A Programming Model and System Support for Disconnected-Aware Applications on Resource-Constrained Devices (FarGo-DA) MSc seminar by Weinsberg Yaron January 6,
Outline Introduction FarGo-DA’s Programming Model A sample FarGo-DA application Runtime infrastructure Status and Future Work Summary
Thesis Domain Modern wide-area networks are characterized by: Dynamic changes of bandwidth, load, machine capacities and availability. Dynamic changes of bandwidth, load, machine capacities and availability. Requires high level of adaptability from distributed software systems. We follow the distributed object programming model and use Object mobility to: increase performance increase performance decrease cost decrease cost attain high availability attain high availability The need: new models and techniques for designing wide-area sw systems.
A generic framework for constructing disconnected- aware applications. Enable developers to encode connection semantics. Enable developers to encode connection semantics. Dynamically changing the application layout upon (re)disconnection. Dynamically changing the application layout upon (re)disconnection. A new model and techniques for designing disconnected-aware applications. Separating the connection layout from the application logic. Separating the connection layout from the application logic. A lightweight mechanism for management and of disconnection and reconnection Research Objectives
Focus on Voluntary disconnection. User initiated User initiated enables the system to prepare for disconnection enables the system to prepare for disconnection One-to-many One-to-many Uni-directional client-server applications. NLPCs act as active clients only, and are not servers of any interaction. NLPCs act as active clients only, and are not servers of any interaction. The disconnection request is always initiated by the NLPC (client), not by any server. The disconnection request is always initiated by the NLPC (client), not by any server. Assumptions
Application objects interact across multiple hosts using remote method invocations (RMI) FarGo adds a dynamic layout Add dynamic relocation while preserving the distributed object abstraction Add dynamic relocation while preserving the distributed object abstraction All references into and out of a migrating object must remain valid. All references into and out of a migrating object must remain valid. site B site A FarGo Overview
site B site A FarGo Overview Application objects interact across multiple hosts using remote method invocations (RMI) FarGo adds a dynamic layout Add dynamic relocation while preserving the distributed object abstraction Add dynamic relocation while preserving the distributed object abstraction All references into and out of a migrating object must remain valid. All references into and out of a migrating object must remain valid.
A unique aspect of FarGo is the ability to associate movement semantics with component references. The semantics can be changed dynamically. FarGo Overview A component reference Source component Target component The semantic goes here… FarGo support the following semantics: Link, Pull, Stamp, Duplicate. Link, Pull, Stamp, Duplicate.
A Pull reference: Pull use r Link A relocator A Sample FarGo Reference When moves, moves along. Useful for frequent or data-intensive interaction. Core A Core B
Pull A Pull reference: Core A Core B use r When moves, moves along. Useful for frequent or data-intensive interaction. Link A Sample FarGo Reference
A new programming model: A framework that enables developers to specify architecture-oriented (re)disconnection policies. A framework that enables developers to specify architecture-oriented (re)disconnection policies. A runtime for interpreting and carrying the policies upon disconnection,reconnection. A runtime for interpreting and carrying the policies upon disconnection,reconnection. FarGo-DA Extends the FarGo implementation with: A whole new class of Disconnected-Aware (or DA) semantics. A whole new class of Disconnected-Aware (or DA) semantics. An extended FarGo invocation mechanism (and FarGo compiler) An extended FarGo invocation mechanism (and FarGo compiler) An extended FarGo serialization mechanism An extended FarGo serialization mechanism FarGo-DA’s Programming Model
