Using Impala through a Proxy for High Availability

For most clusters that have multiple users and production availability requirements, you might set up a proxy server to relay requests to and from Impala.

Currently, the Impala statestore mechanism does not include such proxying and load-balancing features. Set up a software package of your choice to perform these functions.

Overview of Proxy Usage and Load Balancing for Impala

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 schedule queries 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. Select and download a load-balancing proxy software or other load-balancing hardware appliance. It should only need to be installed and configured on a single host, typically on an edge node. Pick a host other than the DataNodes where impalad is running, because the intention is to protect against the possibility of one or more of these DataNodes becoming unavailable.
  2. Configure the load balancer (typically by editing a configuration file). In particular:
  3. If you are using Hue or JDBC-based applications, you typically set up load balancing for both ports 21000 and 21050, because these client applications connect through port 21050 while the impala-shell command connects through port 21000. 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.

Choosing the Load-Balancing Algorithm

Load-balancing software offers a number of algorithms to distribute requests. Each algorithm has its own characteristics that make it suitable in some situations but not others.

Connects sessions to the coordinator with the fewest connections, to balance the load evenly. Typically used for workloads consisting of many independent, short-running queries. In configurations with only a few client machines, this setting can avoid having all requests go to only a small set of coordinators.
Recommended for Impala with F5.
Source IP Persistence
Sessions from the same IP address always go to the same coordinator. A good choice for Impala workloads containing a mix of queries and DDL statements, such as CREATE TABLE and ALTER TABLE. Because the metadata changes from a DDL statement take time to propagate across the cluster, prefer to use the Source IP Persistence algorithm in this case. If you are unable to choose Source IP Persistence, run the DDL and subsequent queries that depend on the results of the DDL through the same session, for example by running impala-shell -f script_file to submit several statements through a single session.
Required for setting up high availability with Hue.
Distributes connections to all coordinator nodes. Typically not recommended for Impala.

You might need to perform benchmarks and load testing to determine which setting is optimal for your use case. Always setup using two load-balancing algorithms: Source IP Persistence for Hue and Leastconn for others.

Special Proxy Considerations for Clusters Using Kerberos

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.
  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. For a cluster managed by Cloudera Manager (5.1 or higher), fill in the Impala configuration setting Impala Daemons Load Balancer with the appropriate host:port combination. Then restart the Impala service. For systems using a recent level of Cloudera Manager, this is all the configuration you need; you can skip the remaining steps in this procedure.
  4. On systems not managed by Cloudera Manager, or systems using Cloudera Manager earlier than 5.1:
    1. 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.
    2. Add an entry impala/actual_hostname@realm to the keytab on each host running the impalad daemon.
    3. 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
    4. Make sure that the impala user has permission to read this merged keytab file.
    5. 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:
      • In the impalad option definition, or the advanced configuration snippet, add:
      • 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.
    6. 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.

Example of Configuring HAProxy Load Balancer for Impala

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:

    # 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 local0
    log local1 notice
    chroot      /var/lib/haproxy
    pidfile     /var/run/
    maxconn     4000
    user        haproxy
    group       haproxy

    # 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.
    mode                    http
    log                     global
    option                  httplog
    option                  dontlognull
    option http-server-close
    option forwardfor       except
    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
    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 check
    server symbolic_name_2 check
    server symbolic_name_3 check
    server symbolic_name_4 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 check
    server symbolic_name_6 check
    server symbolic_name_7 check
    server symbolic_name_8 check