1VMOD_SAINTMODE(3) VMOD_SAINTMODE(3)
2
6 vmod_saintmode - Saint mode backend director
7
9 import saintmode [from "path"] ;
10 VOID blacklist(DURATION expires)
12 STRING status()
14 new xsaintmode = saintmode(BACKEND backend, INT threshold)
16 BACKEND xsaintmode.backend()
18 INT xsaintmode.blacklist_count()
20 BOOL xsaintmode.is_healthy()
22
24 This VMOD provides saintmode functionality for Varnish Cache 4.1 and
25 newer. The code is in part based on Poul-Henning Kamp's saintmode
26 implementation in Varnish 3.0.
27 Saintmode lets you deal with a backend that is failing in random ways
29 for specific requests. It maintains a blacklist per backend, marking
30 the backend as sick for specific objects. When the number of objects
31 marked as sick for a backend reaches a set threshold, the backend is
32 considered sick for all requests. Each blacklisted object carries a
33 TTL, which denotes the time it will stay blacklisted.
34 Saintmode in Varnish 4.1 is implemented as a director VMOD. We instan‐
36 tiate a saintmode object and give it a backend as an argument. The
37 resulting object can then be used in place of the backend, with the
38 effect that it also has added saintmode capabilities.
39 Any director will then be able to use the saintmode backends, and as
41 backends marked sick are skipped by the director, this provides a way
42 to have fine grained health status on the backends, and making sure
43 that retries get a different backend than the one which failed.
44 Example:
46 vcl 4.0;
48 import saintmode;
50 import directors;
51 backend tile1 { .host = "192.0.2.11"; .port = "80"; }
53 backend tile2 { .host = "192.0.2.12"; .port = "80"; }
54 sub vcl_init {
56 # Instantiate sm1, sm2 for backends tile1, tile2
57 # with 10 blacklisted objects as the threshold for marking the
58 # whole backend sick.
59 new sm1 = saintmode.saintmode(tile1, 10);
60 new sm2 = saintmode.saintmode(tile2, 10);
61 # Add both to a director. Use sm0, sm1 in place of tile1, tile2.
63 # Other director types can be used in place of random.
64 new imagedirector = directors.random();
65 imagedirector.add_backend(sm1.backend(), 1);
66 imagedirector.add_backend(sm2.backend(), 1);
67 }
68 sub vcl_backend_fetch {
70 # Get a backend from the director.
71 # When returning a backend, the director will only return backends
72 # saintmode says are healthy.
73 set bereq.backend = imagedirector.backend();
74 }
75 sub vcl_backend_response {
77 if (beresp.status >= 500) {
78 # This marks the backend as sick for this specific
79 # object for the next 20s.
80 saintmode.blacklist(20s);
81 # Retry the request. This will result in a different backend
82 # being used.
83 return (retry);
84 }
85 }
86 VOID blacklist(DURATION expires)
88 Marks the backend as sick for a specific object. Used in vcl_back‐
89 end_response. Corresponds to the use of beresp.saintmode in Varnish
90 3.0. Only available in vcl_backend_response.
91 Example:
93 sub vcl_backend_response {
95 if (beresp.http.broken-app) {
96 saintmode.blacklist(20s);
97 return (retry);
98 }
99 }
100 STRING status()
102 Returns a JSON formatted status string suitable for use in vcl_synth.
103 sub vcl_recv {
105 if (req.url ~ "/saintmode-status") {
106 return (synth(700, "OK"));
107 }
108 }
109 sub vcl_synth {
111 if (resp.status == 700) {
112 synthetic(saintmode.status());
113 return (deliver);
114 }
115 }
116 Example JSON output:
118 {
120 "saintmode" : [
121 { "name": "sm1", "backend": "foo", "count": "3", "threshold": "10" },
122 { "name": "sm2", "backend": "bar", "count": "2", "threshold": "5" }
123 ]
124 }
125 new xsaintmode = saintmode(BACKEND backend, INT threshold)
127 Constructs a saintmode director object. The threshold argument sets the
128 saintmode threshold, which is the maximum number of items that can be
129 blacklisted before the whole backend is regarded as sick. Corresponds
130 with the saintmode_threshold parameter of Varnish 3.0.
131 Example:
133 sub vcl_init {
135 new sm = saintmode.saintmode(b, 10);
136 }
137 BACKEND xsaintmode.backend()
139 Used for assigning the backend from the saintmode object.
140 Example:
142 sub vcl_backend_fetch {
144 set bereq.backend = sm.backend();
145 }
146 INT xsaintmode.blacklist_count()
148 Returns the number of objects currently blacklisted for a saintmode
149 director object.
150 Example:
152 sub vcl_deliver {
154 set resp.http.troublecount = sm.blacklist_count();
155 }
156 BOOL xsaintmode.is_healthy()
158 Checks if the object is currently blacklisted for a saintmode director
159 object. If there are no valid objects available (called from vcl_hit
160 or vcl_recv), the function will fall back to the backend's health func‐
161 tion.
162 VMOD_SAINTMODE(3)