History Array

H: numpy structured array
    A record of runtime attributes and output data for all ensemble members.


libEnsemble uses a NumPy structured array to store information about each point (ensemble member) generated and processed in the ensemble.

The manager maintains a global copy. Each row contains:

  1. Data generated by the gen_f

  2. Resultant output from the sim_f

  3. Reserved fields containing metadata

When the history array is initialized, it creates fields for each gen_specs["out"] and sim_specs["out"] entry. These entries may resemble:

gen_specs["out"] = [("x", float, 2), ("theta", int)]
sim_specs["out"] = [("f", float)]

Therefore, the gen_f and sim_f must return output as NumPy structured arrays for slotting into these fields.

Ensure input/output field names for a function match each other or a reserved field:

gen_specs["out"] = [("x", float, 2), ("theta", int)]  # produces "x" and "theta"
sim_specs["in"] = ["x", "theta", "sim_id"]  # accepts "x", "theta" and "sim_id", a reserved field

Reserved Fields

User fields and reserved fields are combined together in the final History array returned by libEnsemble.

These reserved fields can be modified to adjust how/when a point is evaluated:

  • sim_id [int]: Each unit of work must have a sim_id. This can be set by the generator or by the manager by default. Users should ensure these IDs are sequential and unique when running multiple generators.

  • cancel_requested [bool]: Can be set True in a generator to request attempted cancellation of the corresponding simulation.

The following fields are automatically populated by libEnsemble:

gen_worker [int]: Worker that generated this entry

gen_started_time [float]: Time gen_worker was initiated that produced this entry

gen_ended_time [float]: Time gen_worker requested this entry

sim_worker [int]: Worker that did (or is doing) the sim evaluation for this entry

sim_started [bool]: True if entry was given to sim_worker for sim evaluation

sim_started_time [float]: Time entry was given to sim_worker for a sim evaluation

sim_ended [bool]: True if entry’s sim evaluation completed

sim_ended_time [float]: Time entry’s sim evaluation completed

gen_informed [bool]: True if gen_worker was informed about the sim evaluation of this entry

gen_informed_time [float]: Time gen_worker was informed about the sim evaluation of this entry

kill_sent [bool]: True if a kill signal was sent to worker for this entry

Other than "sim_id" and cancel_requested, these fields cannot be overwritten by user functions unless libE_specs["safe_mode"] is set to False.


Adjusting values in protected fields may crash libEnsemble.

Example Workflow updating History

Step 1: The history array is initialized on the manager

The history array is initialized using the libEnsemble reserved field and the user-provided gen_specs["out"] and sim_specs["out"] entries. In the figure below, only the reserved fields: sim_id, sim_started, and sim_ended are shown for brevity.


gen_f and sim_f functions accept a local history array as the first argument that contains only the rows and fields specified. For new function calls these will be specified by either gen_specs["in"] or sim_specs["in"]. For generators this may be empty.

Step 2: Persistent generator gen_f is called


Step 3: Points are given out for sim_f to evaluate


Step 4: Results returned to persistent generator gen_f