接口描述

fabric/core/deliverservice/deliveryclient.go
        

// DeliverService used to communicate with orderers to obtain
// new block and send the to the committer service
type DeliverService interface {
    // StartDeliverForChannel dynamically starts delivery of new blocks from ordering service
    // to channel peers.
    StartDeliverForChannel(chainID string, ledgerInfo blocksprovider.LedgerInfo) error

    // StopDeliverForChannel dynamically stops delivery of new blocks from ordering service
    // to channel peers.
    StopDeliverForChannel(chainID string) error
    // Stop terminates delivery service and closes the connection
    Stop()
}

根据描述可以很明显的看出来是与order服务进行收发block的接口，接下来阅读以下主要代码，三个函数只有 StartDeliverForChannel是重点，其余两个都不是重点，所以我们主要就搞懂这个就可以了，既然是收发代码，就是找到收发逻辑就可以了。

接口实现

// StartDeliverForChannel starts blocks delivery for channel
// initializes the grpc stream for given chainID, creates blocks provider instance
// that spawns in go routine to read new blocks starting from the position provided by ledger
// info instance.
func (d *deliverServiceImpl) StartDeliverForChannel(chainID string, ledgerInfo blocksprovider.LedgerInfo) error {
    d.lock.Lock()
    defer d.lock.Unlock()
    if d.stopping {
        errMsg := fmt.Sprintf("Delivery service is stopping cannot join a new channel %s", chainID)
        logger.Errorf(errMsg)
        return errors.New(errMsg)
    }
    if _, exist := d.clients[chainID]; exist {
        errMsg := fmt.Sprintf("Delivery service - block provider already exists for %s found, can't start delivery", chainID)
        logger.Errorf(errMsg)
        return errors.New(errMsg)
    } else {
        abc, err := d.clientsFactory.Create()
        if err != nil {
            logger.Errorf("Unable to initialize atomic broadcast, due to %s", err)
            return err
        }
        logger.Debug("This peer will pass blocks from orderer service to other peers")
        d.clients[chainID] = blocksprovider.NewBlocksProvider(chainID, abc, d.gossip)

        if err := d.clients[chainID].RequestBlocks(ledgerInfo); err == nil {
            // Start reading blocks from ordering service in case this peer is a leader for specified chain
            go d.clients[chainID].DeliverBlocks()
        }
    }
    return nil
}

通过代码可以看出来 blocksprovider.NewBlocksProvider 产生了真正的连接，在这个连接上会先 RequestBlocks，然后 DeliverBlocks ，我们只要搞清楚 与 谁建立的连接，RequestBlocks，DeliverBlocks 的逻辑后就弄清楚了 这个service的主要功能了

• 连接的建立

func NewDeliverService(gossip blocksprovider.GossipServiceAdapter, endpoints []string) (DeliverService, error) {
    indices := rand.Perm(len(endpoints))
    for _, idx := range indices {
        logger.Infof("Creating delivery service to get blocks from the ordering service, %s", endpoints[idx])

        dialOpts := []grpc.DialOption{grpc.WithInsecure(), grpc.WithTimeout(3 * time.Second), grpc.WithBlock()}

        if comm.TLSEnabled() {
            dialOpts = append(dialOpts, grpc.WithTransportCredentials(comm.InitTLSForPeer()))
        } else {
            dialOpts = append(dialOpts, grpc.WithInsecure())
        }

        conn, err := grpc.Dial(endpoints[idx], dialOpts...)
        if err != nil {
            logger.Errorf("Cannot dial to %s, because of %s", endpoints[idx], err)
            continue
        }
        return NewFactoryDeliverService(gossip, &blocksDelivererFactoryImpl{conn}, conn), nil
    }
    return nil, fmt.Errorf("Wasn't able to connect to any of ordering service endpoints %s", endpoints)
}

