a common api for structured peer-to-peer overlays
DESCRIPTION
A Common API for Structured Peer-to-Peer Overlays. Frank Dabek, Ben Y. Zhao , Peter Druschel, Ion Stoica. Structured Peer-to-Peer Overlay. They are: Scalable, self-organizing overlay networks Provide routing to location-independent names Examples: CAN, Chord, Pastry, Tapestry, … - PowerPoint PPT PresentationTRANSCRIPT
A Common API for Structured Peer-to-Peer OverlaysFrank Dabek, Ben Y. Zhao,Peter Druschel, Ion Stoica
OceanStore / Sahara Retreat [email protected] January 14, 2003
Structured Peer-to-Peer Overlay They are:
Scalable, self-organizing overlay networks Provide routing to location-independent names Examples: CAN, Chord, Pastry, Tapestry, …
Basic operation: Large sparse namespace N
(integers: 0–2128 or 0–2160) Nodes in overlay network have nodeIds N Given k N, a deterministic function maps k
to its root node (a live node in the network) route(msg, k) delivers msg to root(k)
OceanStore / Sahara Retreat [email protected] January 14, 2003
Current Progress Lots of applications built on top
File systems, archival backup Application level multicast Routing for anonymity, attack resilience
But do we really understand them? What is the core functionality that applications leverage from
them? What are the strengths and weaknesses of each protocol? How
can they be exploited by applications? How can we build new protocols customized to our future
needs?
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Our Goals Protocol comparison
Compare and contrast protocol semantics Identify basic commonalities Isolate and understand differences
Towards a common API Easily supportable by old and new protocols Enables application portability between protocols Enables common benchmarks Provides a framework for reusable components
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Talk Outline
Motivation
DHTs and DOLRs
A Flexible Routing API
Usage Examples
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Decomposing Functional Layers Distributed Hash Tables (DHT)
put(key, data), value = get(key) Hashtable layered across network Handles replication; distributes replicas randomly Routes queries towards replicas by name
Decentralized Object Location and Routing (DOLR) publish(objectId), route(msg, nodeId),
routeObj(msg, objectId, n) Application controls replication and placement Cache location pointers to replicas; queries quickly intersect
pointers and redirect to nearby replica(s)
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DHT Illustrated
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DOLR Illustrated
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Architecture
Routing Mesh Tier 0
Tier 1
Tier 2CFS PAST SplitStream i3 OceanStore
Replication
Bayeux
MulticastCAN,Chord+DHash
DHT TapestryPastry+Scribe
DOLR
OceanStore / Sahara Retreat [email protected] January 14, 2003
Talk Outline
Motivation
DHTs and DOLRs
A Flexible Routing API
Usage Examples
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Flexible API for Routing Goal
Consistent API for leveraging routing mesh Flexible enough to build higher abstractions
Openness promotes new abstractions Allow competitive selection to determine right abstractions
Three main components Invoking routing functionality Accessing namespace mapping properties Open, flexible upcall interface
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API (routing)Data types Key, nodeId = 160 bit integer Node = Address (IP + port #), nodeId Msg: application-specific msg of arbitrary size
Invoking routing functionality Route(key, msg, [node])
route message to node currently responsible for key Non-blocking, best effort – message may be lost or duplicated. node: transport address of the node last associated with key
(proposed first hop, optional)
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API (namespace properties) nextHopSet = local_lookup(key, num, safe)
Returns a set of at most num nodes from the local routing table that are possible next hops towards the key.
Safe: whether choice of nodes is randomly chosen nodehandle[ ] = neighborSet(max_rank)
Returns unordered set of nodes as neighbors of the current node. Neighbor of rank i is responsible for keys on this node should all
neighbors of rank < i fail nodehandle[ ] = replicaSet(key, num)
Returns ordered set of up to num nodes on which replicas of the object with key key can be stored.
Result is subset of neighborSet plus local node boolean = range(node, rank, lkey, rkey)
Returns whether current node would be responsible for the range specified by lkey and rkey, should the previous rank-1 nodes fail.
