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Consistency and Replication

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  1. Consistency and Replication Chapter 6

  2. Object Replication (1) • Organization of a distributed remote object shared by two different clients.

  3. Object Replication (2) • A remote object capable of handling concurrent invocations on its own. • A remote object for which an object adapter is required to handle concurrent invocations

  4. Object Replication (3) • A distributed system for replication-aware distributed objects. • A distributed system responsible for replica management

  5. Data-Centric Consistency Models • The general organization of a logical data store, physically distributed and replicated across multiple processes.

  6. Strict Consistency time • Behavior of two processes, operating on the same data item. • A strictly consistent store. • A store that is not strictly consistent. • Idea: All writes are instantaneously propagated to all processes!

  7. Sequential Consistency (1) Idea: Results appear as if operations of different processes were executed sequentially, and operations within a process appear in the order of its program. • A sequentially consistent data store. P3 and P4 see same sequences • A data store that is not sequentially consistent. • P3 and P4 see different sequences

  8. Sequential Consistency (2) • Four valid execution sequences for the processes. The vertical axis is time.

  9. Causal Consistency (1) concurrent • This sequence is allowed with a causally-consistent store, but not with sequentially or strictly consistent store. • Idea: If writes are causally related, they appear in the same order, otherwise there is no restriction for their order.

  10. Causal Consistency (2) • A violation of a causally-consistent store. • A correct sequence of events in a causally-consistent store.

  11. FIFO Consistency Idea: Writes done by a single process are seen by all other processes in the order in which they were issued, but writes from different processes may be seen in a different order by different processes. • A valid sequence of events of FIFO consistency

  12. Weak Consistency Idea: Use of an explicit synchronization operation S. When called, everything is made consistent: local changes are propagated to other processes and remote changes are propagated to local process. • A valid sequence of events for weak consistency. • An invalid sequence for weak consistency.

  13. Release Consistency Idea: Why only one synchronization operation, better two: Acquire and Release. This enhances performance relatively to Weak Consistency: 1. Acquire: All data are brought from remote sites. 2. Release: All local changes are made visible to others. • A valid event sequence for release consistency.

  14. Entry Consistency Idea: Even Release Consistency is improvable: 1. Delay propagation after a release until a remote process acquires the data  Lazy Release Consistency 2. Do not do that for all data, rather used ones.  Associate locks with (individual) shared data items. • A valid event sequence for entry consistency.

  15. Summary of Consistency Models • Consistency models not using synchronization operations. • Models with synchronization operations.

  16. Eventual Consistency R(x)b ??? Previously written a has not yet been propagated to this replica W(x)a • The principle of a mobile user accessing different replicas of a distributed database. • Eventual consistency: few updates that propagate gradually to all replicas. • High degree of inconsistency is tolerated • E.g. WWW • Problem: mobile users (see figure); solution: client-centric consistency models.

  17. Monotonic Reads • The read operations performed by a single process P at two different local copies of the same data store. • A monotonic-read consistent data store: WS(x1;x2) means that version x1 has been propagated to L2. • A data store that does not provide monotonic reads: WS(x2) does not include version x1! • Idea: If a client reads some value, a subsequent read returns that value or a newer one (but never an older one).

  18. Monotonic Writes {client writes x1} x1 propagated {client writes x2} • The write operations performed by a single process P at two different local copies of the same data store • A monotonic-write consistent data store: previous value x1 has been propagated. • A data store that does not provide monotonic-write consistency: value x1 has not been propagated. • Idea: If a client writes some value A, a subsequent write will operate on the most recently written value A (or a newer one).

  19. Read Your Writes {client writes x1} x1 propagated {client reads x2} • A data store that provides read-your-writes consistency. • A data store that does not. • Idea: If a client writes some value A, a subsequent read will return the most recently written value A (or a newer one).

  20. Writes Follow Reads {client reads x1} x1 propagated {client writes x2 using version x1} • A writes-follow-reads consistent data store • A data store that does not provide writes-follow-reads consistency • Idea: If a client reads some value A, a subsequent write will operate on the most recently read value A (or a newer one).

  21. Replica Placement • The logical organization of different kinds of copies of a data store into three concentric rings. • Permanent replicas: such as replicas in a cluster or mirrors in WAN. • Server-initiated replication: Server keeps track of clients that frequently use data, and it replicates/migrates the data closer to clients, if needed. • push cashes • Client-initiated replication: Client uses a cache for most frequently used data.

  22. Server-Initiated Replicas • Counting access requests from different clients. • In the figure: Server Q owns a copy of file F and counts the accesses originating from • clients C1 and C2 on behalf of server P. If necessary, Q decides to • migrate/replicate the file to/in the new location P.

  23. Pull versus Push Protocols for Update Propagation • A comparison between push-based and pull-based protocols in the case of multiple client, single server systems. • Update propagation: • Push-based: server initiates the update propagation to other replicas (e.g. in • permanent/server-initiated replication). •  Use of “short” leases (time period in which an update is guaranteed) • may help server reduce its state and be more efficient. • Pull-based: Client requests a new update from server (rather with client-initiated • replication).

  24. Remote-Write Protocols (1) • Primary-basedremote-write protocol with a fixed server to which all read and write operations are forwarded. • In this consistency protocol, there is only one copy (i.e. original) of an item x (no replication). • Writes and reads go through this remote copy (well-suited for sequentially consistency)

  25. Remote-Write Protocols (2) • The principle of primary-backup protocol. • In this consistency protocol, there are multiple copies of an item x (replication). • Writes go through a primary copy and are forwarded to backups, reads may work locally (also well-suited for sequential consistency, since primary serializes updates).

  26. Local-Write Protocols (1) • Primary-based local-write protocol in which a single copy is migrated between processes. • In this consistency protocol, there is only one copy of an item x (no replication). • Writes and reads are performed after migrating the copy to the local server.

  27. Local-Write Protocols (2) • Primary-backup protocol in which the primary migrates to the process wanting to perform an update. • In this consistency protocol, there are multiple copies of an item x (replication) • Writes are performed after migrating primary to local server; updates are forwarded to other backup locations. Reads may proceed locally.

  28. Active Replication (1) e.g. withdraw from my account $300 instead of $100 ! • The problem of replicated invocations. • Active replication: Each replica has its own process for carrying updates. • Problems: 1) Updates need to be made in same order ( timestamps). • 2) Invocations cause a problem (see figure).

  29. Active Replication (2) • Forwarding an invocation request from a replicated object. • Returning a reply to a replicated object.

  30. Quorum-Based Protocols N: number of replicas (here 12) NR : number of replicas in read quorum NW: number of replicas in write quorum NR + NW > N (1) NW > N/2 (2) (a) (b) • Examples of the voting algorithm: • A correct choice of read and write set: Any three will include one from write quorum (latest version). • A correct choice, known as ROWA (read one, write all): Read from any replica will lead to latest version, but writes involves all replicas.

  31. Orca OBJECT IMPLEMENTATION stack; top: integer; # variable indicating the top stack: ARRAY[integer 0..N-1] OF integer # storage for the stack OPERATION push (item: integer) # function returning nothing BEGIN GUARD top < N DO stack [top] := item; # push item onto the stack top := top + 1; # increment the stack pointer OD; END; OPERATION pop():integer; # function returning an integer BEGIN GUARD top > 0 DO # suspend if the stack is empty top := top – 1; # decrement the stack pointer RETURN stack [top]; # return the top item OD; END;BEGIN top := 0; # initializationEND; • A simplified stack object in Orca, with internal data and two operations.

  32. Management of Shared Objects in Orca • Four cases of a process P performing an operation on an object O in Orca.