1VCL-PROBE(7) VCL-PROBE(7)
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6 VCL-probe - Configuring Backend Health Probes
7
9 Varnish can be configured to periodically send a request to test if a
10 backend is answering and thus "healthy".
11 Probes can be configured per backend:
13 backend foo {
15 [...]
16 .probe = {
17 [...]
18 }
19 }
20 They can be named and shared between backends:
22 probe light {
24 [...]
25 }
26 backend foo {
28 .probe = light;
29 }
30 backend bar {
32 .probe = light;
33 }
34 Or a default probe can be defined, which applies to all backends with‐
36 out a specific .probe configured:
37 probe default {
39 [...]
40 }
41 The basic syntax is the same as for backends:
43 probe name {
45 .attribute1 = value;
46 .attribute2 = "value";
47 [...]
48 }
49 There are no mandatory attributes, they all have defaults.
51
53 The URL to query. Defaults to /:
54 .url = "/health-probe";
56
58 Can be used to specify a full HTTP/1.1 request to be sent:
59 .request = "GET / HTTP/1.1"
61 "Host: example.com"
62 "X-Magic: We're fine with this."
63 "Connection: close";
64 Each of the strings will have CRNL appended and a final HTTP header
66 block terminating CRNL will be appended as well.
67 Because connection shutdown is part of the health check, Connection:
69 close is mandatory.
70
72 The expected HTTP status, defaults to 200:
73 .expected_response = 418;
75
77 How fast the probe must succeed, default is two seconds:
78 .timeout = 10s;
80
82 Time between probes, default is five seconds:
83 .interval = 1m;
85
87 Backend health probes uses a 64 stage shift register to remember the
88 most recent health probes and to evaluate the total health of the back‐
89 end.
90 In the CLI, a good backend health status looks like this:
92 varnish> backend.list -p boot.backend
94 Backend name Admin Probe Health Last change
95 boot.backend probe 5/5 healthy Wed, 13 Jan 2021 10:31:50 GMT
96 Current states good: 5 threshold: 4 window: 5
97 Average response time of good probes: 0.000793
98 Oldest ================================================== Newest
99 4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
100 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
101 RRRRRRRRRRRRRRRRRRRRRRR----RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
102 HHHHHHHHHHHHHHHHHHHHHHH--------HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
103 Starting from the bottom, the last line shows that this backend has
105 been declared "Happy" for most the 64 health probes, but there were
106 some trouble some while ago.
107 However, in this case the .window is configured to five, and the
109 .threshold is set to four, so at this point in time, the backend is
110 considered fully healthy.
111 An additional .initial fills that many "happy" entries in the shift
113 register when the VCL is loaded, so that backends can quickly become
114 healthy, even if their health is normally considered over many samples:
115 .interval = 1s;
117 .window = 60;
118 .threshold = 45;
119 .initial = 43;
120 This backend will be considered healthy if three out of four health
122 probes in the last minute were good, but it becomes healthy as soon as
123 two good probes have happened after the VCL was loaded.
124 The default values are:
126 • .window = 8
128 • .threshold = 3
130 • .initial = one less than .threshold
132 SEE ALSO
134 • varnishd(1)
135 • vcl(7)
137 • vcl-backend(7)
139 • vmod_directors(3)
141 • vmod_std(3)
143 HISTORY
145 VCL was developed by Poul-Henning Kamp in cooperation with Verdens Gang
146 AS, Redpill Linpro and Varnish Software. This manual page is written
147 by Per Buer, Poul-Henning Kamp, Martin Blix Grydeland, Kristian Lyn‐
148 gstøl, Lasse Karstensen and others.
149 COPYRIGHT
151 This document is licensed under the same license as Varnish itself. See
152 LICENSE for details.
153 • Copyright (c) 2006 Verdens Gang AS
155 • Copyright (c) 2006-2021 Varnish Software AS
157 VCL-PROBE(7)