面试的时候如果聊到缓存，肯定会聊到redis，因为它现在是缓存事实上的标准。
早些年一些互联网公司会用到memcached作为缓存，它是多线程的，用c语言开发，不过现在基本很少了，有兴趣的同学可以学习下它的源码。
那么聊redis，个问题就是它的工作原理，redis重要的工作原理，就是它的线程模型。
redis是单线程、nio、异步的线程模型，你要是这个都不知道，工作中用到redis，出了问题都不知道如何入手排查。
学习redis线程模型之前，必须了解socket网络编程相关知识，如果你不懂socket，肯定搞不懂redis原理。
redis使用文件事件处理器（file event handler)处理所有事件，这个文件事件处理器是单线程的，所以redis才叫单线程的。
文件事件处理器采用IO多路复用器，同时监听多个socket，将产生事件的socket放入内存队列中，事件分发器根据socket上的事件类型，选择对应的事件处理器进行处理。
图1 redis 文件事件处理器过程
文件事件处理器的结构包含 4 个部分：
客户端socket IO 多路复用器 文件事件分派器 事件处理器（连接应答处理器、命令请求处理器、命令回复处理器）
连接应答处理器
在redis server启动的时候，会设置客户端建立连接时的连接应答处理器acceptTcpHandler，并和服务器监听套接字的AE_READABLE 事件关联起来，具体代码在server.c的initServer方法中：
在networking.c 文件中acceptTcpHandler方法实现中，redis的连接应答处理器，用于对连接服务器进行监听，对套接字的客户端进行应答相应。
void acceptTcpHandler(aeEventLoop *el, int fd, void *privdata, int mask) {int cport, cfd, max = MAX_ACCEPTS_PER_CALL;char cip[NET_IP_STR_LEN]; UNUSED(el); UNUSED(mask); UNUSED(privdata); //循环处理连接应答while(max--) { cfd = anetTcpAccept(server.neterr, fd, cip, sizeof(cip), &cport);if (cfd == ANET_ERR) {if (errno != EWOULDBLOCK) serverLog(LL_WARNING,"Accepting client connection: %s", server.neterr);return; } serverLog(LL_VERBOSE,"Accepted %s:%d", cip, cport); acceptCommonHandler(connCreateAcceptedSocket(cfd),,cip); }}
连接应答处理器
void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) { client *c = (client*) privdata; int nread, readlen; size_t qblen;UNUSED(el);UNUSED(mask);
readlen = PROTO_IOBUF_LEN;/* If this is a multi bulk request, and we are processing a bulk reply * that is large enough, try to maximize the probability that the query * buffer contains exactly the SDS string representing the object, even * at the risk of requiring more read(2) calls. This way the function * processMultiBulkBuffer() can avoid copying buffers to create the * Redis Object representing the argument. */if (c->reqtype == PROTO_REQ_MULTIBULK && c->multibulklen && c->bulklen != -1 && c->bulklen >= PROTO_MBULK_BIG_ARG) { ssize_t remaining = (size_t)(c->bulklen+2)-sdslen(c->querybuf);
/* Note that the 'remaining' variable may be zero in some edge case, * for example once we resume a blocked client after CLIENT PAUSE. */if (remaining > && remaining < readlen) readlen = remaining; }
qblen = sdslen(c->querybuf);if (c->querybuf_peak < qblen) c->querybuf_peak = qblen;c->querybuf = sdsMakeRoomFor(c->querybuf, readlen); nread = read(fd, c->querybuf+qblen, readlen);if (nread == -1) {if (errno == EAGAIN) {return; } else { serverLog(LL_VERBOSE, "Reading from client: %s",strerror(errno)); freeClient(c);return; } } else if (nread == ) { serverLog(LL_VERBOSE, "Client closed connection"); freeClient(c);return; } else if (c->flags & CLIENT_MASTER) {/* Append the query buffer to the pending (not applied) buffer * of the master. We'll use this buffer later in order to have a * copy of the string applied by the last command executed. */c->pending_querybuf = sdscatlen(c->pending_querybuf,c->querybuf+qblen,nread); }
sdsIncrLen(c->querybuf,nread);c->lastinteraction = server.unixtime;if (c->flags & CLIENT_MASTER) c->read_reploff += nread; server.stat_net_input_bytes += nread;if (sdslen(c->querybuf) > server.client_max_querybuf_len) { sds ci = catClientInfoString(sdsempty(),c), bytes = sdsempty();
bytes = sdscatrepr(bytes,c->querybuf,64); serverLog(LL_WARNING,"Closing client that reached max query buffer length: %s (qbuf initial bytes: %s)", ci, bytes); sdsfree(ci); sdsfree(bytes); freeClient(c);return; }
/* Time to process the buffer. If the client is a master we need to * compute the difference between the applied offset before and after * processing the buffer, to understand how much of the replication stream * was actually applied to the master state: this quantity, and its * corresponding part of the replication stream, will be propagated to * the sub-slaves and to the replication backlog. */ processInputBufferAndReplicate(c);}
void processInputBufferAndReplicate(client *c) {if (!(c->flags & CLIENT_MASTER)) { processInputBuffer(c); } else { size_t prev_offset = c->reploff; processInputBuffer(c); size_t applied = c->reploff - prev_offset;if (applied) { replicationFeedSlavesFromMasterStream(server.slaves,c->pending_querybuf, applied); sdsrange(c->pending_querybuf,applied,-1); } }}命令回复处理器
以上文章来源于南山的架构笔记 ，作者南山
