Choosing From the Many Flavors of PostgreSQL Replication

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Choosing from the Many Flavors of PostgreSQL Replication Richard Yen PG Conf Silicon Valley 2019

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Disclaimer This is not an EDB talk!

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Why Replication?

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Why Replication? • Because one copy is not enough!

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Why Replication? • Because one copy is not enough! • Fault-Tolerance

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Why Replication? • Because one copy is not enough! • Fault-Tolerance • Scalability

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Why Replication? • Because one copy is not enough! • Fault-Tolerance • Scalability • Recoverability

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Why Replication? • Because one copy is not enough! • Fault-Tolerance • Scalability • Recoverability • People want “real” data to work with

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Which one is for me? • Streaming replication • Logical replication • Slony • Bucardo • Londiste • DRBD • Corosync • 2ndQ BDR • pg_shard • pglogical • Daﬀodil • Mammoth • Pgpool • EDB Replication Server • Spectral Core Replicator Pro • And More!

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It all boils down to…

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It all boils down to… Trigger-based | WAL-based

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Trigger-based Replication

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Trigger-based Replication • Relies on PostgreSQL’s trigger mechanism to log all DML into event/ shadow tables

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Trigger-based Replication • Relies on PostgreSQL’s trigger mechanism to log all DML into event/ shadow tables • Data from shadow tables ingested by another program and replayed to other nodes

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Trigger-based Replication • Relies on PostgreSQL’s trigger mechanism to log all DML into event/ shadow tables • Data from shadow tables ingested by another program and replayed to other nodes • Very reliable: replicating production environments since 2003

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Trigger-based Replication

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Trigger-based Replication • Main players

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Trigger-based Replication • Main players • Slony — written in C

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Trigger-based Replication • Main players • Slony — written in C • Londiste — written in Python

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Trigger-based Replication • Main players • Slony — written in C • Londiste — written in Python • Bucardo — written in Perl

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Trigger-based Pros & Cons

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Trigger-based Pros & Cons • Select which tables to replicate

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Trigger-based Pros & Cons • Select which tables to replicate • Enables row-based/column-based replication

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Trigger-based Pros & Cons • Select which tables to replicate • Enables row-based/column-based replication • Major-version upgrades

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Trigger-based Pros & Cons • Select which tables to replicate • Enables row-based/column-based replication • Major-version upgrades • Master-master replication (Bucardo only)

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Trigger-based Pros & Cons

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Trigger-based Pros & Cons • Primary keys required

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Trigger-based Pros & Cons • Primary keys required • Limited ability to replicate DDL

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Trigger-based Pros & Cons • Primary keys required • Limited ability to replicate DDL • Large object replication not available

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Trigger-based Pros & Cons • Primary keys required • Limited ability to replicate DDL • Large object replication not available • Trigger activity generates additional I/O

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Trigger-based Pros & Cons • Primary keys required • Limited ability to replicate DDL • Large object replication not available • Trigger activity generates additional I/O • Event tracking consumes network bandwidth

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WAL-based Replication

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WAL-based Replication • WAL ﬁles are the mechanism to recover from crashes

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WAL-based Replication • WAL ﬁles are the mechanism to recover from crashes • WAL ﬁles can be used to replay DML on remote servers

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WAL-based Replication • WAL files are the mechanism to recover from crashes • WAL files can be used to replay DML on remote servers • Streaming WAL activity effectively creates an up-to-date clone

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WAL-based Pros & Cons

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WAL-based Pros & Cons • It's fast

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WAL-based Pros & Cons • It's fast • Easy to set up

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WAL-based Pros & Cons • It's fast • Easy to set up • Requires no additional software, I/O, bandwidth

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WAL-based Pros & Cons • It's fast • Easy to set up • Requires no additional software, I/O, bandwidth • Exact replica provides assurance for backups

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WAL-based Pros & Cons • It's fast • Easy to set up • Requires no additional software, I/O, bandwidth • Exact replica provides assurance for backups • Exact replica means all data types are supported

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WAL-based Pros & Cons

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WAL-based Pros & Cons • All or nothing

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WAL-based Pros & Cons • All or nothing • No major version upgrades

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WAL-based Pros & Cons • All or nothing • No major version upgrades • Query cancellations

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WAL-based Pros & Cons • All or nothing • No major version upgrades • Query cancellations • Storage impact on primary • wal_keep_segments • replication slots

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Middle Ground: Logical Replication

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Middle Ground: Logical Replication • Introduced in v. 10

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Middle Ground: Logical Replication • Introduced in v. 10 • Based oﬀ Logical Decoding framework introduced in v. 9.4

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Middle Ground: Logical Replication • Introduced in v. 10 • Based oﬀ Logical Decoding framework introduced in v. 9.4 • WAL ﬁles are reconstructed into SQL statements

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Middle Ground: Logical Replication • Introduced in v. 10 • Based off Logical Decoding framework introduced in v. 9.4 • WAL files are reconstructed into SQL statements • Paves the way to more granular replication filters

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Best of Both Worlds?

