kommunikation und verteilte systeme naming vs. locating ... · naming vs. locating entities a)...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

45Chapter 3: Naming

Naming vs. Locating Entities

a) Direct, single level mapping between names and addressesb) Two-level mapping using identifiers. Needs a location service to resolve identifiers

� Till now: resources with fixed locations (hierarchical, caching, ...)

� Problem: some entity may change its location frequently

� Simple solution: record aliases for the new address or the new name

� But: efficiency, re-use of old names, ...

New approaches are necessary, e.g. identifiers for an resource

Entity ID



46Chapter 3: Naming

Simple Solution for a Location Service

Using Broadcast or Multicast

� Broadcast is typically offered in LANs

� Simple locating process: broadcast identifier and wait on a reply

(principle used in the Internet protocol ARP [Address Resolution Protocol])

� But: inefficient in large systems

� More efficient: using multicast for location

� But: you need to build up and to know the multicast group



47Chapter 3: Naming

Forwarding Pointers (1)More popular approachfor location:

Forwarding Pointers

Principle:

• A moving entity leaves behind a reference to the new location

• Client follows the chain of forwarding pointers

But...

� Long chains make the location process very expensive� Intermediate nodes have to store all pointers as long as needed

� Broken links prohibit location

Short chains and robust pointers are needed

old location

new location



48Chapter 3: Naming

Forwarding Pointers (2)

� When an object moves it leaves behind a proxy having the new location reference

� Location is transparent for the client, request is forwarded along the chain

� Object sends back its new location to the caller, the forwarding pointer is redirected



49Chapter 3: Naming

Home-Based Approaches

• Popular approach for large-scale networks: home location

• principle of Mobile IP• But: increase in communication latency, fixed home location



50Chapter 3: Naming

Hierarchical Approaches� Extending the home-based approach to several layers

� Network is divided into domains, sub-domains, ... (similar to DNS)

� Leaf domains: local area network, cell in a mobile telephone network, ...

� An entity located in domain D is represented by a location record in directory node dir(D)

� Location records on higher hierarchies point to next sub-domain directory node



51Chapter 3: Naming

Information Stored in Nodes

• Entities may have multiple addresses (e.g. replication)

• Higher-level node stores pointers to each location• Scalability problem: root node has to store all information…



52Chapter 3: Naming

Location Lookup

• Looking up a location in a hierarchically organized location service• Client contacts directory node in its own domain

• Go up hierarchy to the first directory node holding the information



53Chapter 3: Naming

Location Update

a) An insert request is forwarded to the first node that knows about entity E.

b) A chain of forwarding pointers to the leaf node is created.

Install a replicate in a new domain: new pointers have to be set

Similar operation: deletion of pointers



54Chapter 3: Naming

Pointer Caches• Caching can be used to store locations of 'stable' nodes

• Location caching: inefficient lookup with each location change

• Pointer caching: Caching a reference to a directory node (dir(D)) of the lowest-level domain in which an entity (E) will reside most of the time.



55Chapter 3: Naming

Invalidation of Pointer Caches

• A cache entry that needs to be invalidated because it returns a non-local address, while such an address is available.



56Chapter 3: Naming

Scalability Issues• Root directory node becomes bottleneck• Solution: placing sub-nodes of a partitioned root across the network

• Spread sub-nodes uniformly; but… new scalability problems: which node to give responsibility???



57Chapter 3: Naming

The Problem of Unreferenced Objects• Problem with forwarding pointers: unreferenced object• Garbage collection for remote objects: hidden from clients and objects itself• How many proxies point to another one?• Reference graph



58Chapter 3: Naming

Solution: Reference Counting• Simply count the references pointing to you• Problem: unreliable communication

� Process P expects to get an acknowledgement when it increases the skeletons counter

� Acknowledgement can get lost

� P sends the increase message again• Necessary to detect duplicates



59Chapter 3: Naming

Reference Counting

a) Copying a reference to another process and incrementing the counter too late

b) Solution by using acknowledgements

Another problem: copying a remote reference to another process

One more problem: performance problems in large-scale systems by communication overhead



60Chapter 3: Naming

Advanced Referencing Counting

a) The initial assignment of weights in weighted reference countingb) Weight assignment when creating a new reference.

Weighted reference counting: each object has

• A fixed total weight

• A partial weight, initialised with the total weight

Creating a remote reference causes transmitting half the partial weight to the referencer



61Chapter 3: Naming

Weighted Referencing Counting

• Copying a reference to P2 causes P1 in transmitting half the weight

• Deleting a reference causes the remote object to subtract the weight of the referencer from its total weight

• When the total weight becomes zero, there are no more references



62Chapter 3: Naming

Weighted Referencing Counting

• Problem: the partial weight of the remote object can become zero. What is with former objects which want to make a reference?

• Make use of indirections when partial weight reaches one

• When copying the reference to P2, P1 creates a local skeleton with some total weight and the same partial weight

• Then transmitting half the partial weight to P2



63Chapter 3: Naming

Generation Referencing Counting• Alternative to the use of indirections: generation reference counting

• Associate a generation and a copy counter with each referencing process• Both counters are initialised with zero• When copying a reference, the copy counter is increased; the new referencer

becomes the next generation compared to the old one• Skeleton maintains the numbers of outstanding copies for each generation; in case

of a decrement request, the counter for the referencer's generation is decreased. The copies of the referencer is added to the next generation. If all generation entries are zero, there are no more references



64Chapter 3: Naming

And much simpler...

Reference listing

• Skeleton keeps track of the proxies having a reference to it, i.e. it has a list of all these proxies (reference list) instead of a counter

� No problems with duplicated increments

� Easy to keep the list consistent in case of process failures

� Problem: copying a reference and deleting it too early (as in reference counting)

� Main drawback: bad scalability in case of many references

• Used in Java RMI



65Chapter 3: Naming

Tracing-based Garbage CollectionHow can isolated referencer groups be located?

� Tracing all entities in a distributed system

� Removing all non-reachable entities

� Scalability problems! → only consider groups of processes



66Chapter 3: Naming

Tracing in Groups1. Marking the skeletons

• Hard mark: reachable from a root object, a hard marked proxy, or an external object

• Soft mark: only reachable from inside the group

2. Propagating marks to proxies

3. Repeating these steps till no more change is made



67Chapter 3: Naming

Tracing in Groups

If there are no more changes: deletion of soft-marked objects

• reduction of objects in groups

• after that: analysis of intergroup references on higher-level groups



68Chapter 3: Naming

Conclusion

Different concepts:

• Naming Services for mapping of logical names to addresses• Directory Services for searching addresses by describing the needed object• Discovery Services as a name database in “dynamic” networks

• Location Services for supporting moving objects• Some close relations to file systems and reference counting

What is the best concept?

There is no general answer – it always depends on the application

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