NoSQL Databases - ce.uniroma2.it · Università degli Studi di Roma “Tor Vergata” Dipartimento...

Università degli Studi di Roma “Tor Vergata” Dipartimento di Ingegneria Civile e Ingegneria Informatica

NoSQL Databases

Corso di Sistemi e Architetture per Big Data A.A. 2016/17

Valeria Cardellini

The reference Big Data stack

Valeria Cardellini - SABD 2016/17

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Resource Management

Data Storage

Data Processing

High-level Interfaces Support / Integration

Traditional RDBMSs

•  RDMBSs: the traditional technology for storing structured data in web and business applications

•  SQL is good –  Rich language and toolset –  Easy to use and integrate –  Many vendors

•  They promise ACID guarantees


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ACID properties

•  Atomicity –  All included statements in a transaction are either executed

or the whole transaction is aborted without affecting the database (“all or nothing” principle)

•  Consistency –  A database is in a consistent state before and after a

transaction

•  Isolation –  Transactions cannot see uncommitted changes in the

database (i.e., the results of incomplete transactions are not visible to other transactions)

•  Durability –  Changes are written to a disk before a database commits a

transaction so that committed data cannot be lost through a power failure.


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RDBMS constraints

•  Domain constraints –  Restricts the domain of each attribute or the set of possible

values for the attribute

•  Entity integrity constraint –  No primary key value can be null

•  Referential integrity constraint –  To maintain consistency among the tuples in two relations:

every value of one attribute of a relation should exist as a value of another attribute in another relation

•  Foreign key –  To cross-reference between multiple relations: it is a key in a

relation that matches the primary key of another relation


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Pros and cons of RDBMS

•  Well-defined consistency model

•  ACID guarantees •  Relational integrity

maintained through entity and referential integrity constraints

•  Well suited for OLTP apps –  OLTP: online transaction

processing •  Sound theoretical foundation •  Stable and standardized

DBMSs available •  Well understood


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•  Performance as major constraint, scaling is difficult

•  Limited support for complex data structures

•  Complete knowledge of DB structure required to create ad hoc queries

•  Commercial DBMSs are expensive

•  Some DBMSs have limits on fields size

•  Data integration from multiple RDBMSs can be cumbersome

Pros Cons

RDBMS challenges

•  Web-based applications caused spikes –  Internet-scale data size –  High read-write rates –  Frequent schema changes

•  Let’s scale RDBMSs –  RDBMS were not designed to be distributed

•  Possible solutions: –  Replication –  Sharding


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Replication

•  Master/slave architecture •  Scales read operations •  Write operations?


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Sharding

•  Horizontal partitioning of data across many separate servers

•  Scales read and write operations •  Cannot execute transactions across shards

(partitions) •  Consistent hashing is one form of sharding


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-  Hash both data and nodes using the same hash function in a same ID space

Scaling RDBMSs is expensive and inefficient


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Source: Couchbase technical report

NoSQL data stores •  NoSQL = Not Only SQL

–  SQL-style querying is not the crucial objective

•  Main features of NoSQL data stores –  Avoid unneeded complexity –  Support flexible schema –  Scale horizontally –  Provide scalability and high availability by storing and

replicating data in distributed systems, often across datacenters

–  Useful when working with Big data when the data’s nature does not require a relational model

•  Traditional join operations cannot be used –  Do not typically support ACID properties, but rather BASE

•  Compromising reliability for better performance Valeria Cardellini - SABD 2016/17

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ACID vs BASE

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•  Two design philosophies at opposite ends of the consistency-availability spectrum -  Keep in mind the CAP theorem! Pick two of Consistency, Availability and Partition tolerance

•  ACID: the traditional approach to address the consistency issue in RDBMS –  A pessimistic approach: prevent conflicts from

occurring •  Usually implemented with write locks managed by the

system

–  But ACID does not scale well when handling petabytes of data (remember of latency!)


