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
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.
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.
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.include-xmins: Add slot catxmin (catalog xmin) and xmin to each changeset. Default is false.include-next-xids: Adds nextxid and epoch (indicates xid wraparound) to each changeset. Default is false.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.*.foomeans table foo in all schemas andbar.*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 aspublic.Foo\ bar.add-tables: include only rows from the specified tables. Default is all tables from all schemas. It has the same rules fromfilter-tables.msg-per-record: opt into an alternate output format where every record creates a new json message, this supercedeswrite-in-chunksand makes parsing large transactions easier. See Alternate Output Format for details
There are two ways to obtain the changes (JSON objects) from wal2json plugin: calling functions via SQL or 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
$ 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
wal2json also has an alternate output format that instead of outputting a message per transaction, it outputs a message per record change.
This can be opted into with msg-per-record option. Additionally this optional probably should NOT be used with pretty-print. With this configuration,
wal2json outputs proper "jsonlines" (http://jsonlines.org/) formatted json.
This has advantages if you reguarly deal with large transactions or with writing to other systems. Intead of needing to use write-in-chunks and stream parsing the json for example, you can instead parse each message normally.
Primarily, this means adding two new "kind"s of messages, begin and commit and breaking up the change array. Additionally, the xid is added to
each message to associate messages to one another.
An example of the output (without pretty-print) is shown below:
{"kind": "begin","xid": 1042,"split": true}
{"kind": "insert","xid": 1042,"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","xid": 1042,"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","xid": 1042,"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","xid": 1042,"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","xid": 1042,"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","xid": 1042, "schema": "public", "table": "table2_without_pk", "columnnames": ["a", "b", "c"], "columntypes": ["integer", "numeric(5,2)", "text"],"columnvalues": [1, 2.34, "Tapir"]}
{"kind": "commit","xid": 1042}
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