NIFI-4987: Added TTL to RedisDistributedMapCacheClientService

[zenfenan]

Note to reviewers

I have replaced the original getAndPutIfAbsent with a new approach. setNX doesn't take TTL so I modified the function to do this: check exists(key) is false then set(key, value). Do let me know, if there are any concerns.

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[bbende]

@zenfenan the getAndPutIfAbsent method needs to be an atomic operation which is the reason we are using the watch and multi step operation so we can't remove that.

As an example, the HBase implementation is implemented with the same approach you have in this PR and creates the problem described here:

https://stackoverflow.com/questions/49905725/getting-duplicates-with-nifi-hbase-1-1-2-clientmapcacheservice

[bbende]

Was looking into this a little more and I believe we can keep all of the origin getAndPutIfAbsent, and just add the following right after the set:

redisConnection.expire(kv.getKey(), 1000);

Replacing 1000 with whatever time variable.

[zenfenan]

@bbende Thanks for pointing it out.

One question: When the Cache update strategy is set to Keep original and the TTL was initially set to 0 secs (meaning TTL set to -1 or forever) and lets say key1 with a value was set in the cache. Later the TTL was modified to something like 1 hour and key1 comes again. What is expected to be the outcome? I think the TTL has to be updated from -1 to the new value and the flowfile should be sent to success relationship. Correct?

Update
Just realized, if we go by that way, it would introduce unnecessary complexity. So I think it's better to go with set in stone approach when Keep original strategy is used i.e. if 0 secs is set, it will live forever any further flowfile carrying that key will be sent to failure regardless of the TTL modified later, if it is set to some time like 1 hour, till 1 hour all flowfiles carrying the existing key will be sent to failure and after 1 hour, that key will expire.

[bbende]

The TTL concept is really specific to the implementation (Redis in this case). From the perspective of the DMC interface, the API for getAndPutIfAbsent is saying it will only do a put if the key wasn't there. So I wouldn't really expect anything about the key to change unless the put was performed, so I lean towards not updating the TTL when the key was already there because to me the TTL is part of the put which didn't happen.

The current logic in PutDistributedMapCache looks like if "keep original" is chosen and there is an existing value for the key, then it routes to failure, so I don't think that should be changed unless someone believes that is a bug and wasn't the intended behavior.

I was also looking at the commands again, and I noticed this as another way to do a put-if-absent with TTL:

redisConnection.set(kv.getKey(), kv.getValue(), Expiration.milliseconds(-1), RedisStringCommands.SetOption.ifAbsent());

The only issue is that "setnx" returns a boolean that tells us if the set was done which we check after the transaction is done, and "set" is void, so I'm not sure if it is better to just use setnx + expire. You'll have to play with it to see how it works out.

[zenfenan]

@bbende I have made the changes.

[bbende]

@zenfenan thanks for the updates, I think there is also a small changed needed in putIfAbsent correct? If setnx returns true then set expire?

[zenfenan]

@bbende Yeah. My bad. I have now added TTL to putIfAbsent() as well.

[bbende]

Looks good, merged, thanks!