概述 proxy模块是通过ups机制实现了反向代理功能的。该模块非常复杂。 在这里不会说ups机制，除非引入的非常深入。其余的均会带过，了解ups机制请看上篇。 proxy模块通过proxy_cache[_xxx]指令控制proxy缓存。 指令： proxy_cache 是否开启缓存，可以为变量，如果是变量则需要配置proxy_cache_path。 proxy_cache_path 配置缓存的路径，和缓存的策略。 proxy_cache_convert_head 是否把head请求转成get请求缓存。 proxy_cache_bypass 有缓存但是不从缓存获取。bypass。 代码 proxy_cache对应的处理函数ngx_http_proxy_cache。 其作用是是否开启缓存，可以配置变量和常量，如果是变量则该变量的值如果等于proxy_cache_path指令的keys_zone的name，则会走缓存。 如果是常量，则所有的请求都会走缓存。 static char * ngx_http_proxy_cache(ngx_conf_t *cf, ngx_command_t *cmd, void *conf) { ngx_http_proxy_loc_conf_t *plcf = conf; ngx_str_t *value; ngx_http_complex_value_t cv; ngx_http_compile_complex_value_t ccv; value = cf->args->elts; if (plcf->upstream.cache != NGX_CONF_UNSET) { return "is duplicate"; } if (ngx_strcmp(value[1].data, "off") == 0) { plcf->upstream.cache = 0; return NGX_CONF_OK; } if (plcf->upstream.store > 0) { return "is incompatible with \"proxy_store\""; } plcf->upstream.cache = 1; ngx_memzero(&ccv, sizeof(ngx_http_compile_complex_value_t)); ccv.cf = cf; ccv.value = &value[1]; ccv.complex_value = &cv; if (ngx_http_compile_complex_value(&ccv) != NGX_OK) { return NGX_CONF_ERROR; } if (cv.lengths != NULL) { // zone 有变量 plcf->upstream.cache_value = ngx_palloc(cf->pool, sizeof(ngx_http_complex_value_t)); if (plcf->upstream.cache_value == NULL) { return NGX_CONF_ERROR; } *plcf->upstream.cache_value = cv; return NGX_CONF_OK; } // zone 有变量 plcf->upstream.cache_zone = ngx_shared_memory_add(cf, &value[1], 0, &ngx_http_proxy_module); if (plcf->upstream.cache_zone == NULL) { return NGX_CONF_ERROR; } return NGX_CONF_OK; } proxy_cache_path 的解析， struct ngx_http_file_cache_s { ngx_http_file_cache_sh_t *sh; ngx_slab_pool_t *shpool; ngx_path_t *path; off_t max_size; size_t bsize; time_t inactive; time_t fail_time; ngx_uint_t files; ngx_uint_t loader_files; ngx_msec_t last; ngx_msec_t loader_sleep; ngx_msec_t loader_threshold; ngx_uint_t manager_files; ngx_msec_t manager_sleep; ngx_msec_t manager_threshold; ngx_shm_zone_t *shm_zone; ngx_uint_t use_temp_path; /* unsigned use_temp_path:1 */ }; char * ngx_http_file_cache_set_slot(ngx_conf_t *cf, ngx_command_t *cmd, void *conf) { char *confp = conf; off_t max_size; u_char *last, *p; time_t inactive; ssize_t size; ngx_str_t s, name, *value; ngx_int_t loader_files, manager_files; ngx_msec_t loader_sleep, manager_sleep, loader_threshold, manager_threshold; ngx_uint_t i, n, use_temp_path; ngx_array_t *caches; ngx_http_file_cache_t *cache, **ce; cache = ngx_pcalloc(cf->pool, sizeof(ngx_http_file_cache_t)); if (cache == NULL) { return NGX_CONF_ERROR; } cache->path = ngx_pcalloc(cf->pool, sizeof(ngx_path_t)); if (cache->path == NULL) { return NGX_CONF_ERROR; } use_temp_path = 1; inactive = 600; loader_files = 100; loader_sleep = 50; loader_threshold = 200; manager_files = 100; manager_sleep = 50; manager_threshold = 200; name.len = 0; size = 0; max_size = NGX_MAX_OFF_T_VALUE; value = cf->args->elts; cache->path->name = value[1]; if (cache->path->name.data[cache->path->name.len - 1] == '/') { cache->path->name.len--; } if (ngx_conf_full_name(cf->cycle, &cache->path->name, 0) != NGX_OK) { return NGX_CONF_ERROR; } for (i = 2; i < cf->args->nelts; i++) { if (ngx_strncmp(value[i].data, "levels=", 7) == 0) { p = value[i].data + 7; last = value[i].data + value[i].len; for (n = 0; n < NGX_MAX_PATH_LEVEL && p < last; n++) { if (*p > '0' && *p < '3') { // level[3] 最多3级子目录，每一级子目录长度为1，2。 // proxy_cache_path /data/nginx/cache levels=1:2 // 最后保存的目录是：/data/nginx/cache/c/29/ cache->path->level[n] = *p++ - '0'; cache->path->len += cache->path->level[n] + 1; if (p == last) { break; } if (*p++ == ':' && n < NGX_MAX_PATH_LEVEL - 1 && p < last) { continue; } goto invalid_levels; } goto invalid_levels; } // level 小于3级，每级加上分隔符长度为3 if (cache->path->len < 10 + NGX_MAX_PATH_LEVEL) { continue; } invalid_levels: ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid \"levels\" \"%V\"", &value[i]); return NGX_CONF_ERROR; } if (ngx_strncmp(value[i].data, "use_temp_path=", 14) == 0) { if (ngx_strcmp(&value[i].data[14], "on") == 0) { use_temp_path = 1; } else if (ngx_strcmp(&value[i].data[14], "off") == 0) { use_temp_path = 0; } else { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid use_temp_path value \"%V\", " "it must be \"on\" or \"off\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "keys_zone=", 10) == 0) { name.data = value[i].data + 10; p = (u_char *) ngx_strchr(name.data, ':'); if (p) { name.len = p - name.data; p++; s.len = value[i].data + value[i].len - p; s.data = p; size = ngx_parse_size(&s); if (size > 8191) { continue; } } ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid keys zone size \"%V\"", &value[i]); return NGX_CONF_ERROR; } if (ngx_strncmp(value[i].data, "inactive=", 9) == 0) { s.len = value[i].len - 9; s.data = value[i].data + 9; inactive = ngx_parse_time(&s, 1); if (inactive == (time_t) NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid inactive value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "max_size=", 9) == 0) { s.len = value[i].len - 9; s.data = value[i].data + 9; max_size = ngx_parse_offset(&s); if (max_size < 0) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid max_size value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "loader_files=", 13) == 0) { loader_files = ngx_atoi(value[i].data + 13, value[i].len - 13); if (loader_files == NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid loader_files value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "loader_sleep=", 13) == 0) { s.len = value[i].len - 13; s.data = value[i].data + 13; loader_sleep = ngx_parse_time(&s, 0); if (loader_sleep == (ngx_msec_t) NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid loader_sleep value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "loader_threshold=", 17) == 0) { s.len = value[i].len - 17; s.data = value[i].data + 17; loader_threshold = ngx_parse_time(&s, 0); if (loader_threshold == (ngx_msec_t) NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid loader_threshold value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "manager_files=", 14) == 0) { manager_files = ngx_atoi(value[i].data + 14, value[i].len - 14); if (manager_files == NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid