= Logical Decoding Output Plug-in Installation for PostgreSQL
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This document describes the database setup required for streaming data changes out of https://www.postgresql.org/[PostgreSQL].
This comprises configuration applying to the database itself as well as the installation of the https://github.com/eulerto/wal2json[wal2json] logical decoding output plug-in.
The installation and the tests are performed at the following environment/configuration:

* https://www.postgresql.org/docs/9.6/static/index.html[PostgreSQL (v9.6.10)]
* https://github.com/eulerto/wal2json[wal2json]
* https://www.centos.org/[CentOS 7]

Similar steps need to be taken for other Postgres and OS versions and the Decoderbufs logical decoding plug-in which also is supported by Debezium.

[NOTE]
====
As of Debezium 0.10, the connector supports PostgreSQL 10+ logical replication streaming using _pgoutput_.
This means that a logical decoding output plug-in is no longer necessary and changes can be emitted directly from the replication stream by the connector.
====

[[logical-decoding-plugin-setup]]
== Logical Decoding Plug-ins

Logical decoding is the process of extracting all persistent changes to a database's tables into a coherent, easy to understand format
which can be interpreted without detailed knowledge of the database's internal state.

As of PostgreSQL 9.4, logical decoding is implemented by decoding the contents of the write-ahead log, which describe changes
on a storage level, into an application-specific form such as a stream of tuples or SQL statements.
In the context of logical replication, a slot represents a stream of changes that can be replayed to a client in the order
they were made on the origin server. Each slot streams a sequence of changes from a single database.
The output plug-ins transform the data from the write-ahead log's internal representation into the format the consumer
of a replication slot desires. Plug-ins are written in C, compiled, and installed on the machine which runs the PostgreSQL server,
and they use a number of PostgreSQL specific APIs, as described by the
https://www.postgresql.org/docs/9.6/static/logicaldecoding-output-plugin.html[PostgreSQL documentation].

Debezium’s PostgreSQL connector works with one of Debezium’s supported logical decoding plug-ins,

* https://github.com/debezium/postgres-decoderbufs/blob/master/README.md[protobuf] or
* https://github.com/eulerto/wal2json/blob/master/README.md[wal2json]

to encode the changes in either https://github.com/google/protobuf[Protobuf] format or http://www.json.org/[JSON] format.

[TIP]
====
For simplicity, Debezium also provides a Docker image based on a vanilla https://github.com/debezium/docker-images/tree/master/postgres/9.6[PostgreSQL server image]
on top of which it compiles and installs the plug-ins.
====

[WARNING]
====
The Debezium logical decoding plug-ins have only been installed and tested on _Linux_ machines. For Windows and other platforms it may
require different installation steps
====

[discrete]
==== Differences between Plug-ins

The plug-ins' behaviour is not completely same for all cases. So far these differences have been identified

* wal2json plug-in is not able to process quoted identifiers (https://github.com/eulerto/wal2json/issues/35[issue])
* wal2json plug-in does not emit events for tables without primary keys
* wal2json plug-in does not support special values (`NaN` or `infinity`) for floating point types

All up-to-date differences are tracked in a test suite
https://github.com/debezium/debezium/blob/master/debezium-connector-postgres/src/test/java/io/debezium/connector/postgresql/DecoderDifferences.java[Java class].

More information about the logical decoding and output plug-ins can be found at:

* https://www.postgresql.org/docs/9.6/static/logicaldecoding-explanation.html[PostgreSQL logical decoding explanation]
* https://www.postgresql.org/docs/9.6/static/logicaldecoding-output-plugin.html[PostgreSQL logical decoding output plug-in]
* https://wiki.postgresql.org/wiki/Logical_Decoding_Plugins[PostgreSQL logical decoding plug-ins]

[[logical-decoding-output-plugin-installation]]
=== Installation

At the current installation example, the https://github.com/eulerto/wal2json[wal2json] output plug-in for logical decoding is used.
The wal2json output plug-in produces a JSON object per transaction. All of the new/old tuples are available in the JSON object.
The plug-in *compilation and installation* is performed by executing the related commands extracted from the
https://github.com/debezium/docker-images/blob/master/postgres/9.6/Dockerfile[Debezium docker image file].