ACID vs BASE (2)

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•  BASE stands for Basically Available, Soft state, Eventual consistency –  An optimistic approach

•  Lets conflicts occur, but detects them and takes action to sort the out

•  Approaches: •  conditional updates: test the value just before updating •  save both updates: record that they are in conflict and

then merge them

–  Basically Available: the system is available most of the time and there could exist a subsystem temporarily unavailable

–  Soft state: data is not durable in the sense that its persistence is in the hand of the user that must take care of refresh them

–  Eventually consistent: the system eventually converge to a consistent state

•  Usually adopted in NoSQL databases Valeria Cardellini - SABD 2016/17

Consistency

•  Biggest change from a centralized relational database to a cluster-oriented NoSQL

•  RDBMS: strong consistency –  Traditional RDBMS are CA systems

•  NoSQL systems: mostly eventual consistency


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Consistency: an example •  Ann is trying to book a room of the Ace Hotel in New

York on a node located in London of a booking system

•  Pathin is trying to do the same on a node located in Mumbai

•  The booking system uses a replicated database with the master located in Mumbai and the slave in London

•  There is only a room available •  The network link between the two servers breaks


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Ann Pathin

London Mumbay

Consistency: an example •  CA system: neither user can book any hotel room

–  No tolerance to network partitions

•  CP system: –  Pathin can make the reservation –  Ann can see the inconsistent room information but cannot

book the room

•  AP: both nodes accept the hotel reservation –  Overbooking!

•  Remember that the tolerance to this situation depends on the application type –  Blog, financial exchange, shopping chart, …


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Pessimistic vs. optimistic approach

Valeria Cardellini - SDCC 2016/17 16

•  Concurrency involves a fundamental tradeoff between: -  Safety (avoiding errors such as update conflicts)

and

-  Liveness (responding quickly to clients)

•  Pessimistic approaches often: -  Severely degrade the responsiveness of a system

-  Leads to deadlocks, which are hard to prevent and debug

NoSQL cost and performance


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Source: Couchbase technical report

Pros and cons of NoSQL

•  Easy to scale-out •  Higher performance for

massive data scale •  Allows sharing of data

across multiple servers •  Most solutions are either

open-source or cheaper •  HA and fault tolerance

provided by data replication •  Supports complex data

structures and objetcs •  No fixed schema, supportrs

unstructured data •  Very fast retrieval of data,

suitable for real-time apps Vale

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•  Do not provide ACID guarantees, less suitable for OLTP apps

•  No fixed schema, no common data storage model

•  Limited support for aggregation (sum, avg, count, group by)

•  Performance for complex join is poor

•  No well defined approach for DB design (different solutions have different data models)

•  Lack of consistent model can lead to solution lock-in

Pros Cons

Barriers to NoSQL

•  Main barriers to NoSQL adoption –  No full ACID transaction support –  Lack of standardized interfaces –  Huge investments already made in existing

RDBMSs

•  A commercial example –  AWS launched two NoSQL services (SimpleDB in

2007 and later DynamoDB in 2012) and one RDBMS service (RDS in 2009)


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NoSQL data models

•  A number of largely diverse data stores not based on the relational data model


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NoSQL data models

•  A data model is a set of constructs for representing the information –  Relational model: tables, columns and rows

•  Storage model: how the DBMS stores and manipulates the data internally

•  A data model is usually independent of the storage model

•  Data models for NoSQL systems: –  Aggregate-oriented models: key-value,

document, and column-family –  Graph-based models


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Aggregates •  Data as units that have a complex structure

–  More structure than just a set of tuples –  E.g.: complex record with simple fields, arrays,

records nested inside •  Aggregate pattern in Domain-Driven Design

–  A collection of related objects that we treat as a unit

–  A unit for data manipulation and management of consistency

•  Advantages of aggregates –  Easier for application programmers to work with –  Easier for database systems to handle operating

on a cluster See http://thght.works/1XqYKB0 Valeria Cardellini - SABD 2016/17

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Transactions?

•  Relational databases do have ACID transactions!

•  Aggregate-oriented databases: –  Support atomic transactions, but only within a

single aggregate –  Don’t have ACID transactions that span multiple

aggregates •  Update over multiple aggregates: possible inconsistent

reads

–  Part of the consideration for deciding how to aggregate data

•  Graph databases tend to support ACID transactions


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Key-value data model •  Simple data model in which data is represented as a

collection of key-value pairs –  Associative array (map or dictionary) as fundamental data

model

•  Strongly aggregate-oriented –  Lots of aggregates –  Each aggregate has a key

•  Data model: –  A set of <key,value> pairs –  Value: an aggregate instance

•  The aggregate is opaque to the database –  Just a big blob of mostly meaningless bit

•  Access to an aggregate: –  Lookup based on its key

•  Richer data models can be implemented on top Valeria Cardellini - SABD 2016/17

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Key-value data model: example