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Deliver(key, msg) Delivers an incoming message to the application. One application
per node. Demultiplexing done by including demux key in msg. Forward(&key, &msg, &nextHopNode)
Synchronous upcall invoked at each node along route On return, will forward msg to nextHopNode App may modify key, msg, nextHopNode, or terminate by setting
nextHopNode to NULL. Update(node, boolean joined)
Upcall invoked to inform app of a change in the local node’s neighborSet, either a new node joining or an old node leaving.
API (upcalls)
msg msgdeliver forward
msgRouting Layer Routing Layer
Application Application
OceanStore / Sahara Retreat [email protected] January 14, 2003
Talk Outline
Motivation
DHTs and DOLRs
A Flexible Routing API
Usage Examples
OceanStore / Sahara Retreat [email protected] January 14, 2003
DHT Implementation Interface
put (key, value) value = get (key)
Implementation (source S, root R) Put: route(key, [PUT,value,S], NULL)
Reply: route(NULL, [PUT-ACK,key], S) Get: route(key, [GET,S], NULL)
Reply: route(NULL, [value,R], S)
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DOLR Implementation Interface
RouteNode(msg, nodeId) Publish(objectId) RouteObj(msg, objectId, n)
Implementation (server S, client C, object O) RouteNode: route(nodeId, msg, NULL) Publish: route(objectId, [“publish”,O,S], NULL)
Upcall: addLocal([O,S]) RouteObj: route(nodeId, [n,msg], NULL)
Upcall: serverSet[] = getLocal(O);if (|serverSet|<n), route(nodeId, [n-|serverSet|,msg], NULL)for first n entries in serverSet, route(serverSet[i], msg, NULL)
OceanStore / Sahara Retreat [email protected] January 14, 2003
Conclusion Very much ongoing work
Feedback valuable and appreciated
Ongoing Work Implementations will support routing API Working towards higher level abstractions
Distributed Hash Table APIDOLR publish/route API
For more information, see IPTPS 2003
Thank you…
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Backup Slides Follow…
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Storage API: Overview linsert(key, value); value = lget(key);
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Storage API linsert(key, value): store the tuple <key,
value> into local storage. If a tuple with key already exists, it is replaced. The insertion is atomic wrt to failures of the local node.
value = lget(key): retrieves the value associated with key from local storage. Returns null if no tuple with key exists.
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To Do
Following slides contain functions that we haven’t decided on yet…
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Basic DHT API: Overview insert(key, value, lease); value = get(key); release(key);
Upcalls: insertData(key, value, lease);
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Basic DHT API
insert(key, value, lease): inserts the tuple <key, value> into the DHT. The tuple is guaranteed to be stored in the DHT only for “lease” time. “value” also includes the type of operations to be performed on insertion. Default operation types include REPLACE: replace value associated with the same
key APPEND: append value to the existing key UPCALL: generate an upcall to application before
inserting …
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Basic DHT API value = get(key): retrieves the value
associated with key. Returns null if no tuple with key exists in the DHT.
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Basic DHT API Release(key): releases any tuples with
key from the DHT. After this operations completes, tuples with key are no longer guaranteed to exist in the DHT.
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Basic DHT API: Open questions Semantics?
Verification/Access control/multiple DHTs? Caching?Replication?
Should we have leases? It makes us dependent on secure time sync.
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Replicating DHT API Insert(key, value, numReplicas); adds a
numReplicas argument to insert. Ensures resilience of the tuple to up to numReplicas-1 “simultaneous” node failures.
Open questions: consistency
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Caching DHT API
Same as basic DHT API. Implementation uses dynamic caching to balance query load.
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Resilient DHT API
Same as replicating DHT API. Implementation uses dynamic caching to balance query load.
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Publish API: Overview Publish(key, object); object = Lookup(key); Remove(key, object):
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Publish API Publish(key, object): ensures that the locally
stored object can be located using the key. Multiple instances of the object may be published under the same key from different locations.
object = Lookup(key): locates the nearest instance of the object associated with key. Returns null if no such object exists.
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Publish API Remove(key, object): after this operation
completes, the local instance of object can no longer be located using key.