  We describe the Zwicky Transient Facility (ZTF) Forced Photometry Service
(ZFPS) as developed and maintained by the ZTF Science Data System Team at
IPAC/Caltech. The service is open for public use following a subscription. The
ZFPS has been operational since early 2020 and has been used to generate
publication quality lightcurves for a myriad of science programs. The ZFPS has
been recently upgraded to allow users to request forced-photometry lightcurves
for up to 1500 sky positions per request in a single web-application
submission. The underlying software has been recoded to take advantage of a
parallel processing architecture with the most compute-intensive component
rewritten in C and optimized for the available hardware. The ZTF processing
cluster consists of 66 compute nodes, each hosting at least 16 physical cores.
The compute nodes are generally idle following nightly real-time processing of
the ZTF survey data and when other ad hoc processing tasks have been completed.
The ZFPS and associated infrastructure at IPAC/Caltech therefore enable
thousands of forced-photometry lightcurves to be generated along with a wealth
of quality metrics to facilitate analyses and filtering of bad quality data
prior to scientific use.
