Google Megastore

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_address] May. 2011

Agenda
Motivation
Architecture
ACID over NOSQL Database
Replication via Paxos
Operational Results

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

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.

Agenda
Motivation
Megastore Architecture
Operational Results

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.

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.

Architecture

Architecture
Scalable replication.

Architecture
Operation across Entity Groups

Agenda
Motivation
Operational Results

Data Model
Somewhere between RDBMS and row-column storage of NOSQL.
Schemas
Tables (Entity group root table/child table, child table must have a single distinguished foreign key referencing root table)
Entities
Properties

Sample Schema

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

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.

Transactions & Concurrency
Entity group is a mini-database providing serializable ACID semantics.
MVCC (MultiVersion Concurrency Control) using transaction timestamp
Reads and Writes are isolated

Three level of reads consistency
Current: 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.

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.

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.

Agenda
Motivation
Operational Results

Paxos
Basic Paxos
Multi-Paxos

Writes

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

Agenda
Motivation
Operational Results

Distribution of Availability

Distribution of Average Latencies

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.

Questions?

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Google Megastore

by bergwolf on Aug 28, 2011

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Google Megastore — Presentation Transcript