Code Migration Russell T Potee III 4 December

Code Migration Russell T. Potee, III 4 December 2007

Overview What is Code Migration? Code Migration Terminology Resource Relocation and Binding Reconfiguration Code Migration Paradigms Incremental Code Migration with XML

What is Code Migration? Code migration is when programs are moved from one machine to another, often moving parts of its execution environment along with it. The intent is for the transferred program to continue execution on the target machine.

What is Code Migration Code migration is often used for load distribution, reducing network bandwidth, dynamic customization, and mobile agents. Code migration increases scalability, improves performance, and provides flexibility. We will gain a more tangible understanding of code migration when we get to the paradigms section.

Code Migration Terminology Code migration has been around for sometime, but traditionally code migration technologies could not agree on terms. To understand code migration properly, we will go over some standardized terms for code migration as defined by Fuggetta et al.

Code Migration Terminology Many code migration technologies provide Computational Environments to support code mobility. Computational environments give applications the ability to move components from one host to another.

Code Migration Terminology Code migration components consist of execution units and resources. Execution units can be divided into the following: Code Segment Data Segment Set of instructions being executed References to external resources Files, printers, devices, et cetera Execution Segment Process state, private data, stack pointer, and instruction pointer

Code Migration Terminology Strong Mobility Described as the ability to transfer both the code segment and execution state of the execution unit from one computational environment to another. Weak Mobility Described as the ability to transfer the code segment, but not the execution state. Typically, weak mobility applications have predefined starting positions For example - Java Applets

Code Migration Terminology Strong code mobility is best characterized by two terms: Migration is when an execution unit is suspended, transferred from one computational environment to another, and resumes execution. Remote Cloning is when a copy of the running execution unit is copied to another computational environment while continuing execution on the original executional environment.

Code Migration Terminology Sender-initiated (proactive) migration Transfer is initiated autonomously by the migrating execution unit Example Uploading programs to a computational server Receiver-initiated (reactive) migration Transfer is initiated by an execution unit different from the migrating execution unit Example Java applets are downloaded from a web server to the client browser at the request of the browser.

Resource Relocation and Binding Reconfiguration When an execution unit migrates, the data space needs to be reconfigured. This could involve rebinding or moving resources depending on the type of resource.

Resource Relocation and Binding Reconfiguration Types of resources: Transferable This type of resources can be transferred from one machine to another Free or Fixed Free resources can be easily moved from one machine to another. For example, data files. Fixed resources can be moved, but not easily. For example, moving a database can be moved, but could slow performance. Non-transferable It is not possible to move this resource For example, a local device such as a printer.

Resource Relocation and Binding Reconfiguration 3 types of binding Binding by identifier (strongest) Binding by value (weaker) Execution units refer to resources by unique identifiers Execution units refer to resources based on the value/content of the resources and want local access to this type of resource Binding by type (weakest) Execution units refer a resource of a given type, often local devices

Resource Relocation and Binding Reconfiguration Binding by identifier Free transferable resource It is best to move this type of resource with the execution unit. Otherwise, the resource stays in the original computational environment, and the execution unit will refer to the resource by a network reference. Network references are the least desirable because the execution unit will be slowed due to network problems. Fixed transferable and non-transferable resources will need to use a network reference.

Resource Relocation and Binding Reconfiguration Binding by value Free transferable resource Fixed transferable resource Can copy or move the resource from one computational environment to another. Moving the resource may cause problems if other execution units are accessing the resource. Network references can be used, but again, this is least desirable. Can be copied or use network reference. Non-transferable Can use network reference.

Resource Relocation and Binding Reconfiguration Binding by type Best to use re-binding. Re-binding involves the execution unit removing the original binding, moving to a new computational environment, and binding to the same type of resource in the new computational environment.

Resource Relocation and Binding Reconfiguration

Code Migration Paradigms Defining a design paradigm, or software architecture, is an important of the design phase of software development. This section will identify four design paradigms related to code migration.

Code Migration Paradigms Client-Server (CS) Paradigm The server component knows how to perform a particular service. The client component at location A requests execution of the service from the server component at location B. The server component executes the request and returns the result to the client component.

