wal2json is an output plugin for logical decoding. It means that the plugin 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.
The wal2json output plugin 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.
wal2json is released under PostgreSQL license.
- PostgreSQL 9.4+
This extension is supported on those platforms that PostgreSQL is. The installation steps depend on your operating system.
You can also keep up with the latest fixes and features cloning the Git repository.
$ git clone https://github.com/eulerto/wal2json.git
Before use this extension, you should build it and load it at the desirable database.
$ git clone https://github.com/eulerto/wal2json.git
$ PATH=/path/to/bin/pg_config:$PATH
$ USE_PGXS=1 make
$ USE_PGXS=1 make install
Sorry, never tried it. ;(
You need to set up at least two parameters at postgresql.conf:
wal_level = logical
max_replication_slots = 1
After changing these parameters, a restart is needed.
There are two ways to obtain the changes (JSON objects) from wal2json plugin: (i) calling functions via SQL or (ii) pg_recvlogical.
Besides the configuration above, it is necessary to configure a replication connection to use pg_recvlogical.
First, add an entry at pg_hba.conf:
local replication myuser trust
host replication myuser 10.1.2.3/32 trust
Also, set max_wal_senders at postgresql.conf:
max_wal_senders = 1
A restart is necessary if you changed max_wal_senders.
You are ready to try wal2json. In one terminal:
$ pg_recvlogical -d postgres --slot test_slot --create-slot -P wal2json
$ pg_recvlogical -d postgres --slot test_slot --start -o pretty-print=1 -o write-in-chunks=0 -f -
In another terminal:
$ cat /tmp/example1.sql
CREATE TABLE table_with_pk (a SERIAL, b VARCHAR(30), c TIMESTAMP NOT NULL, PRIMARY KEY(a, c));
CREATE TABLE table_without_pk (a SERIAL, b NUMERIC(5,2), c TEXT);
BEGIN;
INSERT INTO table_with_pk (b, c) VALUES('Backup and Restore', now());
INSERT INTO table_with_pk (b, c) VALUES('Tuning', now());
INSERT INTO table_with_pk (b, c) VALUES('Replication', now());
DELETE FROM table_with_pk WHERE a < 3;
INSERT INTO table_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 table_without_pk SET c = 'Anta' WHERE c = 'Tapir';
COMMIT;
$ psql -At -f /tmp/example1.sql
CREATE TABLE
CREATE TABLE
BEGIN
INSERT 0 1
INSERT 0 1
INSERT 0 1
DELETE 2
INSERT 0 1
UPDATE 1
COMMIT
The output in the first terminal is:
{
"xid": 34225,
"change": [
]
}
{
"xid": 34226,
"change": [
]
}
WARNING: table "table_without_pk" without primary key or replica identity is nothing
CONTEXTO: slot "test_slot", output plugin "wal2json", in the change callback, associated LSN 0/126E5F70
{
"xid": 34227,
"change": [
{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [1, "Backup and Restore", "2015-08-27 16:46:35.818038"]
}
,{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [2, "Tuning", "2015-08-27 16:46:35.818038"]
}
,{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [3, "Replication", "2015-08-27 16:46:35.818038"]
}
,{
"kind": "delete",
"schema": "public",
"table": "table_with_pk",
"oldkeys": {
"keynames": ["a", "c"],
"keytypes": ["int4", "timestamp"],
"keyvalues": [1, "2015-08-27 16:46:35.818038"]
}
}
,{
"kind": "delete",
"schema": "public",
"table": "table_with_pk",
"oldkeys": {
"keynames": ["a", "c"],
"keytypes": ["int4", "timestamp"],
"keyvalues": [2, "2015-08-27 16:46:35.818038"]
}
}
,{
"kind": "insert",
"schema": "public",
"table": "table_without_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "numeric", "text"],
"columnvalues": [1, 2.34, "Tapir"]
}
]
}
Dropping the slot in the first terminal:
Ctrl+C
$ pg_recvlogical -d postgres --slot test_slot --drop-slot
$ cat /tmp/example2.sql
CREATE TABLE table_with_pk (a SERIAL, b VARCHAR(30), c TIMESTAMP NOT NULL, PRIMARY KEY(a, c));
CREATE TABLE table_without_pk (a SERIAL, b NUMERIC(5,2), c TEXT);
SELECT 'init' FROM pg_create_logical_replication_slot('test_slot', 'wal2json');
BEGIN;
INSERT INTO table_with_pk (b, c) VALUES('Backup and Restore', now());
INSERT INTO table_with_pk (b, c) VALUES('Tuning', now());
INSERT INTO table_with_pk (b, c) VALUES('Replication', now());
DELETE FROM table_with_pk WHERE a < 3;
INSERT INTO table_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 table_without_pk SET c = 'Anta' WHERE c = 'Tapir';
COMMIT;
SELECT data FROM pg_logical_slot_get_changes('test_slot', NULL, NULL, 'pretty-print', '1', 'write-in-chunks', '0');
SELECT 'stop' FROM pg_drop_replication_slot('test_slot');
The script above produces the output below:
$ psql -At -f /tmp/example2.sql
CREATE TABLE
CREATE TABLE
init
BEGIN
INSERT 0 1
INSERT 0 1
INSERT 0 1
DELETE 2
INSERT 0 1
UPDATE 1
COMMIT
psql:/tmp/example2.sql:17: WARNING: table "table_without_pk" without primary key or replica identity is nothing
CONTEXTO: slot "test_slot", output plugin "wal2json", in the change callback, associated LSN 0/12713E40
{
"xid": 34232,
"change": [
{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [1, "Backup and Restore", "2015-08-27 16:49:37.218511"]
}
,{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [2, "Tuning", "2015-08-27 16:49:37.218511"]
}
,{
"kind": "insert",
"schema": "public",
"table": "table_with_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "varchar", "timestamp"],
"columnvalues": [3, "Replication", "2015-08-27 16:49:37.218511"]
}
,{
"kind": "delete",
"schema": "public",
"table": "table_with_pk",
"oldkeys": {
"keynames": ["a", "c"],
"keytypes": ["int4", "timestamp"],
"keyvalues": [1, "2015-08-27 16:49:37.218511"]
}
}
,{
"kind": "delete",
"schema": "public",
"table": "table_with_pk",
"oldkeys": {
"keynames": ["a", "c"],
"keytypes": ["int4", "timestamp"],
"keyvalues": [2, "2015-08-27 16:49:37.218511"]
}
}
,{
"kind": "insert",
"schema": "public",
"table": "table_without_pk",
"columnnames": ["a", "b", "c"],
"columntypes": ["int4", "numeric", "text"],
"columnvalues": [1, 2.34, "Tapir"]
}
]
}
stop
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