Caching and Data Consistency in P2P Dai Bing Tian Zeng Yiming
Feb 22, 2016
Caching and Data Consistency in P2P
Dai Bing TianZeng Yiming
Caching and Data Consistency
Why CachingCaching helps use bandwidth more
efficiently The data consistency in this topic is different
from the consistency in distributed database It refers to the consistency between cached
copy and data on servers.
Introduction Caching is built based on current P2P arch
itectures like CAN, BestPeer, Pastry, etc. Caching layer is between application layer
and P2P layer. Every peer has its cache control unit and it
s local cache, and publish the cache contents
Presentation Order
We will present four papers, they areSquirrelPeerOLAPCaching for Range Queries
With CANWith DAG
Overview
Paper Based on Caching Consistency
Squirrel Pastry Yes Yes
PeerOLAP BestPeer Yes No
RQ with CAN CAN Yes Yes
RQ with DAG Not Specified
Yes Yes
Squirrel
Enables web browsers on desktop machines to share their local caches
Uses a self-organizing, peer-to-peer network Pastry as its object location service
Pastry is fault resilient, so is Squirrel
Web Caching Web browser generate HTTP GET requests If the object is in the local cache, return it if “f
resh” enough “freshness” can be checked by submitting c
GET request If no such object, issue GET request to the s
erver For simplicity, we assume objects are cache
able
Home Node
As described in Pastry, every peer (node) has its nodeID
objectID = SHA-1 (obj URL) This object is assigned to the node whose
ID is numerically nearest to the objectID The node who owns this object is called th
e home node of this object
Two approaches There are two approaches of Squirrel
Home-storeDirectory
Home-store stores the object directly in the cache of the home node
Directory stores the pointer to the nodes who have this object in its cache, these nodes are called delegates
Home-store
Origin ServerRequester
HomeNode
LAN
WAN
RequestRoutedThroughPastry
Request for AIs my copy of A fresh?
Send A over
Yes, it is fresh Request for A
Is my copy of A fresh?
Send A over
Yes, it is fresh
Directory Origin Server
HomeNode
Requester
LANWAN
RequestRoutedThroughPastry
Delegate
Request for A
Get it from D
Request for A
Send A over
No directoryGet it from Server
Request for A
Send A over
Send A over
Request for A
I’m your delegate
Is my copy of A fresh?
Yes, it is fresh
Update Meta-infoKeep the directory
Requester and I are your delegates
Conclusion The home-store approach is less
complicated, but it does not have any collaboration
The directory approach is more collaborative, it has the ability to store more objects in those peers with larger cache capacity, by setting the pointers to these peers in the directory
PeerOLAP OnLine Analytical Processing (OLAP) quer
y typically involves large amounts of data Each peer has a cache containing some re
sults An OLAP query can be answered by combi
ning partial results from many peers PeerOLAP acts as a large distributed cach
e
Data Warehouse & Chunk “A data warehous
e is based on a multidimensional data model which views data in the form of a data cube.”
–Han & Kamber
Date
Product
Country
sum
sum TV
VCRPC
1Qtr 2Qtr 3Qtr 4QtrU.S.A
Canada
Mexico
sum
http://www.cs.sfu.ca/~han/dmbook
PeerOLAP network LIGLO servers provide global name lookup
and maintain a list of active peers
Except for LIGLO servers, the network is fully distributed without any centralized administration point
LIGLO
Data Warehouse Peer
Query Processing
Assumption 1: Only chunks at the same aggregation level as the query are considered
Assumption 2: The selecting predicates is a subset of grouping-by predicates
Cost Model Every chunk is associated with a cost
value, indicating how long it spends to get this chunk
PQTr
csizekQPCnPQcN
,
PQcNQcSPQcT ,,,
Eager Query Processing (EQP) Peer P sends requests for the missing chu
nks to all its neighbors, Q1, Q2, .... Qk
Each Qi provides the desired chunks as many as possible, return to P with a cost associated with each chunk
Qi then propagates the requests to all its neighbors recursively
In order to avoid flooding, hmax is set to limit the depth of the search
EQP (Contd.) P collects (chunk, cost) pairs from all its neigh
bors Random select one chunk ci, and find the peer
who can provide it with lowest cost, Qi For the subsequent chunks, it evaluates the mi
nimum of two cases: the peer with lowest cost is not connected yet, or some existing peer who can also provide this chunk
Ask for chunks from these peers and the rest missing chunks from the warehouse.
Lazy Query Processing (LQP)
Instead of propagating the requests from each Qi to all its neighbors, each Qi selects its most beneficial neighbor, and forward the request.