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Types of key-value stores


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•  Consistency models –  Range from eventual consistency to serializability

•  AP: Dynamo, Voldemort, Riak •  CP: Scalaris, Redis, Berkeley DB

•  Some data stores support ordering of keys

•  Some maintain data in memory (RAM), while others employ solid-state drives or rotating disks

Query features in key-value data stores

•  Only query by the key! –  There is a key and there is the rest of the data (the value)

•  It is not possible to use some attribute of the value column

•  The key needs to be suitably chosen –  E.g., session ID for storing session data

•  What if we don’t know the key? –  Some system allows the search inside the value using a full-

text search (e.g., using Apache Solr)


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Suitable use cases for key-value data stores •  Storing session information in web apps

–  Every session is unique and is assigned a unique sessionId value

–  Store everything about the session using a single put, request or retrieved using get

•  User profiles and preferences –  Almost every user has a unique userId, username, …, as

well as preferences such as language, which products the user has access to, …

–  Put all into an object, so getting preferences of a user takes a single get operation

•  Shopping cart data –  All the shopping information can be put into the value where

the key is the userId


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Examples of key-value stores

•  Amazon’s Dynamo is the most notable example –  Riak: open-source implementation

•  Other key-value stores include: –  Amazon DynamoDB

•  Data model and name from Dynamo, but different implementation –  Berkeley DB (ordered keys) –  Memcached, Redis, Hazelcast (in-memory data stores) –  Oracle NoSQL Database –  Encache (Java-based cache) –  Scalaris –  Voldemort (used by LinkedIn) –  upscaledb –  LevelDB (written by Google’s fellows and open source)


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Amazon’s Dynamo •  Highly available and scalable distributed key-

value data store built for Amazon’s platform –  A very diverse set of Amazon applications with

different storage requirements –  Need for storage technologies that are always

available on a commodity hardware infrastructure •  E.g., shopping cart service: “Customers should be able to view

and add items to their shopping cart even if disks are failing, network routes are flapping, or data centers are being destroyed by tornados”

–  Meet stringent Service Level Agreements (SLAs) •  E.g., “service guaranteeing that it will provide a response within

300ms for 99.9% of its requests for a peak client load of 500 requests per second.”


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G. DeCandia et al., "Dynamo: Amazon's highly available key-value store", Proc. of ACM SOSP 2007.

Dynamo features

•  Simple key-value API –  Simple operations to read (get) and write (put) objects uniquely

identified by a key –  Each operation involves only one object at time

•  Focus on eventually consistent store –  Sacrifices consistency for availability –  BASE rather than ACID

•  Efficient usage of resources •  Simple scale-out schema to manage increasing data

set or request rates

•  Internal use of Dynamo –  Security is not an issue since operation environment is

assumed to be non-hostile


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Dynamo design principles •  Sacrifice consistency for availability (CAP theorem) •  Use optimistic replication techniques •  Possible conflicting changes which must be detected

and resolved: when to resolve them and who resolves them? –  When: execute conflict resolution during reads rather than

writes, i.e. “always writeable” data store –  Who: data store or application; if data store, use simple

policy (e.g., “last write wins”)

•  Other key principles: –  Incremental scalability

•  Scale-out with minimal impact on the system –  Simmetry and decentralization

•  P2P techniques –  Heterogeneity


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Dynamo API

•  Each stored object has an associated key •  Simple API including get() and put() operations to

read and write objects get(key)!

•  Returns single object or list of objects with conflicting versions and context

•  Conflicts are handled on reads, never reject a write put(key, context, object)!

•  Determines where the replicas of the object should be placed based on the associated key, and writes the replicas to disk

•  Context encodes system metadata, e.g., version number –  Both key and object treated as opaque array of bytes –  Key: 128-bit MD5 hash applied to client supplied key


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Techniques used in Dynamo


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Problem Technique Advantage

Partitioning Consistent hashing Incremental scalability

High Availability for writes Vector clocks with reconciliation during reads

Version size is decoupled from update rates

Handling temporary failures

Sloppy Quorum and hinted handoff

Provides high availability and durability guarantee

when some of the replicas are not available

Recovering from permanent failures

Anti-entropy using Merkle trees

Synchronizes divergent replicas in the background

Membership and failure detection

Gossip-based membership protocol and

failure detection

Preserves symmetry and avoids having a

centralized registry for storing membership and

node liveness information

Data partitioning in Dynamo •  Consistent hashing: output range of a hash is treated

as a ring (similar to Chord) –  MD5(key) -> node (position on the ring) –  Differently from Chord: zero-hop DHT

