Citus

Deploy native high-availability Citus horizontally sharded clusters with Pigsty, seamlessly scaling PostgreSQL across multiple shards and accelerating OLTP/OLAP queries.

Concept

PGSQL

Pigsty natively supports Citus. This is a distributed horizontal scaling extension based on the native PostgreSQL kernel.

Installation

Citus is a PostgreSQL extension plugin that can be installed and enabled on a native PostgreSQL cluster following the standard plugin installation process.

./pgsql.yml -t pg_extension -e '{"pg_extensions":["citus"]}'

Configuration

To define a citus cluster, you need to specify the following parameters:

pg_mode must be set to citus instead of the default pgsql
You must define the shard name pg_shard and shard number pg_group on each shard cluster
You must define pg_primary_db to specify the database managed by Patroni
If you want to use postgres from pg_dbsu instead of the default pg_admin_username to execute admin commands, then pg_dbsu_password must be set to a non-empty plaintext password

Additionally, you need extra hba rules to allow SSL access from localhost and other data nodes.

You can define each Citus cluster as a separate group, like standard PostgreSQL clusters. The current complete template is conf/ha/citus.yml:

all:
  children:
    pg-citus0: # citus shard 0
      hosts: { 10.10.10.10: { pg_seq: 1, pg_role: primary } }
      vars: { pg_cluster: pg-citus0 , pg_group: 0 }
    pg-citus1: # citus shard 1
      hosts: { 10.10.10.11: { pg_seq: 1, pg_role: primary } }
      vars: { pg_cluster: pg-citus1 , pg_group: 1 }
    pg-citus2: # citus shard 2
      hosts: { 10.10.10.12: { pg_seq: 1, pg_role: primary } }
      vars: { pg_cluster: pg-citus2 , pg_group: 2 }
    pg-citus3: # citus shard 3
      hosts:
        10.10.10.13: { pg_seq: 1, pg_role: primary }
        10.10.10.14: { pg_seq: 2, pg_role: replica }
      vars: { pg_cluster: pg-citus3 , pg_group: 3 }
  vars:                               # Global parameters for all Citus clusters
    pg_mode: citus                    # pgsql cluster mode must be set to: citus
    pg_shard: pg-citus                # citus horizontal shard name: pg-citus
    pg_primary_db: meta               # citus database name: meta
    pg_dbsu_password: DBUser.Postgres # If using dbsu, you need to configure a password for it
    pg_users: [ { name: dbuser_meta ,password: DBUser.Meta ,pgbouncer: true ,roles: [ dbrole_admin ] } ]
    pg_databases: [ { name: meta ,extensions: [ { name: citus }, { name: postgis }, { name: timescaledb } ] } ]
    pg_hba_rules:
      - { user: 'all' ,db: all  ,addr: 127.0.0.1/32 ,auth: ssl ,title: 'all user ssl access from localhost' }
      - { user: 'all' ,db: all  ,addr: intra        ,auth: ssl ,title: 'all user ssl access from intranet'  }

You can also specify identity parameters for all Citus cluster members within a single group, as shown in conf/ha/citus.yml:

#==========================================================#
# pg-citus: 10 node citus cluster (5 x primary-replica pair)
#==========================================================#
pg-citus: # citus group
  hosts:
    10.10.10.50: { pg_group: 0, pg_cluster: pg-citus0 ,pg_vip_address: 10.10.10.60/24 ,pg_seq: 0, pg_role: primary }
    10.10.10.51: { pg_group: 0, pg_cluster: pg-citus0 ,pg_vip_address: 10.10.10.60/24 ,pg_seq: 1, pg_role: replica }
    10.10.10.52: { pg_group: 1, pg_cluster: pg-citus1 ,pg_vip_address: 10.10.10.61/24 ,pg_seq: 0, pg_role: primary }
    10.10.10.53: { pg_group: 1, pg_cluster: pg-citus1 ,pg_vip_address: 10.10.10.61/24 ,pg_seq: 1, pg_role: replica }
    10.10.10.54: { pg_group: 2, pg_cluster: pg-citus2 ,pg_vip_address: 10.10.10.62/24 ,pg_seq: 0, pg_role: primary }
    10.10.10.55: { pg_group: 2, pg_cluster: pg-citus2 ,pg_vip_address: 10.10.10.62/24 ,pg_seq: 1, pg_role: replica }
    10.10.10.56: { pg_group: 3, pg_cluster: pg-citus3 ,pg_vip_address: 10.10.10.63/24 ,pg_seq: 0, pg_role: primary }
    10.10.10.57: { pg_group: 3, pg_cluster: pg-citus3 ,pg_vip_address: 10.10.10.63/24 ,pg_seq: 1, pg_role: replica }
    10.10.10.58: { pg_group: 4, pg_cluster: pg-citus4 ,pg_vip_address: 10.10.10.64/24 ,pg_seq: 0, pg_role: primary }
    10.10.10.59: { pg_group: 4, pg_cluster: pg-citus4 ,pg_vip_address: 10.10.10.64/24 ,pg_seq: 1, pg_role: replica }
  vars:
    pg_mode: citus                    # pgsql cluster mode: citus
    pg_shard: pg-citus                # citus shard name: pg-citus
    pg_primary_db: test               # primary database used by citus
    pg_dbsu_password: DBUser.Postgres # all dbsu password access for citus cluster
    pg_vip_enabled: true
    pg_vip_interface: auto
    pg_extensions: [ 'citus postgis timescaledb pgvector' ]
    pg_libs: 'citus, timescaledb, pg_stat_statements, auto_explain' # citus will be added by patroni automatically
    pg_users: [ { name: test ,password: test ,pgbouncer: true ,roles: [ dbrole_admin ] } ]
    pg_databases: [ { name: test ,owner: test ,extensions: [ { name: citus }, { name: postgis } ] } ]
    pg_hba_rules:
      - { user: 'all' ,db: all  ,addr: 10.10.10.0/24 ,auth: trust ,title: 'trust citus cluster members'        }
      - { user: 'all' ,db: all  ,addr: 127.0.0.1/32  ,auth: ssl   ,title: 'all user ssl access from localhost' }
      - { user: 'all' ,db: all  ,addr: intra         ,auth: ssl   ,title: 'all user ssl access from intranet'  }

