Rare and random HTTP 503 errors from JPL Horizon on parallax models

[ketozhang]

Less than 1% of the times when I am running a model with parallax (letting pyLIMA fetch ephemerides for me), I get errors from JPL Horizon.

The culprit might be here where too many queries are made to JPL:

pyLIMA/pyLIMA/parallax/JPL_ephemerides.py

Lines 78 to 110 in 92b4f47

	if distances.min() < 0.002: # Low orbits
	# adding exact telescopes dates
	DATES = [dates.tolist()]
	RA = [ra.tolist()]
	DEC = [dec.tolist()]
	DISTANCES = [distances.tolist()]
	start = 0
	while start < len(time_to_treat): # Split the time request in chunk of 50.
	end = start + 50
	obj = Horizons(id=body, location=OBSERVATORY_ID,
	epochs=time_to_treat[start:end])
	ephemerides = obj.ephemerides()
	dates = ephemerides['datetime_jd'].data.data
	ra = ephemerides['RA'].data.data
	dec = ephemerides['DEC'].data.data
	distances = ephemerides['delta'].data.data
	DATES.append(dates)
	RA.append(ra)
	DEC.append(dec)
	DISTANCES.append(distances)
	start = end
	dates = np.concatenate(DATES)
	ra = np.concatenate(RA)
	dec = np.concatenate(DEC)
	distances = np.concatenate(DISTANCES)

However, it is also likely because I am running many models at the same time overloading the Horizon servers.

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A fix that satisfies both issue is to add a "retry with backoff" algorithm (try to query again after waiting an exponentially increasing time)

It should be added to either here...

pyLIMA/pyLIMA/parallax/parallax.py

Lines 181 to 184 in 92b4f47

	from pyLIMA.parallax import JPL_ephemerides
	spacecraft_positions = \
	JPL_ephemerides.horizons_API(satellite_name, time_to_treat,
	observatory='Geocentric')[1]

or as part of JPL_ephemerides.horizons_API itself.

[ketozhang]

I can contribute to this sometime soon.

My idea is to add one of these two libraries

• https://github.com/hynek/stamina
• https://github.com/litl/backoff

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One thing to discuss is whether this is the right place to fix this or should the user handle this themselves (by handling whenever they see a requests.exceptions.HTTPError from pyLIMA)