manager_files value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "manager_sleep=", 14) == 0) { s.len = value[i].len - 14; s.data = value[i].data + 14; manager_sleep = ngx_parse_time(&s, 0); if (manager_sleep == (ngx_msec_t) NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid manager_sleep value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } if (ngx_strncmp(value[i].data, "manager_threshold=", 18) == 0) { s.len = value[i].len - 18; s.data = value[i].data + 18; manager_threshold = ngx_parse_time(&s, 0); if (manager_threshold == (ngx_msec_t) NGX_ERROR) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid manager_threshold value \"%V\"", &value[i]); return NGX_CONF_ERROR; } continue; } ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid parameter \"%V\"", &value[i]); return NGX_CONF_ERROR; } if (name.len == 0 || size == 0) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "\"%V\" must have \"keys_zone\" parameter", &cmd->name); return NGX_CONF_ERROR; } cache->path->manager = ngx_http_file_cache_manager; cache->path->loader = ngx_http_file_cache_loader; cache->path->data = cache; cache->path->conf_file = cf->conf_file->file.name.data; cache->path->line = cf->conf_file->line; cache->loader_files = loader_files; cache->loader_sleep = loader_sleep; cache->loader_threshold = loader_threshold; cache->manager_files = manager_files; cache->manager_sleep = manager_sleep; cache->manager_threshold = manager_threshold; // 添加到cycle这个结构体的path数组中，由nginx框架统一管理 if (ngx_add_path(cf, &cache->path) != NGX_OK) { return NGX_CONF_ERROR; } cache->shm_zone = ngx_shared_memory_add(cf, &name, size, cmd->post); if (cache->shm_zone == NULL) { return NGX_CONF_ERROR; } if (cache->shm_zone->data) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "duplicate zone \"%V\"", &name); return NGX_CONF_ERROR; } cache->shm_zone->init = ngx_http_file_cache_init; cache->shm_zone->data = cache; cache->use_temp_path = use_temp_path; cache->inactive = inactive; cache->max_size = max_size; caches = (ngx_array_t *) (confp + cmd->offset); ce = ngx_array_push(caches); if (ce == NULL) { return NGX_CONF_ERROR; } *ce = cache; return NGX_CONF_OK; } proxy_pass 指令解析ngx_http_proxy_pass函数。 static char * ngx_http_proxy_pass(ngx_conf_t *cf, ngx_command_t *cmd, void *conf) { ngx_http_proxy_loc_conf_t *plcf = conf; size_t add; u_short port; ngx_str_t *value, *url; ngx_url_t u; ngx_uint_t n; ngx_http_core_loc_conf_t *clcf; ngx_http_script_compile_t sc; if (plcf->upstream.upstream || plcf->proxy_lengths) { return "is duplicate"; } clcf = ngx_http_conf_get_module_loc_conf(cf, ngx_http_core_module); clcf->handler = ngx_http_proxy_handler; if (clcf->name.data[clcf->name.len - 1] == '/') { clcf->auto_redirect = 1; } value = cf->args->elts; url = &value[1]; n = ngx_http_script_variables_count(url); if (n) { ngx_memzero(&sc, sizeof(ngx_http_script_compile_t)); sc.cf = cf; sc.source = url; sc.lengths = &plcf->proxy_lengths; sc.values = &plcf->proxy_values; sc.variables = n; sc.complete_lengths = 1; sc.complete_values = 1; if (ngx_http_script_compile(&sc) != NGX_OK) { return NGX_CONF_ERROR; } #if (NGX_HTTP_SSL) plcf->ssl = 1; #endif return NGX_CONF_OK; } if (ngx_strncasecmp(url->data, (u_char *) "http://", 7) == 0) { add = 7; port = 80; } else if (ngx_strncasecmp(url->data, (u_char *) "https://", 8) == 0) { #if (NGX_HTTP_SSL) plcf->ssl = 1; add = 8; port = 443; #else ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "https protocol requires SSL support"); return NGX_CONF_ERROR; #endif } else { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid URL prefix"); return NGX_CONF_ERROR; } ngx_memzero(&u, sizeof(ngx_url_t)); u.url.len = url->len - add; u.url.data = url->data + add; u.default_port = port; u.uri_part = 1; u.no_resolve = 1; plcf->upstream.upstream = ngx_http_upstream_add(cf, &u, 0); if (plcf->upstream.upstream == NULL) { return NGX_CONF_ERROR; } plcf->vars.schema.len = add; plcf->vars.schema.data = url->data; plcf->vars.key_start = plcf->vars.schema; ngx_http_proxy_set_vars(&u, &plcf->vars); plcf->location = clcf->name; if (clcf->named #if (NGX_PCRE) || clcf->regex #endif || clcf->noname) { if (plcf->vars.uri.len) { ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "\"proxy_pass\" cannot have URI part in " "location given by regular expression, " "or inside named location, " "or inside \"if\" statement, " "or inside \"limit_except\" block"); return NGX_CONF_ERROR; } plcf->location.len = 0; } plcf->url = *url; return NGX_CONF_OK; } 执行请求的时候会调用ngx_http_proxy_handler函数。 struct ngx_http_upstream_s { ngx_http_upstream_handler_pt read_event_handler; ngx_http_upstream_handler_pt write_event_handler; ngx_peer_connection_t peer; ngx_event_pipe_t *pipe; ngx_chain_t *request_bufs; ngx_output_chain_ctx_t output; ngx_chain_writer_ctx_t writer; ngx_http_upstream_conf_t *conf; ngx_http_upstream_srv_conf_t *upstream; #if (NGX_HTTP_CACHE) ngx_array_t *caches; #endif ngx_http_upstream_headers_in_t headers_in; ngx_http_upstream_resolved_t *resolved; ngx_buf_t from_client; ngx_buf_t buffer; off_t length; ngx_chain_t *out_bufs; ngx_chain_t *busy_bufs; ngx_chain_t *free_bufs; ngx_int_t (*input_filter_init)(void *data); ngx_int_t (*input_filter)(void *data, ssize_t bytes); void *input_filter_ctx; #if (NGX_HTTP_CACHE) // proxy 模块配置为ngx_http_proxy_create_key ngx_int_t (*create_key)(ngx_http_request_t *r); #endif ngx_int_t (*create_request)(ngx_http_request_t *r); ngx_int_t (*reinit_request)(ngx_http_request_t *r); ngx_int_t (*process_header)(ngx_http_request_t *r); void (*abort_request)(ngx_http_request_t *r); void (*finalize_request)(ngx_http_request_t *r, ngx_int_t rc); ngx_int_t (*rewrite_redirect)(ngx_http_request_t *r, ngx_table_elt_t *h, size_t prefix); ngx_int_t (*rewrite_cookie)(ngx_http_request_t *r, ngx_table_elt_t *h); ngx_msec_t timeout; ngx_http_upstream_state_t *state; ngx_str_t method; ngx_str_t schema; ngx_str_t uri; #if (NGX_HTTP_SSL || NGX_COMPAT) ngx_str_t ssl_name; #endif ngx_http_cleanup_pt *cleanup; unsigned store:1; unsigned cacheable:1; unsigned accel:1; unsigned ssl:1; #if (NGX_HTTP_CACHE) unsigned cache_status:3; #endif unsigned buffering:1; unsigned keepalive:1; unsigned upgrade:1; unsigned request_sent:1; unsigned request_body_sent:1; unsigned