Introduction

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.

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.

Build and Install

This module is supported on those platforms that PostgreSQL is. The installation steps depend on your operating system. PostgreSQL yum repository and PostgreSQL apt repository provide wal2json packages.

In Red Hat/CentOS:

$ sudo yum install wal2json_17

In Debian/Ubuntu:

$ sudo apt-get install postgresql-17-wal2json

You can also keep up with the latest fixes and features cloning the Git repository.

$ git clone https://github.com/eulerto/wal2json.git

Unix based Operating Systems

Before installing wal2json, you should have PostgreSQL 9.4+ installed (including the header files). If PostgreSQL is not in your search path, add it. If you are using PostgreSQL yum repository, install postgresql17-devel and add /usr/pgsql-17/bin to your search path (yum uses 17, 16, 15, 14, 13, 12, 11, 10, 96 or 95). If you are using PostgreSQL apt repository, install postgresql-server-dev-17 and add /usr/lib/postgresql/17/bin to your search path. (apt uses 17, 16, 15, 14, 13, 12, 11, 10, 9.6 or 9.5).

If you compile PostgreSQL by yourself and install it in /home/euler/pg17:

$ tar -zxf wal2json-wal2json_2_6.tar.gz
$ cd wal2json-wal2json_2_6
$ export PATH=/home/euler/pg17/bin:$PATH
$ make
$ make install

If you are using PostgreSQL yum repository:

$ sudo yum install postgresql17-devel
$ tar -zxf wal2json-wal2json_2_6.tar.gz
$ cd wal2json-wal2json_2_6
$ export PATH=/usr/pgsql-17/bin:$PATH
$ make
$ make install

If you are using PostgreSQL apt repository:

$ sudo apt-get install postgresql-server-dev-17
$ tar -zxf wal2json-wal2json_2_6.tar.gz
$ cd wal2json-wal2json_2_6
$ export PATH=/usr/lib/postgresql/17/bin:$PATH
$ make
$ make install

Windows

There are several ways to build wal2json on Windows. If you are build PostgreSQL too, you can put wal2json directory inside contrib, change the contrib Makefile (variable SUBDIRS) and build it following the Installation from Source Code on Windows instructions. However, if you already have PostgreSQL installed, it is also possible to compile wal2json out of the tree. Edit wal2json.vcxproj file and change c:\pg\17 to the PostgreSQL prefix directory. The next step is to open this project file in MS Visual Studio and compile it. Final step is to copy wal2json.dll to the pg_config --pkglibdir directory.

Configuration

postgresql.conf

You need to set up at least two parameters at postgresql.conf:

wal_level = logical
#
# these parameters only need to set in versions 9.4, 9.5 and 9.6
# default values are ok in version 10 or later
#
max_replication_slots = 10
max_wal_senders = 10

After changing these parameters, a restart is needed.

Parameters

• include-xids: add xid to each changeset. Default is false.
• include-timestamp: add timestamp to each changeset. Default is false.
• include-schemas: add schema to each change. Default is true.
• include-types: add type to each change. Default is true.
• include-typmod: add modifier to types that have it (eg. varchar(20) instead of varchar). Default is true.
• include-type-oids: add type oids. Default is false.
• include-domain-data-type: replace domain name with the underlying data type. Default is false.
• include-column-positions: add column position (pg_attribute.attnum). Default is false.
• include-origin: add origin of a piece of data. Default is false.
• include-not-null: add not null information as columnoptionals. Default is false.
• include-default: add default expression. Default is false.
• include-pk: add primary key information as pk. Column name and data type is included. Default is false.
• numeric-data-types-as-string: use string for numeric data types. JSON specification does not recognize Infinity and NaN as valid numeric values. There might be potential interoperability problems for double precision numbers. Default is false.
• pretty-print: add spaces and indentation to JSON structures. Default is false.
• write-in-chunks: write after every change instead of every changeset. Only used when format-version is 1. Default is false.
• include-lsn: add nextlsn to each changeset. Default is false.
• include-transaction: emit records denoting the start and end of each transaction. Default is true.
• include-unchanged-toast (deprecated): Don't use it. It is deprecated.
• filter-origins: 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: exclude 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: 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: 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: include only messages if prefix is in the list. Default is all prefixes. It is a comma separated value. wal2json applies filter-msg-prefixes before this parameter.
• format-version: defines which format to use. Default is 1.
• actions: define 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).

Examples

There are two ways to obtain the changes (JSON objects) from wal2json plugin: calling functions via SQL or pg_recvlogical.

pg_recvlogical

Besides the configuration above, it is necessary to configure a replication connection to use pg_recvlogical. A logical replication connection in version 9.4, 9.5, and 9.6 requires replication keyword in the database column. Since version 10, logical replication matches a normal entry with a database name or keywords such as all.

First, add a replication connection rule at pg_hba.conf (9.4, 9.5, and 9.6):

local    replication     myuser                     trust

If you are using version 10 or later:

local    mydatabase      myuser                     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 add-msg-prefixes=wal2json -f -

In another terminal:

$ cat /tmp/example1.sql
CREATE TABLE table1_with_pk (a SERIAL, b VARCHAR(30), c TIMESTAMP NOT NULL, PRIMARY KEY(a, c));
CREATE TABLE table1_without_pk (a SERIAL, b NUMERIC(5,2), c TEXT);

BEGIN;
INSERT INTO table1_with_pk (b, c) VALUES('Backup and Restore', now());
INSERT INTO table1_with_pk (b, c) VALUES('Tuning', now());
INSERT INTO table1_with_pk (b, c) VALUES('Replication', now());
SELECT pg_logical_emit_message(true, 'wal2json', 'this message will be delivered');
SELECT pg_logical_emit_message(true, 'pgoutput', 'this message will be filtered');
DELETE FROM table1_with_pk WHERE a < 3;
SELECT pg_logical_emit_message(false, 'wal2json', 'this non-transactional message will be delivered even if you rollback the transaction');

INSERT INTO table1_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 table1_without_pk SET c = 'Anta' WHERE c = 'Tapir';
COMMIT;

DROP TABLE table1_with_pk;
DROP TABLE table1_without_pk;

$ psql -At -f /tmp/example1.sql postgres
CREATE TABLE
CREATE TABLE
BEGIN
INSERT 0 1
INSERT 0 1
INSERT 0 1
3/78BFC828
3/78BFC880
DELETE 2
3/78BFC990
INSERT 0 1
UPDATE 1
COMMIT
DROP TABLE
DROP TABLE

The output in the first terminal is:

{
	"change": [
	]
}
{
	"change": [
	]
}
{
    "change": [
        {
            "kind": "message",
            "transactional": false,
            "prefix": "wal2json",
            "content": "this non-transactional message will be delivered even if you rollback the transaction"
        }
    ]
}
WARNING:  table "table1_wit