Parallel command execution – Linux Cluster

The pdsh parallel shell tool allows you and lets you run a shell command across multiple nodes in a cluster.

This is a high performance, parallel pdsh shell remote shell utility for admins. Chaos Pdsh is a multithreaded remote shell client which executes commands on multiple remote hosts in parallel.  A parallel shell permits your clusters Linux Ubuntu RedHat to run the same similar command on many designated hosts or nodes within the hadoop cluster. In this case you do not have to really log in to each node individually.

High-performance and parallel remote shell utility with dshgroup module allows dsh on pdsh (or otherwise known as Dancer’s shell sudo) files from /etc/dsh/group directory. Now download Parallel Distributed Shell free of charge.

What is pdsh?

pdsh is a variant of the rsh(1) command. Unlike rsh(1), which runs commands on a single remote host, pdsh can run multiple remote commands in parallel. pdsh uses a “sliding window” (or fanout) of threads to conserve resources on the initiating host while allowing some connections to time out.

When pdsh receives SIGINT (ctrl-C), it lists the status of current threads. A second SIGINT within one second terminates the program. Pending threads may be canceled by issuing ctrl-Z within one second of ctrl-C. Pending threads are those that have not yet been initiated, or are still in the process of connecting to the remote host.

If a remote command is not specified on the command line, pdsh runs interactively, prompting for commands and executing them when terminated with a carriage return. In interactive mode, target nodes that time out on the first command are not contacted for subsequent commands, and commands prefixed with an exclamation point will be executed on the local system.

The core functionality of pdsh may be supplemented by dynamically loadable modules. The modules may provide a new connection protocol (replacing the standard rcmd(3) protocol used by rsh(1)), filtering options (e.g. removing hosts that are “down” from the target list), and/or host selection options (e.g., -a selects all hosts from a configuration file.). By default, pdsh must have at least one “rcmd” module loaded. See the RCMD MODULES section for more information.

Installing pdsh

Debian based:

apt install pdsh

RHEL based:

yum install pdsh

Running

The following command installs telegraf on all 4 nodes in cluster02

Running multiple commands

Pipe redirection

 

Example

 

When using ssh for remote execution, expect the stderr of ssh to be folded in with that of the remote command. When invoked by pdsh, it is not possible for ssh to prompt for passwords if RSA/DSA keys are configured properly, etc.. For ssh implementations that suppport a connect timeout option, pdsh attempts to use that option to enforce the timeout (e.g. -oConnectTimeout=T for OpenSSH), otherwise connect timeouts are not supported when using ssh. Finally, there is no reliable way for pdsh to ensure that remote commands are actually terminated when using a command timeout. Thus if -u is used with ssh commands may be left running on remote hosts even after timeout has killed local ssh processes.

Output from multiple processes per node may be interspersed when using qshell or mqshell rcmd modules.

The number of nodes that pdsh can simultaneously execute remote jobs on is limited by the maximum number of threads that can be created concurrently, as well as the availability of reserved ports in the rsh and qshell rcmd modules. On systems that implement Posix threads, the limit is typically defined by the constant PTHREADS_THREADS_MAX.

How to Setup Redis Cluster from Source

What is redis

Redis is an open source in-memory database. It stores data in key-value format. Because of residing in memory, redis is an excellent tool for caching. Redis provides a rich set of data types. This gives redis upper hand over Memcached. Apart from caching, redis can be used as distributed message broker.

Redis Cluster and Sentinels

To achieve high availability, redis can be deployed in cluster along with Sentinels. Sentinel is a feature of redis. Multiple sentinels are deployed across redis clusters for monitoring purpose. When redis master goes down, sentinels elect a new master from slaves. When old master comes up again, it is added as slave.

Another use case of clustering is a distribution of load. In high load environment, we can send write requests to master and read request to slaves.

This tutorial is specifically focused on Redis Cluster Master Slave model. We will not cover data sharding across cluster here. In data sharding, keys are distributed across multiple redis nodes.

Setup for tutorial

For this tutorial, we will use 3 (virtual) servers. On one server Redis master will reside while other two servers will be used for slaves. Standard redis port is 6379. To differentiate easily, we will run master on 6379 port and slaves on
6380 and 6381 ports. Same will be applied for sentinel services. Master sentinel will listen on 16379 port while slave sentinels will be on 16380 and 16381.

Lets put this easy way.

This tutorial is tested on CentOS 6.9. For CentOS 7.X, check below Notes Section.

Installation

We will follow same installation steps for setting up of all servers. Only difference will be in configurations.

  • Step 1: Grab redis source, make and install
  • Step 2: Setup required directories
  • Step 3: Configure redis master
  • Step 4: Configure redis master sentinel
  • Step 5: Add low privileged user to run redis
  • Step 6: Setup init scripts
  • Step 7: Start service

Server 1 (Redis Master)


Install Redis

Setup required directories

Configure redis master

Edit config file /etc/redis/6379.conf in your favorite editor and change below options.

Configure redis master sentinel

Add config file for sentinel at /etc/redis/sentinel_6379.conf. Open a file and add below content

Add non-privileged user

Setup init scripts

You can find sample init scripts in Notes section below.

Start service

Server 2 (Redis Slave 1)


Install Redis

Setup required directories

Configure redis slave 1

Edit config file /etc/redis/6380.conf in your favorite editor and change below options.

Configure redis slave 1 sentinel

Add config file for sentinel at /etc/redis/sentinel_6380.conf. Open a file and add below content

Add non-privileged user

Setup init scripts

You can find sample init scripts in Notes section below. Change $HOST and $PORT values accordingly

Start service

Server 3 (Redis Slave 2)


Install Redis

Setup required directories

Configure redis slave 2

Edit config file /etc/redis/6381.conf in your favorite editor and change below options.

Configure redis slave 2 sentinel

Add config file for sentinel at /etc/redis/sentinel_6381.conf. Open a file and add below content

Add non-privileged user

Setup init scripts

You can find sample init scripts in Notes section below. Change $HOST and $PORT values accordingly

Start service

Sentinel Testing

Redis Fail-over Testing

For fail-over testing, we can take down redis-master either using init script or below command.

Also we can force sentinel to run fail over using below command

Sample init scripts

Redis Init Script

Sentinel Init Script

Notes

Security

  • NEVER EVER run redis on public interface
  • If redis is deployed in cloud environment like AWS, set up security groups/firewalls carefully. Most of times, cloud providers use ephemeral ips. Because of ephermal ips, even redis is bound to private ip, it can be accessed over public interface.
  • For more security, dangerous commands can be disabled(renamed). But be careful with them in cluster environment.
  • Redis also provides simple authentication mechanism. It is not covered here because of scope.

Sentinel management

  • During redis fail-over, config files are rewritten by sentinel program. So when restarting redis-cluster, be careful.

Sources

  • https://redis.io/topics/cluster-tutorial
  • https://redis.io/topics/security
  • https://redis.io/commands/debug-segfault