前些天帮别人优化PHP程序，搞得灰头土脸，最后黔驴技穷开启了FastCGI Cache，算是勉强应付过去了吧。不过FastCGI Cache不支持分布式缓存，当服务器很多的时候，冗余的浪费将非常严重，此外还有数据一致性问题，所以它只是一个粗线条的解决方案。

对此类问题而言，SRCache是一个细粒度的解决方案。其工作原理大致如下：

SRCache工作原理

当问题比较简单的时候，通常SRCache和Memc模块一起搭配使用。网上能搜索到一些相关的例子，大家可以参考，这里就不赘述了。当问题比较复杂的时候，比如说缓存键的动态计算等，就不得不写一点代码了，此时Lua模块是最佳选择。

闲言碎语不多讲，表一表Nginx配置文件长啥样：

lua_shared_dict phoenix_status 100m;

lua_package_path '/path/to/phoenix/include/?.lua;/path/to/phoenix/vendor/?.lua;;';

init_by_lua_file /path/to/phoenix/config.lua;

server {
    listen 80;

    server_name foo.com;

    root /path/to/root;

    index index.html index.htm index.php;

    location / {
        try_files $uri $uri/ /index.php$is_args$args;
    }

    location ~ \.php$ {
        set $phoenix_key "";
        set $phoenix_fetch_skip 1;
        set $phoenix_store_skip 1;

        rewrite_by_lua_file /path/to/phoenix/monitor.lua;

        srcache_fetch_skip $phoenix_fetch_skip;
        srcache_store_skip $phoenix_store_skip;

        srcache_fetch GET /phoenix/content key=$phoenix_key;
        srcache_store PUT /phoenix/content key=$phoenix_key;

        add_header X-SRCache-Fetch-Status $srcache_fetch_status;
        add_header X-SRCache-Store-Status $srcache_store_status;

        try_files $uri =404;

        include fastcgi.conf;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_intercept_errors on;

        error_page 500 502 503 504 = /phoenix/failover;
    }

    location = /phoenix/content {
        internal;
        content_by_lua_file /path/to/phoenix/content.lua;
    }

    location = /phoenix/failover {
        internal;
        rewrite_by_lua_file /path/to/phoenix/failover.lua;
    }
}

Nginx启动后，会载入config.lua中的配置信息。请求到达后，缺省情况下，SRCache为关闭状态，在monitor.lua中，会对当前请求进行正则匹配，一旦匹配成功，那么就会计算出缓存键，并且把SRCache设置为开启状态，最后由content.lua完成读写。

看看「config.lua」文件的内容，它主要用来记录一些全局的配置信息：

phoenix = {}

phoenix["memcached"] = {
    default = {
        timeout = "100",
        keepalive = {idle = 10000, size = 100},
    },
    {host = "127.0.0.1", port = "11211"},
    {host = "127.0.0.1", port = "11212"},
    {host = "127.0.0.1", port = "11213"},
}

phoenix["rule"] = {
    default = {
        expire = 600,
        min_uses = 0,
        max_errors = 0,
        query = {
            ["debug"] = false,
        },
    },
    {
        regex = "^/foo/bar",
        query = {
            ["page"] = function(v)
                if v == "" or v == nil then
                    return 1
                end

                return tonumber(v) or false
            end,
            ["limit"] = true,
        },
    },
}

看看「monitor.lua」文件的内容，它主要用来计算缓存键，并开启SRCache模块：

local status = require "status"

local status = status:new(ngx.shared.phoenix_status)

local request_uri_without_args = ngx.re.sub(ngx.var.request_uri, "\\?.*", "")

table.unpack = table.unpack or unpack

for index, rule in ipairs(phoenix["rule"]) do
    if type(rule["regex"]) == "string" then
        rule["regex"] = {rule["regex"], ""}
    end

    local regex, options = table.unpack(rule["regex"])

    if ngx.re.match(request_uri_without_args, regex, options) then
        local default = phoenix["rule"]["default"]

        local expire     = rule["expire"]     or default["expire"]
        local min_uses   = rule["min_uses"]   or default["min_uses"]
        local max_errors = rule["max_errors"] or default["max_errors"]

        local key = {
            ngx.var.request_method, " ",
            ngx.var.scheme, "://",
            ngx.var.host, request_uri_without_args,
        }

        rule["query"] = rule["query"] or {}

        if default["query"] then
            for key, value in pairs(default["query"]) do
                if not rule["query"][key] then
                    rule["query"][key] = value
                end
            end
        end

        local query = {}

        local args = ngx.req.get_uri_args()

        for name, value in pairs(rule["query"]) do
            if type(value) == "function" then
                value = value(args[name])
            end

            if value == true then
                value = args[name]
            end

            if value then
                query[name] = value
            elseif args[name] then
                return
            end
        end

        query = ngx.encode_args(query)

        if query ~= "" then
            key[#key + 1] = "?"
            key[#key + 1] = query
        end

        key = table.concat(key)
        key = ngx.md5(key)

        ngx.var.phoenix_key = key

        local now = ngx.time()

        if ngx.var.arg_phoenix == true then
            ngx.var.phoenix_fetch_skip = 0
        else
            for i = 0, 1 do
                local errors = status:get_errors(index, now - i * 60)

                if errors >= max_errors then
                    ngx.var.phoenix_fetch_skip = 0
                    break
                end
            end
        end

        local uses = status:incr_uses(key, 1)

        if uses >= min_uses then
            local timestamp = status:get_timestamp(key)

            if now - timestamp >= expire then
                ngx.var.phoenix_store_skip = 0
            end
        end

        break
    end
end

看看「content.lua」文件的内容，它主要通过Resty库来读写Memcached：

local memcached = require "resty.memcached"
local status    = require "status"

