redis与连接

Redis处理命令

connection主要方法及与reply关系

connection只支持移动语义，不支持拷贝和赋值
recv使用ReplyUPtr，即unique_ptr<redisReply, ReplyDeleter>,其中ReplyDeleter定义如下

struct ReplyDeleter {
    void operator()(redisReply *reply) const {
        if (reply != nullptr) {
            freeReplyObject(reply);
        }
    }
};

其调用redisGetReply获取响应，当中会处理以下几种异常情况

• redisGetReply返回不是REDIS_OK，抛出异常
• 调用broken判断连接是否断了
• 在handle_error_reply标识为true,并且响应的type值为REDIS_REPLY_ERROR，抛出异常

send方法主要是调用redisAppendCommandArgv，会处理下面情况

• redisAppendCommandArgv如果失败，会抛出异常
• 调用broken()看连接是否断了

   bool broken() const noexcept {
        return !_ctx || _ctx->err != REDIS_OK;
    }

连接池及其管理

SafeConnection:包含连接池的引用，构造函数负责从连接池中得到可用的连接，析构函数中将连接还到连接池中
GuardedConnection：使用ConnectionPoolSPtr _pool指向连接池的指针

解析响应

通过模板函数来作具体类型的的解析转发

template <typename T>
inline T parse(redisReply &reply) {
    return parse(ParseTag<T>(), reply);
}

optional解析

template <typename T>
Optional<T> parse(ParseTag<Optional<T>>, redisReply &reply) {
    if (reply::is_nil(reply)) {
        // Because of a GCC bug, we cannot return {} for -std=c++17
        // Refer to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86465
#if defined REDIS_PLUS_PLUS_HAS_OPTIONAL
        return std::nullopt;
#else
        return {};
#endif
    }

    return Optional<T>(parse<T>(reply));
}

字符串解析

std::string parse(ParseTag<std::string>, redisReply &reply) {
#ifdef REDIS_PLUS_PLUS_RESP_VERSION_3
    if (!reply::is_string(reply) && !reply::is_status(reply)
            && !reply::is_verb(reply) && !reply::is_bignum(reply)) {
        throw ParseError("STRING or STATUS or VERB or BIGNUM", reply);
    }
#else
    if (!reply::is_string(reply) && !reply::is_status(reply)) {
        throw ParseError("STRING or STATUS", reply);
    }
#endif

    if (reply.str == nullptr) {
        throw ProtoError("A null string reply");
    }

    // Old version hiredis' *redisReply::len* is of type int.
    // So we CANNOT have something like: *return {reply.str, reply.len}*.
    return std::string(reply.str, reply.len);
}

设计点

• 在redis层使用函数作为模板类型参数，也就是command层定义的函数。

template <typename Cmd, typename ...Args>
auto Redis::command(Cmd cmd, Args &&...args)
    -> typename std::enable_if<!std::is_convertible<Cmd, StringView>::value, ReplyUPtr>::type {
    if (_connection) {
        // Single Connection Mode.
        // TODO: In this case, should we reconnect?
        auto &connection = _connection->connection();
        if (connection.broken()) {
            throw Error("Connection is broken");
        }

        return _command(connection, cmd, std::forward<Args>(args)...);
    } else {
        assert(_pool);

        // Pool Mode, i.e. get connection from pool.
        SafeConnection connection(*_pool);

        return _command(connection.connection(), cmd, std::forward<Args>(args)...);
    }
}

template <typename ...Args>
auto Redis::command(const StringView &cmd_name, Args &&...args)
    -> typename std::enable_if<!IsIter<typename LastType<Args...>::type>::value, ReplyUPtr>::type {
    auto cmd = [](Connection &connection, const StringView &name, Args &&...params) {
                    CmdArgs cmd_args;
                    cmd_args.append(name, std::forward<Args>(params)...);
                    connection.send(cmd_args);
    };

    return command(cmd, cmd_name, std::forward<Args>(args)...);
}

template <typename Input>
auto Redis::command(Input first, Input last)
    -> typename std::enable_if<IsIter<Input>::value, ReplyUPtr>::type {
    range_check("command", first, last);

    auto cmd = [](Connection &connection, Input start, Input stop) {
                    CmdArgs cmd_args;
                    while (start != stop) {
                        cmd_args.append(*start);
                        ++start;
                    }
                    connection.send(cmd_args);
    };

    return command(cmd, first, last);
}

• 使用可变参数模板

template <typename Cmd, typename ...Args>
    ReplyUPtr _command(Connection &connection, Cmd cmd, Args &&...args);
//command系列
template <typename Cmd, typename ...Args>
    auto command(Cmd cmd, Args &&...args)
        -> typename std::enable_if<!std::is_convertible<Cmd, StringView>::value, ReplyUPtr>::type;

• 在redis层最终会调用不带连接的command方法，调用带连接参数的_command方法（调用command层具体的函数，然后接收响应）

template <typename Cmd, typename ...Args>
ReplyUPtr Redis::_command(Connection &connection, Cmd cmd, Args &&...args) {
    assert(!connection.broken());

    cmd(connection, std::forward<Args>(args)...);

    auto reply = connection.recv();

    return reply;
}