Complet: A mobile java component. DA-Complet: A complet that is aware of the core’s connectivity Only DA-Complets are affected by (re)disconnection Only DA-Complets are affected by (re)disconnection Core: A process that can execute, send, and receive complets. Complet Reference: A reference that points to a complet ( “MetaRef”) Reifies (re)disconnection semantics. Reifies (re)disconnection semantics. Accessible via system methods: Accessible via system methods: FarGo-DA’s Programming Model MetaRef metaRef = Core.getMetaRef(target); metaRef.setDA(new Clone(),new Merge());
Identify complets and their interactions. Complets are “semantically remote to each other” Complets are “semantically remote to each other” object parameters are passed by value complets are passed by reference. Physically, complets may be co-located or remote. Physically, complets may be co-located or remote. Facet 1 Application Logic Design
Mark the DA components. Non-DA components are not affected. Define FarGo-DA’s connection semantics via reference types: Disconnection Semantics: Disconnection Semantics: Clone, Replace, StoreAndForward, Depart Define migration priorities. Reconnection Semantics: Reconnection Semantics: Merge, Purge, Overwrite, Last Facet 2 DA Connection Layout
A Clone reference. Disconnection Semantics Core A Core B When core A disconnects, is cloned to the locality of . The default connection semantics. Within each core, the system notifies all DA-complets about the coming disconnection. Enable to dynamically change the connection semantics Enable to dynamically change the connection semantics Clone I want to Disconnect! Disconnection Event MetaRef metaRef = Core.getMetaRef(beta); metaRef.setDA(new Clone());
A Clone reference. Core A Core B Clone I am the clone complet (replica) I am the original complet The local reference is updated. the clone complet has the same ID as the original complet. Disconnection Semantics
A Replace reference. Core A Core B locates a local instance of ’s type and connects to it. If non-exist, a new one is created. If non-exist, a new one is created. Useful when: reduced functionality (smaller footprint) is sufficient. the origin complet cannot be migrated. Replace lik e 1.same interface same binding-name equivalence method will figure it out… Disconnection Semantics
A Replace reference. Core A Core B Replace lik e Disconnection Event locates a local instance of ’s type and connects to it. If non-exist, a new one is created. If non-exist, a new one is created. Useful when: reduced functionality (smaller footprint) is sufficient. the origin complet cannot be migrated. Disconnection Semantics
A Replace reference. Core A Core B Replace lik e locates a local instance of ’s type and connects to it. If non-exist, a new one is created. If non-exist, a new one is created. Useful when: reduced functionality (smaller footprint) is sufficient. the origin complet cannot be migrated. Disconnection Semantics
A StoreAndForward reference. Core A Core B StoreAndForward Disconnection Event Disconnection Semantics A special kind of reference: Store + Forward. All invocations are one-way, in order to enable the source complet to continue its work. All invocations are stored at the local NLPC, pending for reconnection. The number of invocation stored is limited and can be set by the developer (with a runtime limit).
A StoreAndForward reference. Core A Core B StoreAndForward A special kind of reference: Store + Forward. All invocations are one-way, in order to enable the source complet to continue its work. All invocations are stored at the local NLPC, pending for reconnection. The number of invocation stored is limited and can be set by the developer (with a runtime limit). Local buffer Storing: - method number - parameters. Method invoked… Disconnected Operation Disconnection Semantics
A StoreAndForward reference. Core A Core B StoreAndForward Upon Reconnection, the runtime forwards the pending invocations to the remote complet. No return values. An example use of this reference type is for outgoing messages, CVS commits, instant messaging etc. Local buffer Upon Reconnection… method invoked! Disconnection Semantics
A Depart reference. Core A Core B Depart Disconnection Semantics User Invoker return value method invoked! All invocations are handed over to the remote core prior to disconnection, for remote execution Upon reconnection the results, if any, are transferred to the source complet Useful to spawn heavy remote computations without the need to be continuously connected Partly implemented (based on Miki Abu’s work,ICDCS01) a remote thread created method invoked. return value is available: - can pull it -can be notified return value is saved if is currently disconnected.
