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. Each entry contains multiple named fields; each field can contain different data types, including scalars, arrays, or strings.

The manager maintains the global history array for the ensemble. Each entry stores data generated by the gen_f and then the resultant output from the sim_f, in addition to some reserved fields.

When the history array is initialized, it creates fields in H for each gen_specs["out"] and sim_specs["out"] entry. For a simple example these entries might be:

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

In this example, x is a two-dimensional coordinate, theta represents some integer input parameter, and f is a scalar output of the simulation to be run with the generated x and theta values.

For ease, the gen_f and sim_f are expected to return output as NumPy structured arrays. The output from the gen_f or sim_f functions can be the exact elements declared in the gen/sim_specs["out"] lists, or can be a subset of these lists. (The manager’s history array will update any fields returned to it.)

The names of the input fields for gen_f and sim_f must exist in the manager’s history array (i.e.,~ they must be output fields from gen_f or sim_f or reserved fields). The input fields must be a simple list, for example:

sim_specs["in"] = ["x", "theta"]

Reserved Fields

The manager’s history array also contains several reserved fields. These include a sim_id to globally identify the point (on the manager this is usually the same as the array index). The sim_id can be provided by the user from the gen_f, but is otherwise assigned by the manager as generated points are received.

The reserved boolean field cancel_requested can also be set in a user function to request that libEnsemble cancels evaluation of the point.

The remaining reserved fields are protected (populated by libEnsemble), and store information about each entry. These include boolean fields for the current scheduling status of the point (sim_started when the sim evaluation has started out, sim_ended when sim evaluation has completed, and gen_informed when the sim output has been passed back to the generator). Timing fields give the time (since the epoch) corresponding to each state, and when the point was generated. Other protected fields include the worker IDs on which points were generated or evaluated.

The user fields and the reserved fields together make up the final history array returned by libEnsemble.

The full list of these reserved fields is given below.

  • sim_id [int]: Each unit of work output from gen_f must have an associated sim_id. The generator can assign this, but users must be careful to ensure that points are added in order. For example, if alloc_f allows for two gen_f instances to be running simultaneously, alloc_f should ensure that both don’t generate points with the same sim_id. If the generator does not provide, then a sim_id will be assigned by the manager as generated points are received.

  • cancel_requested [bool]: True if the cancellation of the sim_f evaluation of this entry was requested. This field can be set by the user in a user function.

Protected fields:

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, reserved 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