zk maxClientCnxns参数

在zk模板配置文件中有：

# the maximum number of client connections.
# increase this if you need to handle more clients
maxClientCnxns=60

这个配置的作用就是：一个ip所对应的客户机，只能和zk服务器维持60个连接。

以NIOServerCnxnFactory为例：

public abstract class ServerCnxnFactory {
    //存放zk所有连接
    protected final HashSet<ServerCnxn> cnxns = new HashSet<ServerCnxn>();
}

public class NIOServerCnxnFactory extends ServerCnxnFactory implements Runnable {
    //客户端ip -> 客户端的连接集合
    final HashMap<InetAddress, Set<NIOServerCnxn>> ipMap = new HashMap<InetAddress, Set<NIOServerCnxn>>( );
    //获取指定ip的连接数
    private int getClientCnxnCount(InetAddress cl) {
        // The ipMap lock covers both the map, and its contents
        // (that is, the cnxn sets shouldn't be modified outside of
        // this lock)
        synchronized (ipMap) {
            Set<NIOServerCnxn> s = ipMap.get(cl);
            if (s == null) return 0;
            return s.size();
        }
    }
    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 ((k.readyOps() & SelectionKey.OP_ACCEPT) != 0) {
                        SocketChannel sc = ((ServerSocketChannel) k
                                .channel()).accept();
                        InetAddress ia = sc.socket().getInetAddress();
                        //获取客户端连接数
                        int cnxncount = getClientCnxnCount(ia);
                        //单个客户端连接数超过限制
                        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);
                            NIOServerCnxn cnxn = createConnection(sc, sk);
                            sk.attach(cnxn);
                            addCnxn(cnxn);
                        }
                    } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
                        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");
    }

}

tips: 关心这个参数，是因为之前有人上线时，客户端连接数超过了默认值，导致无法建立连接。