runhash

README

SYNOPSIS

runhash options subcommand [key [...]]

DESCRIPTION

runhash: command line interface for distributed node selection

runhash uses Rendezvous or highest random weight hashing to deterministically choose one or more nodes from a pool of nodes:

Rendezvous or highest random weight (HRW) hashing is an algorithm that allows clients to achieve distributed agreement on a set of k options out of a possible set of n options. A typical application is when clients need to agree on which sites (or proxies) objects are assigned to. (Wikipedia)

runhash deterministically chooses:

• a node from a group of nodes to run a command
• a node or nodes to use for a service

SUBCOMMANDS

sort

Select one or more nodes from a list of nodes. Nodes are read:

• as command line arguments
• space delimited nodes from the RUNHASH_NODES environment variable
• from stdin if - is provided as the argument

Example

runhash sort mykey 127.0.0.1 192.168.1.1 10.0.0.1

RUNHASH_NODES="127.0.0.1 192.168.1.1 10.0.0.1" runhash sort mykey

echo -e "127.0.0.1\n192.168.1.1\n10.0.0.1" | runhash sort mykey -

exec

Execute a command on a subset of nodes chosen from the list of nodes based on the key. The number of nodes chosen is set by the -n option.

The command will always run on this node if either:

• RUNHASH_NODES is empty
• the node is included in RUNHASH_NODES and -n is 0 (all nodes)

Example

RUNHASH_NODES="$(uname -n) foo bar" runhash -n 1 exec mykey ls -al

xargs

Failover a command between a sorted list of nodes. The command is executed sequentially on the sorted list of nodes. If the command exits non-0, the command is run again with the next node in the list.

{} in the command arguments is replaced with the selected node.

Example

RUNHASH_NODES="127.0.0.1 127.1.1.1" runhash xargs mykey nc "{}" 443

# set an environment variable for a command
RUNHASH_NODES="127.0.0.1 127.1.1.1" runhash xargs mykey \
  env TEST_VARIABLE="{}" env

Build

go install go.iscode.ca/runhash/cmd/runhash@latest

To build a reproducible executable from the git repository:

CGO_ENABLED=0 go build -trimpath -ldflags "-w" ./cmd/runhash

# to include the version number
make

OPTIONS

n int : number of nodes to return. Set to 0 to return all nodes.

node string : overrides RUNHASH_NODE environment variable

nodes string : overrides RUNHASH_NODES environment variable

sorted true|false : use the existing sort order for nodes, provided in the environment or command line (default false)

xargs

okexit true|false : the --okexit option does the opposite of bash's set -o errexit: xargs terminates if the command exits with status 0. If --okexit=false, xargs will run the command on all nodes in the list.

replace string : template string (default "{}")

ENVIRONMENT VARIABLES

RUNHASH_NODE="hostname" : Tag used to identify this node in the node list. Usually this will be set to the hostname or an IP address. Defaults to uname -n.

RUNHASH_NODES="" : Space separated list of nodes.