•  “Virtual nodes” –  Each node can be responsible for more than one virtual

node –  Work distribution proportional to the capabilities of the

individual node


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Replication in Dynamo

•  Each object is replicated on N nodes –  N is a parameter configured per-instance by the application

•  Preference list: list of nodes that is responsible for storing a particular key –  More than N nodes to account for node failures –  See figure: object identified by key K is replicated on nodes

B, C and D


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•  Node D will store the keys in the ranges (A, B], (B, C], and (C, D]



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High availability for writes Vector clocks with reconciliation during reads











failure detection




Data versioning in Dynamo •  A put() call may return to its caller before the

update has been applied at all the replicas •  A get() call operation may return an object that

does not have the latest updates •  Version branching can also happen due to node/

network failures •  Problem: multiple versions of an object, that the

system needs to reconcile •  Solution: use vectorial clocks to capture the casuality

among different versions of the same object –  If causal: older version can be forgotten –  If concurrent: conflict exists, requiring reconciliation


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failure detection




Sloppy quorum in Dynamo

•  R/W: minimum numebr of nodes at must participate in a successful read/write operation

•  Setting R + W > N yields a quorum-like system –  The latency of a get() (or put()) operation is dictated by the

slowest of the R (or W) replicas –  R and W are usually configured to be less than N, to provide

better latency –  Typical configuration in Dynamo: (N, R, W) = (3, 2, 2)

•  Balances performance, durability, and availability

•  Sloppy quorum –  Due to partitions, quorums might not exist –  Sloppy quorum: create transient replicas

•  N healthy nodes from the preference list (may not always be the first N nodes encountered while walking the consistent hashing ring)


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Put and get operations •  put operation

–  Coordinator generates new vector clock and writes the new version locally

–  Send to N nodes –  Wait for response from W nodes

•  get operation –  Coordinator requests existing versions from N

•  Wait for response from R nodes

–  If multiple versions, return all versions that are causally unrelated

–  Divergent versions are then reconciled –  Reconciled version written back


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Hinted handoff in Dynamo

•  Consider N = 3; if A is temporarily down or unreachable, put will use D

•  D knows that the replica belongs to A

•  Later, D detects A is alive –  Sends the replica to A –  Removes the replica


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•  Hinted handoff for transient failures

•  Again, “always writeable” principle



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failure detection




Membership management

•  Administrator explicitly adds and removes nodes

•  Gossiping to propagate membership changes –  Eventually consistent view –  O(1) hop overlay


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Failure detection •  Passive failure detection

–  Use pings only for detection from failed to alive

•  In the absence of client requests, node A doesn't need to know if node B is alive

•  Anti-entropy for replica synchronization •  Use Merkle trees for fast inconsistency detection and

minimum transfer of data –  Merkle tree: hash tree where leaves are hashes of the

values of individual keys


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-  Nodes maintain Merkle tree of each key range

-  Exchange root of Merkle tree to check if the key ranges are updated

Riak KV

•  Distributed NoSQL key-value data store inspired by Dynamo –  Open-source version

•  Like Dynamo –  Employs consistent hashing to partition and replicate data

around the ring –  Makes use of gossiping to propagate membership changes –  Uses vector clocks to resolve conflicts –  Nodes can be added and removed from the Riak cluster as

needed –  Two ways of resolving update conflicts:

•  Last write wins •  Both values are returned allowing the client to resolve the

conflict

•  Can run on Mesos Valeria Cardellini - SABD 2016/17

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Column-family data model •  Strongly aggregate-oriented

–  Lots of aggregates –  Each aggregate has a key

•  Similar to a key/value store, but the value can have multiple attributes (columns)

•  Data model: a two-level map structure: –  A set of <row-key, aggregate> pairs –  Each aggregate is a group of pairs <column-key, value> –  Column: a set of data values of a particular type

•  Structure of the aggregate visible •  Columns can be organized in families

–  Data usually accessed together


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Column-family data model


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•  Store and process data by column instead of row –  Can access faster the data needed rather than scanning and

discarding unwanted data in row –  But the primary key is the data

…;Smith:001;Jones:002,004;Johnson:003;…!