Complet Prioritization NLPCs are resource constrained. not all clone jobs can be fulfilled. not all clone jobs can be fulfilled. FarGo-DA provides a simple interface to setting priorities, by an additional parameter to the setDA method. At runtime, the system carries out the disconnection semantics based on the priority order. Invocations on references without target complets are gracefully handled via FarGo-DA exceptions MetaRef metaRef = Core.getMetaRef(target); metaRef.setDA(new Clone(),new Merge(),Priority.High); Disconnection Semantics
A Merge reference. Core A Core B When core A reconnects, ’s clone is merged with the original complet (explained later). The default reconnection semantics. Merge I want to Reconnect the network… Reconnection Event Reconnection… Merging… Reconnection Semantics
A Merge reference. Core A Core B When core A reconnects, ’s clone is merged with the original complet (explained later). The default reconnection semantics. Merge merged!! ! Reconnected!!! Reconnection Semantics
A Purge reference. Core A Core B When core A reconnects, ’s clone is purged. is reconnected to . Analogous to methods [in] parameters in RPC Purge I want to Reconnect the network… Reconnection Event Reconnection… Reconnection Semantics
A Purge reference. Core A Core B Purge Reconnected!!! When core A reconnects, ’s clone is purged. is reconnected to . Analogous to methods [in] parameters in RPC Reconnection Semantics
An Overwrite reference. Core A Core B The opposite of Purge. The state of the cloned complet overwrites the state of the original complet. Analogous to methods [in/out] parameters in RPC is reconnected to . Overwrite I want to Reconnect the network… Reconnection Event Reconnection… Overwrite Reconnection Semantics
An Overwrite reference. Core A Core B Overwrite Reconnection Semantics The opposite of Purge. The state of the cloned complet overwrites the state of the original complet. Analogous to methods [in/out] parameters in RPC is reconnected to . Reconnected!!!
A Last reference. Core A Core B The state of the complet with the latest timestamp wins. The clocks of the NLPC and the remote server should be synchronized. is reconnected to . Last I want to Reconnect the network… Reconnection Event Reconnection… Comparing timestamps… Reconnection Semantics
A Last reference. Core A Core B Last Reconnected!!! I won!!! The state of the complet with the latest timestamp wins. The clocks of the NLPC and the remote server should be synchronized. is reconnected to . Reconnection Semantics
Conflict Resolution In order to resolve conflicting updates between replicas we must assume application specific knowledge. FarGo-DA offers two mechanisms in order to resolve merging conflicts: Callback methods- The Merge callback method. Callback methods- The Merge callback method. void Merge(Complet archive,Complet replica, Log localLog,Log remoteLog) Log localLog,Log remoteLog){ // Application-specific merging code // Application-specific merging code} invoked on the original complet Core A Core B rr original replica aa archive remote log local log
Reconciliation operators encapsulation of primitive types + a state variable that represents a built-in merging method. encapsulation of primitive types + a state variable that represents a built-in merging method. public class BankAccount_ extends DisconnectedAwareComplet extends DisconnectedAwareComplet{ private discInt private discInt balance = new discInt(Operation.Add); balance = new discInt(Operation.Add);} Conflict Resolution FarGo-DA provides the following types: discInt, discLong, discString discInt, discLong, discString And the following operations: Add, Max, Min, Average, Random, Last, First. Add, Max, Min, Average, Random, Last, First.
Each reconciliation operator must implement the discType interface. User defined types are created by implementing the merge method declared at discType interface. FarGo-DA’s Runtime supports the merging of reconciliation operators, contained in Java’s basic container classes (i.e., Vector, Array). Conflict Resolution
Core A Core B Merge How reconciliation operators work ? Upon Reconnection… + Introspection -discInt balance; -discString account; -discInt balance’ ; -discString account ‘ ; -discInt balance; merged!! ! -discString account;
FarGo-DA Example ManageAnywhere the current customer adds a new product to the order ManageAnywhere is a simple tool for collaborative management to be used by remote sales forces. Each sales agent is equipped with a NLPC. The NLPC may or may not be connected to the central facility, depending on the cellular coverage in the visited area. get the available balance calculates the product price according to the quantity quantity
