pgpool-II Tutorial

Welcome to the Tutorial for pgpool-II. From here, you can learn how to install, setup, and run parallel queries or do replication using pgpool-II. We assume that you already know basic PostreSQL operations, so please refer to the PostgreSQL document if needed.

Table of Contents

1. Let's Begin!

1.1. Installing pgpool-II

1.2. Configuration Files

1.3. Configuring PCP commands

1.4. Preparing Database Nodes

1.5. Starting/Stopping pgpool-II

2. Your First Replication

2.1. Configuring Replication

2.2. Checking Replication

3. Your First Parallel Query

3.1. Configuring Parallel Query

3.2. Configuring the System Database

3.3. Partitioning Rule Definition

3.4. Replication Rule Definition

3.5. Checking Parallel Query

1. Let's Begin!

First, we must learn how to install, configure pgpool-II and database nodes before using replication or parallel query.

1.1. Installing pgpool-II

Installing pgpool-II is very easy. In the directory which you have extracted the source tar ball, execute the following commands.

$ ./configure
$ make
$ make install

configure script collects your system information and use it for the compilation procedure. You can pass command line arguments to configure script to change the default behavior, such as the installation directory. pgpool-II will be installed to /usr/local directory by default.

make command compiles the source code, and make install will install the executables. You must have write permission on the installation directory.

In this tutorial, we will install pgpool-II in the default /usr/local directory.

Note: pgpool-II requires libpq library in PostgreSQL 7.4 or later (version 3 protocol). If configure script displays the following error message, libpq library may not be installed, or it is not of version 3.

configure: error: libpq is not installed or libpq is old

If the library is version 3, but above message is still displayed, your libpq library is probably not recognized by configure script.

configure script searches for libpq library under /usr/local/pgsql libaray. If you have installed PostgreSQL to a directory other than /usr/local/pgsql, use --with-pgsql, or --with-pgsql-includedir and --with-pgsql-libdir command line options when you execute configure.

1.2. Configuration Files

pgpool-II configuration parameters are saved in pgpool.conf file. The file is in "parameter = value" per line format. When you install pgpool-II, pgpool.conf.sample is automatically created. We recommend copying and renaming it to pgpool.conf, and edit it as you like.

$ cp /usr/local/etc/pgpool.conf.sample /usr/local/etc/pgpool.conf

pgpool-II only accepts connections from the local host using port 9999. If you wish to receive conenctions from other hosts, set listen_addresses to '*'.

listen_addresses = 'localhost'
port = 9999

We will use the default parameters in thie tutorial.

1.3. Configuring PCP Commands

pgpool-II has an interface for administration purpose to retrieve information on database nodes, shutdown pgpool-II, etc. via network. To use PCP commands, user authentication is required. This authentication is different from PostgreSQL's user authentication. A username and password need to be defined in pcp.conf file. In the file, a username and password are listed as a pair on each line, and they are separated by a colon (:). Passwords are encrypted in md5 hash format.

postgres:e8a48653851e28c69d0506508fb27fc5

When you install pgpool-II, pcp.conf.sample is automatically created. We recommend copying and renaming it to pcp.conf, and edit it.

$ cp /usr/local/etc/pcp.conf.sample /usr/local/etc/pcp.conf

To encrypt your password into md5 hash format, use pg_md5 command, which is installed as a part of pgpool-II executables. pg_md5 takes text as an command line argument, and displays its md5-hashed text.

For example, give "postgres" as the command line argument, at pg_md5 displays md5-hashed text to the standard output.

$ /usr/bin/pg_md5 postgres
e8a48653851e28c69d0506508fb27fc5

PCP commands are executed via network, so the port number must be configured with pcp_port parameter in pgpool.conf file.

We will use the default 9898 for pcp_port in this tutorial.

pcp_port = 9898

1.4. Preparing Database Nodes

Now, we need to set up backend PostgreSQL servers for pgpool-II. These servers can be placed within the same host as pgpool-II, or on separate machines. If you decide to place the servers on the same host, different port numbers must be assigned for each server. If the servers are placed on separate machines, they must be configured properly so that they can accept network connections from pgpool-II.

