PGD-Always-ON v23

Note

This architecture is for Postgres Distributed 5 only. If you require PGD 4 or 3.7 please use BDR-Always-ON.

EDB Postgres Distributed 5 in an Always-ON configuration, suitable for use in test and production.

This architecture requires an EDB subscription. All software will be sourced from EDB Repos 2.0.

Cluster configuration

Overview of configuration options

An example invocation of tpaexec configure for this architecture is shown below.

tpaexec configure ~/clusters/pgd-ao \
         --architecture PGD-Always-ON \
         --edb-postgres-extended 15 \
         --platform aws --instance-type t3.micro \
         --distribution Debian \
         --pgd-proxy-routing global \
         --location-names dc1 dc2 dc3 \
         --witness-only-location dc3 \
         --data-nodes-per-location 2

You can list all available options using the help command.

tpaexec configure --architecture PGD-Always-ON --help

The table below describes the mandatory options for PGD-Always-ON and additional important options. More detail on the options is provided in the following section.

Mandatory Options

Options	Description
`--architecture` (`-a`)	Must be set to `PGD-Always-ON`
Postgres flavour and version (e.g. `--postgresql 15`)	A valid flavour and version specifier.
`--pgd-proxy-routing`	Must be either `global` or `local`.

Additional Options

Options	Description	Behaviour if omitted
`--platform`	One of `aws`, `docker`, `bare`.	Defaults to `aws`.
`--location-names`	A space-separated list of location names. The number of locations is equal to the number of names supplied.	TPA will configure a single location with three data nodes.
`--witness-only-location`	A location name, must be a member of `location-names`.	No witness-only location is added.
`--data-nodes-per-location`	The number of data nodes in each location, must be at least 2.	Defaults to 3.
`--add-proxy-nodes-per-location`	The number of proxy nodes in each location.	PGD-proxy will be installed on each data node.
`--enable-camo`	Sets two data nodes in each location as CAMO partners.	CAMO will not be enabled.
`--bdr-database`	The name of the database to be used for replication.	Defaults to `bdrdb`.
`--enable-pgd-probes`	Enable http(s) api endpoints for pgd-proxy such as `health/is-ready` to allow probing proxy's health.	Disabled by default.
`--proxy-listen-port`	The port on which proxy nodes will route traffic to the write leader.	Defaults to 6432
`--proxy-read-only-port`	The port on which proxy nodes will route read-only traffic to shadow nodes.	Defaults to 6433

More detail about PGD-Always-ON configuration

A PGD-Always-ON cluster comprises a number of locations, preferably odd, each with the same number of data nodes, again preferably odd. If you do not specify any --location-names, the default is to use a single location with three data nodes.

Location names for the cluster are specified as --location-names dc1 dc2 …. A location represents an independent data centre that provides a level of redundancy, in whatever way this definition makes sense to your use case. For example, AWS regions, your own data centres, or any other designation to identify where your servers are hosted.

for AWS users

If you are using TPA to provision an AWS cluster, the locations will be mapped to separate availability zones within the --region you specify. You may specify multiple --regions, but TPA does not currently set up VPC peering to allow instances in different regions to communicate with each other. For a multi-region cluster, you will need to set up VPC peering yourself.

Use --data-nodes-per-location N to specify the number of data nodes in each location. The minimum number is 2, the default is 3.

If you specify an even number of data nodes per location, TPA will add an extra witness node to each location automatically. This retains the ability to establish reliable consensus while allowing cost savings (a witness has minimal hardware requirements compared to the data nodes).

A cluster with only two locations would entirely lose the ability to establish global consensus if one of the locations were to fail. We recommend adding a third witness-only location (which contains no data nodes, only a witness node, again used to reliably establish consensus). Use --witness-only-location loc to designate one of your locations as a witness.

By default, every data node (in every location) will also run PGD-Proxy for connection routing. To create separate PGD-Proxy instances instead, use --add-proxy-nodes-per-location 3 (or however many proxies you want to add).

Depending on your use-case, you must specify --pgd-proxy-routing local or global to configure how PGD-Proxy will route connections to a write leader. Local routing will make every PGD-Proxy route to a write leader within its own location (suitable for geo-sharding applications). Global routing will make every proxy route to a single write leader, elected amongst all available data nodes across all locations.

You may optionally specify --bdr-database dbname to set the name of the database with BDR enabled (default: bdrdb).

You may optionally specify --enable-camo to set two data nodes in each region as CAMO partners.

You may optionally specify --enable-pgd-probes [{http, https}] to enable http(s) api endpoints that will allow to easily probe proxy's health.

You may also specify any of the options described by tpaexec help configure-options.

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