header_sent:1; }; struct ngx_peer_connection_s { ngx_connection_t *connection; struct sockaddr *sockaddr; socklen_t socklen; ngx_str_t *name; ngx_uint_t tries; ngx_msec_t start_time; ngx_event_get_peer_pt get; ngx_event_free_peer_pt free; ngx_event_notify_peer_pt notify; void *data; #if (NGX_SSL || NGX_COMPAT) ngx_event_set_peer_session_pt set_session; ngx_event_save_peer_session_pt save_session; #endif ngx_addr_t *local; int type; int rcvbuf; // 赋值给原请求的log ngx_log_t *log; unsigned cached:1; unsigned transparent:1; /* ngx_connection_log_error_e */ unsigned log_error:2; NGX_COMPAT_BEGIN(2) NGX_COMPAT_END }; struct ngx_event_pipe_s { ngx_connection_t *upstream; ngx_connection_t *downstream; ngx_chain_t *free_raw_bufs; ngx_chain_t *in; ngx_chain_t **last_in; ngx_chain_t *writing; ngx_chain_t *out; ngx_chain_t *free; ngx_chain_t *busy; /* * the input filter i.e. that moves HTTP/1.1 chunks * from the raw bufs to an incoming chain */ ngx_event_pipe_input_filter_pt input_filter; void *input_ctx; ngx_event_pipe_output_filter_pt output_filter; void *output_ctx; #if (NGX_THREADS || NGX_COMPAT) ngx_int_t (*thread_handler)(ngx_thread_task_t *task, ngx_file_t *file); void *thread_ctx; ngx_thread_task_t *thread_task; #endif unsigned read:1; unsigned cacheable:1; unsigned single_buf:1; unsigned free_bufs:1; unsigned upstream_done:1; unsigned upstream_error:1; unsigned upstream_eof:1; unsigned upstream_blocked:1; unsigned downstream_done:1; unsigned downstream_error:1; unsigned cyclic_temp_file:1; unsigned aio:1; ngx_int_t allocated; ngx_bufs_t bufs; ngx_buf_tag_t tag; ssize_t busy_size; off_t read_length; off_t length; off_t max_temp_file_size; ssize_t temp_file_write_size; ngx_msec_t read_timeout; ngx_msec_t send_timeout; ssize_t send_lowat; ngx_pool_t *pool; ngx_log_t *log; ngx_chain_t *preread_bufs; size_t preread_size; ngx_buf_t *buf_to_file; size_t limit_rate; time_t start_sec; ngx_temp_file_t *temp_file; /* STUB */ int num; }; typedef struct { ngx_list_t headers; ngx_uint_t status_n; ngx_str_t status_line; ngx_table_elt_t *status; ngx_table_elt_t *date; ngx_table_elt_t *server; ngx_table_elt_t *connection; ngx_table_elt_t *expires; ngx_table_elt_t *etag; ngx_table_elt_t *x_accel_expires; ngx_table_elt_t *x_accel_redirect; ngx_table_elt_t *x_accel_limit_rate; ngx_table_elt_t *content_type; ngx_table_elt_t *content_length; ngx_table_elt_t *last_modified; ngx_table_elt_t *location; ngx_table_elt_t *accept_ranges; ngx_table_elt_t *www_authenticate; ngx_table_elt_t *transfer_encoding; ngx_table_elt_t *vary; #if (NGX_HTTP_GZIP) ngx_table_elt_t *content_encoding; #endif ngx_array_t cache_control; ngx_array_t cookies; // body 长度 off_t content_length_n; // 最后修改时间（http 缓存使用） time_t last_modified_time; unsigned connection_close:1; unsigned chunked:1; } ngx_http_upstream_headers_in_t; 缓存 static ngx_int_t ngx_http_upstream_cache(ngx_http_request_t *r, ngx_http_upstream_t *u) { ngx_int_t rc; ngx_http_cache_t *c; ngx_http_file_cache_t *cache; c = r->cache; if (c == NULL) { // 是否是定义的缓存的方法 if (!