This page contains definitions for Flux-specific terms and concepts mentioned throughout the various guides and docs on our sites.


An ordered log of timestamped events, stored in the Flux KVS and defined by RFC 18. Eventlogs are used to record job events, capture standard I/O streams, and record resource status changes.

Flux Broker

A distributed message broker that provides communications services within a Flux instance. It is the main controller of the Flux instance and loads all the important job, scheduling, and resource related services for Flux.

Flux IMP (Independent Minister of Privilege)

A system service that allows instance owners to run work on behalf of a guest. In other words, it allows multiple users to use / run jobs on a Flux instance instead of just the owner. More of the nitty gritty details can be found in RFC 15.

Flux Instance

A group of flux-broker processes which are launched via any parallel launch utility that supports PMI (Process Manager Interface).

Flux Shell

A component of Flux that manages the startup and execution of user jobs. You can read more in the flux-shell(1) manpage.

FLUID (Flux Locally Unique ID)

IDs used to represent Flux jobs, defined by RFC 19.

Fractal Scheduling

A term that is sometimes used to describe a tree of Flux instances that are greater than 2 in depth. In other words, a Flux subinstance that has launched additional Flux subinstances.

FSD (Flux Standard Duration)

A Flux standard for human readable time durations. Generally speaking, the suffixes of ‘ms’, ‘s’, ‘m’, ‘h’, ‘d’ are used to indicate time in milliseconds, seconds, minutes, hours, or days respectively. The specifics can be found in RFC 23.


A format supported by many HPC tools to create a compact list of hosts by name, consisting of a common prefix, a number, and an optional suffix. For example, the hosts cow1, cow2, and cow3 could be represented by the hostlist cow[1-3]. 🐮


A compact way of representing an unordered set of integers, defined by RFC 22.


A file which defines the resources, tasks, and other attributes of a single program, defined by RFC 25 and RFC 14. An example can be found in the Example Jobspec section of RFC 25.

Jobtap Plugin

A built-in or external plugin that can be loaded into the job manager broker module. They are an extension of the job manager, arranging for callbacks at different points in a job’s life cycle. Additional information on jobtap plugins can be found in flux-jobtap-plugins(7).

KVS (Key-Value Store)

A component of a Flux instance where information pertaining to the instance is stored in a key-value format. It is used by Flux internally to store data such as resource information, jobspec, logs, and checkpoints. Each job is assigned a KVS namespace, which may also be used by users to store small amounts of data.


Maximum values that are enforced across jobs, queues, and users, such as time and/or resource limits.


A job management data structure that contains sets of jobs in priority order. Multiple queues may be configured for a Flux instance. Queues may differ in what resources are assigned to them, which users are allowed to submit to them, and what defaults/limits may exist for them. Often may be called a “job queue.”

Queue Enable/Disable/Start/Stop

In Flux, the terms “enable”/”disable” are used to describe if jobs can be submitted to a queue. The terms “start”/”stop” are used to describe if jobs can be executed. Under normal operations, a queue is “enabled”/”started,” which allows users to submit jobs and jobs can run. If a queue is “enabled”/”stopped,” it means jobs can be submitted but not allowed to run. This situation is common if resources are temporarily down for maintenance. If a queue is “disabled”/”started,” previously submitted jobs can run, but no more jobs can be submitted going forward. This situation is common if administrators want to take down resources in the future and want currently running jobs to finish.

Queue Drain/Idle

In Flux, the term “drain” is used for waiting for a queue to be empty. It is common to wait for a queue to drain after a queue is disabled. The term “idle” Is used for waiting for all running jobs to finish. It is common to wait for a queue to idle after a queue is stopped.

PMI (Process Manager Interface)

A standard API and wire protocol for communication between MPI runtimes and resource managers.

PMI traditionally provides a key-value store with all-to-all style synchronization to support the bootstrap of parallel programs such as MPI. During wire up, each parallel task can bind to a network endpoint, put its network address into the PMI KVS, synchronize, and then get the addresses of its network peers. All without requiring the process manager (e.g. Flux) to know anything about the network interconnect hardware. Brilliant!

Although there are several variations of PMI, Flux implements the original one from Argonne National Lab which is documented in RFC 13. In addition to providing PMI service to the programs it launches, Flux also uses PMI for its own wire-up when it is launched by Flux or a foreign launcher.

PMI was an unfortunate three letter acronym choice, given that MPI was already a thing. This is known - no whining.


Attributes of a Flux queue that define behavior of the queue, such as time or resource limits.


A resource set defined in JSON (Javascript Object Notation) format, defined by RFC 20.

Shell Plugins

Extensions of a job’s environment that can be configured on a per-job basis using the --setopt option of flux-run(1) and related job submission commands.

Single User Mode

A Flux instance that is used by only one user. This is a common use case when a user runs a subinstance only for themselves.


A Flux instance that is run as a job within another Flux instance. This is commonly done if a user wants to schedule / control a set of resources themselves outside of the parent instance. It is commonly done in single user mode.

System Instance

A Flux instance that is available to all users on a set of resources. Most users will think of this as the “installed” or “default” resource manager/scheduler on a system.

TBON (Tree Based Overlay Network)

The overlay network that Flux brokers wire up amongst themselves in a Flux instance.

URI (Universal Resource Identifier)

Common resource identifier used by many technologies. Used in Flux for identification of Flux brokers. See Wikipedia page for general overview.


The Webster’s dictionary definition is “the sequence of steps involved in moving from the beginning to the end of a working process.” In the context of Flux, this is usually a series or collection of jobs that users execute to accomplish some task. Although not required, typically jobs will be run in a certain order. A number of jobs in the workflow have dependencies and cannot run until prior jobs in the workflow complete.

Last update: Mar 02, 2023