Before executing the commands, make sure that the user has the privileges to write the `wal2json` library at the PostgreSQL `_lib_`
directory (at the test environment, the directory is: `/usr/pgsql-9.6/lib/`).
Also note that the installation process requires the PostgreSQL utility https://www.postgresql.org/docs/9.6/static/app-pgconfig.html[pg_config].
Verify that the `PATH` environment variable is set so as the utility can be found. If not, update the `PATH`
environment variable appropriately. For example at the test environment:

[source,bash]
----
export PATH="$PATH:/usr/pgsql-9.6/bin"
----

.*wal2json* installation commands
[source,bash]
----
$ git clone https://github.com/eulerto/wal2json -b master --single-branch \
&& cd wal2json \
&& git checkout d2b7fef021c46e0d429f2c1768de361069e58696 \
&& make && make install \
&& cd .. \
&& rm -rf wal2json
----

.*wal2json* installation output
[source,bash]
----
Cloning into 'wal2json'...
remote: Counting objects: 445, done.
remote: Total 445 (delta 0), reused 0 (delta 0), pack-reused 445
Receiving objects: 100% (445/445), 180.70 KiB | 0 bytes/s, done.
Resolving deltas: 100% (317/317), done.
Note: checking out 'd2b7fef021c46e0d429f2c1768de361069e58696'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at d2b7fef... Improve style
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fPIC -I. -I./ -I/usr/pgsql-9.6/include/server -I/usr/pgsql-9.6/include/internal -D_GNU_SOURCE -I/usr/include/libxml2  -I/usr/include  -c -o wal2json.o wal2json.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fPIC -L/usr/pgsql-9.6/lib -Wl,--as-needed  -L/usr/lib64 -Wl,--as-needed -Wl,-rpath,'/usr/pgsql-9.6/lib',--enable-new-dtags  -shared -o wal2json.so wal2json.o
/usr/bin/mkdir -p '/usr/pgsql-9.6/lib'
/usr/bin/install -c -m 755  wal2json.so '/usr/pgsql-9.6/lib/'
----

[[fedora-rpm]]
=== Installation on Fedora 30+
Debezium provides https://apps.fedoraproject.org/packages/postgres-decoderbufs[RPM package] for Fedora operating system too.
The package is updated always after a final Debezium release is done.
To use the RPM in question just issue the standard Fedora installation command:
[source,bash]
----
$ sudo dnf -y install postgres-decoderbufs
----
The rest of the configuration is same as described below for `wal2json` plugin.

[[postgresql-server-configuration]]
=== PostgreSQL Server Configuration

Once the *wal2json* plug-in has been installed, the database server should be configured.

[discrete]
=== _Setting up libraries, WAL and replication parameters_

Add the following lines at the end of the `postgresql.conf` PostgreSQL configuration file in order to include the plug-in
at the shared libraries and to adjust some https://www.postgresql.org/docs/9.6/static/runtime-config-wal.html[WAL]
and https://www.postgresql.org/docs/9.6/static/runtime-config-replication.html[streaming replication] settings.
The configuration is extracted from https://github.com/debezium/docker-images/blob/master/postgres/9.6/postgresql.conf.sample[postgresql.conf.sample].
You may need to modify it, if for example you have additionally installed `shared_preload_libraries`.

.*_postgresql.conf_* _, configuration file parameters settings_
[source]
----
############ REPLICATION ##############
# MODULES
shared_preload_libraries = 'wal2json'   //<1>

# REPLICATION
wal_level = logical                     //<2>
max_wal_senders = 4                     //<3>
max_replication_slots = 4               //<4>
----

<1> tells the server that it should load at startup the `wal2json` (use `decoderbufs` for https://github.com/google/protobuf[protobuf]) logical decoding plug-in(s)
(the names of the plug-ins are set in https://github.com/debezium/postgres-decoderbufs/blob/v{debezium-version}/Makefile[protobuf]
and https://github.com/eulerto/wal2json/blob/master/Makefile[wal2json] Makefiles)
<2> tells the server that it should use logical decoding with the write-ahead log
<3> tells the server that it should use a maximum of `4` separate processes for processing WAL changes
<4> tells the server that it should allow a maximum of `4` replication slots to be created for streaming WAL changes

Debezium uses PostgreSQL's logical decoding, which uses replication slots.  Replication slots are guaranteed to retain all WAL required for Debezium even during Debezium outages. It is important for this reason to closely monitor replication slots to avoid too much disk consumption and other conditions that can happen such as catalog bloat if a Debezium slot stays unused for too long. For more information please see the official Postgres docs on https://www.postgresql.org/docs/current/warm-standby.html#STREAMING-REPLICATION-SLOTS[this subject].