(r->method & u->conf->cache_methods)) { return NGX_DECLINED; } // 请求是否要缓存，如果返回NGX_OK 则cache返回要缓存的目录和参数 rc = ngx_http_upstream_cache_get(r, u, &cache); if (rc != NGX_OK) { return rc; } if (r->method == NGX_HTTP_HEAD && u->conf->cache_convert_head) { u->method = ngx_http_core_get_method; } if (ngx_http_file_cache_new(r) != NGX_OK) { return NGX_ERROR; } if (u->create_key(r) != NGX_OK) { return NGX_ERROR; } /* TODO: add keys */ ngx_http_file_cache_create_key(r); if (r->cache->header_start + 256 >= u->conf->buffer_size) { ngx_log_error(NGX_LOG_ERR, r->connection->log, 0, "%V_buffer_size %uz is not enough for cache key, " "it should be increased to at least %uz", &u->conf->module, u->conf->buffer_size, ngx_align(r->cache->header_start + 256, 1024)); r->cache = NULL; return NGX_DECLINED; } u->cacheable = 1; c = r->cache; c->body_start = u->conf->buffer_size; c->min_uses = u->conf->cache_min_uses; c->file_cache = cache; switch (ngx_http_test_predicates(r, u->conf->cache_bypass)) { case NGX_ERROR: return NGX_ERROR; case NGX_DECLINED: u->cache_status = NGX_HTTP_CACHE_BYPASS; return NGX_DECLINED; default: /* NGX_OK */ break; } c->lock = u->conf->cache_lock; c->lock_timeout = u->conf->cache_lock_timeout; c->lock_age = u->conf->cache_lock_age; u->cache_status = NGX_HTTP_CACHE_MISS; } // 打开缓存文件 rc = ngx_http_file_cache_open(r); ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0, "http upstream cache: %i", rc); switch (rc) { case NGX_HTTP_CACHE_STALE: if (((u->conf->cache_use_stale & NGX_HTTP_UPSTREAM_FT_UPDATING) || c->stale_updating) && !r->background && u->conf->cache_background_update) { r->cache->background = 1; u->cache_status = rc; rc = NGX_OK; } break; case NGX_HTTP_CACHE_UPDATING: if (((u->conf->cache_use_stale & NGX_HTTP_UPSTREAM_FT_UPDATING) || c->stale_updating) && !r->background) { u->cache_status = rc; rc = NGX_OK; } else { rc = NGX_HTTP_CACHE_STALE; } break; case NGX_OK: u->cache_status = NGX_HTTP_CACHE_HIT; } switch (rc) { case NGX_OK: return NGX_OK; case NGX_HTTP_CACHE_STALE: c->valid_sec = 0; c->updating_sec = 0; c->error_sec = 0; u->buffer.start = NULL; u->cache_status = NGX_HTTP_CACHE_EXPIRED; break; case NGX_DECLINED: if ((size_t) (u->buffer.end - u->buffer.start) < u->conf->buffer_size) { u->buffer.start = NULL; } else { u->buffer.pos = u->buffer.start + c->header_start; u->buffer.last = u->buffer.pos; } break; case NGX_HTTP_CACHE_SCARCE: u->cacheable = 0; break; case NGX_AGAIN: return NGX_BUSY; case NGX_ERROR: return NGX_ERROR; default: /* cached NGX_HTTP_BAD_GATEWAY, NGX_HTTP_GATEWAY_TIME_OUT, etc. */ u->cache_status = NGX_HTTP_CACHE_HIT; return rc; } if (ngx_http_upstream_cache_check_range(r, u) == NGX_DECLINED) { u->cacheable = 0; } r->cached = 0; return NGX_DECLINED; } 转载请注明：爱开源 » ngx_http_proxy_module 与缓存
概述
proxy模块是通过ups机制实现了反向代理功能的。该模块非常复杂。
在这里不会说ups机制，除非引入的非常深入。其余的均会带过，了解ups机制请看上篇。
proxy模块通过proxy_cache[_xxx]指令控制proxy缓存。
proxy_cache 是否开启缓存，可以为变量，如果是变量则需要配置proxy_cache_path。
proxy_cache_path 配置缓存的路径，和缓存的策略。
proxy_cache_convert_head 是否把head请求转成get请求缓存。
proxy_cache_bypass 有缓存但是不从缓存获取。bypass。
代码
其作用是是否开启缓存，可以配置变量和常量，如果是变量则该变量的值如果等于proxy_cache_path指令的keys_zone的name，则会走缓存。
如果是常量，则所有的请求都会走缓存。
执行请求的时候会调用ngx_http_proxy_handler函数。
缓存