Appendix: Running DOMjudge as a shadow system

It is possible to run DOMjudge as a shadow system behind another system.

Shadowing means that DOMjudge will read events from an external system and mimic all actions described in those events. This is useful if one wants to verify if the results of an external system match DOMjudge, for example to verify both systems give the same judging results. This has been used at the ICPC World Finals for the last few years.

DOMjudge can shadow any system that follows the Contest API specification. It has been tested with recent versions of the ICPC Tools CDS and with DOMjudge itself. Other known systems that implement the specification and that should work are Kattis and PC-Squared.

Configuring DOMjudge

In the DOMjudge admin interface, go to Configuration settings page and modify the settings to mimic the system to shadow from. Also make sure to set data_source to configuration and live data external. This tells DOMjudge that it will be a shadow for an external system. This will:

  • Expose external ID’s in the API for both configuration and live data, i.e. problems, teams, etc. as well as submissions, judgings and runs.
  • Add a Shadow Differences item to the jury menu and homepage for admins.
  • Expose additional information in the submission overview and detail pages.

You can also set the external_ccs_submission_url to a URL template. If you set this, the submission detail page will show a link to the external system. If you use DOMjudge as system to shadow from, you should enter https://url.to.domjudge/jury/submissions/[id].

Importing or creating the contest and problems

The contest needs to exist in DOMjudge and have all problems loaded. It is best to not have any other data for the contest like teams, team categories, team affiliations, submissions and clarifications. The easiest way to create the contest is to import it. For this you need a contest.yaml and a problemset.yaml as described in the CCS requirements specification. Furthermore you need problem directories as described on the problem format page. Place the two YAML files and all directories together in one directory and use the misc-tools/import-contest.sh script to import them. Do not try to import groups, teams or accounts using it, but do import all other things. Note that it will complain that no team is associated with your account so it can’t import jury submissions. That is fine since these submissions will be read using the event feed later on.

Creating the shadowing config file

On the DOMserver, copy the file etc/shadowing.yaml.dist to etc/shadowing.yaml and modify its contents. The .dist file contains comments as to what each field in the file means, but to summarize:

  • As the key pick a recognizable identifier, for example the ID of the contest in the external system.
  • Set id to the (database) ID of the contest in DOMjudge that you created.
  • Set the feed-url, username and password fields to the correct data as provided by the primary system you will be shadowing.
  • Check if you need any of the other options like allow-insecure-ssl. All other options should only be used if you know what you are doing.

Running the event feed import command

After doing all above steps, you can run the event feed import command to start importing events from the primary system. From the DOMserver installation directory, run:

import/import-event-feed key-from-yaml

Where key-from-yaml is the key you configured in the previous step. The command will start running and import events.

If the command loses its connection to the primary system it will automatically reconnect and continue reading where it left off. To stop the command, press ctrl-c. If you rerun the command, it will start again where it left off.

If you ever want to restart reading the feed from the beginning (for example when the primary system had an error and fixed that), you can pass --from-start to the command to start reading from the beginning. Note that the way DOMjudge keeps track of the events that it already processed is by keeping a copy of the event feed in TMPDIR as defined in /etc/domserver-static.php. This feed is named after the key you configured in the YAML file, so if you ever reuse the key, make sure to remove the cached file or pass --from-start.

Viewing shadow differences

In the admin interface, the Shadow Differences page will show all differences between the primary system and DOMjudge. Note that it will only show differences in the final verdict, not on a testcase level.

At the top is a matrix where a summary of the differences is shown. All red cells are actual differences. If they appear in the JU column or row, this simply means one of the systems is not done yet with judging this submission.

Clicking any of the red cells will show detailed information about the differences that correspond to it. You can also use the dropdowns in the Details section. Clicking on a submission will bring you to the submission detail page.

The submission details page shows more information related to the shadow differences:

  • If you set the external_ccs_submission_url configuration option, a link titled View in external CCS will appear taking you to the submission in the primary system. Clicking the external ID just below it will do the same.
  • The external ID and judgement (if any) will be shown.
  • A graph of external testcase runtimes is displayed.
  • The external testcase results are shown, but in a summary line as well as with detailed information.
  • If the primary system has a different verdict than DOMjudge, a warning will be displayed.