local status = status:new(ngx.shared.phoenix_status)

local key = ngx.var.arg_key

local index = ngx.crc32_long(key) % #phoenix["memcached"] + 1

local config  = phoenix["memcached"][index]
local default = phoenix["memcached"]["default"]

local host      = config["host"]      or default["host"]
local port      = config["port"]      or default["port"]
local timeout   = config["timeout"]   or default["timeout"]
local keepalive = config["keepalive"] or default["keepalive"]

local memc, err = memcached:new()

if not memc then
    ngx.log(ngx.ERR, err)
    ngx.exit(ngx.HTTP_SERVICE_UNAVAILABLE)
end

if timeout then
    memc:set_timeout(timeout)
end

local ok, err = memc:connect(host, port)

if not ok then
    ngx.log(ngx.ERR, err)
    ngx.exit(ngx.HTTP_SERVICE_UNAVAILABLE)
end

local method = ngx.req.get_method()

if method == "GET" then
    local res, flags, err = memc:get(key)

    if err then
        ngx.log(ngx.ERR, err)
        ngx.exit(ngx.HTTP_SERVICE_UNAVAILABLE)
    end

    if res == nil and flags == nil and err == nil then
        ngx.exit(ngx.HTTP_NOT_FOUND)
    end

    ngx.print(res)
elseif method == "PUT" then
    local value = ngx.req.get_body_data()
    local expire = ngx.var.arg_expire or 86400

    local ok, err = memc:set(key, value, expire)

    if not ok then
        ngx.log(ngx.ERR, err)
        ngx.exit(ngx.HTTP_SERVICE_UNAVAILABLE)
    end

    status:set_timestamp(key)
else
    ngx.exit(ngx.HTTP_NOT_ALLOWED)
end

if type(keepalive) == "table" then
    if keepalive["idle"] and keepalive["size"] then
        memc:set_keepalive(keepalive["idle"], keepalive["size"])
    end
end

看看「failover.lua」文件的内容，它是为了在出错时激活容灾模式：

ngx.req.set_uri_args(ngx.var.args .. "&phoenix")
ngx.req.set_uri(ngx.var.uri, true)

此外，还有一个「status.lua」文件：

local status = {}

local get_timestamp_key = function(key)
    key = {
        "phoenix", "status", "timestamp", key,
    }

    return table.concat(key, ":")
end

local get_uses_key = function(key, timestamp)
    key = {
        "phoenix", "status", "uses", key, os.date("%Y%m%d%H%M", timestamp),
    }

    return table.concat(key, ":")
end

local get_errors_key = function(key, timestamp)
    key = {
        "phoenix", "status", "errors", key, os.date("%Y%m%d%H%M", timestamp),
    }

    return table.concat(key, ":")
end

local get = function(shared, key)
    return shared:get(key)
end

local set = function(shared, key, value, expire)
    return shared:set(key, value, expire or 86400)
end

local incr = function(shared, key, value, expire)
    value = value or 1

    local counter = shared:incr(key, value)

    if not counter then
        shared:add(key, 0, expire or 86400)
        counter = shared:incr(key, value)
    end

    return counter
end

function status:new(shared)
    return setmetatable({shared = shared}, {__index = self})
end

function status:get_timestamp(key)
    return get(self.shared, get_timestamp_key(key)) or 0
end

function status:set_timestamp(key, value, expire)
    key = get_timestamp_key(key)
    value = value or ngx.time()

    return set(self.shared, key, value, expire)
end

function status:get_uses(key, timestamp)
    timestamp = timestamp or ngx.time()
    key = get_uses_key(key, timestamp)

    return get(self.shared, key) or 0
end

function status:incr_uses(key, value, expire)
    key = get_uses_key(key, ngx.time())
    value = value or 1

    return incr(self.shared, key, value, expire)
end

function status:get_errors(key, timestamp)
    timestamp = timestamp or ngx.time()
    key = get_errors_key(key, timestamp)

    return get(self.shared, key) or 0
end

function status:incr_errors(key, value, expire)
    key = get_errors_key(key, ngx.time())
    value = value or 1

    return incr(self.shared, key, value, expire)
end

return status

最后一个问题：如何判断缓存是否生效了？试试下面的命令：

shell> curl -v "http://foo.com/test?x=123&y=abc"
< X-SRCache-Fetch-Status: HIT
< X-SRCache-Store-Status: BYPASS

目前我主要用srcache来缓存一些接口的json结果集，这些接口同时也支持jsonp，也就是客户端传递一个callback参数之类的，大家应该明白，此时如果不加区分的都缓存，那么有callback的和没有callback的调用结果就都要保存起来了，内存占用直接翻番，可实际上它们的内容大同小异，所以在实际应用时，我们应该仅仅缓存没有callback的数据，而对于有callback的请求，可以用xss-nginx-module来搞定。

关于激活SRCache前后的性能对比，视环境的不同会有所差异，不过绝对是数量级的提升，更重要的是这一切对业务层完全透明，别愣着了，快试试吧！

建议继续学习：

1. 配置Nginx＋uwsgi更方便地部署python应用    (阅读：105405)
2. 搜狐闪电邮箱的 Nginx/Postfix 使用模式    (阅读：32530)
3. 浅析http协议、cookies和session机制、浏览器缓存    (阅读：15808)
4. 分布式缓存系统 Memcached 入门    (阅读：14729)
5. 解析nginx负载均衡    (阅读：14519)
6. Nginx模块开发入门    (阅读：9882)
7. 强制刷新本地 DNS 缓存记录    (阅读：9248)
8. 检查nginx配置，重载配置以及重启的方法    (阅读：9103)
9. Cacti 添加 Nginx 监控    (阅读：8923)
10. Nginx+FastCgi+Php 的工作机制    (阅读：8859)