A Replication Framework for

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A Replication Framework for Program-to-Program
Integration across Unreliable Networks and its
Implementation in a Servlet Container
Alberto Bartoli Milan Prica Etienne Antoniutti
di Muro
DEEI Università di Trieste
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Building Blocks
Programmercode
JMiramare
Replication algorithm
JBora
Group Communication Middleware (GC layer)
Spread
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What JMiramare offers?

• Operations from each client are executed in
  the
• same order in which they were issued by the
  client.

• If the service replica SR receives multiple
  copies of a given
• update request, SR executes the operation
  only once.

• Nota Bene
• Multiple copies of a given request could be
  received by different replicas

• There are NO assumptions about the timing
  policy of client
• retransmissions.

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How it works? (Premises)

• Data structures of the service replica are kept
  in volatile storage.
• Data structures are replicated and kept in sync
  at all servers in the
• primary partition.
• Each client is connected to a single replica at
  a time.
• A replica can execute a request only if it is
  up-to-date for that object.
• If a replica is unable to execute the request,
  it redirects the client to
• another replica.

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... Premises

• Each replica executes read operations locally.
  
• A replica can perform updates only if it is a
  member of the groups
• primary partition.
• Updates involve multicasting to the other
  replicas the new version
• of the object and the last response sent to
  the client.
• A client that does not receive the response for
  an update may
• submit the very same request immediately.
• Replicas that enter the primary partition must
  first gain the
• up-to-date version of server replica data.

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Implementation Issues
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Request

• Method read / update

• Params input params of the method

• objId

• clientId system-wide unique identifier

• serviceHandleobjId counter of the
  successful
• updates from that client

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Response

• Status OK, UNABLE, UNKNOWN

• Result invoked method return value

• NewHandle new value of the successful
• updates counter

• Alternatives network addresses of some
• of the service replicas

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Data Structures of a Server Replica

• obj the actual object replicas

• versionTable number of updates applied to each
  
• object (version number)

• clientTable for each (clientId, objId),
• (lastVersionNumber, lastResponse)

• canUpdate boolean flag, true only if the
  replica can execute updates

• currV the last view received

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Algorithm Overview

• When a replica bootstraps, it spawns
• MainT() thread
• Listening thread

• When a new request comes in, the connection
• is associated with a ClientSessionT() thread

• ClientSessionT() thread processes each
• request as follows

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Execution Path of an HTTP Request
Consistency
Update
Servlet Service
Tomcat AXIS
Not Up To Date
localCall()
Shared data between the MainT and the ClientT.
Main Thread
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Main thread (Principal tasks)

• Receives a message m from ClientSessionT()
• // via grp.localCall() ? JBora

• Multicasts m

• Upon receiving the multicast m, updates the
  object and
• all necessary data structures

• MainT() at the replica that sent the multicast
  also executes
• grp.localRespond() // thus awakening the
  ClientSessionT().

• Handles view changes

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We want to figure out how to integrate our code
with the common replication framework?
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JMiramare?
ComponentMonitor
ComponentHandle
call()
call()
Replication Framework Proposal
Proxy
Normal Axis chain Handlers disabled
static call()
TxHandler
WSMon
Handler
Service Object
handleReq()
handleReq()
handleResp()
handleResp()
AXIS
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?
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Request handling logic (detailed)
Add Entry in ClientTab
New Client ?
Server Up To Date?
Req Read ?
No
No
Yes
req
// update
No
Yes
SERVICE
resp
ltUNABLE, redirectgt
ltOK, resultgt
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Request handling logic (cont.)
Duplicate req?
Can update?
No
Yes
req
// update
No
Yes
resp
ltUNABLE, redirectgt
ltOK, resultgt
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Request handling logic (cont.)
GC
Main Thread propagate()
SERVICE Update (Copy)
// can update
Clone Obj. (Make a Copy)
grp.LocalCall (to MainThr)
req
Yes
No
OK?
resp
Undo
ltOK, resultgt
ltUNKNOWNgt