The wal2sql is an output extension for logical decoding. It means that the extension have access to tuples produced by INSERT and UPDATE. Also, UPDATE/DELETE old row versions can be accessed depending on the configured replica identity. Changes can be consumed using the streaming protocol (logical replication slots) or by a special SQL API.
format version 1 produces a JSON object per transaction.All of the new/old tuples are available in the JSON object. Also, there are options to include properties such as transaction timestamp, schema-qualified, data types, and transaction ids.
format version 2 produces a JSON object per tuple. Optional JSON object for beginning and end of transaction. Also, there are a variety of options to include properties.
You need to set up at least parameters at lightdb.conf
# lightdb.conf
wal_level = logical
include-transaction (boolean)
emit records denoting the start and end of each transaction. Default is true.
include-xids (boolean)
add xid to each changeset. Default is false.
include-timestamp (boolean)
add timestamp to each changeset. Default is false.
include-pk (boolean)
add primary key information as pk. Column name and data type is included. Default is false.
include-origin (boolean)
add origin of a piece of data. Default is false.
include-schemas (boolean)
add schema to each change. Default is true.
include-types (boolean)
add type to each change. Default is true.
include-type-oids (boolean)
add type oids. Default is false.
include-typmod (boolean)
add modifier to types that have it (eg. varchar(20) instead of varchar). Default is true.
include-domain-data-type (boolean)
replace domain name with the underlying data type. Default is false.
include-column-positions (boolean)
add column position (pg_attribute.attnum). Default is false.
include-not-null (boolean)
add not null information as columnoptionals. Default is false.
include-default (boolean)
add default expression. Default is false.
pretty-print (boolean)
dd spaces and indentation to JSON structures. Default is false.
write-in-chunks (boolean)
write after every change instead of every changeset. Only used when format-version is 1. Default is false.
include-lsn (boolean)
add nextlsn to each changeset. Default is false.
actions (boolean)
efine which operations will be sent. Default is all actions (insert, update, delete, and truncate). However, if you are using format-version 1, truncate is not enabled (backward compatibility).
filter-origins (boolean)
exclude changes from the specified origins. Default is empty which means that no origin will be filtered. It is a comma separated value.
filter-tables (boolean)
xclude rows from the specified tables. Default is empty which means that no table will be filtered. It is a comma separated value. The tables should be schema-qualified. *.foo means table foo in all schemas and bar.* means all tables in schema bar. Special characters (space, single quote, comma, period, asterisk) must be escaped with backslash. Schema and table are case-sensitive. Table "public"."Foo bar" should be specified as public.Foo\ bar.
add-tables (boolean)
include only rows from the specified tables. Default is all tables from all schemas. It has the same rules from filter-tables.
filter-msg-prefixes (boolean)
exclude messages if prefix is in the list. Default is empty which means that no message will be filtered. It is a comma separated value.
add-msg-prefixes (boolean)
include only messages if prefix is in the list. Default is all prefixes. It is a comma separated value. wal2sql applies filter-msg-prefixes before this parameter.
format-version (integer)
defines which format to use. Default is 1.
There are two ways to obtain the changes (JSON objects) from wal2sql plugin: calling functions via ltdts_recvlogical or SQL.
Besides the configuration above, it is necessary to configure a replication connection to use ltdts_recvlogical.
Add a replication connection rule at lt_hba.conf
local mydatabase myuser trust
Also, set max_wal_senders at lightdb.conf:
max_wal_senders = 1
A restart is necessary if you changed max_wal_senders. You are ready to try wal2sql. In one terminal:
# create a logical repliction slot name as test_oracle_slot
ltdts_recvlogical -d postgres --slot=test_oracle_slot --create-slot -P wal2sql
# delete a logical repliction slot
ltdts_recvlogical -d postgres --slot=test_oracle_slot --drop-slot
# print output in the first terminal
ltdts_recvlogical -d postgres --slot test_oracle_slot --start -o pretty-print=1 -o add-msg-prefixes=wal2sql -f -
# forward output to oracle。
ltdts_recvlogical --tnsname=TEST --oracle-username=scott --oracle-password=tiger --start --slot=test_oracle_slot -o include-types=false
-o include-type-oids=true -d postgres -h 10.20.30.193 -p 9999 -f debug.dat
Save the following code and execute it
CREATE TABLE table3_with_pk (a SERIAL, b VARCHAR(30), c TIMESTAMP NOT NULL, PRIMARY KEY(a, c));
CREATE TABLE table3_without_pk (a SERIAL, b NUMERIC(5,2), c TEXT);
SELECT 'init' FROM pg_create_logical_replication_slot('test_slot', 'wal2sql');
BEGIN;
INSERT INTO table3_with_pk (b, c) VALUES('Backup and Restore', now());
INSERT INTO table3_with_pk (b, c) VALUES('Tuning', now());
INSERT INTO table3_with_pk (b, c) VALUES('Replication', now());
SELECT pg_logical_emit_message(true, 'wal2sql', 'this message will be delivered');
DELETE FROM table3_with_pk WHERE a < 3;
SELECT pg_logical_emit_message(false, 'wal2sql', 'this non-transactional message will be delivered even if you rollback the transaction');
INSERT INTO table3_without_pk (b, c) VALUES(2.34, 'Tapir');
-- it is not added to stream because there isn't a pk or a replica identity
UPDATE table3_without_pk SET c = 'Anta' WHERE c = 'Tapir';
COMMIT;
SELECT data FROM pg_logical_slot_get_changes('test_slot', NULL, NULL, 'format-version', '2', 'add-msg-prefixes', 'wal2sql');
SELECT 'stop' FROM pg_drop_replication_slot('test_slot');
DROP TABLE table3_with_pk;
DROP TABLE table3_without_pk;
wal2sql is not supported on
LoongArch.
In high availability
please start ltdts_logicalrepl_copier.sh to
copy slot from primary to standby.
In high availability when faiover occur, please restart new primary LightDB to let slot work.