getting smart about the new world of postgresql replication

© Continuent 2008

Getting Smart about the NewWorld of PostgreSQL Replication

PostgreSQL Conference West 2008Robert Hodges – CTO Continuent, Inc.

2 © Continuent PostgreSQL West Conference 2008

Agenda

/ Introductions/ Why Are We Here?/ How Does Database Replication Work?/ What Kinds of Replication Does PostgreSQL Have

Available?/ Summary


About Continuent

/ Company• The leading provider of scale-out and replication solutions for

open source databases

/ Solutions• Tungsten Enterprise Multi-Master, aka uni/cluster• Tungsten Enterprise Master/Slave

/ Value• Help customers build highly capable applications with low cost

databases and commodity hardware/ Open Source

• Tungsten Replicator - Master/slave replication• Tungsten Connector - Proxy for PostgreSQL and MySQL• Sequoia - Generic middleware clustering for JDBC databases• Hedera - Group communications adapters• Bristlecone - Performance benchmarks for database clusters


Why Are We Here?


Possible Reasons

/ You hunger for enlightenment concerning themysteries of data replication

/ You thirst for instruction on the ways ofreplication in the PostgreSQL world

/ The other talks looked really boring


DEEP QUESTIONS about Replication

/ How does it work?/ Does it work over a WAN?/ Where’s multi-master??!/ Do I have to change my application?/ What do I have to change?/ How does it affect performance?/ Does it scale reads?/ Does it scale writes?/ What happens when a database fails?/ What happens when the network fails?/ What’s “split brain?” That sounds really bad.


How Does DatabaseReplication Work?


What is Database Replication?

/ Database replication copies updates automaticallyfrom one database to another

/ The biggest problem in replication is serializingupdates so copies are identical

/ Replication is incredibly useful/ …But also confusing because there are so many

ways to do it/ …And because there are some non-obvious gotchas


Let Me Count the Ways: Bits or SQL?

/ Fun fact: every self-respecting DBMS can apply atleast two kinds of changes

/ Log records -- Databases apply them automaticallyduring recovery

/ SQL statements -- Clients send SQL to make changes

Physical ReplicationReplicate log records/events tocreate bit-for-bit copy

Logical ReplicationReplicate SQL to createequivalent data

Transparent, high performance,hard to open replicas

Flexible, fewer/differentrestrictions, replicas support reads


Replicate Statements or Rows?

/ SQL updates other than DDL can be represented intwo different ways

/ Statements -- What the client said/ Row updates -- What the client actually did

Statement ReplicationReplicate changes as SQLstatements

Row ReplicationReplicate changes other thanDDL as row updates

DDL, only way some DBMS canreplicate

Flexible, fewer weird exceptions


Use Triggers or Read the Log?

/ Another Fun Fact: There are three ways to capturechanges in databases

/ Triggers - For impatient app developers/ Recovery log - For patient developers outside server/ Events - For very patient developers inside server

Log-BasedReplication

Read changes fromrecovery log

Trigger-BasedReplication

Replicate changes asrow updates

Event-BasedReplicationCapture “events”

within server

Quick but lots ofbaggage

High performance butexpensive to implement

Requires server re-engineering


Replicate Now or a Little Later?

/ Replicating is like buying a car--there are lots of waysto pay for it

/ $0 down - Pay later; hope nothing goes wrong/ Down payment - Pay some so less goes wrong later/ Cash - Pay up front and it’s yours forever

AsynchronousReplication

Commit now,replicate later

Semi-SynchronousReplication

Replicate to at leastone other database

SynchronousReplication

Replicate fully toall other databases

Lose data but robustagainst network failure

Trade-off data loss vs.partition handling

Network fails --> youstop


Multi-Master or Master/Slave or…?

OK, now it gets confusing! Should I…/ Update one database and let it serialize all changes?/ Update any database with global serialization?/ Update any database and replicate without global

serialization?

