Connection Pooling

Cloud Native PostgreSQL provides native support for connection pooling with PgBouncer, one of the most popular open source connection poolers for PostgreSQL, through the Pooler CRD.

In a nutshell, a Pooler in Cloud Native PostgreSQL is a deployment of PgBouncer pods that sits between your applications and a PostgreSQL service (for example the rw service), creating a separate, scalable, configurable, and highly available database access layer.

Architecture

The following diagram highlights how the introduction of a database access layer based on PgBouncer changes the architecture of Cloud Native PostgreSQL, like an additional blade in a Swiss Army knife. Instead of directly connecting to the PostgreSQL primary service, applications can now connect to the equivalent service for PgBouncer, enabling reuse of existing connections for faster performance and better resource management on the PostgreSQL side.

Applications writing to the single primary via PgBouncer

Quickstart

The easiest way to explain how Cloud Native PostgreSQL implements a PgBouncer pooler is through an example:

apiVersion: postgresql.k8s.enterprisedb.io/v1
kind: Pooler
metadata:
  name: pooler-example-rw
spec:
  cluster:
    name: cluster-example

  instances: 3
  type: rw
  pgbouncer:
    poolMode: session
    parameters:
      max_client_connections: "1000"
      default_pool_size: "10"

Important

Pooler name should never match with any Cluster name within the same namespace.

This creates a new Pooler resource called pooler-example-rw (the name is arbitrary) that is strictly associated with the Postgres Cluster resource called cluster-example and pointing to the primary, identified by the read/write service (rw, therefore cluster-example-rw).

The Pooler must live in the same namespace of the Postgres cluster. It consists of a Kubernetes deployment of 3 pods running the latest stable image of PgBouncer, configured with the session pooling mode and accepting up to 1000 connections each - with a default pool size of 10 user/database pairs towards PostgreSQL.

Important

The Pooler only sets the * fallback database in PgBouncer, meaning that all parameters in the connection strings passed from the client are relayed to the PostgreSQL server (please refer to "Section [databases]" in PgBouncer's documentation).

Additionally, Cloud Native PostgreSQL automatically creates a secret with the same name of the pooler containing the configuration files used with PgBouncer.

API reference

For details, please refer to PgBouncerSpec section in the API reference.

Pooler resource lifecycle

Pooler resources are not Cluster-managed resources. You are supposed to create poolers manually when they are needed. Additionally, you can deploy multiple poolers per PostgreSQL Cluster.

What is important to note is that the lifecycles of the Cluster and the Pooler resources are currently independent: the deletion of the Cluster doesn't imply the automatic deletion of the Pooler, and viceversa.

Important

Now that you know how a Pooler works, you have full freedom in terms of possible architectures: you can have clusters without poolers, clusters with a single pooler, or clusters with several poolers (i.e. one per application).

Security

Any PgBouncer pooler is transparently integrated with Cloud Native PostgreSQL support for in-transit encryption via TLS connections, both on the client (application) and server (PostgreSQL) side of the pool.

Specifically, PgBouncer automatically reuses the certificates of the PostgreSQL server. Moreover, it uses TLS client certificate authentication to connect to the PostgreSQL server to run the auth_query for clients' password authentication (see the "Authentication" section below).

Containers run as the pgbouncer system user, and access to the pgbouncer database is only allowed via local connections, through peer authentication.

Authentication

Password based authentication is the only supported method for clients of PgBouncer in Cloud Native PostgreSQL.

Internally, our implementation relies on PgBouncer's auth_user and auth_query options. Specifically, the operator:

  • creates a standard user called cnp_pooler_pgbouncer in the PostgreSQL server
  • creates the lookup function in the postgres database and grants execution privileges to the cnp_pooler_pgbouncer user (PoLA)
  • issues a TLS certificate for this user
  • sets cnp_pooler_pgbouncer as the auth_user
  • configures PgBouncer to use the TLS certificate to authenticate cnp_pooler_pgbouncer against the PostgreSQL server
  • removes all the above when it detects that a cluster does not have any pooler associated to it

PodTemplates

You can take advantage of pod templates specification in the template section of a Pooler resource. For details, please refer to PoolerSpec section in the API reference.

Through templates you can configure pods as you like, including fine control over affinity and anti-affinity rules for pods and nodes. By default, containers use images from quay.io/enterprisedb/pgbouncer.