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•  In many analytical databases queries, few attributes are needed –  Column values are stored contiguously on disk:

reduces I/O

•  Both rows and columns are split over multiple nodes to achieve scalability –  Sharding: ability to distribute the content of a

collection among different nodes

•  So column stores are suitable for read-mostly, read-intensive, large data repositories

Properties of column-family stores •  Operations also allow picking out a particular column

get('1234', 'name')!•  Each column:

–  Has to be part of a single column family –  Acts as unit for access

•  You can add any column to any row, and rows can have very different columns

•  You can model a list of items by making each item a separate column

•  Two ways to look at data: –  Row-oriented

•  Each row is an aggregate •  Column families represent useful chunks of data within that

aggregate –  Column-oriented

•  Each column family defines a record type •  Row as the join of records in all column families


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Suitable use cases for column-family stores

•  Queries that involve only a few columns •  Aggregation queries against vast amounts of

data -  E.g., average age of all of your users

•  Column-wise compression •  Well-suited for OLAP-like workloads (e.g.,

data warehouses) which typically involve highly complex queries over all data (possibly petabytes)


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Examples of column-family stores •  Google’s Bigtable is the most notable example •  Other column-family stores include:

–  Apache HBase: open-source implementation providing Bigtable-like capabilities on top of Hadoop and HDFS

–  Apache Accumulo: based on the design of BigTable and powered by Apache Hadoop, Zookeeper, and Thrift

•  Different APIs and different nomenclature from Hbase, but same in operational and architectural standpoint

•  Better security –  Cassandra –  Hypertable –  Amazon Redshift


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Google’s Bigtable


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•  Built on Google File System (GFS), Chubby lock service, SSTable (log-structured storage like LevelDB) and a few other Google technologies –  Data storage organized in tables, whose rows are distributed

over the GFS

•  In 2015 made available as a service on Google Cloud Platform: Cloud Bigtable

•  Underlies Google Cloud Datastore •  Used by a number of Google applications, including:

–  Web indexing, MapReduce, Google Maps, Google Earth, YouTube and Gmail

•  LevelDB is based on concepts from Bigtable –  But widely noted for being unreliable Chang et al., "Bigtable: A Distributed Storage System for Structured Data", ACM Trans. Comput. Syst., 2008.

Bigtable: motivation

•  Lots of (semi-)structured data at Google –  URLs, geographical locations, ...

•  Big data –  Billions of URLs, hundreds of millions of users,

100+TB of satellite image data, …


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Bigtable: main features

•  Distributed multi-level map

•  Fault-tolerant

•  Scalable and self-managing

•  CP system: strong consistency and partition tolerance


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Bigtable: data model •  Table

–  Distributed multi-dimensional sparse map

•  Rows –  Every read or write in a row is atomic –  Rows sorted in lexicographical order by row key

•  Columns –  The basic unit of data access –  Column family: group of (the same type of) column keys –  Column key naming: family:qualifier!


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-  Column family allows for specific optimization for better access control, storage and data indexing

Bigtable data model •  Timestamp

–  Each column value may contain multiple versions

•  Tablet: contiguous ranges of rows stored together •  Tablets are split by the system when they become

too large •  Auto-sharding


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•  Each tablet is served by exactly one tablet server

Bigtable API

•  Metadata operations –  Create/delete tables, column families, change

metadata •  Writes: single-row, atomic

–  Write/delete cells in a row, delete all cells in a row •  Reads: read arbitrary cells in a Bigtable table

–  Each row read is atomic –  One row, all or specific columns, certain

timestamps, and ...


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Writing and reading examples


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•  Main components: –  Master server –  Tablet server –  Client library

Bigtable architecture


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Master server

•  One master server •  Assigns tablets to tablet servers •  Balances tablet server load •  Garbage collection of unneeded files in GFS •  Handles schema changes, e.g., table and

column family creations


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Tablet server

•  Multiple tablet servers •  Can be added or removed dynamically •  Each manages a set of tablets (typically

10-1000 tablets/server) •  Handles read/write requests to tablets •  Splits tablets when too large


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Client library

•  Library that is linked into every client •  Client data do not move through the master •  Clients communicate directly with tablet

servers for reads/writes


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Building blocks

•  The building blocks of Bigtable are: –  Google File System (GFS): raw storage –  Chubby: distributed lock manager –  Scheduler: schedules jobs onto machines


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Chubby lock service •  Distributed lock service used in many Google’s

products –  File system {directory/file} for locking –  Uses Paxos algorithm to solve consensus –  A client leases a session with the service

