TCP Health Checks

Introduction

NGINX and F5 NGINX Plus can continually test your TCP upstream servers, avoid the servers that have failed, and gracefully add the recovered servers into the load‑balanced group.

Prerequisites

You have configured an upstream group of TCP servers in the stream context, for example:

stream {
    #...
    upstream stream_backend {
        server backend1.example.com:12345;
        server backend2.example.com:12345;
        server backend3.example.com:12345;
   }
    #...
}

You have configured a server that passes TCP connections to the server group:

stream {
    #...
    server {
        listen     12345;
        proxy_pass stream_backend;
    }
    #...
}

Passive TCP Health Checks

If an attempt to connect to an upstream server times out or results in an error, NGINX Open Source or NGINX Plus can mark the server as unavailable and stop sending requests to it for a defined amount of time. To define the conditions under which NGINX considers an upstream server unavailable, include the following parameters to the server directive

fail_timeout – The amount of time within which a specified number of connection attempts must fail for the server to be considered unavailable. Also, the amount of time that NGINX considers the server unavailable after marking it so.
max_fails – The number of failed attempts that happen during the specified time for NGINX to consider the server unavailable.

The default values are 10 seconds and 1 attempt. So if a connection attempt times out or fails at least once in a 10‑second period, NGINX marks the server as unavailable for 10 seconds. The example shows how to set these parameters to 2 failures within 30 seconds:

upstream stream_backend {
    server backend1.example.com:12345 weight=5;
    server backend2.example.com:12345 max_fails=2 fail_timeout=30s;
    server backend3.example.com:12346 max_conns=3;
}

Server Slow Start

A recently recovered upstream server can be easily overwhelmed by connections, which may cause the server to be marked as unavailable again. Slow start allows an upstream server to gradually recover its weight from zero to its nominal value after it has been recovered or became available. This can be done with the slow_start parameter of the upstream server directive:

upstream backend {
    server backend1.example.com:12345 slow_start=30s;
    server backend2.example.com;
    server 192.0.0.1 backup;
}

Note that if there is only a single server in a group, the slow_start parameter is ignored and the server is never marked unavailable. Slow start is exclusive to NGINX Plus.

Active TCP Health Checks

Health checks can be configured to test a wide range of failure types. For example, NGINX Plus can continually test upstream servers for responsiveness and avoid servers that have failed.

NGINX Plus sends special health check requests to each upstream server and checks for a response that satisfies certain conditions. If a connection to the server cannot be established, the health check fails, and the server is considered unhealthy. NGINX Plus does not proxy client connections to unhealthy servers. If several health checks are configured for an upstream group, the failure of any check is enough to consider the corresponding server unhealthy.

To enable active health checks:

Specify a shared memory zone – a special area where the NGINX Plus worker processes share state information about counters and connections. Add the zone directive to the upstream server group and specify the zone name (here, stream_backend) and the amount of memory (64 KB):

stream {
    #...
    upstream stream_backend {
        zone   stream_backend 64k;
        server backend1.example.com:12345;
        server backend2.example.com:12345;
        server backend3.example.com:12345;
    }
    #...
}

Enable active health checks for the upstream group with the health_check directive:

stream {
    #...
    server {
        listen        12345;
        proxy_pass    stream_backend;
        health_check;
        #...
    }
}

If necessary, reduce a timeout between two consecutive health checks with the health_check_timeout directive. This directive overrides the proxy_timeout value for health checks, as for health checks this timeout needs to be significantly shorter:

stream {
    #...
    server {
        listen               12345;
        proxy_pass           stream_backend;
        health_check;
        health_check_timeout 5s;
    }
}

By default, NGINX Plus sends health check messages to the port specified by the server directive in the upstream block. You can specify another port for health checks, which is particularly helpful when monitoring the health of many services on the same host. To override the port, specify the port parameter of the health_check directive:
```
stream {
    #...
    server {
        listen               12345;
        proxy_pass           stream_backend;
        health_check         port=12346;
        health_check_timeout 5s;
    }
}
```

Fine-Tuning TCP Health Checks

By default, NGINX Plus tries to connect to each server in an upstream server group every 5 seconds. If the connection cannot be established, NGINX Plus considers the health check failed, marks the server as unhealthy, and stops forwarding client connections to the server.

To change the default behavior, include parameters to the health_check directive:

interval – How often (in seconds) NGINX Plus sends health check requests (default is 5 seconds)
passes – Number of consecutive health checks the server must respond to to be considered healthy (default is 1)
fails – Number of consecutive health checks the server must fail to respond to to be considered unhealthy (default is 1)
```
stream {
    #...
    server {
        listen       12345;
        proxy_pass   stream_backend;
        health_check interval=10 passes=2 fails=3;
    }
    #...
}
```
In the example, the time between TCP health checks is increased to 10 seconds, the server is considered unhealthy after 3 consecutive failed health checks, and the server needs to pass 2 consecutive checks to be considered healthy again.

The “match {}” Configuration Block

You can create your own tests to verify server responses to health checks. These tests are defined with the match {} configuration block placed in the stream {} context.

On the stream {} level, specify the match {} block and name it, for example, tcp_test:
```
stream {
    #...
    match tcp_test {
        #...
    }
}
```
This block will contain tests described in Step 3.

Refer to the block from the health_check directive by specifying the match parameter and the name of the match block:

stream {
    #...
    server {
        listen       12345;
        health_check match=tcp_test;
        proxy_pass   stream_backend;
    }
    #...
}

Within the match block, specify the conditions or tests under which a health check succeed. The block can accept the following parameters:
- send – The text string or hexadecimal literals (“\x” followed by two hex digits) to send to the server
- expect – Literal string or regular expression that the data returned by the server needs to match
These parameters can be used in different combinations, but no more than one send and one expect parameter can be specified at a time:
- If no send or expect parameters are specified, the ability to connect to the server is tested.
- If the expect parameter is specified, the server is expected to unconditionally send data first:
```
match pop3 {
    expect ~* "\+OK";
}
```
- If the send parameter is specified, it is expected that the connection will be successfully established and the specified string will be sent to the server:
```
match pop_quit {
    send QUIT;
}
```
- If both the send and expect parameters are specified, then the string from the send parameter must match the regular expression from the expect parameter:
```
stream {
    #...
    upstream   stream_backend {
        zone   upstream_backend 64k;
        server backend1.example.com:12345;
    }
    match http {
        send      "GET / HTTP/1.0\r\nHost: localhost\r\n\r\n";
        expect ~* "200 OK";
    }
    server {
        listen       12345;
        health_check match=http;
        proxy_pass   stream_backend;
    }
}
```
The example shows that in order for a health check to pass, the HTTP request must be sent to the server, and the expected result from the server contains 200 OK to indicate a successful HTTP response.

Last modified August 22, 2024