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Minimal downtime major PostgreSQL version upgrades with pg_easy_replicate

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Gem Version

pg_easy_replicate is a CLI orchestrator tool that simplifies the process of setting up logical replication between two PostgreSQL databases. pg_easy_replicate also supports switchover. After the source (primary database) is fully replicating, pg_easy_replicate puts it into read-only mode and via logical replication flushes all data to the new target database. This ensures zero data loss and minimal downtime for the application. This method can be useful for performing minimal downtime (up to <1min, depending) major version upgrades between two PostgreSQL databases, load testing with blue/green database setup and other similar use cases.

Battle tested in production at Tines 🚀

Installation

Add this line to your application’s Gemfile:

And then execute:

Or install it yourself as:

$ gem install pg_easy_replicate

This will include all dependencies accordingly as well. Make sure the following requirements are satisfied.

Or via Docker:

docker pull shayonj/pg_easy_replicate:latest

https://hub.docker.com/r/shayonj/pg_easy_replicate

Requirements

  • PostgreSQL 10 and later
  • Ruby 2.7 and later
  • Database user should have permissions for SUPERUSER
  • Both databases should have the same schema

Limits

All Logical Replication Restrictions apply.

Usage

Ensure SOURCE_DB_URL and TARGET_DB_URL are present as environment variables in the runtime environment. The URL are of the postgres connection string format. Example:

$ export SOURCE_DB_URL="postgres://USERNAME:PASSWORD@localhost:5432/DATABASE_NAME"
$ export TARGET_DB_URL="postgres://USERNAME:PASSWORD@localhost:5433/DATABASE_NAME"

Any pg_easy_replicate command can be run the same way with the docker image as well. As long the container is running in an environment where it has access to both the databases. Example

docker run -e SOURCE_DB_URL="postgres://USERNAME:PASSWORD@localhost:5432/DATABASE_NAME"  
  -e TARGET_DB_URL="postgres://USERNAME:PASSWORD@localhost:5433/DATABASE_NAME" 
  -it --rm shayonj/pg_easy_replicate:latest 
  pg_easy_replicate config_check

CLI

$  pg_easy_replicate
pg_easy_replicate commands:
  pg_easy_replicate bootstrap -g, --group-name=GROUP_NAME    # Sets up temporary tables for information required during runtime
  pg_easy_replicate cleanup -g, --group-name=GROUP_NAME      # Cleans up all bootstrapped data for the respective group
  pg_easy_replicate config_check                             # Prints if source and target database have the required config
  pg_easy_replicate help [COMMAND]                           # Describe available commands or one specific command
  pg_easy_replicate start_sync -g, --group-name=GROUP_NAME   # Starts the logical replication from source database to target database provisioned in the group
  pg_easy_replicate stats  -g, --group-name=GROUP_NAME       # Prints the statistics in JSON for the group
  pg_easy_replicate stop_sync -g, --group-name=GROUP_NAME    # Stop the logical replication from source database to target database provisioned in the group
  pg_easy_replicate switchover  -g, --group-name=GROUP_NAME  # Puts the source database in read only mode after all the data is flushed and written
  pg_easy_replicate version                                  # Prints the version

Replicating all tables with a single group

You can create as many groups as you want for a single database. Groups are just a logical isolation of a single replication.

Config check

$ pg_easy_replicate config_check

✅ Config is looking good.

Bootstrap

Every sync will need to be bootstrapped before you can set up the sync between two databases. Bootstrap creates a new super user to perform the orchestration required during the rest of the process. It also creates some internal metadata tables for record keeping.

$ pg_easy_replicate bootstrap --group-name database-cluster-1

{"name":"pg_easy_replicate","hostname":"PKHXQVK6DW","pid":21485,"level":30,"time":"2023-06-19T15:51:11.015-04:00","v":0,"msg":"Setting up schema","version":"0.1.0"}
...

Start sync

Once the bootstrap is complete, you can start the sync. Starting the sync sets up the publication, subscription and performs other minor housekeeping things.

$ pg_easy_replicate start_sync --group-name database-cluster-1

{"name":"pg_easy_replicate","hostname":"PKHXQVK6DW","pid":22113,"level":30,"time":"2023-06-19T15:54:54.874-04:00","v":0,"msg":"Setting up publication","publication_name":"pger_publication_database_cluster_1","version":"0.1.0"}
...

