About Naming Concepts in Distributed systems.
More about its services, its types & the approaches of implementation for Name Space & Name Resolution and Locating Entities Approaches with example diagrams.
2. Contents
Naming Entities
Names, Identifiers and Address
Name Spaces
Name Resolution
Closure Mechanism
Linking and Mounting
Implementation of Name Space
Implementation of Resolution
Conclusion
3. Why naming is important?
Names are used to
Share resources
Uniquely identify entities
To refer locations, and so on…
Name resolution allows a process to access the named
entity
4. Naming Entities
Name string of characters used to refer to an
entity
Entity in DS can be anything, e.g., hosts, printers, disks,
files, mailboxes, web pages, etc
Access Point To access an entity
Address name of access point
Access points of an entity may change
5. Identifier and True Identifiers
We need
single name of entity independent from the address of
that entity location independent
Identifiers name that uniquely identifies an
entity
True Identifier has three properties
Refers to at most one entity
Each entity is referred to by at most one identifier
Never reused
Differentiating point for Address and Identifier
6. Name Space
Names in DS are organized into Name Spaces
Name Space represented as labeled, directed
graph
Leaf node no outgoing edges
Directory node number of labeled outgoing
edges
Stores directory table containing entries for each
outgoing edge as a pair (edge label, node identifier)
Root Node only outgoing edges
Path Name sequence of labels
Absolute Path first node in path name is root
Relative Path the opposite case
8. Name Resolution
The process of looking up a name
Closure Mechanism Knowing how and where to start
name resolution
Mounting transparent way for name resolution with
different name spaces
Mounted File System letting a directory node store
the identifier of a directory node from a different name
space (foreign name space)
Mount point directory node storing the node
identifier
Mounting point directory node in the foreign name
space
Normally the mounting point is root
9. Mounted File System
During resolution, mounting point is looked up &
resolution proceeds by accessing its directory
table
Mounting requires at least
Name of an access protocol (for communication)
Name of the server (resolved to address)
Name of mounting point in foreign name space (resolved
to node identifier in foreign NS)
Each of these names needs to be resolved
Three names can be represented as URL
nfs://oslab.khu.ac.kr/home/faraz
11. Global Name Service (GNS)
Another way to merge different name spaces
Mechanism add a new root node and make the
exiting root node its children
Problem
Existing names need to be changed. E.g.,
home/faraz people/home/faraz
Expansion is generally hidden from user
Has a significant performance overhead when
merging 100s or 1000s of name spaces
13. Implementation of Name Space
For large scale DS, name spaces are organized
hierarchically
Name Spaces are partitioned into three logical layers
Global Layer formed by highest-level nodes
Administration Layer formed by directory nodes
managed within a single organization
Managerial Layer formed by nodes that may
typically change regularly
15. Implementation of Name Space
Item Global Administrational Managerial
Geographical scale of network Worldwide Organization Department
Total number of nodes Few Many Vast numbers
Responsiveness to lookups Seconds Milliseconds Immediate
Update propagation Lazy Immediate Immediate
Number of replicas Many None or few None
Is client-side caching applied? Yes Yes Sometimes
16. Implementation of Name Resolution
Assumptions
No replication of name servers
No client side caching
Each client has access to a local name server
Two possible implementations
Iterative Name Resolution
Server will resolve the path name as far as it can, and return
each intermediate result to the client
Recursive Name Resolution
A name server passes the result to the next name server found
by it
18. Recursive Name Resolution
Advantages
Caching result is more effective
Reduced communication cost
Disadvantage
Demands high performance on each name server
19. Domain Name System (DNS)
An example implementation of name resolution
Primarily used for looking up host address and
mail servers
DNS name space is hierarchically organized as a
rooted tree
A label is a case sensitive string with max. length
of 63 characters
Max. length of complete path name is 255
characters
The root is represented by a dot
We generally omit this dot for readability
20.
21. Naming versus Locating Entities
Entities are named for lookup and subsequent
access
Human-friendly Names
Identifiers
Addresses
Virtually all naming systems maintain mapping
from Human-friendly names to addresses
Partitioning of Name space
Global Level
Administrator Level
Managerial Level
23. Naming versus Locating Entities
Possible Solutions
Record the address of new machine
Lookup operation shall work
Another update shall be required to database in case it changes
again
Record the name of the new machine
Less efficient
Find the name of new machine
Lookup the address associated with the name
Addition of step to lookup operation
For highly mobile entities, it becomes only worse
24. Naming versus Locating Entities
Direct, single level mapping between names and addresses.
T-level mapping using identities.
25. Simple solutions: Broadcasting and multicasting
A location service accepts an identifier as input and
returns the current address of the identified entity.
Simple solutions exist to work in local area network.
Address Resolution Protocol (ARP) to map the IP address
of a machine to its data-link address, which uses
broadcasting.
Multicasting can be used to locate entities in point-to-
point networks (such as the Internet).
Each multicasting address can be associated with multiple
replicated entities.
32. Hierarchical Approaches (4)
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.
33. Pointer Caches (1)
Caching a reference to a directory node of the
lowest-level domain in which an entity will reside
most of the time.
34. Pointer Caches (2)
A cache entry that needs to be invalidated
because it returns a nonlocal address, while such
an address is available.
35. Scalability Issues
The scalability issues related to uniformly placing subnodes of a
partitioned root node across the network covered by a location
service.
36. The Problem of Unreferenced
Objects
An example of a graph representing objects
containing references to each other.
