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combined-upstreams: NGINX Combined Upstreams module

Installation

CentOS/RHEL 6, 7, 8 or Amazon Linux 2

yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install nginx-module-combined-upstreams

Enable the module by adding the following at the top of /etc/nginx/nginx.conf:

load_module modules/ngx_http_combined_upstreams_module.so;

This document describes nginx-module-combined-upstreams v2.1 released on Aug 27 2020.


The module introduces three directives add_upstream, combine_server_singlets, and extend_single_peers available inside upstream configuration blocks, and a new configuration block upstrand for building super-layers of upstreams. Additionally, directive dynamic_upstrand is introduced for choosing upstrands in run-time.

Directive add_upstream

Populates the host upstream with servers listed in an already defined upstream specified by the mandatory 1st parameter of the directive. Optional 2nd parameter may have only value backup which marks all servers of the sourced upstream as backups.

An example

upstream  combined {
    add_upstream    upstream1;            # src upstream 1
    add_upstream    upstream2;            # src upstream 2
    server          some_another_server;  # if needed
    add_upstream    upstream3 backup;     # src upstream 3
}

Directive combine_server_singlets

Produces multiple singlet upstreams from servers so far defined in the host upstream. A singlet upstream contains only one active server whereas other servers are marked as backups. If no parameters were passed then the singlet upstreams will have names of the host upstream appended by the ordering number of the active server in the host upstream. Optional 2 parameters can be used to adjust their names. The 1st parameter is a suffix added after the name of the host upstream and before the ordering number. The 2nd parameter must be an integer value which defines zero-alignment of the ordering number, for example if it has value 2 then the ordering numbers could be '01', '02', ..., '10', ... '100' ...

An example

upstream  uhost {
    server                   s1;
    server                   s2;
    server                   s3 backup;
    server                   s4;
    # build singlet upstreams uhost_single_01,
    # uhost_single_02, uhost_single_03 and uhost_single_04
    combine_server_singlets  _single_ 2;
    server                   s5;
}

Why numbers, not names?

In the example above singlet upstreams will have names like uhost_single_01, but names that contain server names like uhost_single_s1 would look better and more convenient. Why not use them instead ordering numbers? Unfortunately nginx does not remember server names after a server has been added into an upstream, therefore we cannot simply fetch them.

Update. There is a good news! Since version 1.7.2 nginx remembers server names in upstream data and now we can use them when referring to a special keyword byname. For example

    combine_server_singlets  byname;
    # or
    combine_server_singlets  _single_ byname;

All colons (:) in the server names are replaced with underscores (_).

Where this can be useful

Hmm, I do not know. Anyway a singlet upstream is a prominent category because it defines a single server with fallback mode. We can use them to provide robust HTTP session management when backend servers identify themselves using a known mechanism like HTTP cookies.

upstream  uhost {
    server  s1;
    server  s2;
    combine_server_singlets;
}

server {
    listen       8010;
    server_name  main;
    location / {
        proxy_pass http://uhost$cookie_rt;
    }
}
server {
    listen       8020;
    server_name  server1;
    location / {
        add_header Set-Cookie "rt=1";
        echo "Passed to $server_name";
    }
}
server {
    listen       8030;
    server_name  server2;
    location / {
        add_header Set-Cookie "rt=2";
        echo "Passed to $server_name";
    }
}

In this configuration the first client request will choose backend server randomly, the chosen server will set cookie rt to a predefined value (1 or 2) and all further requests from this client will be proxied to the chosen server automatically until it goes down. Say it was server1, then when it goes down the cookie rt on the client side will still be 1. Directive proxy_pass will route the next client request to a singlet upstream uhost1 where server1 is declared active and server2 is backed up. As soon as server1 is not reachable any longer nginx will route the request to server2 which will rewrite the cookie rt and all further client requests will be proxied to server2 until it goes down.

