  Smart contracts are a new paradigm that emerged with the rise of the
blockchain technology. They allow untrusting parties to arrange agreements.
These agreements are encoded as a programming language code and deployed on a
blockchain platform, where all participants execute them and maintain their
state. Smart contracts are promising since they are automated and
decentralized, thus limiting the involvement of third trusted parties, and can
contain monetary transfers. Due to these features, many people believe that
smart contracts will revolutionize the way we think of distributed
applications, information sharing, financial services, and infrastructures.
  To release the potential of smart contracts, it is necessary to connect the
contracts with the outside world, such that they can understand and use
information from other infrastructures. For instance, smart contracts would
greatly benefit when they have access to web content. However, there are many
challenges associated with realizing such a system, and despite the existence
of many proposals, no solution is secure, provides easily-parsable data,
introduces small overheads, and is easy to deploy.
  In this paper we propose PDFS, a practical system for data feeds that
combines the advantages of the previous schemes and introduces new
functionalities. PDFS extends content providers by including new features for
data transparency and consistency validations. This combination provides
multiple benefits like content which is easy to parse and efficient
authenticity verification without breaking natural trust chains. PDFS keeps
content providers auditable, mitigates their malicious activities (like data
modification or censorship), and allows them to create a new business model. We
show how PDFS is integrated with existing web services, report on a PDFS
implementation and present results from conducted case studies and experiments.