Usage

You can access any node just like accessing a regular cluster:

pgbench -i postgres://test:test@pg-citus0/test
pgbench -nv -P1 -T1000 -c 2 postgres://test:test@pg-citus0/test

By default, changes you make to one Shard only occur on that cluster and are not synchronized to other Shards.

If you want to distribute writes across all Shards, you can use the API functions provided by Citus to mark tables as:

Distributed tables (automatic partitioning, requires specifying partition key)
Reference tables (full replication: does not require specifying partition key)

Starting from Citus 11.2, any Citus database node can play the role of coordinator, meaning any primary node can write:

psql -h pg-citus0 -d test -c "SELECT create_distributed_table('pgbench_accounts', 'aid'); SELECT truncate_local_data_after_distributing_table('public.pgbench_accounts');"
psql -h pg-citus0 -d test -c "SELECT create_reference_table('pgbench_branches')         ; SELECT truncate_local_data_after_distributing_table('public.pgbench_branches');"
psql -h pg-citus0 -d test -c "SELECT create_reference_table('pgbench_history')          ; SELECT truncate_local_data_after_distributing_table('public.pgbench_history');"
psql -h pg-citus0 -d test -c "SELECT create_reference_table('pgbench_tellers')          ; SELECT truncate_local_data_after_distributing_table('public.pgbench_tellers');"

After distributing the tables, you can also access them on other nodes:

psql -h pg-citus1 -d test -c '\dt+'

For example, a full table scan will show that the execution plan has become a distributed plan:

vagrant@meta-1:~$ psql -h pg-citus3 -d test -c 'explain select * from pgbench_accounts'
                                               QUERY PLAN
---------------------------------------------------------------------------------------------------------
 Custom Scan (Citus Adaptive)  (cost=0.00..0.00 rows=100000 width=352)
   Task Count: 32
   Tasks Shown: One of 32
   ->  Task
         Node: host=10.10.10.52 port=5432 dbname=test
         ->  Seq Scan on pgbench_accounts_102008 pgbench_accounts  (cost=0.00..81.66 rows=3066 width=97)
(6 rows)

You can initiate writes from several different primary nodes:

pgbench -nv -P1 -T1000 -c 2 postgres://test:test@pg-citus1/test
pgbench -nv -P1 -T1000 -c 2 postgres://test:test@pg-citus2/test
pgbench -nv -P1 -T1000 -c 2 postgres://test:test@pg-citus3/test
pgbench -nv -P1 -T1000 -c 2 postgres://test:test@pg-citus4/test

When a node fails, the native high availability support provided by Patroni will promote the standby node and automatically take over.

test=# select * from  pg_dist_node;
 nodeid | groupid |  nodename   | nodeport | noderack | hasmetadata | isactive | noderole | nodecluster | metadatasynced | shouldhaveshards
--------+---------+-------------+----------+----------+-------------+----------+----------+-------------+----------------+------------------
      1 |       0 | 10.10.10.51 |     5432 | default  | t           | t        | primary  | default     | t              | f
      2 |       2 | 10.10.10.54 |     5432 | default  | t           | t        | primary  | default     | t              | t
      5 |       1 | 10.10.10.52 |     5432 | default  | t           | t        | primary  | default     | t              | t
      3 |       4 | 10.10.10.58 |     5432 | default  | t           | t        | primary  | default     | t              | t
      4 |       3 | 10.10.10.56 |     5432 | default  | t           | t        | primary  | default     | t              | t