可以看到连接是建立到了，endpoints，通过追踪代码可以发现就是 order 节点的地址，这里需要注意一点， 传过来的 endpoints 是一个数组，但是只随机的与某一个节点建立连接。
现在我们知道了是与order通信，由于fabric是基于grpc的，所以我们还要找出来与order的哪个服务进行通信，blocksDelivererFactoryImpl 这个struct定义了与哪个服务通信，我们一起看下

fabric/core/deliverservice/deliveryclient.go

type blocksDelivererFactoryImpl struct {
    conn *grpc.ClientConn
}

// Create a factory method which is capable to instantiate new BlocksDeliverer
func (factory *blocksDelivererFactoryImpl) Create() (blocksprovider.BlocksDeliverer, error) {
    var abc orderer.AtomicBroadcast_DeliverClient
    var err error
    abc, err = orderer.NewAtomicBroadcastClient(factory.conn).Deliver(context.TODO())
    if err != nil {
        return nil, err
    }

    return abc, nil
}

明显是AtomicBroadcast 服务的 Deliver 接口通信

• RequestBlocks

fabric/core/deliverservice/blocksprovider/blocksprovider.go

func (b *blocksProviderImpl) RequestBlocks(ledgerInfoProvider LedgerInfo) error {
    height, err := ledgerInfoProvider.LedgerHeight()
    if err != nil {
        logger.Errorf("Can't get legder height from committer [%s]", err)
        return err
    }

    if height > 0 {
        logger.Debugf("Starting deliver with block [%d]", height)
        if err := b.seekLatestFromCommitter(height); err != nil {
            return err
        }
    } else {
        logger.Debug("Starting deliver with olders block")
        if err := b.seekOldest(); err != nil {
            return err
        }
    }

    return nil
}

func (b *blocksProviderImpl) seekLatestFromCommitter(height uint64) error {
    seekInfo := &orderer.SeekInfo{
        Start:    &orderer.SeekPosition{Type: &orderer.SeekPosition_Specified{Specified: &orderer.SeekSpecified{Number: height}}},
        Stop:     &orderer.SeekPosition{Type: &orderer.SeekPosition_Specified{Specified: &orderer.SeekSpecified{Number: math.MaxUint64}}},
        Behavior: orderer.SeekInfo_BLOCK_UNTIL_READY,
    }

    //TODO- epoch and msgVersion may need to be obtained for nowfollowing usage in orderer/configupdate/configupdate.go
    msgVersion := int32(0)
    epoch := uint64(0)
    env, err := utils.CreateSignedEnvelope(common.HeaderType_CONFIG_UPDATE, b.chainID, localmsp.NewSigner(), seekInfo, msgVersion, epoch)
    if err != nil {
        return err
    }
    return b.client.Send(env)
}

明显 可以看出来只是在请求 start - stop 之间的block

• DeliverBlocks

func (b *blocksProviderImpl) DeliverBlocks() {
    for !b.isDone() {
        msg, err := b.client.Recv()   // 接收Request请求的代码
        if err != nil {
            logger.Warningf("Receive error: %s", err.Error())
            return
        }
        switch t := msg.Type.(type) {
        case *orderer.DeliverResponse_Status:
            if t.Status == common.Status_SUCCESS {
                logger.Warning("ERROR! Received success for a seek that should never complete")
                return
            }
            logger.Warning("Got error ", t)
        case *orderer.DeliverResponse_Block:
            seqNum := t.Block.Header.Number

            numberOfPeers := len(b.gossip.PeersOfChannel(gossipcommon.ChainID(b.chainID)))
            // Create payload with a block received
            payload := createPayload(seqNum, t.Block)
            // Use payload to create gossip message
            gossipMsg := createGossipMsg(b.chainID, payload)

            logger.Debugf("Adding payload locally, buffer seqNum = [%d], peers number [%d]", seqNum, numberOfPeers)
            // Add payload to local state payloads buffer
            b.gossip.AddPayload(b.chainID, payload)

            // Gossip messages with other nodes
            logger.Debugf("Gossiping block [%d], peers number [%d]", seqNum, numberOfPeers)
            b.gossip.Gossip(gossipMsg)  // gossip 发送
        default:
            logger.Warning("Received unknown: ", t)
            return
        }
    }
}

可以看出来，request 来的 block，在这边都通过 gossip 广播出去了

总结

这个这个代码很清晰了，DeliverService 就是从 order 请求 block，然后广播出去。 当然在整个fabric中的业务作用目前看不出来，还需要深入分析