37. Reference Counting (1)
The problem of maintaining a proper reference
count in the presence of unreliable
communication.
38. Reference Counting (2)
a) Copying a reference to another process
and incrementing the counter too late
b) A solution.
39. Advanced Referencing Counting (1)
a) The initial assignment of weights in weighted
reference counting
b) Weight assignment when creating a new reference.
43. Reference Listing (1)
Skeleton Keeps track of Proxies
Instead of counting them maintain an explicit list of references
Adding/removing references to the list have no effect on the
fact the proxy is already exists/removed
Idempotent Operations
Repeatable without affecting the end result
Increment/decrement operation are clearly not
idempotent
44. Reference Listing (2)
Advantages
Don’t require reliable communication
Duplicate messages need not to be detected
Only insertion/deletion should be acknowledged
Easier to keep system consistent in case of process failures
Drawback
Scale badly
Solution
Leasing
45. Identifying Unreachable Entities
Trace based garbage collection
Scalability problems
Naïve tracing
Mark and sweep collectors
White, Grey, Black marks
Drawbacks
Reachability graphs need to remain same during both
phases
No process can run when GC is running
49. Conclusion
Naming, organization of names and name
resolution are key issue in any distributed systems
Locating entities is an open research issues. There
are few methods like Forwarding pointers,
hierarchical approaches, home based approaches
and pointer caches but each has its own short
comings
Reference counting, advanced reference counting
and Reference listing are few methods that can be
used for unreferenced objects
Fall-back mechanism for location services based on “Forwarding Pointers”
Draw Backs:
Increased Communication Latency: One has to Contact the Home even if the Host is present in Local network.
Home location must always exist
Solution: Two-Tiered Scheme, Locate the entity in local registry first, then contact the Entity’s Home location. (Mohan and Jain, 1994) applied it in Mobile Telephony.
Home Location be kept at traditional Naming Service and let the client first look up the location of Home. That location can be cached.
Global Location Service (Van Steen et al, 1998) representative of many Personal Communication Systems (Pitoura and Samaras, 2001, Wang 1993)
Network is divided into Hierarchy of Domains similar as DNS
Domain - Sub Domains Leaf Domain (LAN/Cell)
Each Entity present in a Domain D is represented by a Location Record in the directory node dir(D)
Each Location record stores a pointer to the directory node of the next entity, where each location record stores a pointer to the directory node of next lower level sub-domain.
Lookup operations exploit LOCALITY
Insertion is Installing a Chain of Pointers in top-Down fashion
Deletion is Analogous to Insertion.
Delete process continues until a pointer is removed from a location record that remains nonempty afterwards
Storing lookup results in traditional Location Services is highly effective because the entities are STATIONARY.
For Mobile Entities, caching is not effective. But E moves in D regularly, then a reference to dir(D), can, in principal, be cached at every node along the path from the leaf node where the lookup was initiated
Pointer Caching Approach is described by (Jain, 1996) Global Location Service (Van Steen, 1998); (Baggio et al, 2000)
Improvements:
By letting dir(D) store actual location of E, instead of a pointer to sub-domain. It shall make lookup operation in only two steps 1) get appropriate directory node 2) get the actual location of the E
Open Questions:
Which Domain pointer should be cached if E moves in two domains regularly.
When to invalidate the cache entry
Problem:
Root node is required to store a location record for each entity and to process requests for entity
Storage: Location record 1 KB, Billion records take on tera byte 10 100 GB disks
Looup/update request processing: single root node becomes bottleneck
Solution:
Partition the root node/high-level directories into sub nodes. Each sub node is reponsible for a specific sub set of all entities.
Question: Where to physically place each sub node in the network.
Answer 1) centralized approach. Keep all at the same place. And root node is implemented by means of parallel computer
Having a remote reference to an object doesn’t mean that the object will always be accessible
Uni-processor systems VS distributed systems
(Plainfosse and Shpiro, 1995) and (Abdullah and RingWood, 1998).
Popular Method in Uni-processor systems.
Problems:
Unreliable Communication
If no special measures are taken to detect duplicate messages then skeleton may falsely increment its reference counter again.
When a remote reference is to be removed and message is lost again.
Passing a reference requires three messages. That is performance impact in distributed systems.
Only Decrement counting can be used to maintain reference integrity. Weighted Reference Counting
Problem: Only a limited number of references can be made.
Use of Indirection:
This is similar to forwarding pointers and suffer from the same problems.
Chains are performance degrading
Chains are more Susceptible to failure
Generation Reference Counting:
Each remote reference is created as Proxy/Skeleton pair. (p, s)
Each proxy has a generation number. When created it is set to zero. When reference is copied and new proxy is made, this number adds up.
Skeleton maintains a table G in which G[i] denotes the outstanding copies of generation i.
When a proxy is removed, the Copy Counter (n) and Generation number (k) is sent to Skeleton.
Skeleton adjusts G by decrementing G[k] by one and incrementing G[k+1] by n
Advantages: Handle duplicate references without the need to contact skeleton at proxy creation time
Java RMI based on (Birrell et al, 1993)
Naïve tracing in Distributed Systems: Emerald Systems, (jul et al, 1998)
(Lang et al, 1992)
Only for (proxy, skeleton) sets. Distributed GC.
Address Scalability. Basic idea is to let low level groups collect garbage, and leave the analysis of inter group references
Garbage Reclamation is actually be performed by the Local GC.