Merge branch '278-rewrite-events-queue' into 'devel'

Resolve "Rewrite events queue"

Closes #278

See merge request wgp/dougal/software!46

etc/db/upgrades/upgrade30-v0.4.3-large-notification-payloads.sql

@@ -0,0 +1,164 @@
+-- Support notification payloads larger than Postgres' NOTIFY limit.
+--
+-- New schema version: 0.4.3
+--
+-- ATTENTION:
+--
+-- ENSURE YOU HAVE BACKED UP THE DATABASE BEFORE RUNNING THIS SCRIPT.
+--
+--
+-- NOTE: This upgrade affects the public schema only.
+-- NOTE: Each application starts a transaction, which must be committed
+-- or rolled back.
+--
+-- This creates a new table where large notification payloads are stored
+-- temporarily and from which they might be recalled by the notification
+-- listeners. It also creates a purge_notifications() procedure used to
+-- clean up old notifications from the notifications log and finally,
+-- modifies notify() to support these changes. When a large payload is
+-- encountered, the payload is stored in the notify_payloads table and
+-- a trimmed down version containing a notification_id is sent to listeners
+-- instead. Listeners can then query notify_payloads to retrieve the full
+-- payloads. It is the application layer's responsibility to delete old
+-- notifications.
+--
+-- To apply, run as the dougal user:
+--
+-- psql <<EOF
+-- \i $THIS_FILE
+-- COMMIT;
+-- EOF
+--
+-- NOTE: It can be applied multiple times without ill effect.
+--
+
+BEGIN;
+
+CREATE OR REPLACE PROCEDURE pg_temp.show_notice (notice text) AS $$
+BEGIN
+	RAISE NOTICE '%', notice;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE PROCEDURE pg_temp.upgrade_schema () AS $outer$
+BEGIN
+
+	RAISE NOTICE 'Updating public schema';
+	-- We need to set the search path because some of the trigger
+	-- functions reference other tables in survey schemas assuming
+	-- they are in the search path.
+	EXECUTE format('SET search_path TO public');
+
+	CREATE TABLE IF NOT EXISTS public.notify_payloads (
+		id SERIAL,
+		tstamp timestamptz NOT NULL DEFAULT CURRENT_TIMESTAMP,
+		payload text NOT NULL DEFAULT '',
+		PRIMARY KEY (id)
+	);
+
+	CREATE INDEX IF NOT EXISTS notify_payload_tstamp ON notify_payloads (tstamp);
+
+	CREATE OR REPLACE FUNCTION public.notify() RETURNS trigger
+		LANGUAGE plpgsql
+		AS $$
+	DECLARE
+	channel text := TG_ARGV[0];
+	pid text;
+	payload text;
+	notification text;
+	payload_id integer;
+	BEGIN
+
+	SELECT projects.pid INTO pid FROM projects WHERE schema = TG_TABLE_SCHEMA;
+
+	payload := json_build_object(
+	'tstamp', CURRENT_TIMESTAMP,
+	'operation', TG_OP,
+	'schema', TG_TABLE_SCHEMA,
+	'table', TG_TABLE_NAME,
+	'old', row_to_json(OLD),
+	'new', row_to_json(NEW),
+	'pid', pid
+	)::text;
+
+	IF octet_length(payload) < 1000 THEN
+	PERFORM pg_notify(channel, payload);
+	ELSE
+	-- We need to find another solution
+	-- FIXME Consider storing the payload in a temporary memory table,
+	-- referenced by some form of autogenerated ID. Then send the ID
+	-- as the payload and then it's up to the user to fetch the original
+	-- payload if interested. This needs a mechanism to expire older payloads
+	-- in the interest of conserving memory.
+
+	INSERT INTO notify_payloads (payload) VALUES (payload) RETURNING id INTO payload_id;
+
+	notification := json_build_object(
+	'tstamp', CURRENT_TIMESTAMP,
+	'operation', TG_OP,
+	'schema', TG_TABLE_SCHEMA,
+	'table', TG_TABLE_NAME,
+	'pid', pid,
+	'payload_id', payload_id
+	)::text;
+
+	PERFORM pg_notify(channel, notification);
+	RAISE INFO 'Payload over limit';
+	END IF;
+	RETURN NULL;
+	END;
+	$$;
+
+	CREATE PROCEDURE public.purge_notifications (age_seconds numeric DEFAULT 120) AS $$
+		DELETE FROM notify_payloads WHERE EXTRACT(epoch FROM CURRENT_TIMESTAMP - tstamp) > age_seconds;
+	$$ LANGUAGE sql;
+
+
+
+END;
+$outer$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE PROCEDURE pg_temp.upgrade () AS $outer$
+DECLARE
+	row RECORD;
+	current_db_version TEXT;
+BEGIN
+
+	SELECT value->>'db_schema' INTO current_db_version FROM public.info WHERE key = 'version';
+
+	IF current_db_version >= '0.4.3' THEN
+		RAISE EXCEPTION
+			USING MESSAGE='Patch already applied';
+	END IF;
+
+	IF current_db_version != '0.4.2' THEN
+		RAISE EXCEPTION
+			USING MESSAGE='Invalid database version: ' || current_db_version,
+				HINT='Ensure all previous patches have been applied.';
+	END IF;
+
+	-- This upgrade modified the `public` schema only, not individual
+	-- project schemas.
+	CALL pg_temp.upgrade_schema();
+
+END;
+$outer$ LANGUAGE plpgsql;
+
+CALL pg_temp.upgrade();
+
+CALL pg_temp.show_notice('Cleaning up');
+DROP PROCEDURE pg_temp.upgrade_schema ();
+DROP PROCEDURE pg_temp.upgrade ();
+
+CALL pg_temp.show_notice('Updating db_schema version');
+INSERT INTO public.info VALUES ('version', '{"db_schema": "0.4.3"}')
+ON CONFLICT (key) DO UPDATE
+	SET value = public.info.value || '{"db_schema": "0.4.3"}' WHERE public.info.key = 'version';
+
+
+CALL pg_temp.show_notice('All done. You may now run "COMMIT;" to persist the changes');
+DROP PROCEDURE pg_temp.show_notice (notice text);
+
+--
+--NOTE Run `COMMIT;` now if all went well
+--