Master/Slave

Single masterserializes and

replicates

Multi-Master

Multiple masterswith globalserialization

Master-Master

Multiple masterswith no globalserialization

Fast serialization,SPOF, no split brain

Good scaling but reallyhard to implement

Convenient for WAN buthard for applications


Master/Slave Replication is Way Flexible!

MasterMaster SlaveSlave

Master-Slave Pair forbasic availability

MasterMaster SlaveSlave

Fan-Out forread scaling SlaveSlave

SlaveSlave MasterMaster

Fan-In for datawarehouseloading

MasterMaster

MasterMasterMaster/Master/SlaveSlave

Chaining to off-load master or goacross slow link

SlaveSlave

MasterMaster MasterMasterCircular Replicationfor cross-siteoperation


What Kinds of ReplicationDoes PostgreSQL Support?


An Amazing Number of Ways to Replicate!

Most of the “active” replication methods

Log-based master/slaveMammothReplicator

Heterogeneous log-basedmaster/slave

TungstenReplicator

Master-master replicationBucardoCertification-based multi-masterPostgres RClient-driver-based multi-masterHA-JDBCMiddleware-based multi-masterPg-Pool IIMiddleware-based multi-masterSequoiaTrigger-based master/slaveLondistTrigger-based master/slaveSLONY

DescriptionNameWAL shipping to standby instanceWarm Standby


Warm Standby

/ Warm standby--your friend for basic availability/ Replicates the write-ahead log (WAL) to standby

server/ Key Characteristics

• Physical replication - Copies WAL blocks to standbyhost in permanent recovery mode

• Asynchronous - Applications do not wait• Log-based - Reads completed WAL files

/ So…• It’s completely transparent• It’s very fast• It does not have serialization or split brain problems


Warm Standby Implementation

WALWALSegmentsSegments

PostgreSQLPostgreSQL

Master InstanceMaster Instance

pg_xlogspg_xlogsDirectoryDirectory

ArchivedArchivedWALWAL

SegmentsSegments

ArchiveArchiveDirectoryDirectory

PostgreSQLPostgreSQL

StandbyStandby


Warm Standby Self Defense

/ Warm standby helps with availability, not scaling/ It works over a WAN for disaster recovery/ You cannot open the standby or otherwise use it in

any way without doing a failover/ You are virtually guaranteed to lose data if your

system is busy and you don’t replicate pg_xlogs/ Stupid warm standby tricks you might want to know

• Creating throw-away reporting databases with ZFS• Avoiding data loss using DRBD

/ You should also know about the following:• pgstandby - Makes standby setup very easy• Ucarp and Heartbeat - Automates failover• Other tools?


PostgreSQL 8.4 Will Be Better!

/ Well maybe…Here’s the plan/ WAL streaming - Copy log records in real time/ Read-only standby - Open standby for reads/ Synchronous and asynchronous modes

• Async - Don’t wait for standby acknowledgement• Syncnet - Wait for acknowledgement that event is received• Syncdisk - Wait for standby to process fully

/ Monitoring hooks to allow Heartbeat, Ucarp, etc. tointegrate easily and manage failover

/ Unclear how much will make it into the 8.4 release


SLONY

/ SLONY--Table to table replication for PostgreSQL• HA and disaster recovery• Database upgrades• Workload scaling• Geographic distribution of data

/ Key Characteristics• Logical replication - Replicates using SQL• Asynchronous - Replication occurs after commits• Trigger-based - Triggers on all replicated tables• Row-based - Separate mechanismc for DDL

/ So…• It’s flexible• It’s reasonably fast• No split-brain• It handles any DML• Replicas open for reads


Master / Slave Replication (SLONY)

MasterDB

Backups

Client Updates

Slave

Slave

Read-OnlyApps

LegendReplication:Client Access:

Slon Daemon

Slon Daemon

Slon Daemon

SLONY Cluster


SLONY Self Defense

/ Clean failover design; “flip” between master & slave/ Handles sequences/ Flexible topologies like chaining and fan-out/ Permits complex schemes where some tables are

replicated and others are not/ Complex administration/ Does not automatically replicate DDL/ If you plan to use SLONY, you will need to study its

habits carefully


Sequoia Middleware Clustering

/ Multi-master replication using middleware/ Key Characteristics

• Logical replication - Replicates SQL• Statement-based - Distributes SQL statements send

by clients• Synchronous - Updates all databases at once• Multi-master - All nodes update at once

/ So…• No master SPOF• Automatic scaling of reads• Excellent design for daylight maintenance• Automatic scaling of reads without any application

changes


DB DB ServerServer

Java Clients

Sequoia Architecture

Client ApplicationsNative Clients

(PHP, Perl, C, Ruby, etc.)