•  Ensure there is only one active master •  Store bootstrap location of Bigtable data •  Discover tablet servers •  Store Bigtable schema information •  Store access control lists


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Master startup

•  The master executes the following steps at startup –  Grabs a unique master lock in Chubby, which

prevents concurrent master instantiations –  Scans the servers directory in Chubby to find the

live servers –  Communicates with every live tablet server to find

what tablets are already assigned to each server –  Scans the METADATA table to find unassigned

tablets


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Tablet assignment

•  1 tablet -> 1 tablet server •  Master uses Chubby to keep tracks of set of

live tablet serves and unassigned tablets •  When a tablet server starts, it creates and

acquires an exclusive lock in Chubby •  Master detects the status of the lock of each

tablet server by checking periodically •  Master is responsible for finding when tablet

server is no longer serving its tablets and reassigning those tablets as soon as possible


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Finding a tablet

•  Three-level hierarchy •  Root tablet contains location of all tablets in a special

METADATA table •  METADATA table contains location of each tablet

under a row •  The client library caches tablet locations


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SSTable

•  SSTable file format used internally to store Bigtable data

•  Immutable, sorted file of key-value pairs •  Each SSTable is stored in a GFS file •  Chunks of data plus a block index •  A block index is used to locate blocks •  The index is loaded into memory when the SSTable is opened


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Tablet serving

•  Updates committed to a commit log –  Recently committed updates are stored in memory

(memtable) –  Older updates are stored in a sequence of

SSTables


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Loading tablets

•  To load a tablet, a tablet server does the following: –  Finds location of tablet through its METADATA

•  Metadata for a tablet includes list of SSTables and set of redo points

–  Read SSTables index blocks into memory –  Read the commit log since the redo point and

reconstructs the memtable


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Bigtable consistency and availability

•  Strong consistency –  CP system –  Only one tablet server is responsible for a given

piece of data. –  Replication is handled on the GFS layer

•  Tradeoff with availability –  If a tablet server fails, its portion of data is

temporarily unavailable until a new server is assigned


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Comparing Dynamo and Bigtable


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Dynamo Bigtable Data model Key-value, row store Column store API Single value Single value and range Data partition Random Ordered Optimized for Writes Writes Consistency Eventual Atomic Multiple version Version Timestamp Replication Quorum GFS Data center aware Yes Yes Persistency Local and pluggable Replicated and

distributed file system Architecture Decentralized Hierarchical (master/

worker) Client library Yes Yes

Cassandra


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•  Some large production deployments: -  Apple: over 75,000, over 10 PB of data -  Netflix: 2,500 nodes, 420 TB, over 1 trillion requests per day

•  Initially developed at Facebook •  A mixture of Amazon’s Dynamo and Google’s

BigTable

Dynamo BigTable P2P architecture (replication & partitioning), gossip-based discovery and error detection

Sparse column-oriented data model, storage architecture

(SSTable disk storage)

Cassandra

Cassandra: features

•  High availability and incremental scalability •  Robust support for clusters spanning multiple data

centers –  Asynchronous masterless replication allowing low latency

operations

•  Data model: structured key-value store where columns are added only to specified keys –  Different keys can have different number of columns as in

BigTable –  Emphasizes denormalization instead of normalization and joins –  See example at http://bit.ly/2n1Vfua

•  Write-oriented system –  BigTable designed for intensive read workloads


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Consistency in Cassandra

•  AP system –  No master node to coordinate reads and writes

•  Tunable read and write consistency for each query SELECT points! FROM fantasyfootball.playerpoints USING CONSISTENCY QUORUM! WHERE playername = ‘Tom Brady’;!

•  Achieved through quorum-based protocol –  If N+W > R and W >= N/2 +1 you have strong consistency –  Some available consistency levels (see http://bit.ly/2n26EdE)

•  ONE: only a single replica must respond •  QUORUM: a majority of the replicas must respond •  ALL: all of the replicas must respond •  LOCAL_QUORUM: a majority of the replicas in the local datacenter must

respond •  Tunable tradeoffs between consistency and latency


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Cassandra Query Language (CQL) •  An SQL-like language CREATE COLUMNFAMILY Customer (!KEY varchar PRIMARY KEY,!name varchar,!city varchar,!web varchar);!!INSERT INTO Customer (KEY,name,city,web)!VALUES ('mfowler',!'Martin Fowler',!'Boston',!'www.martinfowler.com');!!SELECT * FROM Customer!SELECT name,web FROM Customer!SELECT name,web FROM Customer WHERE city='Boston'!