Given the expected number of neighbors a peer has is k, EQP will visit O(k^hmax) nodes, LQP only visit O(khmax)
Chunk Replacement Least Benefit First (LBF)
csize
QPHaPQcTPcB
,,
Similar to LRU, every chunk has a weight
Once the chunk is used by P, its weight is set back to the original benefit value
Every time there is a new chunk come in, the weight of old chunks will reduce
Collaboration
LBF gives local chunk replacement algorithm 3 variations of global behavior
Isolated Caching Policy: non-collaborativeHit Aware Caching Policy: collaborativeVoluntary Caching: highly collaborative
Network Reorganization
Optimization can be done by creating virtual neighborhoods of peers with similar query patterns
So that there is a high probability for P to get missing chunks directly from neighbors
Each connection is assigned a benefit value and the most beneficial connections are selected to be the peer’s neighbors
Conclusion
PeerOLAP is a distributed caching system for OLAP results
By sharing the contents of individual caches, PeerOLAP constructs a large virtual cache which can benefit all peers
PeerOLAP is fully distributed and highly scalable
Caching For Range Queries Range Query:
E.g. SELECT Student.name
WHERE 20<Student.age<30 Why Cache?
Data source too far away from the requesting node Data source overloaded with queries Data source is a single point of failure
What to cache? All tuples falling in the range
Who cache? Peers responsible for the range
Problem Definition
Given a relation R, and a range attribute A, we assume that the results of prior range-selection queries of the form R.A(LOW, HIGH) are stored at the peers. When a query is issued at a peer which requires the retrieval of tuples from R in the range R.A(low, high), we want to locate a peer in the system which already stores tuples that can be accessed to compute the answer.
A P2P Framework for Caching Range Queries Based on CAN. Map data into 2d virtual space, where d is # dim
ensions of the relation. For every dimension/attribute, say its domain is
[a, b], it is mapped to a square virtual hash space whose corner coordinates are (a,a), (b,a), (b,b) and (a,b).
The virtual hash space is further partitioned into rectangular areas, each of which is called a zone.
Example Virtual hash space for an
attribute whose domain is [10,70]
zone-1: <(10,56),(15,70)>zone-5: <(10,48),(25,56)>zone-8: <(47,10),(70,54)>
Terminology Each zone is assigned to a peer. Active Peer
Owns a zone Passive Peer
Not participate in the partitioning, register itself with an active peer Target Point
A range [low,high] is hashed to a point with coordinates (low,high) Target Zone
Where the target point resides Target Node
The peer that owns the target zone “Stores” the tuples falling into the range which is mapped to the its
zone Caches the tuples in the local cache; OR Stores a pointer to the peer who caches the tuples
Zone Maintenance
Initially, only the data source is the active node and the entire virtual hash space is its zone
A zone split happens under two conditions:Heavy Answering LoadHeavy Routing Load
Example of Zone Splits If a zone has too
many queries to answer It finds the x-median
and y-median of the stored results. Determine if a split at x-median or y-median results in even distribution of stored answers and the space.
If a zone is overloaded because of routing queries It splits the zone from
the midpoint of the longer side.
Answering A Range Query
If an active node poses the query, the query is initiated from the corresponding zone; if a passive node poses the query, it contacts any active node from where the query starts routing.
2 steps involvedQuery RoutingQuery Forwarding
Query Routing
If the target point falls in this zone
Return this zone Else
Route the query to the neighbor who is closest to the target point
(26,30)
Query Routing
If the target point falls in this zone
Return this zone Else
Route the query to the neighbor who is closest to the target point
(26,30)
Query Routing
If the target point falls in this zone
Return this zone Else
Route the query to the neighbor who is closest to the target point
(26,30)
Forwarding
If the results are stored in the target node, then the results are sent back to the querying node
Else, it is still possible that zones lie in the upper left area of the target point store the results. So we need to forward the query to these zones too.
Example If no results are found
in zone-7, the shaded region may still contains the results.
Reason: Any prior range query q whose range subsumes (x,y) must be hashed into the shaded region.
Forwarding (Cont.)
How far should it go? For a range (low,high), w
e want to restrict to results falling in (low-offset,high+offset), where offset = AcceptableFit x |domain|.
AcceptabelFit [0,1] The shaded square defin
ed by the target point and offset is called the Acceptable Region
offset
Forwarding (Cont.) Flood Forwarding
A naïve approach. Forward to the left and top neighbors if they fall in the acceptable region
Directed Forwarding Forward to the neighbor
that maximally overlaps with the acceptable region
Can bound the number of forwards by specifying a limit d, which is decremented for every forward.
