Google Megastore

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Megastore - Providing Scalable, Highly Available

J. Baker, C. Bond,

J.C. Corbett, JJ Furman,

A. Khorlin, J. Larson,

J-M Léon, Y. Li, A. Lloyd,

V. Yushprakh

Google Inc.

[email protected]. 2011

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Agenda

Motivation

Architecture

ACID over NOSQL Database

Replication via Paxos

Operational Results

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Motivation

Build a system to please everyone (users, admins, developers).

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Motivation

High availability – Fully functional during planned maintenance periods, as well as most unplanned infrastructure issues.

Scalability – Service huge audience of potential users.

ACID – Easier for writing and deploying applications.

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Agenda

Motivation

Megastore Architecture



Operational Results

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Megastore Overview

Widely deployed in Google for several years.

Used on more than 100 production applications.

Handles more than 3 billion write and 20 billion read transactions daily.

Stores nearly a petabyte of primary data across many global datacenters.

Available on GAE since Jan 2011.

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Architecture

Built on top of Bigtable and Chubby.

Blends the scalability of a NoSQL datastore with the convenience of a traditional RDBMS

Synchronous replication based on Paxos across datacenters.

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Architecture

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Architecture

Scalable replication.

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Architecture

Operation across Entity Groups

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Agenda

Motivation




Operational Results

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Data Model

Somewhere between RDBMS and row-column storage of NOSQL.SchemasTables (Entity group root table/child table, child table must have a single distinguished foreign key referencing root table)EntitiesProperties

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Sample Schema

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Mapping to Bigtable

Primary Keys are chosen to cluster entities that will be read together.

Each entity is mapped into a single Bigtable row.

“IN TABLE” instructs to colocate tables into the same Bigtable, and key ordering ensures Photo entities are stored adjacent to corresponding User.

Bigtable column name = Megastore table name + property name

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Indexes

Two level of indexes:Local index: Separate indexes for each entity group. Stored in entity group and updated atomically and consistently.Global index: Span entity groups. Not guaranteed to reflect all recent updates.

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Transactions & Concurrency

Entity group is a mini-database providing serializable ACID semantics.

MVCC (MultiVersion Concurrency Control) using transaction timestamp

Reads and Writes are isolated

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Three level of reads consistencyCurrent: apply all previous committed logs before read within a single entity group.Snapshot: pick the last known fully applied transaction to read, within a single entity group.Inconsistent: ignore the state of log and read the latest value directly.

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Write transaction:Current read: Obtain the timestamp and log position of the last committed transaction.Application logic: Read from Bigtable and gather writes into a log entry.Commit: Use Paxos to achieve consensus for appending the log entry to log.Apply: Write mutations to the entities and indexes in Bigtable.Clean up: Delete temp data.

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Queues provide transactional messaging between entity groups. Declaring a queue automatically creates an inbox on each entity group (scale automatically).

Two phase commit

Queue is recommended over two phase commit.

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Agenda

Motivation




Operational Results

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Paxos

Basic Paxos

Multi-Paxos

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Reads

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Writes

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Failure Detection

Coordinators obtain specific Chubby locks in remote datacenters at startup.

If it ever loses a majority of its locks from a crash or network partition, it will consider all entity groups in its purview to be out-of-date.

reads at the replica must query the log position from a majority of replicas until the locks are regained and its coordinator entries are revalidated.

all writers must wait for the coordinator's Chubby locks to expire before writes can complete

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Agenda

Motivation




Operational Results

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Distribution of Availability

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Distribution of Average Latencies

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Conclusion

Most users see five nines availability

Average read latencies are tens of milliseconds, indicating most reads are local.

Most writes costs 100-400 milliseconds.

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Questions?

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Transcript of Google Megastore