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Document data model

•  Strongly aggregate-oriented –  Lots of aggregates –  Each aggregate has a key

•  Similar to a key-value store (unique key), but API or query/update language to query or update based on the internal structure in the document –  The document content is no longer opaque

•  Similar to a column-family store, but values can have complex documents, instead of fixed format

•  Document: encapsulates and encodes data in some standard formats or encodings –  XML, YAML, JSON, BSON, …


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Document data model •  Data model:

–  A set of <key, document> pairs –  Document: an aggregate instance

•  Structure of the aggregate is visible –  Limits on what we can place in it

•  Access to an aggregate –  Queries based on the fields in the aggregate

•  Flexible schema –  No strict schema to which documents must conform, which

eliminates the need of schema migration efforts


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Document data model


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•  The data model resembles the JSON object

Document data store API

•  Usual CRUD operations (although not standardized) –  Creation (or insertion) –  Retrieval (or query, search, finds)

•  Not only simple key-to-document lookup •  API or query language that allows the user to retrieve

documents based on content (or metadata)

–  Update (or edit) •  Replacement of the entire document or individual

structural pieces of the document

–  Deletion (or removal)


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Examples of document stores

•  MongoDB and CouchDB are the two major representatives –  You will use MongoDB in lab –  Documents grouped together to form collections –  Collections organized into databases

•  Other popular document stores include: –  Couchbase –  DocumentDB: PaaS service in Microsoft’s Azure cloud platform –  RethinkDB –  RavenDB (for .NET)


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Key-value vs document stores •  Key-value store

–  A key plus a big blob of mostly meaningless bits –  We can store whatever we like in the aggregate –  We can only access an aggregate by lookup based on its

key

•  Document store –  A key plus a structured aggregate –  More flexibility in access –  We can submit queries to the database based on the fields

in the aggregate –  We can retrieve part of the aggregate rather than the whole

thing –  Indexes based on the contents of the aggregate


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Suitable use cases for document stores

•  Good for storing and managing big data-size collections of semi-structured data with a varying number of fields –  Text and XML documents, email messages –  Conceptual documents like de-normalized

(aggregate) representations of DB entities such as product or customer

–  Sparse data in general, i.e., irregular (semi-structured) data that would require an extensive use of nulls in RDBMS

•  Nulls being placeholders for missing or nonexistent values


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When not to use document stores

•  Complex transactions spanning different operations –  Document stores unsuited for atomic cross-

document operations

•  Queries against varying aggregate structure –  Data is saved as an aggregate in the form of

application entities. If the design of the aggregate is constantly changing, aggregate is saved at the lowest level of granularity. In this scenario, document stores may not work


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Graph data model •  Uses graph structures with nodes, edges, and

properties to represent stored data –  Nodes are the entities and have a set of attributes –  Edges are the relationships between the entities

•  E.g.: an author writes a book –  Edges can be directed or undirected –  Nodes and edges also have individual properties consisting of

key-value pairs

•  Replaces relational tables with structured relational graphs of interconnected key-value pairs

•  Powerful data model –  Differently from other types of NoSQL stores, it concerns itself

with relationships –  Focus on visual representation of information (more human-

friendly than other NoSQL stores) –  Other types of NoSQL stores are poor for interconnected data


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Graph data model: example

•  Small example from Neo4j


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Graph database

•  Explicit graph structure •  Each node knows its adjacent nodes

–  As the number of nodes increases, the cost of local step (or hop) remains the same

•  Plus an index for lookups

•  Cons: –  Sharding: data partitioning is difficult –  Horizontal scalability

•  When related nodes are stored on different servers, traversing multiple servers is not performance-efficient

–  Require rewiring your brain Valeria Cardellini - SABD 2016/17

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Examples of graph databases

•  Neo4j •  OrientDB •  InfiniteGraph •  AllegroGraph •  HyperGraphDB


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Suitable use cases for graph databases •  Good for applications where you need to model

entities and relationships between them –  Social networking applications –  Pattern recognition –  Dependency analysis –  Recommendation systems –  Solving path finding problems raised in navigation systems –  …

•  Good for applications in which the focus is on querying for relationships between entities and analyzing relationships –  Computing relationships and querying related entities is

simpler and faster than in RDBMS


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Neo4j

•  Support for ACID •  Properties of nodes and relationships captured in the

form of multiple attributes (key-value pairs) –  Relationships are unidirectional

