JSON output plugin for changeset extraction
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eulerto Refactor wal2json output
Almost all writes were handled in two different ways: with or without
pretty print. It means more lines of code and an error-prone code. The
new code avoids all of the pretty-print tests and also uses only one
setence per output. It reduces the code by more than 150 lines.

I also fix some comments (use only C89 comments) and improve style.
Latest commit d4c0e81 May 22, 2018

README.md

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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.

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.

Requirements

  • PostgreSQL 9.4+

Build and Install

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

Unix based Operating Systems

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

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:\postgres\pg103 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
max_replication_slots = 1

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-not-null: add not null information as columnoptionals. 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. Default is false.
  • include-lsn: add nextlsn to each changeset. Default is false.
  • include-unchanged-toast: add TOAST value even if it was not modified. Since TOAST values are usually large, this option could save IO and bandwidth if it is disabled. Default is true.
  • 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.

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.

First, add a replication connection rule at pg_hba.conf:

local    replication     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 -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 postgres
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:

{
	"change": [
	]
}
{
	"change": [
	]
}
WARNING:  table "table_without_pk" without primary key or replica identity is nothing
{
	"change": [
		{
			"kind": "insert",
			"schema": "public",
			"table": "table_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [1, "Backup and Restore", "2018-03-27 11:58:28.988414"]
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [2, "Tuning", "2018-03-27 11:58:28.988414"]
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [3, "Replication", "2018-03-27 11:58:28.988414"]
		}
		,{
			"kind": "delete",
			"schema": "public",
			"table": "table_with_pk",
			"oldkeys": {
				"keynames": ["a", "c"],
				"keytypes": ["integer", "timestamp without time zone"],
				"keyvalues": [1, "2018-03-27 11:58:28.988414"]
			}
		}
		,{
			"kind": "delete",
			"schema": "public",
			"table": "table_with_pk",
			"oldkeys": {
				"keynames": ["a", "c"],
				"keytypes": ["integer", "timestamp without time zone"],
				"keyvalues": [2, "2018-03-27 11:58:28.988414"]
			}
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table_without_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "numeric(5,2)", "text"],
			"columnvalues": [1, 2.34, "Tapir"]
		}
	]
}

Dropping the slot in the first terminal:

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

SQL functions

$ cat /tmp/example2.sql
CREATE TABLE table2_with_pk (a SERIAL, b VARCHAR(30), c TIMESTAMP NOT NULL, PRIMARY KEY(a, c));
CREATE TABLE table2_without_pk (a SERIAL, b NUMERIC(5,2), c TEXT);

SELECT 'init' FROM pg_create_logical_replication_slot('test_slot', 'wal2json');

BEGIN;
INSERT INTO table2_with_pk (b, c) VALUES('Backup and Restore', now());
INSERT INTO table2_with_pk (b, c) VALUES('Tuning', now());
INSERT INTO table2_with_pk (b, c) VALUES('Replication', now());
DELETE FROM table2_with_pk WHERE a < 3;

INSERT INTO table2_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 table2_without_pk SET c = 'Anta' WHERE c = 'Tapir';
COMMIT;

SELECT data FROM pg_logical_slot_get_changes('test_slot', NULL, NULL, 'pretty-print', '1');
SELECT 'stop' FROM pg_drop_replication_slot('test_slot');

The script above produces the output below:

$ psql -At -f /tmp/example2.sql postgres
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 "table2_without_pk" without primary key or replica identity is nothing
{
	"change": [
		{
			"kind": "insert",
			"schema": "public",
			"table": "table2_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [1, "Backup and Restore", "2018-03-27 12:05:29.914496"]
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table2_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [2, "Tuning", "2018-03-27 12:05:29.914496"]
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table2_with_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "character varying(30)", "timestamp without time zone"],
			"columnvalues": [3, "Replication", "2018-03-27 12:05:29.914496"]
		}
		,{
			"kind": "delete",
			"schema": "public",
			"table": "table2_with_pk",
			"oldkeys": {
				"keynames": ["a", "c"],
				"keytypes": ["integer", "timestamp without time zone"],
				"keyvalues": [1, "2018-03-27 12:05:29.914496"]
			}
		}
		,{
			"kind": "delete",
			"schema": "public",
			"table": "table2_with_pk",
			"oldkeys": {
				"keynames": ["a", "c"],
				"keytypes": ["integer", "timestamp without time zone"],
				"keyvalues": [2, "2018-03-27 12:05:29.914496"]
			}
		}
		,{
			"kind": "insert",
			"schema": "public",
			"table": "table2_without_pk",
			"columnnames": ["a", "b", "c"],
			"columntypes": ["integer", "numeric(5,2)", "text"],
			"columnvalues": [1, 2.34, "Tapir"]
		}
	]
}
stop

License

Copyright (c) 2013-2018, 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.