https://www.varnish-cache.org/docs/4.0/whats-new/index.html#whats-new-indexhttps://www.varnish-cache.org/docs/4.0/reference/vcl.html#reference-vcl
优雅模式(Garce mode)
Varnish中的请求合并
当几个客户端请求同一个页面的时候，varnish只发送一个请求到后端服务器，然后让其他几个请求挂起并等待返回结果；获得结果后，其它请求再复制后端的结果发送给客户端；
但如果同时有数以千计的请求，那么这个等待队列将变得庞大，这将导致2类潜在问题：
惊群问题(thundering herd problem)，即突然释放大量的线程去复制后端返回的结果，将导致负载急速上升；
没有用户喜欢等待；
故为了解决这类问题，可以配置varnish在缓存对象因超时失效后再保留一段时间，以给那些等待的请求返回过去的文件内容(stale content)，配置案例如下：
sub vcl_recv {if (! req.backend.healthy) {set req.grace = 5m;} else {set req.grace = 15s;}}sub vcl_fetch {set beresp.grace = 30m;}# 以上配置表示varnish将会将失效的缓存对象再多保留30分钟，此值等于最大的req.grace值即可；# 而根据后端主机的健康状况，varnish可向前端请求分别提供5分钟内或15秒内的过期内容；
安装配置
# 安装包下载地址：yum localinstall –nogpgcheck varnish-4.0.0-1.el6.x86_64.rpm varnish-libs-4.0.0-1.el6.x86_64.rpm varnish-docs-4.0.0-1.el6.x86_64.rpmvi /etc/sysconfig/varnish # 编辑配置文件，修改如下项VARNISH_STORAGE_SIZE=100M # 此值根据自身情况调整，测试环境可调低此值VARNISH_STORAGE=”malloc,${VARNISH_STORAGE_SIZE}” # Varnish 4中默认使用malloc(即内存)作为缓存对象存储方式；service varnish start # 启动varnish，，默认外部请求的监听端口6081，管理端口6082，后端主机127.0.0.1:80===========varnishadm -S /etc/varnish/secret -T 127.0.0.1:6082 # 登录管理命令行varnish> vcl.list# 列出所有的配置varnish> vcl.load test1 ./test.vcl # 加载编译新配置，test1是配置名，test.vcl是配置文件varnish> vcl.use test1# 使用配置，需指定配置名，当前使用的配置以最后一次vcl.use为准varnish> vcl.show test1# 显示配置内容，需指定配置名
实例解析
## This is an example VCL file for Varnish.## It does not do anything by default, delegating control to the# builtin VCL. The builtin VCL is called when there is no explicit# return statement.## See the VCL chapters in the Users Guide at https://www.varnish-cache.org/docs/# and for more examples.# Marker to tell the VCL compiler that this VCL has been adapted to the# new 4.0 format.vcl 4.0;import directors; # 导入directors模块；probe backend_healthcheck { # 创建健康监测机制；.url = “/health.html”;.window = 5;.threshold = 2;.interval = 3s;}backend web1 { # 创建后端主机；.host = “192.168.0.25”;.port = “80”;.probe = backend_healthcheck;}backend web2 {.host = “192.168.0.35”;.port = “80”;.probe = backend_healthcheck;}backend img1 {.host = “192.168.0.25”;.port = “4040”;.probe = backend_healthcheck;}backend img2 {.host = “192.168.0.35”;.port = “4040”;.probe = backend_healthcheck;}sub vcl_init { # 创建后端主机组，即directors；new web_cluster = directors.random();web_cluster.add_backend(web1,1.0);web_cluster.add_backend(web2,1.0);new img_cluster = directors.random();img_cluster.add_backend(img1,1.0);img_cluster.add_backend(img2,1.0);}acl purgers { # 定义可访问来源IP；”127.0.0.1″;”192.168.0.0″/24;}sub vcl_recv {if (req.method == “GET” && req.http.cookie) { # 带cookie首部的GET请求也缓存；return(hash);}if (req.url ~ “test.html”) { # test.html文件禁止缓存；return(pass);}if (req.method == “PURGE”) { # PURGE请求的处理；if (!client.ip ~ purgers) {return(synth(405,”Method not allowed”));}return(hash);}if (req.http.X-Forward-For) { # 为发往后端主机的请求添加X-Forward-For首部；set req.http.X-Forward-For = req.http.X-Forward-For + “,” + client.ip;} else {set req.http.X-Forward-For = client.ip;}if (req.http.host ~ “(?i)^()?lnmmp.com$”) { # 根据不同的访问域名，分发至不同的后端主机组；set req.http.host = “www.lnmmp.com”;set req.backend_hint = web_cluster.backend();} elsif (req.http.host ~ “(?i)^images.lnmmp.com$”) {set req.backend_hint = img_cluster.backend();}return(hash);}sub vcl_hit {if (req.method == “PURGE”) { # PURGE请求的处理；purge;return(synth(200,”Purged”));}}sub vcl_miss {if (req.method == “PURGE”) { # PURGE请求的处理；purge;return(synth(404,”Not in cache”));}}sub vcl_pass {if (req.method == “PURGE”) { # PURGE请求的处理；return(synth(502,”PURGE on a passed object”));}}sub vcl_backend_response {# 自定义缓存文件的缓存时长，即TTL值；if (bereq.url ~ “\.(jpg|jpeg|gif|png)$”) {set beresp.ttl = 7200s;}if (bereq.url ~ “\.(html|css|js)$”) {set beresp.ttl = 1200s;}if (beresp.http.Set-Cookie) { # 定义带Set-Cookie首部的后端响应不缓存，直接返回给客户端；return(deliver);}}sub vcl_deliver {if (obj.hits > 0) { # 为响应添加X-Cache首部，显示缓存是否命中；set resp.http.X-Cache = “HIT from ” + server.ip;} else {set resp.http.X-Cache = “MISS”;}}
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