Using Impala through a Proxy for High Availability

Using a load-balancing proxy server for Impala has the following advantages:

• Applications connect to a single well-known host and port, rather than keeping track of the hosts where the impalad daemon is running.
• If any host running the impalad daemon becomes unavailable, application connection requests still succeed because you always connect to the proxy server rather than a specific host running the impalad daemon.
• The coordinator node for each Impala query potentially requires more memory and CPU cycles than the other nodes that process the query. The proxy server can issue queries using round-robin scheduling, so that each connection uses a different coordinator node. This load-balancing technique lets the Impala nodes share this additional work, rather than concentrating it on a single machine.

The following setup steps are a general outline that apply to any load-balancing proxy software.

1. Download the load-balancing proxy software. It should only need to be installed and configured on a single host. Pick a host other than the DataNodes where impalad is running, because the intention is to protect against the possibility of these DataNodes becoming unavailable.
2. Configure the software (typically by editing a configuration file). In particular:

   • Set up a port that the load balancer will listen on to relay Impala requests back and forth.

   • Consider enabling "sticky sessions". Cloudera recommends enabling this setting so that stateless client applications such as impalad and Hue are not disconnected from long-running queries. Evaluate whether this setting is appropriate for your combination of workload and client applications.

   • For Kerberized clusters, follow the instructions in Special Proxy Considerations for Clusters Using Kerberos.

3. Specify the host and port settings for each Impala node. These are the hosts that the load balancer will choose from when relaying each Impala query. See Ports Used by Impala for when to use port 21000, 21050, or another value depending on what type of connections you are load balancing.
4. Run the load-balancing proxy server, pointing it at the configuration file that you set up.
5. On systems managed by Cloudera Manager, on the page Impala > Configuration > Impala Daemon Default Group, specify a value for the Impala Daemons Load Balancer field. Specify the address of the load balancer in host:port format. This setting lets Cloudera Manager route all appropriate Impala-related operations through the proxy server.
6. For any scripts, jobs, or configuration settings for applications that formerly connected to a specific datanode to run Impala SQL statements, change the connection information (such as the -i option in impala-shell) to point to the load balancer instead.

In a cluster using Kerberos, applications check host credentials to verify that the host they are connecting to is the same one that is actually processing the request, to prevent man-in-the-middle attacks. To clarify that the load-balancing proxy server is legitimate, perform these extra Kerberos setup steps:

1. This section assumes you are starting with a Kerberos-enabled cluster. See Enabling Kerberos Authentication for Impala for instructions for setting up Impala with Kerberos. See the CDH Security Guide for general steps to set up Kerberos: CDH 5 instructions or CDH 4 instructions.
2. Choose the host you will use for the proxy server. Based on the Kerberos setup procedure, it should already have an entry impala/proxy_host@realm in its keytab. If not, go back over the initial Kerberos configuration steps for the keytab on each host running the impalad daemon.
3. Copy the keytab file from the proxy host to all other hosts in the cluster that run the impalad daemon. (For optimal performance, impalad should be running on all DataNodes in the cluster.) Put the keytab file in a secure location on each of these other hosts.
4. On systems not managed by Cloudera Manager, add an entry impala/actual_hostname@realm to the keytab on each host running the impalad daemon.
5. For each impalad node, merge the existing keytab with the proxy’s keytab using ktutil, producing a new keytab file. For example:

   $ ktutil
   ktutil: read_kt proxy.keytab
   ktutil: read_kt impala.keytab
   ktutil: write_kt proxy_impala.keytab
   ktutil: quit

6. Make sure that the impala user has permission to read this merged keytab file.
7. Change some configuration settings for each host in the cluster that participates in the load balancing. Follow the appropriate steps depending on whether you use Cloudera Manager or not:
   • Change some configuration settings for each host in the cluster that participates in the load balancing. In the impalad option definition, or the Cloudera Manager safety valve (Cloudera Manager 4) or advanced configuration snippet (Cloudera Manager 5), add:

     --principal=impala/proxy_host@realm
     --be_principal=impala/actual_host@realm
     --keytab_file=path_to_merged_keytab

   • On a cluster managed by Cloudera Manager, create a role group to set the configuration values from the preceding step on a per-host basis.
   • On a cluster not managed by Cloudera Manager, see Modifying Impala Startup Options for the procedure to modify the startup options.
8. Restart Impala to make the changes take effect. Follow the appropriate steps depending on whether you use Cloudera Manager or not:
   • On a cluster managed by Cloudera Manager, restart the Impala service.
   • On a cluster not managed by Cloudera Manager, restart the impalad daemons on all hosts in the cluster, as well as the statestored and catalogd daemons.

