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Support connection load balancing in libpq

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This adds support for load balancing connections with libpq using a
connection parameter: load_balance_hosts=<string>. When setting the
param to random, hosts and addresses will be connected to in random
order. This then results in load balancing across these addresses and
hosts when multiple clients or frequent connection setups are used.

The randomization employed performs two levels of shuffling:

  1. The given hosts are randomly shuffled, before resolving them
     one-by-one.
  2. Once a host its addresses get resolved, the returned addresses
     are shuffled, before trying to connect to them one-by-one.

Author: Jelte Fennema <postgres@jeltef.nl>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Michael Banck <mbanck@gmx.net>
Reviewed-by: Andrey Borodin <amborodin86@gmail.com>
Discussion: https://postgr.es/m/PR3PR83MB04768E2FF04818EEB2179949F7A69@PR3PR83MB0476.EURPRD83.prod.outlook.
This commit is contained in:
Daniel Gustafsson
2023-03-29 21:53:38 +02:00
parent 44d85ba5a3
commit 7f5b19817e
10 changed files with 431 additions and 3 deletions
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61
doc/src/sgml/libpq.sgml
View File

@ -2115,6 +2115,67 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-load-balance-hosts" xreflabel="load_balance_hosts">
<term><literal>load_balance_hosts</literal></term>
<listitem>
<para>
Controls the order in which the client tries to connect to the available
hosts and addresses. Once a connection attempt is successful no other
hosts and addresses will be tried. This parameter is typically used in
combination with multiple host names or a DNS record that returns
multiple IPs. This parameter can be used in combination with
<xref linkend="libpq-connect-target-session-attrs"/>
to, for example, load balance over standby servers only. Once successfully
connected, subsequent queries on the returned connection will all be
sent to the same server. There are currently two modes:
<variablelist>
<varlistentry>
<term><literal>disable</literal> (default)</term>
<listitem>
<para>
No load balancing across hosts is performed. Hosts are tried in
the order in which they are provided and addresses are tried in
the order they are received from DNS or a hosts file.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>random</literal></term>
<listitem>
<para>
Hosts and addresses are tried in random order. This value is mostly
useful when opening multiple connections at the same time, possibly
from different machines. This way connections can be load balanced
across multiple <productname>PostgreSQL</productname> servers.
</para>
<para>
While random load balancing, due to its random nature, will almost
never result in a completely uniform distribution, it statistically
gets quite close. One important aspect here is that this algorithm
uses two levels of random choices: First the hosts
will be resolved in random order. Then secondly, before resolving
the next host, all resolved addresses for the current host will be
tried in random order. This behaviour can skew the amount of
connections each node gets greatly in certain cases, for instance
when some hosts resolve to more addresses than others. But such a
skew can also be used on purpose, e.g. to increase the number of
connections a larger server gets by providing its hostname multiple
times in the host string.
</para>
<para>
When using this value it's recommended to also configure a reasonable
value for <xref linkend="libpq-connect-connect-timeout"/>. Because then,
if one of the nodes that are used for load balancing is not responding,
a new node will be tried.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>

13
doc/src/sgml/regress.sgml
View File

@ -256,7 +256,7 @@ make check-world -j8 >/dev/null
<varname>PG_TEST_EXTRA</varname> to a whitespace-separated list, for
example:
<programlisting>
make check-world PG_TEST_EXTRA='kerberos ldap ssl'
make check-world PG_TEST_EXTRA='kerberos ldap ssl load_balance'
</programlisting>
The following values are currently supported:
<variablelist>
@ -290,6 +290,17 @@ make check-world PG_TEST_EXTRA='kerberos ldap ssl'
</listitem>
</varlistentry>
<varlistentry>
<term><literal>load_balance</literal></term>
<listitem>
<para>
Runs the test <filename>src/interfaces/libpq/t/004_load_balance_dns.pl</filename>.
This requires editing the system <filename>hosts</filename> file and
opens TCP/IP listen sockets.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>wal_consistency_checking</literal></term>
<listitem>