eos中客户端跟区块链的通信采用基于http-json作为传输的RPC（Remote Procedure Call ），下面简述一下客户端通过RPC调用callA这个过程：
1）client:callA(arg1, arg2...) // client stub 存放server的地址消息，再将client的callA请求参数序列化打包成json数据格式然后通过http发送给server
2）server:callA(arg1, arg2...) // server stub 接收client发送过来的json格式参数，然后将参数反序列化解包成二进制格式，并调用本地的方法callA最终再将结果序列化成json数据格式通过http返回给client
下面以push_transaction为例：
cleos --print-request --print-response -v push action eosio.token transfer '[ "eosio", "hellohello22", "100.0000 SYS", "m" ]' -p eosio@active
客户端发出的http请求： post参数为json数据格式
POST /v1/chain/push_transaction HTTP/1.0
Host: 127.0.0.1:8888
content-length: 380
Accept: */*
Connection: close
{
"signatures": [
"SIG_K1_KXikhe2K5KSbuvwc38EahnxwzGA1gFcsy6AoN9tdThxbdxob3kVYQoUZGrsKC7PMraEjEBKKaFfUtoBmoEHvUntF2bDxzh"
],
"compression": "none",
"packed_context_free_data": "",
"packed_trx": "5023d05b2c3d5d2c778e000000000100a6823403ea3055000000572d3ccdcd010000000000ea305500000000a8ed3232220000000000ea305520048d51351aa36a40420f00000000000453595300000000016d00"
}
客户端收到的响应：返回的响应为json格式
{
"transaction_id": "c6dece9a8c2abf83d5b83e8c9c8f74179ab35c7bfeedcc0e41b1bc90681ea331",
"processed": {
"id": "c6dece9a8c2abf83d5b83e8c9c8f74179ab35c7bfeedcc0e41b1bc90681ea331",
"receipt": {
"status": "executed",
"cpu_usage_us": 2889,
"net_usage_words": 16
},
"elapsed": 76187,
"net_usage": 128,
"scheduled": false,
"action_traces": [{
"receipt": {
"receiver": "eosio.token",
"act_digest": "ff3f09bbc56a0c9a7a06871c2f66c723c8befa90b837e22985ab65bfc0bf7032",
"global_sequence": 278021,
"recv_sequence": 51,
"auth_sequence": [[
"eosio",
277915
]
],
"code_sequence": 1,
"abi_sequence": 1
},
"act": {
"account": "eosio.token",
"name": "transfer",
"authorization": [{
"actor": "eosio",
"permission": "active"
}
],
"data": {
"from": "eosio",
"to": "hellohello22",
"quantity": "100.0000 SYS",
"memo": "m"
},
...
}
}
- url handler的注册
区块链中所有需要提供http api的插件都依赖于http_plugin，http_plugin实现了一个简单http服务器（websocketpp），要提供一个url接口只需要调用http_plugin中的add_handler函数添加一个处理该url的handler
// 在handler 中需要调用的回调函数
using url_response_callback = std::function<void(int,string)>;
// 相应url的handler，http_plugin会调用
using url_handler = std::function<void(string,string,url_response_callback)>;
void add_handler(const string& url, const url_handler&);
下面以chain_api_plugin.cpp中的push_transaction handler为例
void chain_api_plugin::plugin_startup() {
ilog( "starting chain_api_plugin" );
my.reset(new chain_api_plugin_impl(app().get_plugin<chain_plugin>().chain()));
auto ro_api = app().get_plugin<chain_plugin>().get_read_only_api();
auto rw_api = app().get_plugin<chain_plugin>().get_read_write_api();
app().get_plugin<http_plugin>().add_api({
...
CHAIN_RW_CALL_ASYNC(push_transaction, chain_apis::read_write::push_transaction_results, 202),
...
});
}
push_transaction handler对应的uri为/v1/chain/push_transaction，下面push_transaction handler代码为上述CHAIN_RW_CALL_ASYNC宏展开
[this, rw_api](string, string body, url_response_callback cb) mutable {
if (body.empty()) body = "{}";
// 调用本地RPC函数push_transaction
rw_api.push_transaction(fc::json::from_string(body).as<chain_apis::read_write::push_transaction_params>(),
[cb, body](const fc::static_variant<fc::exception_ptr, chain_apis::read_write::push_transaction_results>& result){
if (result.contains<fc::exception_ptr>()) {
try {
// 如果处理结果中包含异常则直接抛出异常
result.get<fc::exception_ptr>()->dynamic_rethrow_exception();
} catch (...) {
http_plugin::handle_exception("chain", "push_transaction", body, cb);
}
} else {
// 没有异常的情况下相应码为202，第二个参数为返回的序列化结果
cb(202, result.visit(async_result_visitor()));
}
}
);
}
上述调用本地RPC函数push_transaction，第一个参数把接收到的json数据格式反序列化为push_transaction_params参数，第二个参数为一个匿名函数，push_transaction处理完之后会调用该匿名函数
- http请求的处理
http_plugin处理http请求，找到相应uri对于的handler然后调用，最终handler处理完成之后会调用url_response_callback返回相应码跟body
template<class T>
void handle_http_request(typename websocketpp::server<detail::asio_with_stub_log<T>>::connection_ptr con) {
try {
...
auto body = con->get_request_body();
auto resource = con->get_uri()->get_resource();
// 是否有该uri的handler
auto handler_itr = url_handlers.find( resource );
if( handler_itr != url_handlers.end()) {
con->defer_http_response();
// 调用该uri的handler，这个handler就是我们上面通过add_handler添加的handler，第三个参数url_response_callback最终处理完成之后调用（响应码，body）
handler_itr->second( resource, body, [con]( auto code, auto&& body ) {
con->set_body( std::move( body ));
con->set_status( websocketpp::http::status_code::value( code ));
con->send_http_response();
} );
}
...
} catch( ... ) {
handle_exception<T>( con );
}
}
关于eos中的参数的序列化与反序列化，eos fc中实现了反射机制，上层应用只需要为某一类型定义一个FC_REFLECT/FC_REFLECT_DERIVED就能实现该类型的序列化跟反序列化
