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Cristian Lumezanu Neil Spring Bobby Bhattacharjee Decentralized Message Ordering for Publish/Subscribe Systems

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Ordering? ABCD P1P2 m1< m2 Publishers Subscribers m2 < m1 Subscribers may observe an ambiguous order of messages m1m2

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Applications Network Games Subscribers = players Messages = events in the region of the game world to which the player belongs Common events must be seen in the same order for consistency Messaging Chat rooms, buddy lists Example of messages in a chat room Alice : Who wants to go to Sydney? Bob : I do Connor : Who wants to go to Melbourne? Diane: I am going Bob goes to Sydney, Diane goes to Melbourne Diane goes to Sydney, Bob goes to Melbourne

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Naive solution ABCD P1P2 Publishers Subscribers

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Naive solution ABCD P1P2 Publishers Subscribers Sequencer Not scalable Central point of failure Distribute the task of ordering to many sequencers

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Our solution ABCD P1P2 Publishers Subscribers Sequencer Network Scalable | Practical

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Groups GROUP: all subscribers with the same subscription Order among messages is enforced across groups RULE 1: A sequencer (ingress-only sequencer) is associated with each group and establishes order among all messages addressed to the group except for… ABCD G0 G1 EFG m0, m1, …

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Double Overlapped Groups DOUBLE OVERLAPPED GROUPS: groups that have at least two subscribers in common Receivers may make inconsistent decisions about message order when they belong to double overlaps RULE 2: A sequencer is associated with each double overlap ABCD G0 G1 EFG m0, m1, … D: m0 < m0 < m1 < m1 E: m0 < m1 < m0 < m1

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencing scheme SEQUENCING NETWORK A sequencer is created for each double overlap between groups and for each group that has no double overlaps MESSAGE TRANSMISSION Messages traverse the sequencing network and receive sequence numbers from all sequencers associated with the destination group MESSAGE RECEPTION Subscribers order messages unambiguously according to the sequence numbers

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencing Network: Construction Q0 Q1 G0 = {A, B, C} G1 = {B, C, E} G2 = {A, B, D} G3 = {B, E} to G0 to G2to G1 Q2 to G3 1.All members of the same group see the common messages in the same order 2. All destinations can make an immediate decision of whether to deliver or buffer arriving messages Properties

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencing Network: Operation Q0 Q1 G0 = {A, B, C} G1 = {B, C} G2 = {A, B, D} to G0 to G2to G1 m0| | When a message arrives, the receiver checks the sequence numbers assigned by the relevant sequencers and decides whether to deliver or buffer the message Q0 Q1 m0 m1 m2 1 2 12 m1| |m2| |m2| | 1m0| 1 | 2 m#| Q0 | Q1 m0| 1 |m1| 2 |

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 C1: A single path must connect sequencers associated to each group C2: The undirected sequencing graph must be loop free Sequencing Network: Conditions Q0 Q1 Q2 Conditions

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Loop-free sequencing network Q0 Q1 G0 = {A, B, D} G1 = {A, B, C} G2 = {B, C, D} to G0 to G2 to G1 Q2 Q0 Q1 Q2 m0 m1 m2 1 2 1 2 1 2 B: m0 < m1 AMBIGUOUS m0| | |m1| | |m1| 2 | |m1| 2 | | 1m2| | |m2| | 1 |m2| | 1 | 2 m0| 1 | 2 | m0| 1 | | m#| Q0 | Q1 | Q2 < m2< m0

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Loop-free sequencing network Q0 Q1 G0 = {A, B, D} G1 = {A, B, C} G2 = {B, C, D} to G0 to G2 to G1 Q2 Q0 Q1 Q2 m0 m1 m2 2 1 1 2 1 2 B: m2 < m0 < m1 UNAMBIGUOUS m0| | |m1| | |m2| | |m2| | 1 |m2| | 1 | 1 m0| 1 | 2 | m0| 1 | | m#| Q0 | Q1 | Q2 m1| 2 | | 2m1| 2 | |

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Results QUESTIONS What is the delay penalty incurred by the sequencing network? How many sequence numbers does each message receive? EXPERIMENT SETUP Packet-level simulator over a 10,000 node topology End-hosts arranged into similar sized clusters distributed uniformly at random through the topology Each host belongs to zero or more groups The size of groups is generated from a Zipf distribution Sequencers are assigned to physical nodes using a heuristic that minimizes the distance between sequencers on the same path

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Latency Stretch ratio between the time taken for a message to traverse the sequencing network and time taken using the direct unicast path expresses the delay penalty of an individual node when unambiguous delivery is required worst case results since shortest unicast paths are rarely followed in publish/subscribe systems How is the increase in delay distributed? sub-linear growth

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Distribution of latency increase The highest ratios correspond to pairs in which sender and destination are very close to each other

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencers on a Path How many sequence numbers a message must collect Vector timestamp approaches Sender belongs to the destination group Append to a message information about the last message received from all the other members of the group, for each group O(n x g) information [n nodes, g groups] Our approach Appends to a message information for each sequencer traversed O(g)

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencers on a Path The number of sequencers on a path is less than half of the total number of nodes that participate

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Conclusions and Future Work CONCLUSIONS Method for ordering messages in a publish/subscribe system Practical and scalable Key insight: only messages to groups with two or more common members must be ordered FUTURE WORK Scheme for optimizing the sequencing network and the placement of sequencers on physical nodes Dynamic behavior Different models for group membership

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Thank You!

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Backup slides

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Sequencer state State maintained by a sequencer Sequence number Group-local sequence number Forwarding table Reverse-path table Output retransmission buffer Buffer for messages from previous sequencers

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Placing sequencers Co-locating sequencers on the same physical node 1. Place on the same physical node any sequencers whose corresponding overlaps have a subset relationship between them 2. Co-locate sequencers whose overlaps do not have a subset relationship but share at least a node Mapping co-located sequencers (sequencing node) to physical machines 1. If no sequencing node associated with a group has been mapped, map one at random 2. If there are sequencing nodes already mapped to a physical node, pick the closest unassigned sequencing node on the path associated to the group and map it to neighboring physical nodes

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Decentralized Message Ordering for Publish/Subscribe Systems Middleware 2006 Stress Stress of a sequencing node – ratio between the number of groups for which it has to forward messages and the total number of groups

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