wal2json

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.

Since PostgreSQL 18.6, 17.11, 16.15, 15.19, and 14.24, an output plugin can only be loaded if its library is listed in the output_plugin_libraries parameter (this restriction was added as a fix for CVE-2026-6471). The default value contains only the output plugins that are shipped with PostgreSQL (pgoutput and test_decoding), hence, wal2json has to be added to it:

output_plugin_libraries = 'pgoutput, test_decoding, wal2json'

The library name must be exactly wal2json. Otherwise, creating or reading from a slot fails with:

ERROR:  library "wal2json" may not be used as an output plugin

This parameter is only read when the output plugin is loaded, so a reload (pg_ctl reload or SELECT pg_reload_conf()) is sufficient. Older versions (including PostgreSQL 13 and prior minor versions of the branches listed above) do not have this parameter and do not require this setting.

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.
• partition-root: use the root partitioned table as the table name (similar to publish_via_partition_root in CREATE PUBLICATION). If false, use the partition name. filter-tables and add-tables match the same name that is used as the table name, hence, if this parameter is true, they should specify the root partitioned table. Default is false.
• 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:

…

The output in the first terminal is:

…

Dropping the slot in the first terminal:

Ctrl+C
$ pg_recvlogical -d postgres --slot test_slot --drop-slot

SQL functions

…

The script above produces the output below:

…

Let's repeat the same example with format-version 2:

…

The script above produces the output below:

…

License

Copyright (c) 2013-2024, Euler Taveira de Oliveira All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

Neither the name of the Euler Taveira de Oliveira nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.