In this tutorial, we will place three servers within the same host as pgpool-II, and assign 5432, 5433, 5434 port numbers respectively. To configure pgpool-II, edit pgpool.conf as follows.

backend_hostname0 = 'localhost'
backend_port0 = 5432
backend_weight0 = 1
backend_hostname1 = 'localhost'
backend_port1 = 5433
backend_weight1 = 1
backend_hostname2 = 'localhost'
backend_port2 = 5434
backend_weight2 = 1

For backend_hostname, backend_port, backend_weight, set the node's hostname, port number, and ratio for load balancing. At the end of each parameter string, node ID must be specified by adding positive integers starting with 0 (i.e. 0, 1, 2, …).

backend_weight parameters are all 1, meaning that SELECT queries are equally distributed among three servers.

1.5. Starting/Stopping pgpool-II

To fire up pgpool-II, execute the following command on a terminal.

$ pgpool

The above command, however, prints no log messages because pgpool detaches the terminal. If you want to show pgpool log messages, you pass -n option to pgpool command. pgpool-II is executed as non-daemon process, and the terminal will not be detached.

$ pgpool -n &

The log messages are printed on the terminal, so the recommended options to use are like the following.

$ pgpool -n -d > /tmp/pgpool.log 2>&1 &

-d option enables debug messages to be generated.

The above command keeps appending log messages to /tmp/pgpool.log. If you need to rotate log files, pass the logs to a external command which have log rotation function. For example, you can use rotatelogs coming with Apache2:

$ pgpool -n 2>&1 | /usr/local/apache2/bin/rotatelogs \
  -l -f /var/log/pgpool/pgpool.log.%A 86400 &

55 23 * * * /usr/bin/find /var/log/pgpool -type f -mtime +5 -exec /bin/rm -f '{}' \;

Also cronolog helps you.

$ pgpool -n 2>&1 | /usr/sbin/cronolog \
  --hardlink=/var/log/pgsql/pgpool.log \
  '/var/log/pgsql/%Y-%m-%d-pgpool.log' &

To stop pgpool-II process, execute the following command.

$ pgpool stop

If any client is still connected, pgpool-II waits for them to disconnect, and then terminate itself. Execute the following command instead if you want to shutdown pgpool-II forcibly.

$ pgpool -m fast stop

2. Your First Replication

Replication enables the same data to be copied to multiple database nodes.

In this section, we'll use three database nodes, which we have already set up in section "1. Let's Begin!", and takes you step by step to create a database replication system. Sample data to be replicated will be generated by pgbench benchmark program.

2.1. Configuring Replication

To enable the database replication function, set replication_mode to true in pgpool.conf file.

replication_mode = true

When replication_mode is set to true, pgpool-II will send a copy of a received query to all the database nodes.

When load_balance_mode is set to true, pgpool-II will distribute SELECT queries among the database nodes.

load_balance_mode = true

In this section, we enable both replication_mode and load_balance_mode.

2.2. Checking Replication

To reflect the changes in pgpool.conf, pgpool-II must be restarted. Please refer to section "1.5 Starting/Stopping pgpool-II".

After configuring pgpool.conf and restarting pgpool-II, let's try the actual replication and see if everything is working OK.

First, we need to create a database to be replicated. We will name it "bench_replication". This database needs to be created on all the nodes. Use createdb commands through pgpool-II, and the database will be created on all the nodes.

$ createdb -p 9999 bench_replication

Then, we'll execute pgbench with -i option. -i option initializes the database with pre-defined tables and data.

$ pgbench -i -p 9999 bench_replication

The following table is the summary of tables and data, which will be created by pgbench -i. If, on all the nodes, the listed tables and data are created, replication is working correctly.

Let's use a simple shell script to check the above on all the nodes. The following script will display the number of rows in branches, tellers, accounts, and history tables on all the nodes (5432, 5433, 5434).

$ for port in 5432 5433 5434; do
>     echo $port
>     for table_name in branches tellers accounts history; do
>         echo $table_name
>         psql -c "SELECT count(*) FROM $table_name" -p $port bench_replication
>     done
> done

3. Your First Parallel Query

Data within the different range is stored in two or more data base nodes in a parallel Query. This is called a partitioning. Moreover you could replicate some of tables among database nodes even in the parallel query mode.

To enable parallel query in pgpool-II, you must set up another database called "System Database" (we will denote it as SystemDB from this point).

SystemDB holds the user-defined rules to decide what data will be saved in which database node. Another use of SystemDB is to merge results sent back from the database nodes using dblink.

In this section, we will use three database nodes which we have set up in section "1. Let's Begin!", and takes you step by step to create a parallel query database system. We will use pgbench again to create sample data.

