Fabric網絡節點發現及成員管理

一個新節點通過已知的節點加入到網絡中，此時，它所知的網絡節點信息是非常有限的，需要通過節點發現獲知更多的節點，建立起足夠的連接。另外，當一個新節點加入到網絡時，原有網絡節點也需要通過節點發現感知到新節點的加入。

分布在各地的網絡節點總是會有上線離線的變化，有這就需要Fabric網絡必須動態維護一個節點成員列表，這就需要節點成員管理。

一、節點發現與成員管理

1. 節點發現

一個節點要加入Fabric網絡，必須要知道至少一個已知Fabric節點作為啟動節點。

相關配置如下：

    # Gossip related configuration
    gossip:
        # Bootstrap set to initialize gossip with
        bootstrap: 127.0.0.1:7051

Fabric節點發現與成員管理流程如下圖所示：

2. 網絡連接層次的節點成員管理（在線，掉線）

在線節點(Peer)通過持續不斷地廣播“活着”的消息，來表明他們的可用性。

這一部分相當於心跳檢測，如果節點離線，就在channel成員列表中刪除節點。

3. 相關消息定義

// AliveMessage is sent to inform remote peers
// of a peer's existence and activity
message AliveMessage {
    Member membership  = 1;
    PeerTime timestamp = 2;
    bytes identity     = 4;
}

// MembershipRequest is used to ask membership information
// from a remote peer
message MembershipRequest {
    Envelope self_information = 1;
    repeated bytes known         = 2;
}

// MembershipResponse is used for replying to MembershipRequests
message MembershipResponse {
    repeated Envelope alive = 1;
    repeated Envelope dead  = 2;
}

二、節點間消息傳播（Gossip）

1. 消息發送方式：

• 點對點發送（end to end）
• gossip方式——發送消息時會根據消息類型對節點進行過濾篩選（另外還會去除掉發送節點）后再隨機（具體實現上是隨機就近原則）選擇\(k\)個節點發送消息。

這里采用的是push和pull方式。

2. push

節點有了新消息后，隨機選擇\(k\)個節點（例如，3），向它們發送新消息。\(k\)個節點收到后，繼續隨機選擇\(k\)個節點發送新信息，直到所有節點都知道該新信息。

3. pull

所有節點周期性的隨機選取\(k\)個（默認配置=3）個節點，向它們獲取數據。Fabric中gossip協議pull操作如下：

/* PullEngine is an object that performs pull-based gossip, and maintains an internal state of items
   identified by string numbers
.
   The protocol is as follows:
   1) The Initiator sends a Hello message with a specific NONCE to a set of remote peers.
   2) Each remote peer responds with a digest of its messages and returns that NONCE.
   3) The initiator checks the validity of the NONCEs received, aggregates the digests,
      and crafts a request containing specific item ids it wants to receive from each remote peer and then
      sends each request to its corresponding peer.
   4) Each peer sends back the response containing the items requested, if it still holds them and the NONCE.

    Other peer				   			   Initiator
	 O	<-------- Hello <NONCE> -------------------------	O
	/|\	--------- Digest <[3,5,8, 10...], NONCE> -------->     /|\
	 |	<-------- Request <[3,8], NONCE> -----------------      |
	/ \	--------- Response <[item3, item8], NONCE>------->     / \

*/

4. pull相關消息定義

// GossipHello is the message that is used for the peer to initiate
// a pull round with another peer
message GossipHello {
    uint64 nonce         = 1;
    bytes metadata       = 2;
    PullMsgType msg_type = 3;
}

// DataDigest is the message sent from the receiver peer
// to the initator peer and contains the data items it has
message DataDigest {
    uint64 nonce             = 1;
    repeated bytes digests  = 2; // Maybe change this to bitmap later on
    PullMsgType msg_type     = 3;
}

// DataRequest is a message used for a peer to request
// certain data blocks from a remote peer
message DataRequest {
    uint64 nonce             = 1;
    repeated bytes digests  = 2;
    PullMsgType msg_type     = 3;
}

// DataUpdate is the final message in the pull phase
// sent from the receiver to the initiator
message DataUpdate {
    uint64 nonce                = 1;
    repeated Envelope data      = 2;
    PullMsgType msg_type        = 3;
}