In this article, we will analyze the source code of the server when zookeeper establishes the connection. Here we skip the process of the server startup (because it contains too much content, one article can not finish), and look directly at the code of the server to process the request.

There are two kinds of NIOServer CnxnFactory and Netty Server CnxnFactory for server-side processing requests. By default, NIOServer CnxnFactory can be modified by specifying zookeeper. server CnxnFactory parameters.

These two kinds of logic are the same, just a native NIO using java and a netty. Here we will analyze NIOServer CnxnFactory.

NIOServer CnxnFactory implements the Runnable interface. Look at its run method, which handles requests in a loop.

//NIOServerCnxnFactory.java
//200th elements
public void run() {
    while (!ss.socket().isClosed()) {
        try {
            selector.select(1000);
            Set<SelectionKey> selected;
            synchronized (this) {
                selected = selector.selectedKeys();
            }
            ArrayList<SelectionKey> selectedList = new ArrayList<SelectionKey>(
                selected);
            Collections.shuffle(selectedList);
            for (SelectionKey k : selectedList) {
                //If it is a connection request
                if ((k.readyOps() & SelectionKey.OP_ACCEPT) != 0) {
                    SocketChannel sc = ((ServerSocketChannel) k
                                        .channel()).accept();
                    InetAddress ia = sc.socket().getInetAddress();
                    //Get the number of client connections corresponding to the IP address
                    int cnxncount = getClientCnxnCount(ia);
                    //Close if exceeded
                    if (maxClientCnxns > 0 && cnxncount >= maxClientCnxns){
                        LOG.warn("Too many connections from " + ia
                                 + " - max is " + maxClientCnxns );
                        sc.close();
                    } else {
                        LOG.info("Accepted socket connection from "
                                 + sc.socket().getRemoteSocketAddress());
                        sc.configureBlocking(false);
                        SelectionKey sk = sc.register(selector,
                                                      SelectionKey.OP_READ);
                        //Each connection is a NIOServer Cnxn
                        NIOServerCnxn cnxn = createConnection(sc, sk);
                        sk.attach(cnxn);
                        addCnxn(cnxn);
                    }
                } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
                    //In the second loop, you enter here to process the real connection request.
                    NIOServerCnxn c = (NIOServerCnxn) k.attachment();
                    c.doIO(k);
                } else {
                    if (LOG.isDebugEnabled()) {
                        LOG.debug("Unexpected ops in select "
                                  + k.readyOps());
                    }
                }
            }
            selected.clear();
        } catch (RuntimeException e) {
            LOG.warn("Ignoring unexpected runtime exception", e);
        } catch (Exception e) {
            LOG.warn("Ignoring exception", e);
        }
    }
    closeAll();
    LOG.info("NIOServerCnxn factory exited run method");
}

//NIOServerCnxn.java
//237th elements
void doIO(SelectionKey k) throws InterruptedException {
    try {
        if (isSocketOpen() == false) {
            LOG.warn("trying to do i/o on a null socket for session:0x"
                     + Long.toHexString(sessionId));

            return;
        }
        if (k.isReadable()) {
            int rc = sock.read(incomingBuffer);
            if (rc < 0) {
                throw new EndOfStreamException(
                    "Unable to read additional data from client sessionid 0x"
                    + Long.toHexString(sessionId)
                    + ", likely client has closed socket");
            }
            if (incomingBuffer.remaining() == 0) {
                boolean isPayload;
                if (incomingBuffer == lenBuffer) { // start of next request
                    incomingBuffer.flip();
                    isPayload = readLength(k);
                    incomingBuffer.clear();
                } else {
                    // continuation
                    isPayload = true;
                }
                if (isPayload) { // not the case for 4letterword
                    readPayload();
                }
                else {
                    // four letter words take care
                    // need not do anything else
                    return;
                }
            }
        }
        //Omit part of the code
    } catch (CancelledKeyException e) {
        
    } catch (CloseRequestException e) {
        
    } catch (EndOfStreamException e) {
        
    } catch (IOException e) {
        
    }
}

//NIOServerCnxn.java
//194th elements
private void readPayload() throws IOException, InterruptedException {
    if (incomingBuffer.remaining() != 0) { // have we read length bytes?
        int rc = sock.read(incomingBuffer); // sock is non-blocking, so ok
        if (rc < 0) {
            throw new EndOfStreamException(
                "Unable to read additional data from client sessionid 0x"
                + Long.toHexString(sessionId)
                + ", likely client has closed socket");
        }
    }

    if (incomingBuffer.remaining() == 0) { // have we read length bytes?
        packetReceived();
        incomingBuffer.flip();
        if (!initialized) {
            readConnectRequest();
        } else {
            readRequest();
        }
        lenBuffer.clear();
        incomingBuffer = lenBuffer;
    }
}