[TIP]
====
We strongly recommend reading and understanding https://www.postgresql.org/docs/9.6/static/wal-configuration.html[the official documentation] regarding the mechanics and configuration of the PostgreSQL write-ahead log.
====


[discrete]
[[setting_replication_permissions]]
=== _Setting up replication permissions_

Replication can only be performed by a database user that has appropriate permissions and only for a configured number of hosts.
In order to give a user replication permissions, define a PostgreSQL role that has _at least_ the `REPLICATION` and `LOGIN` permissions.
For example:

[source,sql]
----
CREATE ROLE name REPLICATION LOGIN;
----

[TIP]
====
Superusers have by default both of the above roles.
====

Add the following lines at the end of the `pg_hba.conf` PostgreSQL configuration file, so as to configure the
https://www.postgresql.org/docs/9.6/static/auth-pg-hba-conf.html[client authentication] for the database replication.
The PostgreSQL server should allow replication to take place between the server machine and the host on which the
Debezium PostgreSQL connector is running.

Note that the authentication refers to the database superuser `postgres`. You may change this accordingly,
if some other user with `REPLICATION` and `LOGIN` permissions has been created.

[[pg_hba_conf]]
.*_pg_hba.conf_* _, configuration file parameters settings_
[source]
----
############ REPLICATION ##############
local   replication     postgres                          trust		//<1>
host    replication     postgres  127.0.0.1/32            trust		//<2>
host    replication     postgres  ::1/128                 trust		//<3>
----

<1> tells the server to allow replication for `postgres` locally (i.e. on the server machine)
<2> tells the server to allow `postgres` on `localhost` to receive replication changes using `IPV4`
<3> tells the server to allow `postgres` on `localhost` to receive replication changes using `IPV6`

[TIP]
====
See https://www.postgresql.org/docs/9.6/static/datatype-net-types.html[the PostgreSQL documentation] for more information on network masks.
====


[[database-test-environment-setup]]
=== Database Test Environment Set-up

For the testing purposes, a database named *`test`* with a table named *`test_table`* are created
with the following DDL commands:

._Database SQL commands for test database/table creation_
[source,sql,indent=0]
----
CREATE DATABASE test;

CREATE TABLE test_table (
    id char(10) NOT NULL,
    code        char(10),
    PRIMARY KEY (id)
);
----

[[decoding-output-plugin-test]]
=== Decoding Output Plug-in Test

Test that the `wal2json` is working properly by obtaining the `test_table` changes using the
https://www.postgresql.org/docs/9.6/static/app-pgrecvlogical.html[pg_recvlogical] PostgreSQL client application
that controls PostgreSQL logical decoding streams.

Before starting make sure that you have logged in as a user with database replication permissions, as configured at a link:#setting_replication_permissions[previous step].
Otherwise, the slot creation and streaming fails with the following error message:
[source,bash]
----
pg_recvlogical: could not connect to server: FATAL:  no pg_hba.conf entry for replication connection from host "[local]", user "root", SSL off
----
At the test environment, the user with replication permission is the `postgres`.

Also, make sure that the `PATH` environment variable is set so as the `pg_recvlogical` can be found.
If not, update the `PATH` environment variable appropriately. For example at the test environment:
[source,bash]
----
export PATH="$PATH:/usr/pgsql-9.6/bin"
----

* *Create a slot* named `test_slot` for the database named `test`, using the logical output plug-in `wal2json`

[source,bash]
----
$ pg_recvlogical -d test --slot test_slot --create-slot -P wal2json
----

* *Begin streaming changes* from the logical replication slot `test_slot` for the database `test`

[source,bash]
----
$ pg_recvlogical -d test --slot test_slot --start -o pretty-print=1 -f -
----

* *Perform some basic DML* operations at `test_table` to trigger `INSERT`/`UPDATE`/`DELETE` change events

._Interactive PostgreSQL terminal, SQL commands_
[source,sql]
----
test=# INSERT INTO test_table (id, code) VALUES('id1', 'code1');
INSERT 0 1
test=# update test_table set code='code2' where id='id1';
UPDATE 1
test=# delete from test_table where id='id1';
DELETE 1
----

Upon the `INSERT`, `UPDATE` and `DELETE` events, the `wal2json` plug-in outputs the table changes as captured by `pg_recvlogical`.