3.1. Configuring Parallel Query

To enable the parallel query function, set parallel_mode to true in pgpool.conf file.

parallel_mode = true

Setting paralle_mode to true does not start parallel query automatically. pgpool-II needs SystemDB and the rules to know how to distribute data to the database nodes.

Also, dblink used by SystemDB makes connections to pgpool-II. Therefore, listen_addresses needs to be configured so that pgpool-II accepts those connections.

listen_addresses = '*'

Attention: The replication is not done for the table that does the partitioning though a parallel Query and the replication can be made effective at the same time. Attention: You can have both partitioned tables and replicated tables. However a table cannot be a partioned and replicated one at the same time. Because the data structure of partioned tables and replicated tables are different, "bench_replication" database created in section "2. Your First Replication" cannot be reused in parallel query mode.

replication_mode = true
load_balance_mode = false

OR

replication_mode = false
load_balance_mode = true

In this section, we will set parallel_mode and load_balance_mode to true, listen_addresses to '*', replication_modeto false.

3.2. Configuring SystemDB

"System database" is just an ordinaly database. The only requirement is, dblink functions and the dist_def table, which describes partioning rule, must be installed in the system database. You could have system database on a database node, or you could have multiple node having system database by using cascade configuration of pgpool-II.

In this section, we will create SystemDB within the 5432 port node. The following list is the configuration parameters for SystemDB

system_db_hostname = 'localhost'
system_db_port = 5432
system_db_dbname = 'pgpool'
system_db_schema = 'pgpool_catalog'
system_db_user = 'pgpool'
system_db_password = ''

Actually, the above are the default settings of pgpool.conf. Now, we must create a user called "pgpool", and a database called "pgpool" owned by user "pgpool".

$ createuser -p 5432 pgpool
$ createdb -p 5432 -O pgpool pgpool

3.2.1. Installing dblink

Next, we must install dblink into "pgpool" database. dblink is one of the tools included in contrib directory in the PostgreSQL source code.

To install dblink to your system, execute the following commands.

$ USE_PGXS=1 make -C contrib/dblink
$ USE_PGXS=1 make -C contrib/dblink install

After dblink has been installed into your system, we will define dblink functions in "pgpool" database. If PostgreSQL is installed in /usr/local/pgsql, dblink.sql (a file with function definitions) should have been installed in /usr/local/pgsql/share/contrib. Now, execute the following command to define dblink functions.

$ psql -f /usr/local/pgsql/share/contrib/dblink.sql -p 5432 pgpool

3.2.2. Defining dist_def table

Define a table called "dist_def", which has the partitioning rule, in database called "pgpool". After installing pgpool-II, you will have system_db.sql, which is the psql script to generate the system database.

$ psql -f /usr/local/share/system_db.sql -p 5432 -U pgpool pgpool

dist_def table is created in pgpool_catalog schema. If you have configured system_db_schema to use other schema, you need to edit system_db.sql accordingly.

The definition for "dist_def" is as shown here, and the table name cannot be changed.

CREATE TABLE pgpool_catalog.dist_def (
    dbname text, -- database name
    schema_name text, -- schema name
    table_name text, -- table name
    col_name text NOT NULL CHECK (col_name = ANY (col_list)), -- distribution key-column
    col_list text[] NOT NULL, -- list of column names
    type_list text[] NOT NULL, -- list of column types
    dist_def_func text NOT NULL, -- distribution function name
    PRIMARY KEY (dbname, schema_name, table_name)
);

A tuple stored in "dist_def" can be classified into two types.

• Distribution Rule (col_name, dist_def_func)
• Table's meta-information (dbname, schema_name, table_name, col_list, type_list)

A distribution rule decides how to distribute data to a particular node. Data will be distributed depending on the value of "col_name" column. "dist_def_func" is a function that takes the value of "col_name" as its argument, and returns an integer which points to the appropriate database node ID where the data should be stored.

A meta-information is used to rewrite queries. Parallel query must rewrite queries so that the results sent back from the backend nodes can be merged into one result.

3.2.3. Defining replicate_def table

If you want to use replicated tables in SELECT in parallel mode, you need to register information of such tables(replication rule) to a table called replicate_def. The replicate_def table has already been made when making it from the system_db.sql file when dist_def is defined. The replicate_def table is defined as follows.

CREATE TABLE pgpool_catalog.replicate_def (
    dbname text, -- database name
    schema_name text, -- schema name
    table_name text, -- table name
    col_list text[] NOT NULL, -- list of column names
    type_list text[] NOT NULL, -- list of column types
    PRIMARY KEY (dbname, schema_name, table_name)
);

replicate_def includes table's meta data information(dbname, schema_name, table_name, col_list, type_list).

