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Replication

Replication. March 16, 2001. Replication. What is Replication? A technique for increasing availability, fault tolerance and sometimes, performance Replication can occur at many levels: Data Computation Communication Issues: Consistency and Coordination. Replication, cont.

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Replication

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  1. Replication March 16, 2001

  2. Replication • What is Replication? • A technique for increasing availability, fault tolerance and sometimes, performance • Replication can occur at many levels: • Data • Computation • Communication • Issues: Consistency and Coordination

  3. Replication, cont. • Replication Examples • Atomic writes via file logging • Performance hit - more than double the time to write • DNS vs. Grapevine • Consistency limits performance and scaling • RAID • Replication increases performance, but costs money • AFS • Possibility that merge is not practical

  4. Replication, cont. • Replication of Data in Transactions • One-copy semantics • To a transaction, a set of replicated objects should behave as one • Big Tradeoff - Performance under less than perfect conditions • Network failures • Server downtime

  5. Replication, cont. • Replication Model • Simple: Read-one/write all • Use locks across all replicas to ensure consistency • Problem - availability • If a replica is down, its objects cannot be updated until it comes back up again • Worse - if the network is partitioned, a whole set of objects will be affected

  6. Replication, cont. • Network Failures • Failures Partition a set of replicas • You have a variety of options to choose from: • Optimistic • Pessimistic • Key metrics for choice: • Resource contention • Frequency and duration of network failures

  7. Replication, cont. • Optimistic Approach • Allow any replica to act as a proxy for the whole • As partitions dissolve, merge updates • Use normal commit semantics if you can always merge safely • Alternative approach: manage inconsistencies outside of the transaction • e.g., ATMs, date book

  8. Replication, cont. • Example - Available Copies Replication • Each object that is replicated is managed by a replica manager • Any available replica manager can process requests • Reads from any one replica OK, writes propagate to available replicas • Transactions can lock objects locally at replicas • Two-phase commit at replica level to ensure consistency among replicas • Requires ability to detect server failures

  9. Replication, cont. • Pessimistic Approach - Quorum • One partition can be the proxy for the whole • Other partitions must wait for network failure to be resolved • Each object replica has a value + timestamp when the value was last updated • To read or write, you must have a quorum: • Writes - more than half the votes • Reads - more than (total votes minus minimum needed to write)

  10. Replication, cont. • Virtual Partition • Combine Quorum with available copies replication • Transaction acts in a virtual partition, artificially created • Virtual Partition is smallest quorum • Assumption - Virtual Partition small enough to practically allow available copies to succeed

  11. Transaction Layers • Putting it all together • Transactions • Atomicity • Recoverability • Implemented on top of: • Locks (mutual exclusion algorithms) • Non-volatile storage with atomic writes (logging) • To achieve performance and availability • Distribution and Replication Transaction implementation come in many flavors

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