15-replication - Replication Howd we get here? Failures...

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Unformatted text preview: Replication Howd we get here? Failures & single systems; fault tolerance techniques added redundancy (ECC memory, RAID, etc.) Conceptually, ECC & RAID both put a master in front of the redundancy to mask it from clients -- ECC handled by memory controller, RAID looks like a very reliable hard drive behind a (special) controller Simpler examples... Replicated web sites e.g., Yahoo! or Amazon: DNS-based load balancing (DNS returns multiple IP addresses for each name) Hardware load balancers put multiple machines behind each IP address (Diagram. :) Read-only content Easy to replicate - just make multiple copies of it. Performance boost: Get to use multiple servers to handle the load; Perf boost 2: Locality. Well see this later when we discuss CDNs, can often direct client to a replica near it Availability boost: Can fail-over (done at both DNS level -- slower, because clients cache DNS answers -- and at front-end hardware level) But for read-write data... Must implement write replication, typically with some degree of consistency Important ?: What consistency model? Just like in Flesystems, want to look at the consistency model you supply R/L example: Google mail. Sending mail is replicated to ~2 physically separated datacenters (users hate it when they think they sent mail and it got lost); mail will pause while doing this replication. Q: How long would this take with 2-phase commit? in the wide area? Marking mail read is only replicated in the background - you can mark it read, the replication can fail, and youll have no clue (re-reading a read email once in a while is no big deal) Weaker consistency is cheaper if you can get away with it. Strict transactional consistency (you saw before) sequentially consistent : if client a executes operations {a1, a2, a3, ...} , b executes {b1, b2, b3, ...}, then you could create some serialized version (as if the ops had been performed through a single server) a1, b1, b2, a2, ... (or whatever) executed by the clients using a central server Note this is not transactional consistency - we didnt enforce preserving happens-before. Its just per-program ailure model Well assume for today that failures and disconnections are relatively rare events - they may happen pretty often, but, say, any server is up more than 90% of the time....
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15-replication - Replication Howd we get here? Failures...

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