If you are not already using a load-balancing proxy, you can experiment with HAProxy a free, open source load balancer. This example shows how you might install and configure that load balancer on a Red Hat Enterprise Linux system.

• Install the load balancer: yum install haproxy

• Set up the configuration file: /etc/haproxy/haproxy.cfg. See the following section for a sample configuration file.

• Run the load balancer (on a single host, preferably one not running impalad):

  /usr/sbin/haproxy –f /etc/haproxy/haproxy.cfg

• In impala-shell, JDBC applications, or ODBC applications, connect to the listener port of the proxy host, rather than port 21000 or 21050 on a host actually running impalad. The sample configuration file sets haproxy to listen on port 25003, therefore you would send all requests to haproxy_host:25003.

This is the sample haproxy.cfg used in this example:

global
    # To have these messages end up in /var/log/haproxy.log you will
    # need to:
    #
    # 1) configure syslog to accept network log events.  This is done
    #    by adding the '-r' option to the SYSLOGD_OPTIONS in
    #    /etc/sysconfig/syslog
    #
    # 2) configure local2 events to go to the /var/log/haproxy.log
    #   file. A line like the following can be added to
    #   /etc/sysconfig/syslog
    #
    #    local2.*                       /var/log/haproxy.log
    #
    log         127.0.0.1 local0
    log         127.0.0.1 local1 notice
    chroot      /var/lib/haproxy
    pidfile     /var/run/haproxy.pid
    maxconn     4000
    user        haproxy
    group       haproxy
    daemon

    # turn on stats unix socket
    #stats socket /var/lib/haproxy/stats

#---------------------------------------------------------------------
# common defaults that all the 'listen' and 'backend' sections will
# use if not designated in their block
#
# You might need to adjust timing values to prevent timeouts.
#---------------------------------------------------------------------
defaults
    mode                    http
    log                     global
    option                  httplog
    option                  dontlognull
    option http-server-close
    option forwardfor       except 127.0.0.0/8
    option                  redispatch
    retries                 3
    maxconn                 3000
    contimeout 5000
    clitimeout 50000
    srvtimeout 50000

#
# This sets up the admin page for HA Proxy at port 25002.
#
listen stats :25002
    balance
    mode http
    stats enable
    stats auth username:password

# This is the setup for Impala. Impala client connect to load_balancer_host:25003.
# HAProxy will balance connections among the list of servers listed below.
# The list of Impalad is listening at port 21000 for beeswax (impala-shell) or original ODBC driver.
# For JDBC or ODBC version 2.x driver, use port 21050 instead of 21000.
listen impala :25003
    mode tcp
    option tcplog
    balance leastconn

    server symbolic_name_1 impala-host-1.example.com:21000 check
    server symbolic_name_2 impala-host-2.example.com:21000 check
    server symbolic_name_3 impala-host-3.example.com:21000 check
    server symbolic_name_4 impala-host-4.example.com:21000 check

# Setup for Hue or other JDBC-enabled applications.
# In particular, Hue requires sticky sessions.
# The application connects to load_balancer_host:21051, and HAProxy balances
# connections to the associated hosts, where Impala listens for JDBC
# requests on port 21050.
listen impalajdbc :21051
    mode tcp
    option tcplog
    balance source
    server symbolic_name_5 impala-host-1.example.com:21050
    server symbolic_name_6 impala-host-2.example.com:21050
    server symbolic_name_7 impala-host-3.example.com:21050
    server symbolic_name_8 impala-host-4.example.com:21050