High Availability (HA)

Thanks to Kubernetes' deployments, you can configure your pooler to run on a single instance or over multiple pods. The exposed service will make sure that your clients are randomly distributed over the available pods running PgBouncer - which will then automatically manage and reuse connections towards the underlying server (if using the rw service) or servers (if using the ro service with multiple replicas).

Warning

Please be aware of network hops in case your infrastructure spans multiple availability zones with high latency across them. Consider for example the case of your application running in zone 2, connecting to PgBouncer running in zone 3, pointing to the PostgreSQL primary in zone 1.

PgBouncer configuration options

The operator manages most of the configuration options for PgBouncer, allowing you to modify only a subset of them.

Warning

You are responsible to correctly set the value of each option, as the operator does not validate them.

Below you can find a list of the PgBouncer options you are allowed to customize. Each of them contains a link to the PgBouncer documentation for that specific parameter. Unless differently stated here, the default values are the ones directly set by PgBouncer:

Customizations of the PgBouncer configuration are written declaratively in the .spec.pgbouncer.parameters map.

The operator reacts to the changes in the Pooler specification, and every PgBouncer instance reloads the updated configuration without disrupting the service.

Warning

Every PgBouncer pod will have the same configuration, aligned with the parameters in the specification. A mistake in these parameters could disrupt the operability of the whole Pooler. The operator does not validate the value of any option.

Monitoring

The PgBouncer implementation of the Pooler comes with a default Prometheus exporter that automatically makes available several metrics having the cnp_pgbouncer_ prefix, by running:

  • SHOW LISTS (prefix: cnp_pgbouncer_lists)
  • SHOW POOLS (prefix: cnp_pgbouncer_pools)
  • SHOW STATS (prefix: cnp_pgbouncer_stats)

Similarly to the Cloud Native PostgreSQL instance, the exporter runs on port 9187 of each pod running PgBouncer, and also provides metrics related to the Go runtime (with prefix go_*). You can debug the exporter on a pod running PgBouncer through the following command:

kubectl exec -ti <PGBOUNCER_POD> -- curl 127.0.0.1:9187/metrics

An example of the output for cnp_pgbouncer metrics:

# HELP cnp_pgbouncer_collection_duration_seconds Collection time duration in seconds
# TYPE cnp_pgbouncer_collection_duration_seconds gauge
cnp_pgbouncer_collection_duration_seconds{collector="Collect.up"} 0.002443168

# HELP cnp_pgbouncer_collections_total Total number of times PostgreSQL was accessed for metrics.
# TYPE cnp_pgbouncer_collections_total counter
cnp_pgbouncer_collections_total 1

# HELP cnp_pgbouncer_last_collection_error 1 if the last collection ended with error, 0 otherwise.
# TYPE cnp_pgbouncer_last_collection_error gauge
cnp_pgbouncer_last_collection_error 0

# HELP cnp_pgbouncer_lists_databases Count of databases.
# TYPE cnp_pgbouncer_lists_databases gauge
cnp_pgbouncer_lists_databases 1

# HELP cnp_pgbouncer_lists_dns_names Count of DNS names in the cache.
# TYPE cnp_pgbouncer_lists_dns_names gauge
cnp_pgbouncer_lists_dns_names 0

# HELP cnp_pgbouncer_lists_dns_pending Not used.
# TYPE cnp_pgbouncer_lists_dns_pending gauge
cnp_pgbouncer_lists_dns_pending 0

# HELP cnp_pgbouncer_lists_dns_queries Count of in-flight DNS queries.
# TYPE cnp_pgbouncer_lists_dns_queries gauge
cnp_pgbouncer_lists_dns_queries 0

# HELP cnp_pgbouncer_lists_dns_zones Count of DNS zones in the cache.
# TYPE cnp_pgbouncer_lists_dns_zones gauge
cnp_pgbouncer_lists_dns_zones 0

# HELP cnp_pgbouncer_lists_free_clients Count of free clients.
# TYPE cnp_pgbouncer_lists_free_clients gauge
cnp_pgbouncer_lists_free_clients 49

# HELP cnp_pgbouncer_lists_free_servers Count of free servers.
# TYPE cnp_pgbouncer_lists_free_servers gauge
cnp_pgbouncer_lists_free_servers 0

# HELP cnp_pgbouncer_lists_login_clients Count of clients in login state.
# TYPE cnp_pgbouncer_lists_login_clients gauge
cnp_pgbouncer_lists_login_clients 0

