Every ParaDRAM simulation generates an output restart file whose name ends with either _restart.bin (the default suffix) or _restart.txt. When the file extension is .bin, the contents of the file are written in binary format. Although the binary format is NOT human-readable, it has several advantages to the human-readable ASCII-formatted file suffixed by _restart.txt. The format of the output restart file can be specified via the input specification variable restartFileFormat described here.

The output restart file contains all information that is needed to restart a simulation should runtime interruptions happen.

## How to restart an interrupted ParaDRAM simulation?

Simply rerun the simulation with the same configuration (i.e., the same input specifications). Most importantly, ensure the value of the input variable outputFileName, is set to the same output filename’s prefix by which the simulation output files are generated.

To understand the mechanism behind the restart functionality of the ParaDRAM sampler, you need to know the following facts about the inner-workings of it:

• Every time a ParaDRAM simulation begins, the sampler checks for the value of the simulation specification variable outputFileName which is provided by the user. This serves as the common prefix in the names of all files that are output by the ParaDRAM sampler.
• For example, a user-input value of outputFileName=./out/temp will lead to the generation of a set of ParaDRAM output files with the following names (by the first processor in the simulation),
temp_process_1_progress.txt
temp_process_1_restart.txt
temp_process_1_report.txt
temp_process_1_sample.txt
temp_process_1_chain.txt


which are all stored in the folder ./out/ relative to the current working directory from which you called the sampler.

• If the value of outputFileName ends with a forward- (/) or backward- (\) slash on Windows OS, or with a forward-slash (/) on Linux and Darwin (Mac) OS, then the user-provided value will be treated as the folder name in which the output files will have to be stored. In this case, the ParaDRAM sampler will assign a common random filename-prefix to all of the generated output files (which always starts with ParaDRAM_run_), for example, ParaDRAM_run_20200312_060333_408 in the following filenames,
ParaDRAM_run_20200312_060333_408_process_1_progress.txt

• Every time a ParaDRAM simulation begins, the sampler either gets the output files’ prefix from the user or generates a fresh new prefix as described above. Then it checks for the existence of any collection of output files with this prefix in the working directory of the simulation. Then,
• Fresh simulation – If none of the output files already exist, the simulation begins as a fresh new simulation.
• Simulation crash – If all of the output files with the given prefix already exist in the output path, the simulation will abruptly stop with an error message stating that it cannot overwrite an already existing simulation.
• Restart mode – If all output files exist with the given prefix except the sample file (suffixed with _sample.txt), then the ParaDRAM sampler assumes that this simulation has prematurely ended in the past and that it must restart this unfinished simulation. The reason for this assumption is the following,

Scenario 1 – Suppose you set the specification variable outputFileName = "./output/MyRestartSimulation". Upon starting the simulation, the sampler will generate the following output files ( there will be more than one group of files if parallelizationModel = "multiChain" ),

MyRestartSimulation_process_1_progress.txt
MyRestartSimulation_process_1_restart.txt
MyRestartSimulation_process_1_report.txt
MyRestartSimulation_process_1_sample.txt
MyRestartSimulation_process_1_chain.txt


Then, the simulation gets interrupted and ends prematurely for some reason. To restart this simulation, just rerun the simulation as done in the first time.

Scenario 2 – Suppose you do not provide an input value for the specification variable outputFileName, in which case, the sampler will assign a random prefix to the output files, like the following,

ParaDRAM_run_20200312_060333_408_process_1_progress.txt

You run the simulation which is then interrupted and ends prematurely. In these filenames, anything that appears before _process_ is part of the common prefix that the sampler randomly generates: ParaDRAM_run_20200312_060333_408. Thus, to restart the simulation you will have to set outputFileName = "./ParaDRAM_run_20200312_060333_408" in your restart-simulation-specifications so that the old output files can be found by the sampler upon the simulation restart.