1. Alternatives to Mobile Agents
An appendix to AFRL talk
David Kotz

2. Other techniques work ... Remote Procedure Calls (RPC)
Stored Procedures
Java Applets and servlets
Load balancing in homogeneous networks
Application-specific solutions ( eg. adaptive meshes)
Distributed document retrieval
Distributed simulation

3. Call to server procedure
RPC and queued RPC
RPC
Call to server procedure
Dataset
Client
Server
Procedure
results
Dataset
Client
Server 3 5 4 1 2
Queued calls
Queued results
Queued
RPC
No mobile code
Client constrained to server’s interface
All requests and results over client’s network link
No latency or bandwidth reduction, and blocked if link goes down

4. Stored procedures and REV
Dataset
Client
Server
Proc
1. Send procedure code to server
2. Call procedure
Dataset
Server
Client
Proc
2. Send results (and maybe a server procedure)
1. Send procedure code and arguments to server - procedure called immediately
3. Send results
Serv
Proc
Client
Perfect if accessing one server
Difficulties if accessing more than one server
Procedures usually can not communicate with each other
Procedures usually can not send out their own procedures (e.g., no proxy)

5. Applets and servlets Applets Servlets Web Pages Server Servlet Browser
2. Receive page and “embedded” applet; execute applet
Web
Pages
Browser
Server
Applet
1. Request Web page
Servlet
1. Send servlet
2. Get results
Servlets
Same limitations as stored procedures and REV
Implementations pay more attention to security than stored procedures and REV

6. Load balancing MPI, Condor and Legion: Load balancing can be done
- statically (for a heterogeneous network);
- dynamically through data movement.
Much programmer effort required to achieve even limited solutions.
Not much security support.
Often pre-loaded code or homogeneous network.

7. Application-specific solutions
Example: Modeling of complex mechanical structures
Predefined, fixed
distributed solution
3. Domain
decomposition
2. Grid/mesh
generation
4. Visualization
1. Database
of physical
components
“Hardwired” approach….nothing dynamic.

8. TR without agents ... ... 1. Lot of work.
Queued
RPC
Queued
RPC
...
Pre-installed application-specific proxy
Higher-level database interface
2. Much less work, but now try extending the TR app.
Client
Code
“Queued”
REV
“Queued”
REV
...
Client
Code
Client
Code
Proxy

9. … but takes several techniques
That is, the research community is developing
several different approaches to deal with problems
of efficiency, robustness, adaptability and
flexibility for distributed computing and
information systems.
We propose a single technology, mobile agents
based on strong code mobility, to solve many
problems in a unified framework.

10. Why not extend other techniques?
MPI - dynamic process creation
Legion - checkpointing to support process migration
Java - process serialization and state capture for
persistence (rumor!)
Are these all going to end up supporting code mobility?
Many different systems are heading in the direction of mobile code. Let’s go there directly!

11. Big picture: A uniform framework
SUMMARY (so far)
There are several different strengths of mobile agents.
Mobile code helps solve various performance and
robustness problems.
Idiosyncratic solutions exist in each problem domain.
Mobile agents provide a unified framework, a single
technology for solving many problems.
Let’s go directly to where many different systems are
evolving, now.