•  Nodes tagged with labels –  Labels used to represent different roles in the modeled

domain

•  Neo4j architecture –  Clustered installation

•  Single read/write master and multiple read-only slaves –  Improve data throughput via a multilevel caching scheme


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Neo4j: Twitter graph example


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Neo4j: Cypher •  Cypher: declarative SQL-like language for performing

CRUD operations and more –  See http://bit.ly/2o0kytc –  ASCII-Art to represent patterns –  Relationships are an arrow --> between two nodes

•  Some examples: –  Create a node with labels and properties !CREATE (movie: Movie {title: “Shrek”})!

–  Search for a specified pattern: MATCH corresponds to SELECT in SQL !MATCH (me {name: “Rick”})-[:KNOWS *2..3]-> (remote_friend) RETURN remote_friend.name!•  Nodes that are a variable number of relationship→node hops

away can be found using the following syntax: ! !-[:TYPE*minHops..maxHops]�!


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Neo4j: Cypher

•  Native support for shortest path queries •  Example: find the shortest path between two airports

(SFO and MSO)

–  shortestPath returns a single shortest path between two nodes

•  In the example, the path length is between 0 and 2 –  allShortestPath returns all the shortest paths between

two nodes Valeria Cardellini - SABD 2016/17

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Neo4j: Cypher

•  Shortest path queries (see Twitter graph on slide 93)

•  More queries on Twitter graph at http://network.graphdemos.com –  You can map your Twitter network and analyze it with Neo4j


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Find all the shortest paths between two Twitter users linked by Follows as long as the path is min 0 and max 3 long

Find all the shortest paths between two Twitter users as long as the path is min 0 and max 5 long

Performance comparison of NoSQL data stores

•  Unsolved issue: no standard benchmark –  Yahoo Cloud Serving Benchmark (YCSB): open source

workload generator used in some studies

•  We consider two papers: 1.  “Solving Big Data Challenges for Enterprise Application

Performance Management”, VLDB 2012 http://bit.ly/1vYYihO 2.  “Is Elasticity of Scalable Databases a Myth?”, BigData 2016

•  Overall conclusion: –  No single “winner takes all” among NoSQL data store –  Results depend on use case, workload and deployment

conditions


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Performance comparison @VLDB’12 •  Target: Application Performance Monitoring (APM)

platform for monitoring big data •  Workload R: 95% reads and only 5% writes


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Throughput Read latency

Write latency •  Cassandra: linear scalability in terms of throughput but slow at reading and writing

•  HBase: suffers in scalability, slow at reading but fast at writing

•  Redis: being in-memory, the fastest at reading

Performance comparison @VLDB’12

•  Workload WR: 50% reads and 50% writes


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Study conclusions: -  Linear scalability for Cassandra, HBase, and Voldemort in

most of the tests -  Cassandra’s throughput dominated in all the tests,

however high latency

-  HBase achieved least throughput but exhibited a low write latency at the cost of a high read latency

Throughput Read latency

Performance comparison @BigData’16


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•  Couchbase achieves the highest throughput and lowest latency results

•  Cassandra benefits the most from larger cluster sizes in contrast to MongoDB

Poliglot persistence

•  Different data stores designed to solve different problems

•  Using a single database engine for all of the requirements… –  storing transactional data –  caching session information –  traversing graph of customers –  performing OLAP operations

•  … usually leads to non-performing solutions •  Different needs for availability, consistency, or

backup requirements


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Poliglot persistence: consistency


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Poliglot persistence •  When storing data, it is best to use multiple

data storage technologies –  Choose them based upon the way data is being

used by individual applications or components of a single application

–  Different kinds of data are best dealt with different data stores: pick the right tool for the right use case


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•  See http://bit.ly/2plRJYq

References

•  Golinger at al., “Data management in cloud environments: NoSQL and NewSQL data stores”, J. Cloud Comp., 2013. http://bit.ly/2oRKA5R

•  DeCandia et al., "Dynamo: Amazon's highly available key-value store", ACM SOSP 2007. http://bit.ly/2pmXzsr

•  Chang et al., “Bigtable: a distributed storage system for structured data”, 2007. http://bit.ly/2nywNRg

•  Lakshman and Malik, “Cassandra - a decentralized structured storage system”, LADIS 2009. http://bit.ly/2nyGSxE


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