Is the European Court of Justice best understood as an “agent” or a “trustee”?

Is the European Court of Justice best understood as an “agent” or a “trustee”? How do the member states, as principals, seek to control the ECJ? Is this exercise in control best classified as a success or a failure?
Sources: Alter 1996; Kelemen 2012; Tallberg 2002; Thatcher and Stone Sweet 2002.
Teacher advise:
Stick to the topic when writing. Focus on HOW? And WHY? (These are the big questions addressed)
Do not talk about other examples or anything out of text. Specifically on the text at hand explaining how and why.
Format instructions:
You will be expected to fully incorporate course readings in your responses. Each paper must be 3-5 pages in length, double-spaced, 12 point Times New Roman font, standard margins (1” top-bottom, 1.25” left-right), include page numbers and cite all references and conform to either the APA (https://owl.english.purdue.edu/owl/section/2/10/) or MLA (https://owl.english.purdue.edu/owl/resource/747/01/) style guides. Papers that do not meet all of the formatting requirements will not be accepted.