Code Migration Paradigms Remote Evaluation (REV) Paradigm One software component (A) knows how to perform a particular service, but does not have the resources to do so. The software component (A) sends the code component to perform the service to another software component (B) that has access to the necessary resources. Software component (B) performs executes the code component and returns the results to software component (A).

Code Migration Paradigms Code on Demand (COD) Software component (A) has accesses to resources at its location, but does not know how to interact with the resources. Software component (A) requests a code component from software component (B). Software component (B) sends the code to (A), and (A) executes the code component.

Code Migration Paradigms Mobile Agent (MA) Software component (A) knows how to perform a particular service, but does not have the necessary resources. To access the resources, (A) becomes a mobile agent (MA) by migrating the entire computational environment (state, code, resources) to another location that has the necessary resources. The MA executes accesses the resources at its new location and executes the service code component.

Incremental Code Migration with XML Incremental code migration is the ability to move, add, remove, or replace code sections in a remote program. Mobile Agents are considered to be at the highest level of code mobility, where entire code and data segments can be moved together. However, incremental code migration decomposes mobile agents into individual statements, thus increasing flexibility and scalability.

Incremental Code Migration with XML By supporting incremental code migration, code migration can be more applicable for mobile thin clients, such as PDAs, user interfaces, etc. The remainder of this section will explain how XML can be used to provide incremental code mobility.

Incremental Code Migration with XML allows users to define their own document type definitions (DTDs), a grammar defining the syntax of the document. XML documents always reference a DTD so parsers know the grammar of the document.

Incremental Code Migration with XML Elements of XML DTDs can be attributed to store the values of identifiers, constants, and semantic information. Thus, XML DTDs can be used to define the syntax (instruction set) of programming languages. XML documents that use the syntax defined in the DTD to specify program functionality are called XML programs. Moving an XML program from one machine to another achieves code mobility.

Incremental Code Migration with XML

Incremental Code Migration with XML ON – turn on microwave TIME – Number of minutes POPCORN – pre-set time/temperature for heating popcorn DEFROST – pre-set temperature for defrosting meat TEMPERATURE – can set to high, medium or low BEGIN – start heating food OFF – turn off microwave

Incremental Code Migration with XML

Incremental Code Migration with XML programs can be transferred as source code and interpreted by the remote host, thus establishing code migration. Using XML we can change or add small fragments of code to our programs instead of resending the entire program.

Incremental Code Migration with XML

Incremental Code Migration with XML How do we specify where the replacement or insertion of XML code increments take place? One solution is to use XPointer is an XML-related standard used to navigate through XML documents. By using XPointer, we can navigate through our XML program and identify a code section where we would like to insert our code, thus enabling incremental code migration.

Incremental Code Migration with XML By specifying the location of the popcorn instruction, we can replace the instruction with the burn popcorn instruction that was previously defined.

Incremental Code Migration with XML Overall design view Interpreter implementation Code Mobility By using an XML parser to establish a syntax tree and the DTD, an interpreter can be written to execute the commands of the XML program represented in the syntax tree. With the DTD and interpreter on the target machine, the XML program can be sent over to the target machine to be parsed into a syntax tree and executed by the interpreter. Incremental Code Mobility With the DTD, interpreter, and XML program on the target machine, XML code increments can be sent along with the XPointer navigation to update the XML syntax tree supported by the XML parser and interpreter.

Sources Emmerich, W. , Mascolo, C. , Finkelstein, A. , Fuggetta, A. , “Implementing Incremental Code Migration with XML. ” International Conference on Software Engineering, 2000. Picco, G. P. , and Vigna, G: “Understanding Code Mobility. ” IEEE Trans. Soft. Eng. , May 1998. Milojicic, D. , Douglis, F. , Paindeveine, Y. , Wheeler, R. , Zhou, S. : “Process Migration. ” ACM Computing Surv. , Sept 2000. Tanenbaum, A. , Van Steen, M. : Distributed Systems: Principles and Paradigms, Upper Saddle River, NJ: Prentice Hall, 2007