SAGA result is
• or iff the translation performs no label
  throw(c)
• iff the translation performs a label throw(c)

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Example

• Consider the SAGA
• Suppose A, B and D succeed and C fails
• with
  observation AB
• Then the translation has a computation whose LTS
  has no label throw(c) and with abstract LTS

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The saga

• The world faults and compensations in SOC
• The weapon SOCK
• The treasure SAGAs
• The quest a mapping
• Conclusion and all the activities were
  compensated

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Comments

• SAGAs can be used to program compensation
  policies in SOCK
• SOCK adds communication/distribution
• Exploits automatic fault notification
• SOCK allows to program other kinds of recovery
• Different recovery policies for external faults
  and internal faults
• Compensations can be composed in different orders

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Future work

• Complete our quest
• Understand relationship between different
  approaches to compensations
• Static vs dynamic
• Hierarchical vs flat

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The end

• ...and all the activities
• were compensated !