._Output for `INSERT` event_
[source,json,indent=0,subs="attributes"]
----
{
  "change": [
    {
      "kind": "insert",
      "schema": "public",
      "table": "test_table",
      "columnnames": ["id", "code"],
      "columntypes": ["character(10)", "character(10)"],
      "columnvalues": ["id1       ", "code1     "]
    }
  ]
}
----

[[update-table-change-event]]
._Output for `UPDATE` event_
[source,json,indent=0,subs="attributes"]
----
{
  "change": [
    {
      "kind": "update",
      "schema": "public",
      "table": "test_table",
      "columnnames": ["id", "code"],
      "columntypes": ["character(10)", "character(10)"],
      "columnvalues": ["id1       ", "code2     "],
      "oldkeys": {
        "keynames": ["id"],
        "keytypes": ["character(10)"],
        "keyvalues": ["id1       "]
      }
    }
  ]
}
----

._Output for `DELETE` event_
[source,json,indent=0,subs="attributes"]
----
{
  "change": [
    {
      "kind": "delete",
      "schema": "public",
      "table": "test_table",
      "oldkeys": {
        "keynames": ["id"],
        "keytypes": ["character(10)"],
        "keyvalues": ["id1       "]
      }
    }
  ]
}
----

[TIP]
====
Note that the link:#replica-identity[REPLICA IDENTITY] of the table `test_table` is set to `DEFAULT`.
====

When the test is finished, the slot `test_slot` for the database `test` can be removed by the following command:
[source,bash]
----
$ pg_recvlogical -d test --slot test_slot --drop-slot
----

[[replica-identity]]
[NOTE]
====
https://www.postgresql.org/docs/9.6/static/sql-altertable.html#SQL-CREATETABLE-REPLICA-IDENTITY[REPLICA IDENTITY],
is a PostgreSQL specific table-level setting which determines the amount of information that is available
to logical decoding in case of `UPDATE` and `DELETE` events.

There are 4 possible values for `REPLICA IDENTITY`:

* *DEFAULT* - `UPDATE` and `DELETE` events will only contain the previous values for the primary key columns of a table
* *NOTHING* - `UPDATE` and `DELETE` events will not contain any information about the previous value on any of the table columns
* *FULL* - `UPDATE` and `DELETE` events will contain the previous values of all the table's columns
* *INDEX* `index name` - `UPDATE` and `DELETE` events will contains the previous values of the columns contained in the index definition named `index name`

You can modify and check the replica `REPLICA IDENTITY` for a table with the following commands:

[source,sql]
----
ALTER TABLE test_table REPLICA IDENTITY FULL;
test=# \d+ test_table
                         Table "public.test_table"
 Column |     Type      | Modifiers | Storage  | Stats target | Description
 -------+---------------+-----------+----------+--------------+------------
 id     | character(10) | not null  | extended |              |
 code   | character(10) |           | extended |              |
Indexes:
    "test_table_pkey" PRIMARY KEY, btree (id)
Replica Identity: FULL
----

Here is the output of `wal2json` plug-in on `DELETE` event and `REPLICA IDENTITY` set to `FULL`.
Compare with the link:#update-table-change-event[respective output] when `REPLICA IDENTITY` is set to `DEFAULT`.

._Output for `UPDATE`_
[source,json]
----
{
  "change": [
    {
      "kind": "update",
      "schema": "public",
      "table": "test_table",
      "columnnames": ["id", "code"],
      "columntypes": ["character(10)", "character(10)"],
      "columnvalues": ["id1       ", "code2     "],
      "oldkeys": {
        "keynames": ["id", "code"],
        "keytypes": ["character(10)", "character(10)"],
        "keyvalues": ["id1       ", "code1     "]
      }
    }
  ]
}
----
====
