Apache HTTP Server Version 2.4

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# Apache Module mod_proxy_hcheck

Description: | Dynamic health check of Balancer members (workers) for `mod_proxy`
Status: | Extension
Module Identifier: | proxy_hcheck_module
Source File: | mod_proxy_hcheck.c
Compatibility: | Available in Apache 2.4.21 and later

### Summary

This module provides for dynamic health checking of balancer members (workers). This can be enabled on a worker-by-worker basis. The health check is done independently of the actual reverse proxy requests.

This module requires the service of `mod_watchdog`.

### Parameters

The health check mechanism is enabled via the use of additional BalancerMember parameters, which are configured in the standard way via `ProxyPass`:

A new BalancerMember status state (flag) is defined via this module: "`C`". When the worker is taken offline due to failures as determined by the health check module, this flag is set, and can be seen (and modified) via the `balancer-manager`.

Parameter | Default | Description
--- | --- | ---
hcmethod | None | No dynamic health check performed. Choices are:
Method | Description | Note
--- | --- | ---
None | No dynamic health checking done |
TCP | Check that a socket to the backend can be created: e.g. "are you up" |
OPTIONS | Send an `HTTP OPTIONS` request to the backend | *
HEAD | Send an `HTTP HEAD` request to the backend | *
GET | Send an `HTTP GET` request to the backend | *
*: Unless `hcexpr` is used, a 2xx or 3xx HTTP status will be interpreted as passing the health check
hcpasses | 1 | Number of successful health check tests before worker is re-enabled
hcfails | 1 | Number of failed health check tests before worker is disabled
hcinterval | 30 | Period of health checks in seconds (e.g. performed every 30 seconds)
hcuri |  | Additional URI to be appended to the worker URL for the health check.
hctemplate |  | Name of template, created via `ProxyHCTemplate` to use for setting health check parameters for this worker
hcexpr |  | Name of expression, created via `ProxyHCExpr`, used to check response headers for health. If not used, 2xx thru 3xx status codes imply success

### Topics

- Usage examples

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

### See also

- `mod_proxy`

## Usage examples

The following example shows how one might configured health checking for various backend servers:

    ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
    ProxyHCExpr gdown {%{REQUEST_STATUS} =~ /^[5]/}
    ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}

    <Proxy balancer://foo>
      BalancerMember http://www.example.com/  hcmethod=GET hcexpr=in_maint hcuri=/status.php
      BalancerMember http://www2.example.com/  hcmethod=HEAD hcexpr=ok234 hcinterval=10
      BalancerMember http://www3.example.com/ hcmethod=TCP hcinterval=5 hcpasses=2 hcfails=3
      BalancerMember http://www4.example.com/
    </Proxy>

    ProxyPass "/" "balancer://foo"
    ProxyPassReverse "/" "balancer://foo"

In this scenario, `http://www.example.com/` is health checked by sending a `GET /status.php` request to that server and seeing that the returned page does not include the string Under maintenance. If it does, that server is put in health-check fail mode, and disabled. This dynamic check is performed every 30 seconds, which is the default.

`http://www2.example.com/` is checked by sending a simple `HEAD` request every 10 seconds and making sure that the response status is 2xx, 3xx or 4xx. `http://www3.example.com/` is checked every 5 seconds by simply ensuring that the socket to that server is up. If the backend is marked as "down" and it passes 2 health check, it will be re-enabled and added back into the load balancer. It takes 3 back-to-back health check failures to disable the server and move it out of rotation. Finally, `http://www4.example.com/` is not dynamically checked at all.

## ProxyHCExpr Directive

Description: | Creates a named condition expression to use to determine health of the backend based on its response.
Syntax: | `ProxyHCExpr name {ap_expr expression}`
Context: | server config, virtual host
Status: | Extension
Module: | mod_proxy_hcheck

The `ProxyHCExpr` directive allows for creating a named condition expression that checks the response headers of the backend server to determine its health. This named condition can then be assigned to balancer members via the `hcexpr` parameter

### ProxyHCExpr: Allow for 2xx/3xx/4xx as passing

    ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
    ProxyPass "/apps"     "http://backend.example.com/" hcexpr=ok234

The expression can use curly-parens ("{}") as quoting deliminators in addition to normal quotes.

If using a health check method (eg: `GET`) which results in a response body, that body itself can be checked via `ap_expr` using the `hc()` expression function, which is unique to this module.

In the following example, we send the backend a `GET` request and if the response body contains the phrase Under maintenance, we want to disable the backend.

### ProxyHCExpr: Checking response body

    ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
    ProxyPass "/apps"     "http://backend.example.com/" hcexpr=in_maint hcmethod=get hcuri=/status.php

NOTE: Since response body can quite large, it is best if used against specific status pages.

## ProxyHCTemplate Directive

Description: | Creates a named template for setting various health check parameters
Syntax: | `ProxyHCTemplate name parameter=setting <...>`
Context: | server config, virtual host
Status: | Extension
Module: | mod_proxy_hcheck

The `ProxyHCTemplate` directive allows for creating a named set (template) of health check parameters that can then be assigned to balancer members via the `hctemplate` parameter

### ProxyHCTemplate

    ProxyHCTemplate tcp5 hcmethod=tcp hcinterval=5
    ProxyPass "/apps"     "http://backend.example.com/" hctemplate=tcp5

## ProxyHCTPsize Directive

Description: | Sets the total server-wide size of the threadpool used for the health check workers.
Syntax: | `ProxyHCTPsize <size>`
Context: | server config
Status: | Extension
Module: | mod_proxy_hcheck

If Apache httpd and APR are built with thread support, the health check module will offload the work of the actual checking to a threadpool associated with the Watchdog process, allowing for parallel checks. The `ProxyHCTPsize` directive determines the size of this threadpool. If set to `0`, no threadpool is used at all, resulting in serialized health checks. The default size is 16.

### ProxyHCTPsize

    ProxyHCTPsize 32