Directive extend_single_peers

Peers in upstreams fail according to the rules listed in directive proxy_next_upstream. If an upstream has only one peer in its main or backup part then this peer will never fail. This can be a serious problem when writing a custom algorithm for active health checks of upstream peers. Directive extend_single_peers, being declared in an upstream block, adds a fake peer marked as down in the main or the backup part of the upstream if the part originally contains only one peer. This makes nginx mark the original single peer as failed when it fails to pass the rules of proxy_next_upstream just like in general case of multiple peers.

An example

upstream  upstream1 {
    server  s1;
    extend_single_peers;
}

upstream  upstream2 {
    server  s1;
    server  s2;
    server  s3 backup;
    extend_single_peers;
}

Notice that if a part (the main or the backup) of an upstream contains more than one peer (like the main part in upstream2 from the example) then the directive has no effect: particularly, in the upstream2 it only affects the backup part of the upstream.

Block upstrand

Is aimed to configure a super-layer of upstreams that do not lose their identities. Accepts a number of directives including upstream, order, next_upstream_statuses and others. Upstreams with names starting with tilde (~) match a regular expression. Only upstreams that already have been declared before the upstrand block definition are regarded as candidates.

An example

upstrand us1 {
    upstream ~^u0 blacklist_interval=60s;
    upstream b01 backup;
    order start_random;
    next_upstream_statuses error timeout non_idempotent 204 5xx;
    next_upstream_timeout 60s;
    intercept_statuses 5xx /Internal/failover;
}

Upstrand us1 will combine all upstreams whose names start with u0 and upstream b01 as backup. Backup upstreams are checked if all normal upstreams fail. The failure means that all upstreams in normal or backup cycles have responded with statuses listed in directive next_upstream_statuses or been blacklisted. Here the upstream's response means the status returned by the last server of the upstream, which is strongly affected by value of directive proxy_next_upstream. An upstream is set as blacklisted when it has parameter blacklist_interval and responds with a status listed in the next_upstream_statuses. Blacklisting state is not shared between nginx worker processes.

The next four upstrand directives are akin to those from the nginx proxy module.

Directive next_upstream_statuses accepts 4xx and 5xx statuses notation and values error and timeout to distinguish between cases when errors happen with the upstream's peer connections from those when backends send statuses 502 or 504 (plain values 502 and 504 as well as 5xx refer to both cases). It also accepts value non_idempotent to allow further processing of non-idempotent requests when they were responded by the last server from an upstream but failed according to other statuses listed in the directive. Requests are considered to be non-idempotent when their methods are POST, LOCK or PATCH just like in directive proxy_next_upstream.

Directive next_upstream_timeout limits the overall duration time the upstrand cycles through all of its upstreams. If the time elapses while the upstrand is ready to pass to a next upstream, the last upstream cycle result is returned.

Directive intercept_statuses allows upstrand failover by intercepting the final response in location that matches the given URI. Interceptions must happen even when the upstrand times out. Notice also that walking through upstreams in an upstrand and the upstrand failover URI are not interceptable. Speaking more generally, any internal redirection (by error_page, proxy_intercept_errors, X-Accel-Redirect etc.) will break nested subrequests on which the upstrand's implementation is based which leads to returning empty responses. These are extremely bad cases, and this is why walking through upstreams was protected against interceptions. The upstrand failover URI is more affected by this as the implementation has less control over its location. Particularly, the upstrand failover has only protection against interceptions by error_page and proxy_intercept_errors. This means that the upstrand failover URI location must be as simple as possible (e.g. using simple directives like return or echo).

Directive order currently accepts only one value start_random which means that starting upstreams in normal and backup cycles after worker fired up will be chosen randomly. Starting upstreams in further requests will be cycled in round-robin manner. Additionally, a modifier per_request is also accepted in the order directive: it turns off the global per-worker round-robin cycle. The combination of per_request and start_random makes the starting upstream in every new request be chosen randomly.

