Public Data Access Overview / Discussion Forum

Dear IllustrisTNG Team,
I am using the TNG API to download stellar particle cutouts from TNG100-1 for a study of galaxy structural evolution. Today, 15 October 2026, I have been encountering frequent intermittent HTTP 503 errors.
The requests are made sequentially using Python requests, with automatic retries and increasing delays of 15, 30, and up to 60 seconds. Some downloads succeed, while others repeatedly fail or take several minutes, even for files of only a few MB.
One example of a failed download concerns TNG100-1, snapshot 44, subhalo 36536. The requested stellar fields are Coordinates, Velocities, Masses, GFM_InitialMass, GFM_StellarFormationTime, GFM_Metallicity, and ParticleIDs.
Could you please check whether there is an issue affecting the API or cutout service? Is there a recommended request rate or an alternative way to download these cutouts more reliably?
I can provide a minimal reproducing script and additional error logs if helpful.
Thank you for your assistance.
Best regards,
Mauro D’Onofrio
Dipartimento di Fisica e Astronomia
Università degli Studi di Padova

Here the output:
(.venv) donofrio@MBP-di-Mauro TNG_expanded_controls_2026-10-15 % python3 -u download.py --max-new 0
2/104: TNG100-1_snap034_sub166739_stars.hdf5: exists_valid; 0.8 MiB; 0.0 s
2/104: TNG100-1_snap035_sub171767_stars.hdf5: exists_valid; 0.9 MiB; 0.0 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
3/104: TNG100-1_snap036_sub181837_stars.hdf5: downloaded_valid; 0.9 MiB; 58.5 s
4/104: TNG100-1_snap037_sub149880_stars.hdf5: downloaded_valid; 1.1 MiB; 22.7 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
Errore temporaneo di rete/server. Riprovo tra 30 secondi...
5/104: TNG100-1_snap025_sub64227_stars.hdf5: downloaded_valid; 0.2 MiB; 98.7 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
6/104: TNG100-1_snap026_sub62494_stars.hdf5: downloaded_valid; 0.3 MiB; 87.9 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
7/104: TNG100-1_snap027_sub68982_stars.hdf5: downloaded_valid; 0.7 MiB; 57.7 s
8/104: TNG100-1_snap028_sub81057_stars.hdf5: exists_valid; 0.6 MiB; 0.0 s
9/104: TNG100-1_snap035_sub85513_stars.hdf5: exists_valid; 5.3 MiB; 0.0 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
Errore temporaneo di rete/server. Riprovo tra 30 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
10/104: TNG100-1_snap036_sub87467_stars.hdf5: downloaded_valid; 5.5 MiB; 379.5 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
Errore temporaneo di rete/server. Riprovo tra 30 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
11/104: TNG100-1_snap037_sub95903_stars.hdf5: downloaded_valid; 5.7 MiB; 292.7 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
12/104: TNG100-1_snap038_sub99458_stars.hdf5: downloaded_valid; 5.9 MiB; 62.7 s
13/104: TNG100-1_snap069_sub302473_stars.hdf5: exists_valid; 10.0 MiB; 0.0 s
Errore temporaneo di rete/server. Riprovo tra 15 secondi...
Errore temporaneo di rete/server. Riprovo tra 30 secondi...
Errore temporaneo di rete/server. Riprovo tra 60 secondi...
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The diagnostic log confirms that HTTP 503 is returned by data-eu.tng-project.org, after the API redirect. No Retry-After header is provided. Some requests succeed intermittently, while others repeatedly fail despite sequential requests and increasing retry delays.

I got the same server error in the last few hours trying to download some halo cutouts.

Dear IllustrisTNG Team,

I would like to update my previous report. Today, 19 October 2026, I am also encountering repeated HTTP 503 errors and occasional HTTP 403 responses when accessing the TNG100-1 Stellar Assembly catalogue:

https://www.tng-project.org/api/TNG100-1/files/stellar_assembly.hdf5

I am making sequential HTTP byte-range requests using Python requests, with authentication supplied through the api-key header. My aim is to extract selected catalogue rows without downloading the entire file.

The diagnostic results are:

• The simulation metadata endpoint, https://www.tng-project.org/api/TNG100-1/, returns HTTP 200 with JSON content.
• Some catalogue requests succeed with HTTP 206. We received approximately 11.5 MiB and successfully checked two existing reference measurements.
• Subsequent requests repeatedly return HTTP 503, and sometimes HTTP 403, from data-eu.tng-project.org.
• In the latest diagnostic, even Range: bytes=0-7 returned HTTP 503. This request did not include an If-Range header.
• No Retry-After header was provided.

Requests are sequential, with no concurrent downloads. Transient failures are retried with increasing delays, and successfully received blocks are cached locally.

Could you please check whether there is an issue affecting file access on data-eu.tng-project.org? Are HTTP byte-range requests supported for this catalogue, and is there a recommended request rate?

If this access method is unsuitable, could you recommend an alternative way to extract the in-situ/ex-situ mass fields for a list of 937 snapshot–SubfindID combinations, for example through the TNG Lab?
I can provide the diagnostic JSON and a minimal reproducing script if helpful.
Thank you for your assistance

I am also getting the same error trying to download TNG-SKIRT Atlas