Discussion
ImprovementsLookup During RoutingWarm up queries
Peer soft-departure & Failure event Update—cache consistency
Say a tuple t with range attribut a=k is updated in the data source, then the target zone of point (k,k) and all zones lie in the upper left region have to update their cache.
Range Addressable Network: A P2P Cache Architecture for Data Ranges Assumption:
Tuples stored in the system are labeled 1,2,…,N according to the range attribute
A range [a,b] is a contiguous subset of {1,2,…,N}, where 1<=a<=b<=N
Objective: Given a query range [a,b], peers cooperativel
y find results falling in the shortest superset of [a,b], if they are cached somewhere.
Overview
Based on Range Addressable DAG (Directed Acyclic Graph)
Map every active node in the P2P system to a group of nodes in the DAG
A node is responsible for storing results and answering queries falling into a specific range
Range Addressable DAG
The entire universe [1,N] is mapped to the root.
Recursively divide one node into 3 overlapping intervals of equal length.
Range LookupInput: a query range q=[a,b],
a node v in DAGOutput: the shortest range in
DAG that contains qboolean down=true;search (q, v){
if q i(v)search (q, parent(v));
if q i(child(v)) & downsearch (q, child(v));
elseif some range stored at v is a superset of q
return the shortest range containing q that is stored at v or parent(v); (*)
elsedown=false;search(q,parent(v));
}
Q: [7,10]
[5,12]
[7,13]
Peer Protocol
Maps the logical DAG structure to physical peers
Two componentsPeer Management
Handles peer joining, leaving, failureRange Management
Deals with query routing and updates
Peer Management
It ensures that at any time, every node in the DAG is assigned to some
peer the nodes belonging to one peer, called a
zone, is a connected component of the DAG This is done by handling Join Request,
Leave Request, Failure Event properly.
Join Request
The first peer joining the system takes over the entire DAG
A new peer joining the system contacts one of the peers in the system to take over one of its child zones. Default strategy: left child, then mid child, then right child.
Join Request
The first peer joining the system takes over the entire DAG
A new peer joining the system contacts one of the peers in the system to take over one of its child zones. Default strategy: left child, then mid child, then right child.
Join Request
The first peer joining the system takes over the entire DAG
A new peer joining the system contacts one of the peers in the system to take over one of its child zones. Default strategy: left child, then mid child, then right child.
Join Request
The first peer joining the system takes over the entire DAG
A new peer joining the system contacts one of the peers in the system to take over one of its child zones. Default strategy: left child, then mid child, then right child.
Leave Request
When a peer wants to leave (soft departure), it hands over its zone to the smallest neighboring zone.
Neighboring zones: there is a parent-child relationship among any nodes in the zones
Leave Request
When a peer wants to leave (soft departure), it hands over its zone to the smallest neighboring zone.
Neighboring zones: there is a parent-child relationship among any nodes in the zones
Failure Event
A zone maintains info on all its ancestors. So in case it finds out one of its parents failed, it contacts the nearest alive ancestor for zone takeover.
Range Management
Range Lookup Range Update
When a tuple is updated in the data source, we locate the peer with the shortest range containing that tuple, then update this peer and all its ancestors.
Improvement
Cross Pointers For a node v, if it’s the
left child of its parent, then it keeps cross pointers to all the left children of nodes that are in its parent’s level.
Similarly for mid child.
Improvement (Cont.) Load Balancing by Peer Sampl
ing Collapsed DAG: collapse each
peer’s zone to a single node. The system is balanced if the
collapsed DAG is balanced. Lookup time is O(h) where h is
the height of the collapsed DAG. Hence a balanced system leads to optimal performance.
When a new peer joins, it polls k peers randomly, and send join request to the one whose zone is rooted nearest to the root.
P2
P1
P3
Collapsed DAG: P1
P2 P3
Improvement (Cont.) Load Balancing by Peer Sampl
ing Collapsed DAG: collapse each
peer’s zone to a single node. The system is balanced if the
collapsed DAG is balanced. Lookup time is O(h) where h is
the height of the collapsed DAG. Hence a balanced system leads to optimal performance.
When a new peer joins, it polls k peers randomly, and send join request to the one whose zone roots nearest to the root.
Collapsed DAG:
Conclusion
Caching Range Queries based on CANMaps every attribute into a 2D spaceThe space is divided into zonesPeers manage their respective zonesA range [low,high] is mapped to a point (low,hi
gh) in the 2D spaceQuery Routing & Query Forwarding
Conclusion (Cont.)
Range Addressable NetworkModel ranges as DAGEvery peer takes responsibility of a group of
nodes in DAGQuerying involves traversal of the DAG