Such a failover between failure statuses can be reached during a single request by feeding a special variable that starts with upstrand_ to the proxy_pass directive like so:

location /us1 {
    proxy_pass http://$upstrand_us1;
}

But be careful when accessing this variable from other directives! It starts up the subrequests machinery which may be not desirable in many cases.

Upstrand status variables

There are a number of upstrand status variables available: upstrand_addr, upstrand_cache_status, upstrand_connect_time, upstrand_header_time, upstrand_response_length, upstrand_response_time and upstrand_status. They all are counterparts of corresponding upstream variables and contain the values of the latter for all upstreams passed through a request and all subrequests chronologically. Variable upstrand_path contains path of all upstreams visited during request.

Where this can be useful

The upstrand looks very similar to a simple combined upstream but it also has a crucial difference: the upstreams inside of an upstrand do not get flattened and keep holding their identities. This gives a possibility to configure a failover status for a group of servers associated with a single upstream without need to check them all by turn. In the above example upstrand us1 may hold a list of upstreams like u01, u02 etc. Imagine that upstream u01 holds 10 servers inside and represents a part of a geographically distributed backend system. Let upstrand us1 combine all those geographical parts and we have an application that polls the parts for doing some tasks. Let backends send HTTP status 204 if they do not have new tasks. In a flat combined upstream all 10 servers may be polled before the application will finally receive a new task from another upstream. The upstrand us1 allows skipping to the next upstream after checking the first server in an upstream that does not have tasks. This machinery is apparently suitable for upstreams broadcasting, when messages are being sent to all upstreams in an upstrand.

The examples above show that an upstrand can be regarded as a 2-dimensional upstream that comprises a number of clusters representing natural upstreams and allows short-cycling over them.

Directive dynamic_upstrand

Allows choosing an upstrand from passed variables in run-time. The directive can be set in server, location and location-if clauses.

In the following configuration

    upstrand us1 {
        upstream ~^u0;
        upstream b01 backup;
        order start_random;
        next_upstream_statuses 5xx;
    }
    upstrand us2 {
        upstream ~^u0;
        upstream b02 backup;
        order start_random;
        next_upstream_statuses 5xx;
    }

    server {
        listen       8010;
        server_name  main;

        dynamic_upstrand $dus1 $arg_a us2;

        location / {
            dynamic_upstrand $dus2 $arg_b;
            if ($arg_b) {
                proxy_pass http://$dus2;
                break;
            }
            proxy_pass http://$dus1;
        }
    }

upstrands returned in variables dus1 and dus2 are to be chosen from values of variables arg_a and arg_b. If arg_b is set then the client request will be sent to an upstrand with name equal to the value of arg_b. If there is not an upstrand with this name then dus2 will be empty and proxy_pass will return HTTP status 500. To prevent initialization of a dynamic upstrand variable with empty value, its declaration must be terminated with a literal name that corresponds to an existing upstrand. In this example dynamic upstrand variable dus1 will be initialized by the upstrand us2 if arg_a is empty or not set. Altogether if arg_b is not set or empty and arg_a is set and has a value equal to an existing upstrand, the request will be sent to this upstrand, otherwise (if arg_b is not set or empty and arg_a is set but does not refer to an existing upstrand) proxy_pass will most likely return HTTP status 500 (except there is a variable composed from literal string upstrand_ and the value of arg_a that points to a valid destination), otherwise (both arg_b and arg_a are not set or empty) the request will be sent to the upstrand us2.

See also

There are several articles about the module in my blog, in chronological order:

  1. Простой модуль nginx для создания комбинированных апстримов (in Russian). A comprehensive article discovering details of implementation of directive add_upstream which can also be regarded as a small tutorial for nginx modules development.
  2. nginx upstrand to configure super-layers of upstreams. An overview of block upstrand usage and some details on its implementation.
  3. Не такой уж простой модуль nginx для создания комбинированных апстримов (in Russian). An overview of all features of the module with configuration examples and testing session samples.

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-combined-upstreams.

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