Stats

You can inspect or watch stats any time during the sync process. The stats give you can an idea of when the sync started, current flush/write lag, how many tables are in replicating, copying or other stages, and more.

You can poll these stats to perform any other after the switchover is done. The stats include a switchover_completed_at which is updated once the switch over is complete.

$ pg_easy_replicate stats --group-name database-cluster-1

{
  "lag_stats": [
    {
      "pid": 66,
      "client_addr": "192.168.128.2",
      "user_name": "jamesbond",
      "application_name": "pger_subscription_database_cluster_1",
      "state": "streaming",
      "sync_state": "async",
      "write_lag": "0.0",
      "flush_lag": "0.0",
      "replay_lag": "0.0"
    }
  ],
  "message_lsn_receipts": [
    {
      "received_lsn": "0/1674688",
      "last_msg_send_time": "2023-06-19 19:56:35 UTC",
      "last_msg_receipt_time": "2023-06-19 19:56:35 UTC",
      "latest_end_lsn": "0/1674688",
      "latest_end_time": "2023-06-19 19:56:35 UTC"
    }
  ],
  "sync_started_at": "2023-06-19 19:54:54 UTC",
  "sync_failed_at": null,
  "switchover_completed_at": null

  ....

Performing switchover

pg_easy_replicate doesn’t kick off the switchover on its own. When you start the sync via start_sync, it starts the replication between the two databases. Once you have had the time to monitor stats and any other key metrics, you can kick off the switchover.

switchover will wait until all tables in the group are replicating and the delta for lag is <200kb (by calculating the pg_wal_lsn_diff between sent_lsn and write_lsn) and then perform the switch.

The switch is made by putting the user on the source database in READ ONLY mode, so that it is not accepting any more writes and waits for the flush lag to be 0. It is up to user to kick of a rolling restart of your application containers or failover DNS (more on these below in strategies) after the switchover is complete, so that your application isn’t sending any read/write requests to the old/source database.

$ pg_easy_replicate switchover  --group-name database-cluster-1

{"name":"pg_easy_replicate","hostname":"PKHXQVK6DW","pid":24192,"level":30,"time":"2023-06-19T16:05:23.033-04:00","v":0,"msg":"Watching lag stats","version":"0.1.0"}
...

Replicating single database with custom tables

By default all tables are added for replication but you can create multiple groups with custom tables for the same database. Example

$ pg_easy_replicate bootstrap --group-name database-cluster-1
$ pg_easy_replicate start_sync --group-name database-cluster-1 --schema-name public --tables "users, posts, events"

...

$ pg_easy_replicate bootstrap --group-name database-cluster-2
$ pg_easy_replicate start_sync --group-name database-cluster-2 --schema-name public --tables "comments, views"

...
$ pg_easy_replicate switchover  --group-name database-cluster-1
$ pg_easy_replicate switchover  --group-name database-cluster-2
...

Switchover strategies with minimal downtime

For minimal downtime, it’d be best to watch/tail the stats and wait until switchover_completed_at is updated with a timestamp. Once that happens you can perform any of the following strategies. Note: These are just suggestions and pg_easy_replicate doesn’t provide any functionalities for this.

Rolling restart strategy

In this strategy, you have a change ready to go which instructs your application to start connecting to the new database. Either using an environment variable or similar. Depending on the application type, it may or may not require a rolling restart.

Next, you can set up a program that watches the stats and waits until switchover_completed_at is reporting as true. Once that happens it kicks off a rolling restart of your application containers so they can start making connections to the DNS of the new database.

DNS Failover strategy

In this strategy, you have a weighted based DNS system (example AWS Route53 weighted records) where 100% of traffic goes to a primary origin and 0% to a secondary origin. The primary origin here is the DNS host for your source database and secondary origin is the DNS host for your target database. You can set up your application ahead of time to interact with the database using DNS from the weighted group.

Next, you can set up a program that watches the stats and waits until switchover_completed_at is reporting as true. Once that happens it updates the weight in the DNS weighted group where 100% of the requests now go to the new/target database. Note: Keeping a low ttl is recommended.

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