Building Blocks: A Guide to ZooKeeper Operations

This section surveys all the operations a developer can perform against a ZooKeeper server. It is lower level information than the earlier concepts chapters in this manual, but higher level than the ZooKeeper API Reference.

Handling Errors

Both the Java and C client bindings may report errors. The Java client binding does so by throwing KeeperException, calling code() on the exception will return the specific error code. The C client binding returns an error code as defined in the enum ZOO_ERRORS. API callbacks indicate result code for both language bindings. See the API documentation (javadoc for Java, doxygen for C) for full details on the possible errors and their meaning.

Connecting to ZooKeeper

Before we begin, you will have to set up a running Zookeeper server so that we can start developing the client. For C client bindings, we will be using the multithreaded library(zookeepermt) with a simple example written in C. To establish a connection with Zookeeper server, we make use of C API - _zookeeper_init with the following signature:

int zookeeper_init(const char *host, watcher_fn fn, int recv_timeout, const clientid_t *clientid, void *context, int flags);

• *host : Connection string to zookeeper server in the format of host:port. If there are multiple servers, use comma as separator after specifying the host:port pairs. Eg: "127.0.0.1:2181,127.0.0.1:3001,127.0.0.1:3002"
• fn : Watcher function to process events when a notification is triggered.
• recv_timeout : Session expiration time in milliseconds.
• *clientid : We can specify 0 for a new session. If a session has already establish previously, we could provide that client ID and it would reconnect to that previous session.
• *context : Context object that can be associated with the zkhandle_t handler. If it is not used, we can set it to 0.
• flags : In an initiation, we can leave it for 0.

We will demonstrate client that outputs "Connected to Zookeeper" after successful connection or an error message otherwise. Let's call the following code zkClient.cc:

#include <stdio.h>
#include <zookeeper/zookeeper.h>
#include <errno.h>
using namespace std;

// Keeping track of the connection state
static int connected = 0;
static int expired   = 0;

// *zkHandler handles the connection with Zookeeper
static zhandle_t *zkHandler;

// watcher function would process events
void watcher(zhandle_t *zkH, int type, int state, const char *path, void *watcherCtx)
{
    if (type == ZOO_SESSION_EVENT) {

        // state refers to states of zookeeper connection.
        // To keep it simple, we would demonstrate these 3: ZOO_EXPIRED_SESSION_STATE, ZOO_CONNECTED_STATE, ZOO_NOTCONNECTED_STATE
        // If you are using ACL, you should be aware of an authentication failure state - ZOO_AUTH_FAILED_STATE
        if (state == ZOO_CONNECTED_STATE) {
            connected = 1;
        } else if (state == ZOO_NOTCONNECTED_STATE ) {
            connected = 0;
        } else if (state == ZOO_EXPIRED_SESSION_STATE) {
            expired = 1;
            connected = 0;
            zookeeper_close(zkH);
        }
    }
}

int main(){
    zoo_set_debug_level(ZOO_LOG_LEVEL_DEBUG);

    // zookeeper_init returns the handler upon a successful connection, null otherwise
    zkHandler = zookeeper_init("localhost:2181", watcher, 10000, 0, 0, 0);

    if (!zkHandler) {
        return errno;
    }else{
        printf("Connection established with Zookeeper. \n");
    }

    // Close Zookeeper connection
    zookeeper_close(zkHandler);

    return 0;
}

Compile the code with the multithreaded library mentioned before.

> g++ -Iinclude/ zkClient.cpp -lzookeeper_mt -o Client

Run the client.

> ./Client

From the output, you should see "Connected to Zookeeper" along with Zookeeper's DEBUG messages if the connection is successful.