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Best of Both Worlds? • Like Trigger-based Replication: • Still need uniqueness/PK • DDL is not replicated • Sequences are not replicated

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Best of Both Worlds? • Like Trigger-based Replication: • Still need uniqueness/PK • DDL is not replicated • Sequences are not replicated • Like WAL-based Replication • Still need to monitor storage impact • Can’t create column-level ﬁlters (yet)

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Additional Gotchas

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Additional Gotchas • WAL events are unrolled into individual SQL queries • COPY statements are translated into INSERTs • UPDATE on 1000 rows are translated into 1000 individual UPDATEs

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Additional Gotchas • WAL events are unrolled into individual SQL queries • COPY statements are translated into INSERTs • UPDATE on 1000 rows are translated into 1000 individual UPDATEs • TRUNCATEs are replicated, but not cascaded

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Other options

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Other options • DRBD, Corosync, Windows Cluster • Basically, mirrors a disk or other block-level device • Doesn't support read-only standby • Use cases for these are somewhat rare

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Other options • DRBD, Corosync, Windows Cluster • Basically, mirrors a disk or other block-level device • Doesn't support read-only standby • Use cases for these are somewhat rare • Use at your own discretion

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How do you choose?

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It all depends on your requirements Tolerances, privacy, etc.

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Process of Elimination

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Start with built-in Streaming Replication

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Start with built-in Streaming Replication • Super-simple to set up

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Start with built-in Streaming Replication • Super-simple to set up • No additional software

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Start with built-in Streaming Replication • Super-simple to set up • No additional software • Best performance, least impact

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Start with built-in Streaming Replication • Super-simple to set up • No additional software • Best performance, least impact • Good for most use-cases

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Streaming Replication X

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Next, try Logical Replication

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Next, try Logical Replication • Pick and choose which tables to replicate

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Next, try Logical Replication • Pick and choose which tables to replicate • WAL-based replication performance

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Next, try Logical Replication • Pick and choose which tables to replicate • WAL-based replication performance • Zero-downtime major-version upgrades

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Next, try Logical Replication • Pick and choose which tables to replicate • WAL-based replication performance • Zero-downtime major-version upgrades • Only available in v. 10 and later

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Streaming Replication Logical Replication X X

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Next, try pglogical or EPRS

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Next, try pglogical or EPRS • Replication engines supporting Logical Decoding

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Next, try pglogical or EPRS • Replication engines supporting Logical Decoding • pglogical

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Next, try pglogical or EPRS • Replication engines supporting Logical Decoding • pglogical • EnterpriseDB Postgres Replication Server 7 (EPRS7 — Limited Availability)

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Next, try pglogical or EPRS • Replication engines supporting Logical Decoding • pglogical • EnterpriseDB Postgres Replication Server 7 (EPRS7 — Limited Availability) • Row-level and column-level ﬁlters

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Streaming Replication Logical Replication pglogical | EPRS7 X X X

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Use a Trigger-based Solution

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Use a Trigger-based Solution

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Use a Trigger-based Solution • But please upgrade soon!

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Use a Trigger-based Solution • But please upgrade soon! • Impacts performance

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Use a Trigger-based Solution • But please upgrade soon! • Impacts performance • Often diﬃcult to administration

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Use a Trigger-based Solution • But please upgrade soon! • Impacts performance • Often diﬃcult to administration • Logical Replication is the future

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RDS?

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RDS? Try Bucardo

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Multi-Master Replication?

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Multi-Master Replication? • Holy Grail of replication solutions? • First, ask yourself if you REALLY need it • Consistency • Performance • Conﬂict Resolution • Often, the other options are good enough ?

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Multi-Master Replication • Trigger-based • Bucardo • XDB by EnterpriseDB • WAL-based • Bi-Directional Replication (BDR) by 2nd Quadrant • requires a custom compile/installation of PG • XDB/EPRS7 by EnterpriseDB

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Q & A

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Thank You! Richard Yen Principal Support Engineer, EnterpriseDB support@enterprisedb.com richyen.com