NLPC Server Logic Configurator Inventory Product Accounts CompanyAccount GUI BasicConfigurator 1 n 1 n Application Logic Design The agents user interface. Handles all Handles all service requests. service requests. - financial calc - financial calc - product’s price calc - product’s price calc - smaller footprint service provider service provider products inventory products inventory customers pre-paid accounts public class ComapnyAccount_ extends DisconnectedAwareComplet extends DisconnectedAwareComplet{ private Integer prePaidBalance= 60102; private discInt totalPurchase = new discInt(Operation.Add); }
Connection Layout Design public class Logic_extends DisconnectedAwareComplet { MetaRef mr = Core.getMetaRef(accounts); MetaRef mr = Core.getMetaRef(accounts); mr.setDA(new Clone(),new Merge(),Priority.LOW); mr.setDA(new Clone(),new Merge(),Priority.LOW); mr = Core.getMetaRef(configurator); mr = Core.getMetaRef(configurator); mr.setDA(new Replace(),new Purge(),Priority.High mr.setDA(new Replace(),new Purge(),Priority.High mr = Core.getMetaRef(inventory); mr = Core.getMetaRef(inventory); mr.setDA(new StoreAndForward(),null,Priority.LOW mr.setDA(new StoreAndForward(),null,Priority.LOW} A look at the Logic complet
Disconnection Exemplified
System Design Core Architecture DM - Manages the Reconnection and Disconnection procedures. and Disconnection procedures. - Realized by the DisconnectionManager (DM) DisconnectionManager (DM) The FarGo-DA’s Implementation enables disconnected operation. operation. FarGo-DA’s Implementation provide a general parameter passing scheme passing scheme
System Design Complet Reference Structure The connection reference type is encapsulated in a meta-reference object. same interface as holds the connection semantics. - points to or to a remote tracker a remote tracker - RMI object
System Design FarGo-DA’s invocation model In order to support disconnected operation, a new invocation scheme was implemented in FarGo-DA. FarGo assumed that the complet is “always- connected”: FarGo assumed that the complet is “always- connected”: available on the local core available on the remote core through a remote tracker. Upon invocation, FarGo always try to invoke the complet. In FarGo-DA this is a reference specific property (for example, StoreAndForward stops the invocation flow) Upon invocation, FarGo always try to invoke the complet. In FarGo-DA this is a reference specific property (for example, StoreAndForward stops the invocation flow) s Tc s T c Tremote
System Design FarGo-DA’s invocation model Invocation starts here… connector’s preInvoke method is activated: -continue invocation ? -store data ? -log ? etc… Invocation continues to complet! decision returned.
System Design The Disconnection Procedure Core A Clone Replace Disconnection Event User Asks to Disconnect… Core B
System Design The Disconnection Procedure Core A Clone Replace DM is loaded…………. DMDM Core B
System Design The Disconnection Procedure Core A Clone Replace Creating Internal Database… DMDM New Job: - : should be cloned Core B
System Design The Disconnection Procedure Core A Clone Replace Cloning… DMDM Core B disconnection buffer Is serialized… type Is serialized… type Looking for … Create a new one!!! DM discovers that… - : should be replaced
System Design The Disconnection Procedure Core A Clone Replace Update References… DMDM Core B Replace Disconnection Completed! ready to reconnect!
System Design The Reconnection Procedure Core A Merge Purge DMDM Core B Purge User Reconnects…. DM use the already created database. + merged =
System Design The Reconnection Procedure Core A Merge Purge DMDM Core B Purge User Reconnects…. merged
System Design The Reconnection Procedure Core A Merge Purge DMDM Core B merged Update References… Reconnection Completed!
Status and Future Work Publications: A programming Model and System Support – to be published in ICSE’02 (May 2002) A programming Model and System Support – to be published in ICSE’02 (May 2002) Simulation for NLPC using FarGo’s monitoring API (Hovav' s thesis, MA2000) System is fully implemented (using JDK1.8) and will soon be available for download. Ongoing work: DA-Application profiling. DA-Application profiling. Weakly-Connected Operation. Weakly-Connected Operation. More applications. More applications.
Summary A new programming model: Design time (logical) structural rules. Design time (logical) structural rules. Loose coupling between application-logic and connection layout (re)design. Loose coupling between application-logic and connection layout (re)design. Keeping a uniform (and familiar) language, tools, and object model. Keeping a uniform (and familiar) language, tools, and object model. System Design: Create a new invocation model to support disconnected operation. Create a new invocation model to support disconnected operation. A system component to handle the (re)disconnections. A system component to handle the (re)disconnections. Implementation of the disconnection semantics is built into a new serialization mechanism Implementation of the disconnection semantics is built into a new serialization mechanism Simple yet powerful conflict resolution mechanisms. Simple yet powerful conflict resolution mechanisms. An implemented model and system.