Cache

Question 1

remember: network calls are…

Answer 1

blocking

Question 2

primary use case for caching

Answer 2

you find yourself frequently accessing the same data from a remote server.

Question 3

define a cache ‘miss’

miss-rate

Answer 3

data is requested from cache, and it doesn’t have it

miss-rate: the percentage of requests that were misses

Question 4

define a cache ‘hit’

hit-rate

Answer 4

data is requested from cache, and it has the data

hit-rate: the percentage of requests that were hits

Question 5

in Guava cache framework, the 3 core classes

Answer 5

LoadingCache
CacheBuilder
CacheLoader

Question 6

Loading Cache (Guava framework)

Answer 6

This class manages a cache, keeping objects in a Map and automatically retrieving new data from the data store on a cache miss

Question 7

CacheBuilder (Guava framework)

Answer 7

This factory class creates properly configured instances of LoadingCache

Question 8

CacheLoader (Guava framework)

Answer 8

LoadingCache uses a CacheLoader that we configure to retrieve new data from the data store.

Question 9

high miss-rates indicate that…

Answer 9

…many requests make the expensive request AND add overhead to check and update the cache.

Question 10

Cache is most helpful when…

Answer 10

… the system requests the same information repeatedly. You benefit from the cache only if you request the same data again and resuse the previous result.

Question 11

Programs with high miss-rates may improve the cache performance by…

Answer 11

…adjusting the cache configuration size
adjusting the cache TTL
evaluating if users reuse the same data
“sweet spot”

Question 12

amount of RAM that Guava defaults to use

Answer 12

Guava defaults to using all available RAM. Devs always set an upper limit on the amount of memory the cache can use, via the CacheBuilder’s method maximumSize()

Question 13

maximumSize()

Answer 13

method in Guava’s CacheBuilder that sets the maximum amount of memory the cache will consume, in number of entries (of the pair)

Question 14

where is maximumSize() set?

Answer 14

this is set in your CustomizedCacheDao constructor:

public CustomCacheDao(DataAccessDao accessDao)
loadingCacheVariable = CacheBuilder.newBuilder()
.maximumSize(50)
.build(CacheLoader.from(accessDao::getData)
);

Question 15

how does Cache evict data?

Answer 15

When the cache already contains the maximum number of entries, and we request a new key, the cache evicts the least recently used entry to make space.

Question 16

staleness

Answer 16

Data that is out of date and no longer valid. The data has changed on the remote source, and is different from what we have in our cache.

Question 17

cache invalidation

Answer 17

difficult process of trying to remove stale data

Question 18

Guava’s expire based on time methods

Answer 18

in CacheBuilder:
.expireAfterAccess(6, TimeUnit.HOURS)
or
.expireAfterWrite(6, TimeUnit.HOURS)

Question 19

Guava’s expire based on time methods

Answer 19

in CacheBuilder:
.expireAfterAccess(6, TimeUnit.HOURS)
or
.expireAfterWrite(6, TimeUnit.HOURS)

used when instantiating the cache, just like .maximumSize() and .build(CacheLoader.from(accessDao::getData)

Question 20

When a cache entry is expired…

Answer 20

…Guava will re-load the information from the remote data store even on a cache hit.

Question 21

expired data term

Answer 21

In caching terminology, the maximum “age” is called Time To Live (TTL), as in “ the item will live in the cache for this amount of time”

Question 22

TTL

Answer 22

depends of your application’s need for fresh data. A tradeoff between hit-rate and staleness

Question 23

.expireAfterAccess(num, TimeUnit.unit)

Answer 23

specifies the amount of time (num) that can elapse after the last time an item in the cache has been accessed before it is invalidated.
“access” refers to writing to the cache and reading from the cache.
As long as the callers request the item more often that that TTL, it won’t be invalidated.
(for num=6, if the cached item hasn’t been used in 6 hours it will be invalidated)
example: frequent items in cart

Question 24

.expireAfterWrite(num, TimeUnit.unit)

Answer 24

invalidates an item in the cache a specified amount of time after it was written to the cache ragardless of how recently it was read.
example: movie recommendations (want to forward recent trends to the customer)

Question 25

Data that remains relatively static

Answer 25

product description
addresses
informational type of records
–>longer TTL

Question 26

Data that remains relatively static

Answer 26

product description
addresses
informational type of records
–>longer TTL
if data has a low chance of becoming stale, or has a minimal impact when stale
–>use .expireAfterAccess with a long duration

Question 27

Data that has a high liklihood of change

Answer 27

product ratings
prices
popular products
–>lower TTL
for data that changes frequently, or which is more sensitive to staleness
–>use .expireAfterWrite with shorter duration

Question 28

If data has the potential to have a negative impact when it becomes stale, or that it might become stale more often…

Answer 28

…use .expireAfterWrite

We shouldn’t make the problem worse by continuing to extend its life each time someone requests it (.expireAfterAccess)