协议
网络协议是通信计算机双方必须共同遵从的一组约定。如怎么样建立连接、怎么样互相识别等。只有遵守这个约定，计算机之间才能相互通信交流。
为了使数据在网络上从源到达目的，网络通信的参与方必须遵循相同的规则，这套规则称为协议（protocol），它最终体现为在网络上传输的数据包的格式。
通俗的说，就是大家统一遵守的特定规则。
无论是4层模型、还是7层模型，在每一层都是为了完成一种功能。为了实现这些功能，就需要大家都遵守共同的规则。
而大家都遵守的规则，就叫做"协议"（protocol）。
协议有很多特点:
- 协议中的每个人都必须了解协议，并且预先知道所要完成的所有的步骤。
- 协议中的每个人都必须同意并遵循它。
- 协议必须是清楚的，每一步必须明确定义，并且不会引起误解。
Redis协议
节选一段来自Redis文档中协议部分的描述如下：
Redis定义了一套属于自己的规则，比如开头第一个字节不同的值代表不同的含义。
Spring中的Redis协议类
Protocol 是Spring Boot Redis中关于协议的一个类。
public final class Protocol {
private static final String ASK_RESPONSE = "ASK";
private static final String MOVED_RESPONSE = "MOVED";
private static final String CLUSTERDOWN_RESPONSE = "CLUSTERDOWN";
private static final String BUSY_RESPONSE = "BUSY";
private static final String NOSCRIPT_RESPONSE = "NOSCRIPT";
public static final String DEFAULT_HOST = "localhost";
public static final int DEFAULT_PORT = 6379;
public static final int DEFAULT_SENTINEL_PORT = 26379;
public static final int DEFAULT_TIMEOUT = 2000;
public static final int DEFAULT_DATABASE = 0;
public static final String CHARSET = "UTF-8";
public static final byte DOLLAR_BYTE = '$';
public static final byte ASTERISK_BYTE = '*';
public static final byte PLUS_BYTE = '+';
public static final byte MINUS_BYTE = '-';
public static final byte COLON_BYTE = ':';
public static final String SENTINEL_MASTERS = "masters";
public static final String SENTINEL_GET_MASTER_ADDR_BY_NAME = "get-master-addr-by-name";
public static final String SENTINEL_RESET = "reset";
public static final String SENTINEL_SLAVES = "slaves";
public static final String SENTINEL_FAILOVER = "failover";
public static final String SENTINEL_MONITOR = "monitor";
public static final String SENTINEL_REMOVE = "remove";
public static final String SENTINEL_SET = "set";
public static final String CLUSTER_NODES = "nodes";
public static final String CLUSTER_MEET = "meet";
public static final String CLUSTER_RESET = "reset";
public static final String CLUSTER_ADDSLOTS = "addslots";
public static final String CLUSTER_DELSLOTS = "delslots";
public static final String CLUSTER_INFO = "info";
public static final String CLUSTER_GETKEYSINSLOT = "getkeysinslot";
public static final String CLUSTER_SETSLOT = "setslot";
public static final String CLUSTER_SETSLOT_NODE = "node";
public static final String CLUSTER_SETSLOT_MIGRATING = "migrating";
public static final String CLUSTER_SETSLOT_IMPORTING = "importing";
public static final String CLUSTER_SETSLOT_STABLE = "stable";
public static final String CLUSTER_FORGET = "forget";
public static final String CLUSTER_FLUSHSLOT = "flushslots";
public static final String CLUSTER_KEYSLOT = "keyslot";
public static final String CLUSTER_COUNTKEYINSLOT = "countkeysinslot";
public static final String CLUSTER_SAVECONFIG = "saveconfig";
public static final String CLUSTER_REPLICATE = "replicate";
public static final String CLUSTER_SLAVES = "slaves";
public static final String CLUSTER_FAILOVER = "failover";
public static final String CLUSTER_SLOTS = "slots";
public static final String PUBSUB_CHANNELS = "channels";
public static final String PUBSUB_NUMSUB = "numsub";
public static final String PUBSUB_NUM_PAT = "numpat";
public static final byte[] BYTES_TRUE = toByteArray(1);
public static final byte[] BYTES_FALSE = toByteArray(0);
public static final byte[] POSITIVE_INFINITY_BYTES = "+inf".getBytes();
public static final byte[] NEGATIVE_INFINITY_BYTES = "-inf".getBytes();
}
自定义协议
我们日常最常用的是HTTP协议，而很多分布式框架为了实现高性能，也会自定义自己的协议，比如Dubbo中就定义了自己的协议用于服务之间的交互。
参见 Dubbo 协议详解
而我们日常的开发中，也经常使用一些约定，比如API返回结果统一化，这也是一种协议的体现。
{
"code": 200,
"message": "success",
"data": {}
}