All the query analysis and query rewriting process are depending on the information (table, column and type) stored in dist_def and/or replicate_def table. If the information is not correct, analysis and query rewriting process will produce wrong results.

3.3. Defining Distribution Rules

In this tutorial, we will define rules to distribute pgbench's sample data into three database nodes. The sample data will be created by "pgbench -i -s 3" (i.e. scale factor of 3). We will create a new database called "bench_parallel" for this section.

In pgpool-II's source code, you can find dist_def_pgbench.sql file in sample directoy. We will use this sample file here to create distribution rules for pgbench. Execute the following command in extracted pgpool-II source code directory.

$ psql -f sample/dist_def_pgbench.sql -p 5432 pgpool

Here is the explanation of dist_def_pgbench.sql.

Inside dist_def_pgbench.sql, we are inserting one row into "dist_def" table. There is a distribution function for accounts table. For key-column, aid is defined for accounts respectively (which is primary keys)

INSERT INTO pgpool_catalog.dist_def VALUES (
    'bench_parallel',
    'public',
    'accounts',
    'aid',
    ARRAY['aid', 'bid', 'abalance', 'filler'],
    ARRAY['integer', 'integer', 'integer', 'character(84)'],
    'pgpool_catalog.dist_def_accounts'
);

Now, we must define the distribution function for accounts table. Note that you can use the same function from different tables. Also, you can define functions using languages other than SQL (e.g. PL/pgSQL, PL/Tcl, etc.).

The accounts table when data is initialized specifying 3 scale factor, The value of the aid is 1 to 300000. The function is defined so that data is evenly distributed to three data base nodes.

SQL function will be defined as the return of the number of the data base node.

CREATE OR REPLACE FUNCTION pgpool_catalog.dist_def_branches(anyelement)
RETURNS integer AS $$
    SELECT CASE WHEN $1 > 0 AND $1 <= 1 THEN 0
        WHEN $1 > 1 AND $1 <= 2 THEN 1
        ELSE 2
    END;
$$ LANGUAGE sql;

3.4. Defining Replication Rules

The replication rule is the one that which table decides the replication whether to be done.

Here, it is made with pgbench With the branches table and tellers table are registered. As a result, the accounts table and the inquiry that uses the branches table and the tellers table become possible.

INSERT INTO pgpool_catalog.replicate_def VALUES (
    'bench_parallel',
    'public',
    'branches',
    ARRAY['bid', 'bbalance', 'filler'],
    ARRAY['integer', 'integer', 'character(88)']
);

INSERT INTO pgpool_catalog.replicate_def VALUES (
    'bench_parallel',
    'public',
    'tellers',
    ARRAY['tid', 'bid', 'tbalance', 'filler'],
    ARRAY['integer', 'integer', 'integer', 'character(84)']
);

Replicate_def_pgbench.sql is prepared in sample directory. In the directory that progresses the source code to define a replicate rule by using this as follows The psql command is executed.

$ psql -f sample/replicate_def_pgbench.sql -p 5432 pgpool

3.5. Checking Parallel Query

To reflect the changes in pgpool.conf, pgpool-II must be restarted. Please refer to section "1.5 Starting/Stopping pgpool-II".

After configuring pgpool.conf and restarting pgpool-II, let's try and see if parallel query is working OK.

First, we need to create a database to be distributed. We will name it "bench_parallel". This database needs to be created on all the nodes. Use createdb commands through pgpool-II, and the database will be created on all the nodes.

$ createdb -p 9999 bench_parallel

Then, we'll execute pgbench with -i -s 3 options. -i option initializes the database with pre-defined tables and data. -s option specifies the scale factor for initialization.

$ pgbench -i -s 3 -p 9999 bench_parallel

The tables and data created are shown in "3.3. Defining Distribution Rules".

One way to check if the data have been distributed correctly is to execute a SELECT query via pgpool-II and directly on the backend, and compare two results. If everything is configured right, "bench_parallel" should be distributed as follows.

Let's use a simple shell script to check the above on all the nodes and via pgpool-II. The following script will display the minimum and maximum values in accounts table using port 5432, 5433, 5434, and 9999.

$ for port in 5432 5433 5434 9999; do
>     echo $port
>     psql -c "SELECT min(aid), max(aid) FROM accounts" -p $port bench_parallel
> done