Optimize Image Storage / Get Requests from Amazon S3 with Picasso - amazon-web-services

Question

I am creating a polling app, and each poll is going to have an associated image of the particular topic.
I am using Firebase to dynamically update polls as events occur. In Firebase, I am storing the relevant Image URL (referencing the URL in Amazon S3), and I am then using Picasso to load the image onto the client's device (see code below).
I have already noticed that I may be handling this data inefficiently, resulting in unnecessary Get requests to my Amazon files in S3. I was wondering what options I have with Picasso (i.e. I am thinking some caching) to pull the images for each client just once and them store them locally (I do not want them to remain on the client's device permanently, however). My goal is to minimize costs but not compromise performance. Below is my current code:
mPollsRef.child(mCurrentDateString).child(homePollFragmentIndexConvertedToFirebaseReferenceImmediatelyBelowDate).addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
int numberOfPollAnswersAtIndexBelowDate = (int) dataSnapshot.child("Poll_Answers").getChildrenCount();
Log.e("TAG", "There are " + numberOfPollAnswersAtIndexBelowDate + " polls answers at index " + homePollFragmentIndexConvertedToFirebaseReferenceImmediatelyBelowDate);
addRadioButtonsWithFirebaseAnswers(dataSnapshot, numberOfPollAnswersAtIndexBelowDate);
String pollQuestion = dataSnapshot.child("Poll_Question").getValue().toString();
mPollQuestion.setText(pollQuestion);
//This is where the image "GET" from Amazon S3 using Picasso begins; the URL is in Firebase and then I use that URL
//with the Picasso.load method
final String mImageURL = (String) dataSnapshot.child("Image").getValue();
Picasso.with(getContext())
.load(mImageURL)
.fit()
.into((ImageView) rootView.findViewById(R.id.poll_image));
}
#Override
public void onCancelled(FirebaseError firebaseError) {
}
});

Answer 1

First, the Picasso instance will hold a memory cache by default (or you can configure it).
Second, disk caching is done by the HTTP client. You should use OkHttp 3+ in 2016. By default, Picasso will make a reasonable default cache with OkHttp if you include OkHttp in your dependencies. You can also set the Downloader when creating the Picasso instance (make sure to set the cache on the client and use OkHttpDownloader or comparable).
Third, OkHttp will respect cache headers, so make sure the max-age and max-stale have appropriate values.