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Advanced Topics in Distributed Systems The Chubby lock service for loosely- coupled distributed systems Mike Burrows, Google Inc.Presented by: Ioanna Tsalouchidou 1

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Outline● Introduction● Design● Scaling● Usage● Summary 2

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Introduction● What is Chubby? – An engineering effort by Google – Coarse-grained lock service – Loosely-coupled distributed systems – Large number of small machines – High-speed network (1Gbit/s Ethernet) – Availability and Reliability Vs. Performance 3

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Outline● Introduction - Motivation● Design● Scaling● Usage● Summary 4

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Design le iona Rat ks Se Loc qu en Sy st em ce str rs uc tur rs e handle File Fa Backup es il-o pAliv ve Kee rs Dir Mirroring ec tor Events ies nsSessio Ca se ch a Ba API ing Dat 5

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Rationale● Lock service library Vs. Paxos client library – Maintain existing program structure – Advertising the results – Familiar interface to programmers – Reduces the # of servers needed for progress● Design decision – Small-files service (for primaries) – Event notification – Consistent caching 6

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Rationale● Coarse-grained locks – Less load in the lock server – Less delay caused by lock server unavailability – Survive lock server failures – Many clients served by less servers● Fine-grained locks – Lock-server unavailability → many clients stall 7

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System Structure● One Chubby cell → five servers● Client ↔ Server: via client library● Server and library communicate via RPC 8● Optional: proxy server

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Chubby Cell● Set of five servers (replicas) :-)● Distributed consensus protocol● One master among replicas● Master lease → periodically renewed 9

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Chubby Client ● Clients find the master between replicas ● Client requests to master ● Write requests via consensus protocol among replicas ● Read requests handled by masterApp Library 10

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Files, directories and handles● Interface: similar to UNIX file system /ls/foo/wombat/pouch 11

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Files, directories and handles● Interface: similar to UNIX file system /ls/foo/wombat/pouch Stands for lock service. Common to all Chubby names 12

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Files, directories and handles● Interface: similar to UNIX file system /ls/foo/wombat/pouch Stands for lock service. Common to all Chubby names The name of a Chubby cell, which is resolved to one or more servers through a DNS lookup 13

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Files, directories and handles● Interface: similar to UNIX file system /ls/foo/wombat/pouch Stands for lock service. Common to all Chubby names The name of a Chubby cell, which is resolved to one or more servers through a DNS lookup The name of the directory and the file 14

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Files, directories and handles● Interface: similar to UNIX file system /ls/foo/wombat/pouch● Name space: files and directories called nodes – Ephemeral – Permanent● Clients open nodes to obtain Handles – ~ UNIX file descriptors – Subscription to events 15

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Locks● Any node can act as a lock● Advisory locks (no mandatory locks)● Client handles hold – Exclusive locks (write) – Shared locks (read)● No write permission → no influence to writers 16

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Sequencers● Sequence numbers to interactions that use locks● Lock holder requests a sequencer● Opaque bite-string → state of lock – Name of lock – Mode which was acquired – Lock generation number 17

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Caching● To reduce read traffic – File data – Meta-data – File absence – Consistent, write-through cache 18

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Fail-overs 19

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Fail-overs 20

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Fail-oversOOOPS 21

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Fail-oversOOOPS 22

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Outline● Introduction - Motivation● Design● Scaling● Usage● Summary 24

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Current scaling techniques● Reduce communication with the master – Client caching – Open handles● Use a nearby cell (found with DNS)● Increase lease time (12s → 60s)● Protocol-conversion servers – Translate Chubby to less-complex 25

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Future scaling techniques● Proxies – Handling KeepAlive requests – Read requests● Partitioning – Enable large Chubby cells – Little communication between partitions 26

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Outline● Introduction● Design● Scaling● Usage● Summary 27

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Usage● Initially as a lock service● Most popular use as a name service● GFS: to appoint a master server● Bigtable – Master election – Master discovers its controlling servers – Clients to find their master 28

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Summary● Distributed lock service – Coarse grained synchronization● Based on existing ideas and algorithms● Primary internal name service● Reliable but low-volume storage 29

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Advanced Topics in Distributed Systems The Chubby lock service for loosely- coupled distributed systems Mike Burrows, Google Inc.Presented by: Ιωάννα Τσαλουχίδου 30