//NIOServerCnxn.java
//434th elements
private void readConnectRequest() throws IOException, InterruptedException {
    if (!isZKServerRunning()) {
        throw new IOException("ZooKeeperServer not running");
    }
    zkServer.processConnectRequest(this, incomingBuffer);
    initialized = true;
}

//ZookeeperServer.java
//886th elements
public void processConnectRequest(ServerCnxn cnxn, ByteBuffer incomingBuffer) throws IOException {
    BinaryInputArchive bia = BinaryInputArchive.getArchive(new ByteBufferInputStream(incomingBuffer));
    ConnectRequest connReq = new ConnectRequest();
    connReq.deserialize(bia, "connect");
    if (LOG.isDebugEnabled()) {
        LOG.debug("Session establishment request from client "
                  + cnxn.getRemoteSocketAddress()
                  + " client's lastZxid is 0x"
                  + Long.toHexString(connReq.getLastZxidSeen()));
    }
    boolean readOnly = false;
    try {
        readOnly = bia.readBool("readOnly");
        cnxn.isOldClient = false;
    } catch (IOException e) {
        // this is ok -- just a packet from an old client which
        // doesn't contain readOnly field
        LOG.warn("Connection request from old client "
                 + cnxn.getRemoteSocketAddress()
                 + "; will be dropped if server is in r-o mode");
    }
    //If the client does not set readOnly, but the server is read-only, throw an exception to close the connection.
    if (readOnly == false && this instanceof ReadOnlyZooKeeperServer) {
        String msg = "Refusing session request for not-read-only client "
            + cnxn.getRemoteSocketAddress();
        LOG.info(msg);
        throw new CloseRequestException(msg);
    }
    if (connReq.getLastZxidSeen() > zkDb.dataTree.lastProcessedZxid) {
        String msg = "Refusing session request for client "
            + cnxn.getRemoteSocketAddress()
            + " as it has seen zxid 0x"
            + Long.toHexString(connReq.getLastZxidSeen())
            + " our last zxid is 0x"
            + Long.toHexString(getZKDatabase().getDataTreeLastProcessedZxid())
            + " client must try another server";

        LOG.info(msg);
        throw new CloseRequestException(msg);
    }
    //Negotiating session timeout
    int sessionTimeout = connReq.getTimeOut();
    byte passwd[] = connReq.getPasswd();
    int minSessionTimeout = getMinSessionTimeout();
    if (sessionTimeout < minSessionTimeout) {
        sessionTimeout = minSessionTimeout;
    }
    int maxSessionTimeout = getMaxSessionTimeout();
    if (sessionTimeout > maxSessionTimeout) {
        sessionTimeout = maxSessionTimeout;
    }
    cnxn.setSessionTimeout(sessionTimeout);
    // We don't want to receive any packets until we are sure that the
    // session is setup
    cnxn.disableRecv();
    long sessionId = connReq.getSessionId();
    if (sessionId != 0) {
        //If session Id is not zero, it means that the client that has been connected before is reconnected for reasons such as dropping the line.
        long clientSessionId = connReq.getSessionId();
        LOG.info("Client attempting to renew session 0x"
                 + Long.toHexString(clientSessionId)
                 + " at " + cnxn.getRemoteSocketAddress());
        serverCnxnFactory.closeSession(sessionId);
        cnxn.setSessionId(sessionId);
        reopenSession(cnxn, sessionId, passwd, sessionTimeout);
    } else {
        LOG.info("Client attempting to establish new session at "
                 + cnxn.getRemoteSocketAddress());
        createSession(cnxn, passwd, sessionTimeout);
    }
}

//ZookeeperServer.java
//617th elements
long createSession(ServerCnxn cnxn, byte passwd[], int timeout) {
    //Create a session, zookeeper session management is more complex, under specific circumstances, a chapter analysis
    long sessionId = sessionTracker.createSession(timeout);
    Random r = new Random(sessionId ^ superSecret);
    r.nextBytes(passwd);
    ByteBuffer to = ByteBuffer.allocate(4);
    to.putInt(timeout);
    cnxn.setSessionId(sessionId);
    //Response Client
    submitRequest(cnxn, sessionId, OpCode.createSession, 0, to, null);
    return sessionId;
}