# HELP cnp_pgbouncer_lists_pools Count of pools.
# TYPE cnp_pgbouncer_lists_pools gauge
cnp_pgbouncer_lists_pools 1

# HELP cnp_pgbouncer_lists_used_clients Count of used clients.
# TYPE cnp_pgbouncer_lists_used_clients gauge
cnp_pgbouncer_lists_used_clients 1

# HELP cnp_pgbouncer_lists_used_servers Count of used servers.
# TYPE cnp_pgbouncer_lists_used_servers gauge
cnp_pgbouncer_lists_used_servers 0

# HELP cnp_pgbouncer_lists_users Count of users.
# TYPE cnp_pgbouncer_lists_users gauge
cnp_pgbouncer_lists_users 2

# HELP cnp_pgbouncer_pools_cl_active Client connections that are linked to server connection and can process queries.
# TYPE cnp_pgbouncer_pools_cl_active gauge
cnp_pgbouncer_pools_cl_active{database="pgbouncer",user="pgbouncer"} 1

# HELP cnp_pgbouncer_pools_cl_cancel_req Client connections that have not forwarded query cancellations to the server yet.
# TYPE cnp_pgbouncer_pools_cl_cancel_req gauge
cnp_pgbouncer_pools_cl_cancel_req{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_cl_waiting Client connections that have sent queries but have not yet got a server connection.
# TYPE cnp_pgbouncer_pools_cl_waiting gauge
cnp_pgbouncer_pools_cl_waiting{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_maxwait How long the first (oldest) client in the queue has waited, in seconds. If this starts increasing, then the current pool of servers does not handle requests quickly enough. The reason may be either an overloaded server or just too small of a pool_size setting.
# TYPE cnp_pgbouncer_pools_maxwait gauge
cnp_pgbouncer_pools_maxwait{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_maxwait_us Microsecond part of the maximum waiting time.
# TYPE cnp_pgbouncer_pools_maxwait_us gauge
cnp_pgbouncer_pools_maxwait_us{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_pool_mode The pooling mode in use. 1 for session, 2 for transaction, 3 for statement, -1 if unknown
# TYPE cnp_pgbouncer_pools_pool_mode gauge
cnp_pgbouncer_pools_pool_mode{database="pgbouncer",user="pgbouncer"} 3

# HELP cnp_pgbouncer_pools_sv_active Server connections that are linked to a client.
# TYPE cnp_pgbouncer_pools_sv_active gauge
cnp_pgbouncer_pools_sv_active{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_sv_idle Server connections that are unused and immediately usable for client queries.
# TYPE cnp_pgbouncer_pools_sv_idle gauge
cnp_pgbouncer_pools_sv_idle{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_sv_login Server connections currently in the process of logging in.
# TYPE cnp_pgbouncer_pools_sv_login gauge
cnp_pgbouncer_pools_sv_login{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_sv_tested Server connections that are currently running either server_reset_query or server_check_query.
# TYPE cnp_pgbouncer_pools_sv_tested gauge
cnp_pgbouncer_pools_sv_tested{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_pools_sv_used Server connections that have been idle for more than server_check_delay, so they need server_check_query to run on them before they can be used again.
# TYPE cnp_pgbouncer_pools_sv_used gauge
cnp_pgbouncer_pools_sv_used{database="pgbouncer",user="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_avg_query_count Average queries per second in last stat period.
# TYPE cnp_pgbouncer_stats_avg_query_count gauge
cnp_pgbouncer_stats_avg_query_count{database="pgbouncer"} 1

# HELP cnp_pgbouncer_stats_avg_query_time Average query duration, in microseconds.
# TYPE cnp_pgbouncer_stats_avg_query_time gauge
cnp_pgbouncer_stats_avg_query_time{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_avg_recv Average received (from clients) bytes per second.
# TYPE cnp_pgbouncer_stats_avg_recv gauge
cnp_pgbouncer_stats_avg_recv{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_avg_sent Average sent (to clients) bytes per second.
# TYPE cnp_pgbouncer_stats_avg_sent gauge
cnp_pgbouncer_stats_avg_sent{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_avg_wait_time Time spent by clients waiting for a server, in microseconds (average per second).
# TYPE cnp_pgbouncer_stats_avg_wait_time gauge
cnp_pgbouncer_stats_avg_wait_time{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_avg_xact_count Average transactions per second in last stat period.
# TYPE cnp_pgbouncer_stats_avg_xact_count gauge
cnp_pgbouncer_stats_avg_xact_count{database="pgbouncer"} 1

