Locating Mobile Agents in Distributed Computing Environment

1. Locating Mobile Agents in Distributed Computing Environment

2. Reference [1] A. Di Stefano, and C. Santoro, “Locating Mobile Agents in a Wide Distributed Environment,” IEEE T-PDS, vol 13, no. 8, Aug 2002, p. 844 – 864.

3. Problem Context • Agents are mobile • Agents work with other agents • Cooperation, coordination, competition,.. • Agents must find other agents • Addressing technique for unique identification • Applications use names not identifiers • Naming scheme, that permits programmers autonomous name selection

4. Static vs Mobile Agents • Static entities • Binding protocol that binds names to addresses • Address includes the current location of the addressed entity • Mobile • Dynamic nature of the agents increases complexity • Discover the location • Additional problem: agents can be created, cloned, and terminated quite easily • Need dynamic naming scheme

5. Dynamic Naming • Agent name must indicate the start of the search process • Name space structure • Closely related to location protocol • Transparency, scalability, reliability, efficiency,…. • In general, the approaches meet some of these properties

6. Naming Scheme Properties • What does the Agent Name represent? • May be follow the internet convention • Service.organization.domain • www.gmu.edu • Existing schemes can be classified using the following properties: • Transparency – to achieve transparency the agent name should NOT contain site specific information • cs.gmu.edu/~cs365 is not transparent, but CS365 is transparent. • Location independence – name does not include info regarding current position • Database location + agent name, is location independent but not transparent. • Selectability – a naming scheme is selectable if a programmer can autonomously select a name.

7. Implications of Properties • Transparency – non-transparent systems require that the user have complete knowledge of the distributed system. Need to know the agent birth node. • Locating independence – facilitates agent interaction without knowing where the agent is located. Requires a location protocol. • Selectability – ensures interaction with the parent and child agent without system wide publication. Requires an approach to binding.

8. Finding an Agent Location • Name and Location Base (NLB) • Tuple (m, a, l), where name (m), agent (a), current location(l) • Four operations on the NLB • Bind (m, a, l) performed when a name is assigned to agent. Tuple added to NLB. • Newloc (m, a, l `) l ` is the new location. In NLB (m, a, l `) replaces (m, a, l). • Find (m) extracts a from the NLB. • Unbind (m) when agent name is no longer used – remove from NLB. • Locating a mobile agent requires implementation of NLB and the 4 operations.

9. Implementation Issues • Centralized NLB? • Scalable, reliable, efficient??? • Each implementation approach must assess the scalability, reliability and efficiency of the operations • Binding at the beginning and end – so less important • Newloc and Find – repeated use – so more important • Find may require repetitive application • Location finding + agent catching • For rapidly moving agents, repeat the process several times

10. Implementation Performance Measures • Availability A = (MTTF)/(MTTF+MTTR) • Scalability • Migration overhead Qm = (tnewloc)/(tmigr + tnewloc) tmigr = duration of agent migration tnewloc = time to execute newloc • Interaction overhead Qi = (tcatch)/(tcatch + tinteract) tcatch = time for finding and catching phase tinteract = time for interaction • Scheme should optimize all these!! Utility function! Multiple objectives leads to compromise

11. Internet like Naming Scheme [1] • Human Readable • Agent location is characterized by • Birth location • Current location • Divide the environment into: • Places – context where agents can execute • Agent Systems • Regions • Address (Global Location Identifier – GLI) is a combination of Place, Agent System, Region. • BGLI (Birth GLI), CGLI (Current GLI) • Agent name m :: = “agent:” localname “@” BGLI.region • BGLI.region = region of birth; localname = assigned by programmer • Unique names are required. • Some “authority” ensures uniqueness of region name. • Require a binding protocol to register the new agent and ensure uniqueness of the “localname”.

12. Naming Scheme [1] Properties • Location independence • Points to birth GLI • Selectability • Programmer is able to select a local unique name • Transparency • Limited. Birth place is include in name • Similar to wireless naming approach

13. Naming Schemes - Comparison

14. Location Technique – Database Logging(Objective: Find the current agent location) Location database server Site 3 Migration Location Db Update Site 1 Site2 At each agent migration, the new location info is sent to db server. Given agent name, location db server provides agent current location. Adv: Only one access to the db. Disadv: Failure at server. Delays when the source and destination are far from the server.

15. Location Technique – Path proxies(Objective: Find the current agent location) Migration Proxy Reference Site 1 Site 2 Site 3 As each agent leaves a site, a proxy object is created that points to the destination site. Locate agents by following the proxy path. Adv: Communication between neighboring – distance between server and source / destination sites is not important. Disadv: Access time can be long – catching up to the agent. Many “hops”. Improvement: After time out, Nth proxy object sends info about N+1 proxy object to the N-1 proxy object, and Nth proxy object is deleted. Reduces number of hops.

16. Mobile Agents vs Mobile Hosts (Wireless Networks) • Wireless network incorporate the ability to locate the mobile host • The mobile hosts attachment point (base station) is changing • Mobile Agents – networks are fixed, agent is moving • Protocol related differences • In the mobile host case, the location protocols are handled in the network layer • Mobile agent location is done at a higher level – transportation or even the application layer

17. Mobile Agents vs Mobile Hosts (Wireless Networks) • Distance and speed constraints • Mobile host speeds are limited. Limited region of mobility. Location limited to a “neighborhood”. • Possibly broadcast paging messages in neighboring cells. • Hierarchical view of the network: cellssubnetsregions…. • Mobile agents migrate at the speed of light. Regions of operation are more widespread • Hierarchical approach may work, but for long distances, will require traversal of several layers. This leads to inefficiencies

18. Framework of Proposed Solution • Each region has a site acting as Agent Name Server (ANS) and on each location there is a Site Agent Register (SAR) • ANS • Manages the Region Agent Register (RAR) db • Each entry in RAR is (m, ): agent m at location  •  is a GLI or GLI.region • Each RAR entry indicates an agent that is in the region or has transited through the region • ANS permits remote access to RAR db, but rules of concurrency must be maintained • Mutual exclusion between concurrent requests

19. Framework of Proposed Solution - 2 • SAR • An SAR on each location • Agents at the site or transited through the site • Entry form (m, , ): • Agent  with name m at location  •  is the agent identifier •  is a GLI (agent found or transited location), • GLI.region (agent found or transited region), or • nil (agent is at the same location as the SAR)

20. Figure 3: RAR/SAR tuple meaning

21. Search-by-Path-Chase Protocol • For each SAR (similarly for RAR) • If agent m reaches location at tl, then query to SAR at tq (tq >= tl) will yield • Agent if it at  • If agent has migrated then the location (region) reached at the next migration after tl. • This implies that at tq by following a visited location before tq we can catch up with m. • Since there could be many hops, SPC design is focused on reducing the chase.

22. Steps to Locate an Agent • Extract birth location from m. • Contact the relevant RAR. • If in the RAR, find the SAR. RAR will point to SAR. • If the resulting tuple has an identifier (a ) then we are done, else start the search again. • If not in RAR, contact the ANS of the new region. • Register always contains a pointer, unless there was a crash. • Locks are used to ensure concurrency and correct updates.

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