@graphile/subscriptions-lds

PostGraphile schema plugin to provide live updates powered by PostgreSQL logical decoding. Used as part of PostGraphile's live queries support.

For more background, see postgraphile issue #92.

Installation:

Install alongside postgraphile, e.g.:

yarn add @graphile/subscriptions-lds

Usage:

To use this plugin:

• Ensure your PostgreSQL server has wal_level = logical and the wal2json plugin (see "Setting up PostgreSQL" below)
• Load this plugin with --append-plugins (or appendPlugins)
• Enable PostGraphile live with --live (or live: true)
• If you don't use a superuser or database owner PostgreSQL user with PostGraphile normally (or if you pass a pool to the PostGraphile library), you must provide a superuser or database owner connection string via --owner-connection (or ownerConnectionString)

CLI:

postgraphile \
  --live \
  --owner-connection postgres://db_owner:db_owner_pass@host/db \
  --append-plugins @graphile/subscriptions-lds \
  ...

Library:

app.use(
  postgraphile(DB, SCHEMA, {
    // ...

    // Enable live support in PostGraphile
    live: true,
    // We need elevated privileges for logical decoding
    ownerConnectionString: "postgres://db_owner:db_owner_pass@host/db",
    // Add this plugin
    appendPlugins: [
      //...
      require("@graphile/subscriptions-lds").default,
    ],
  })
);

Setting up PostgreSQL

We currently only support PG10+; if you need support for 9.x please get in touch.

TL;DR: set wal_level = logical in postgresql.conf and ensure wal2json is installed.

This plugin uses logical decoding and wal2json, so you must configure your PostgreSQL database to support this.

Setting wal_level = logical

In your postgresql.conf you need to enable wal_level = logical. You should ensure that the following settings are set (the 10s can be any number greater than 1; set them to how many PostGraphile instances you're expecting to run, plus a little buffer for regular replication needs):

wal_level = logical
max_wal_senders = 10
max_replication_slots = 10

Installing wal2json

You also need to ensure that wal2json is installed. This comes as standard in many managed PostgreSQL services, such as Amazon RDS, but to install it locally:

1. Ensure that which pg_config returns the path to the correct pg_config binary - the one related to your PostgreSQL install. (For example, if on a Mac you've installed both Postgres.app and PostgreSQL from homebrew then you must modify your PATH variable to point at whichever one you use, e.g. export PATH="/Applications/Postgres.app/Contents/Versions/10/bin/:$PATH")
2. Run the below code (it takes about 10 seconds):

git clone https://github.com/eulerto/wal2json.git
cd wal2json
USE_PGXS=1 make
USE_PGXS=1 make install

3. (optional) delete the wal2json folder

Optimising

Please note that the defaults shown below are likely to change over time based on user feedback. This document will not necessarily be updated with the new defaults.

LD_WAIT (default 125)

This environmental variable controls how often in milliseconds we check for changes from the database. Setting it smaller leads to more timely updates but increases overhead. Setting it larger increases efficiency but means each batch takes longer to process which may slow the Node.js event loop.

LIVE_THROTTLE (default 500)

This environmental variable is the minimum duration in milliseconds between live updates to the same subscription.

If your server is getting overwhelmed, you may increase this to increase the period between live updates sent to clients.

If your application is not responsive enough, you may decrease this to get closer to real-time updates.

(Throttle fires on both the leading and trailing edge, so decreasing this only affects successive updates, not the initial update.)

LD_TABLE_PATTERN (default "*.*")

Set this envvar to e.g. app_public.* to only monitor tables in the app_public schema. See filter-tables in the wal2json documentation

Running LDS externally

If you reach sufficient scale that running @graphile/lds on its own server makes sense (rather than using the embedded version) then you can do so easily. Follow the steps in the @graphile/lds README to get the server up and running, and then set environmental variable LDS_SERVER_URL to the full websocket URL to your LDS server, e.g. ws://127.0.0.1:9876 (default) before loading this plugin.

Compatibility check

You can determine if your PostgreSQL instance is configured correctly with this:

DO $$
BEGIN
  if current_setting('wal_level') is distinct from 'logical' then
    raise exception 'wal_level must be set to ''logical'', your database has it set to ''%''. Please edit your `%` file and restart PostgreSQL.', current_setting('wal_level'), current_setting('config_file');
  end if;
  if (current_setting('max_replication_slots')::int >= 1) is not true then
    raise exception 'Your max_replication_slots setting is too low, it must be greater than 1. Please edit your `%` file and restart PostgreSQL.', current_setting('config_file');
  end if;
  if (current_setting('max_wal_senders')::int >= 1) is not true then
    raise exception 'Your max_wal_senders setting is too low, it must be greater than 1. Please edit your `%` file and restart PostgreSQL.', current_setting('config_file');
  end if;
  perform pg_create_logical_replication_slot('compatibility_test', 'wal2json');
  perform pg_drop_replication_slot('compatibility_test');
  raise notice 'Everything seems to be in order.';
end;
$$ LANGUAGE plpgsql;

If you see the following message then all should be good:

NOTICE:  00000: Everything seems to be in order.