# HELP cnp_pgbouncer_stats_avg_xact_time Average transaction duration, in microseconds.
# TYPE cnp_pgbouncer_stats_avg_xact_time gauge
cnp_pgbouncer_stats_avg_xact_time{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_total_query_count Total number of SQL queries pooled by pgbouncer.
# TYPE cnp_pgbouncer_stats_total_query_count gauge
cnp_pgbouncer_stats_total_query_count{database="pgbouncer"} 3

# HELP cnp_pgbouncer_stats_total_query_time Total number of microseconds spent by pgbouncer when actively connected to PostgreSQL, executing queries.
# TYPE cnp_pgbouncer_stats_total_query_time gauge
cnp_pgbouncer_stats_total_query_time{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_total_received Total volume in bytes of network traffic received by pgbouncer.
# TYPE cnp_pgbouncer_stats_total_received gauge
cnp_pgbouncer_stats_total_received{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_total_sent Total volume in bytes of network traffic sent by pgbouncer.
# TYPE cnp_pgbouncer_stats_total_sent gauge
cnp_pgbouncer_stats_total_sent{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_total_wait_time Time spent by clients waiting for a server, in microseconds.
# TYPE cnp_pgbouncer_stats_total_wait_time gauge
cnp_pgbouncer_stats_total_wait_time{database="pgbouncer"} 0

# HELP cnp_pgbouncer_stats_total_xact_count Total number of SQL transactions pooled by pgbouncer.
# TYPE cnp_pgbouncer_stats_total_xact_count gauge
cnp_pgbouncer_stats_total_xact_count{database="pgbouncer"} 3

# HELP cnp_pgbouncer_stats_total_xact_time Total number of microseconds spent by pgbouncer when connected to PostgreSQL in a transaction, either idle in transaction or executing queries.
# TYPE cnp_pgbouncer_stats_total_xact_time gauge
cnp_pgbouncer_stats_total_xact_time{database="pgbouncer"} 0

Logging

Logs are directly sent to standard output, in JSON format, like in the following example:

{
  "level": "info",
  "ts": SECONDS.MICROSECONDS,
  "msg": "record",
  "pipe": "stderr",
  "record": {
    "timestamp": "YYYY-MM-DD HH:MM:SS.MS UTC",
    "pid": "<PID>",
    "level": "LOG",
    "msg": "kernel file descriptor limit: 1048576 (hard: 1048576); max_client_conn: 100, max expected fd use: 112"
  }
}

Pausing connections

The Pooler specification allows you to take advantage of PgBouncer's PAUSE and RESUME commands, using only declarative configuration - via the paused option, by default set to false. When set to true, the operator internally invokes the PAUSE command in PgBouncer, which:

  1. closes all active connections towards the PostgreSQL server, after waiting for the queries to complete
  2. pauses any new connection coming from the client

When the paused option is set back to false, the operator will invoke the RESUME command in PgBouncer, re-opening the taps towards the PostgreSQL service defined in the Pooler.

PAUSE

For further information, please refer to the PAUSE section in the PgBouncer documentation.

Important

In future versions, the switchover operation will be fully integrated with the PgBouncer pooler, and take advantage of the PAUSE/RESUME features to reduce the perceived downtime by client applications. At the moment, you can achieve the same results by setting the paused attribute to true, then issuing the switchover command through the cnp plugin, and finally restoring the paused attribute to false.

Limitations

Single PostgreSQL cluster

The current implementation of the pooler is designed to work as part of a specific Cloud Native PostgreSQL cluster (a service, to be precise). It is not possible at the moment to create a pooler that spans over multiple clusters.

Controlled configurability

Cloud Native PostgreSQL transparently manages several configuration options that are used for the PgBouncer layer to communicate with PostgreSQL. Such options are not configurable from outside and include TLS certificates, authentication settings, databases section, and users section. Also, considering the specific use case for the single PostgreSQL cluster, the adopted criteria is to explicitly list the options that can be configured by users.

Note

We have reasons to believe that the adopted solution addresses the majority of use cases, while leaving room for the future implementation of a separate operator for PgBouncer to complete the gamma with more advanced and customized scenarios.