Controller – Virtual DBs

JDBC Driver JDBC Driver

Tungsten Connector

Controller – Virtual DBs

Driver Driver

Group Comm

Java ClientsSequoia JDBC Driver

Sequoia JDBC Driver

DB DB ServerServer

DB DB ServerServer

DB DB ServerServer


Sequoia Self-Defense

/ Middleware gets multi-master without changingdatabase internals

/ Read scaling fully transparent/ Updates slow/not fully transparent

• SQL statements like nextval() hard to serialize• To make it faster, relax ordering = no serialization!!

/ Middleware replication subject to distributeddeadlock

/ Does not support WAN• If your “ping” is 10ms, max updates/sec = 1000/10 = 100• Metropolitan networks are possible if ping < 1-2ms

/ If a database fails or network partitions, you reloadfrom a backup


Postgres-R Clustering

/ In-core multi-master replication using certification toensure global serialization

/ Key Characteristics• Physical replication - Replicates change sets (events)• Synchronous - All databases in sync• Multi-master - Connect to any node to update• Event replication - Copies change sets

/ So…• Good update performance• Write scaling!• Excellent read scaling (linear)• No master SPOF


Postgres-R Replication using Certification

Client

submitupdate

Tx: 2Update foo set total = 2.3 where id = 25

Tx: 1Update foo set total = 2.5 where id = 25

Abort!

Client

submitupdate

Apply

Group Communications(Totally ordered logical queue)


Postgres-R Self-Defense

/ Postgres-R approach scales writes!/ Read scaling is fully transparent/ Postgres-R subject to aborts on hot spots

• It gets bad fast on “small” data sets or structures like queues/ Sequences are problematic/ DDL changes can lock the cluster/ Provisioning new nodes is complex/ Does not work over a WAN

• (See the Sequoia slides for reason why)

/ Very sensitive to group communication performance/ Nobody has ever gotten the certification approach to

work! This is vaporware!


Or Is It???


Tungsten Replicator

/ Heterogeneous master/slave replication/ Key Characteristics

• Logical replication - Replicates using SQL• Asynchronous - Replication occurs after commits• Log-based - If we can read the log, we do• Row- *or* statement-based - Handles DDL

/ So…• It’s flexible• It’s reasonably fast• No split-brain• Handles DML and DDL• Replicas open for reads


Tungsten ReplicatorMaster

Replicator

Extractor

TransactionHistory Log

SlaveReplicator

Applier

TransactionHistory Log

BinlogsBinlogsPostgreSQLPostgreSQL

““Brand XBrand X””DatabaseDatabase

Master DBMaster DB

Rows +Rows +StatementsStatements

Slave DBSlave DB

JDBC SQLJDBC SQLRequestsRequests

Platform-Platform-IndependentIndependentSQL EventsSQL Events

Log RecordsLog Records

[Filters] [Filters]


Tungsten Replicator

/ Support for writing into PostgreSQL on the way/ Nice heterogeneous features

• Fungible row update event with standard datatypes• Event filtering - dropping based on conditions• Transformations like changing db name, dropping columns, etc.

/ Supports fan-in, fan-out, chaining/ Oracle replication available as commercial extension/ Easy to set up and use/ We can’t read PostgreSQL logs -- that will be a

while…


In Summary… A New Day for PostgreSQLReplication


Questions?

Robert HodgesCTOContinuent, Inc.

http://www.continuent.comhttp://community.continuent.com

getting smart about the new world of postgresql replication

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