chibenwa opened a new pull request, #2740:
URL: https://github.com/apache/james-project/pull/2740
# Why ?
Some mail user agents relies on *partial body retrieval* in order to
simulate "streaming" (and catching back upon transmission errors within a
message scope)
This is most notably true for IOS clients which issues commands like:
```
UID FETCH xx FetchData{items=[UID], setSeen=false,
bodyElements=[BodyFetchElement{firstOctet=0, numberOfOctets=10024,
name=BODY[1.1], sectionType=CONTENT}]
```
(so streaming not the bytes of the message but of it's individual subparts -
we simply cannot work offset into S3)
Using chuncks of 10024 bytes means that in order to download a 10MB files ~
1000 requests needs to be done and thus 1000 S3 reads.
# Proposal
Cache for FETCH of partial body content. (we can bet on locality as a given
connection will be used)
If the fetch concerns a single email and if the fetch includes "streaming of
a body part" via the numberOfOctets + firstOctet either of them being non zero
then cache !
If querying such a request lookup the cache first before reading the store.
Implementation details:
- Use the map of the IMAPSession in order to store the body of the last
partial fetched message
- Use a global atomiclong to track the cache size and enforce it do not
exceed a configured maximum
- Schedule a netty task to cleanup the cache entry after 30 seconds
- Values MUST be wrapped in Weak references in order not to be moved into
old generation by the GC where it would cause memory problems.
- Debug logs upon cache hits, misses in order to review the efficiency of
this mechanism
Let's not use a "global cache" eg with caffeine in order to by design
prevent cross account data leaks.
Default behaviour: NEVER cache.
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