<?xml version="1.0"?>

<!--
  Copyright (C) 2014 Yaroslav Zhuravlev
  Copyright (C) Nginx, Inc.
  -->

<!DOCTYPE module SYSTEM "../../../../dtd/module.dtd">

<module name="Module ngx_stream_upstream_module"
        link="/en/docs/stream/ngx_stream_upstream_module.html"
        lang="en"
        rev="1">

<section id="summary">

<para>
The <literal>ngx_stream_upstream_module</literal> module (1.7.7)
is used to define groups of servers that can be referenced
by the <link doc="ngx_stream_module.xml" id="proxy_pass"/>
directive.
</para>

<para>
<note>
This module is available as part of our
<commercial_version>commercial subscription</commercial_version>.
</note>
</para>

</section>


<section id="example" name="Example Configuration">

<para>
<example>
stream {
    upstream backend {
        hash $remote_addr consistent;
        server backend1.example.com:8000;
        server backend2.example.com:8000;
        server backend3.example.com:8001;
    }

    server {
        listen 9000;
        proxy_pass backend;
    }
}
</example>
</para>

</section>


<section id="directives" name="Directives">

<directive name="upstream">
<syntax block="yes"><value>name</value></syntax>
<default/>
<context>stream</context>

<para>
Defines a group of servers.
Servers can listen on different ports.
In addition, servers listening on TCP and UNIX-domain sockets
can be mixed.
</para>

<para>
Example:
<example>
upstream backend {
    server backend1.example.com:8080 weight=5;
    server 127.0.0.1:8080            max_fails=3 fail_timeout=30s;
    server unix:/tmp/backend3;

    server backup1.example.com:8080  backup;
}
</example>
</para>

<para>
By default, connections are distributed between the servers using a
weighted round-robin balancing method.
In the above example, each 7 connections will be distributed as follows:
5 connections go to <literal>backend1.example.com</literal>
and one connection to each of the second and third servers.
If an error occurs during communication with a server, the connection will
be passed to the next server, and so on until all of the functioning
servers will be tried.
If communication with all servers fails, the connection will be closed.
</para>

</directive>


<directive name="server">
<syntax><value>address</value> [<value>parameters</value>]</syntax>
<default/>
<context>upstream</context>

<para>
Defines the <value>address</value> and other <value>parameters</value>
of a server.
The address can be specified as a domain name or IP address
with an obligatory port, or as a UNIX-domain socket path
specified after the “<literal>unix:</literal>” prefix.
A domain name that resolves to several IP addresses defines
multiple servers at once.
</para>

<para>
The following parameters can be defined:
<list type="tag">

<tag-name id="weight">
<literal>weight</literal>=<value>number</value>
</tag-name>
<tag-desc>
sets the weight of the server, by default, 1.
</tag-desc>

<tag-name id="max_fails">
<literal>max_fails</literal>=<value>number</value>
</tag-name>
<tag-desc>
sets the number of unsuccessful attempts to communicate with the server
that should happen in the duration set by the <literal>fail_timeout</literal>
parameter to consider the server unavailable for a duration also set by the
<literal>fail_timeout</literal> parameter.
By default, the number of unsuccessful attempts is set to 1.
The zero value disables the accounting of attempts.
Here, an unsuccessful attempt is an error or timeout
while establishing a connection with the server.
</tag-desc>

<tag-name id="fail_timeout">
<literal>fail_timeout</literal>=<value>time</value>
</tag-name>
<tag-desc>
sets
<list type="bullet">

<listitem>
the time during which the specified number of unsuccessful attempts to
communicate with the server should happen to consider the server unavailable;
</listitem>

<listitem>
and the period of time the server will be considered unavailable.
</listitem>

</list>
By default, the parameter is set to 10 seconds.
</tag-desc>

<tag-name id="backup">
<literal>backup</literal>
</tag-name>
<tag-desc>
marks the server as a backup server.
Connections to the backup server will be passed
when the primary servers are unavailable.
</tag-desc>

<tag-name id="down">
<literal>down</literal>
</tag-name>
<tag-desc>
marks the server as permanently unavailable; used along with
the <link id="hash"/> directive.
</tag-desc>

<tag-name id="max_conns">
<literal>max_conns</literal>=<value>number</value>
</tag-name>
<tag-desc>
limits the maximum <value>number</value> of simultaneous connections to the
proxied server.
Default value is zero, meaning there is no limit.
</tag-desc>

<tag-name id="slow_start">
<literal>slow_start</literal>=<value>time</value>
</tag-name>
<tag-desc>
sets the <value>time</value> during which the server will recover its weight
from zero to a nominal value,
or when the server becomes available after a period of time
it was considered <link id="fail_timeout">unavailable</link>.
Default value is zero, i.e. slow start is disabled.
</tag-desc>

</list>
</para>

<para>
<note>
If there is only a single server in a group, <literal>max_fails</literal>,
<literal>fail_timeout</literal> and <literal>slow_start</literal> parameters
are ignored, and such a server will never be considered unavailable.
</note>
</para>

</directive>


<directive name="hash">
<syntax><value>key</value> [<literal>consistent</literal>]</syntax>
<default/>
<context>upstream</context>

<para>
Specifies a load balancing method for a server group
where client-server mapping is based on the hashed <value>key</value> value.
Currently, the only supported value for the <literal>key</literal>
is the client remote address specified as <literal>$remote_addr</literal>.
Note that adding or removing a server from the group
may result in remapping most of the keys to different servers.
The method is compatible with the
<link url="http://search.cpan.org/perldoc?Cache%3A%3AMemcached">Cache::Memcached</link>
Perl library.
</para>

<para>
If the <literal>consistent</literal> parameter is specified,
the <link url="http://www.last.fm/user/RJ/journal/2007/04/10/392555/">ketama</link>
consistent hashing method will be used instead.
The method ensures that only a few keys
will be remapped to different servers
when a server is added to or removed from the group.
This helps to achieve a higher cache hit ratio for caching servers.
The method is compatible with the
<link url="http://search.cpan.org/perldoc?Cache%3A%3AMemcached%3A%3AFast">Cache::Memcached::Fast</link>
Perl library with the <value>ketama_points</value> parameter set to 160.
</para>

</directive>


<directive name="least_conn">
<syntax/>
<default/>
<context>upstream</context>

<para>
Specifies that a server group should use a load balancing method
where a connection
is passed to the server with the least number of active connections,
taking into account weights of servers.
If there are several such servers, they are tried in turn using a
weighted round-robin balancing method.
</para>

</directive>

</section>

</module>