[id='async_return_values-{context}']
= Return Values in Asynchronous Communication
Because the `Cache` interface extends `java.util.Map`, write methods like
`put(key, value)` and `remove(key)` return the previous value by default.

In some cases, the return value may not be correct:

1. When using `AdvancedCache.withFlags()` with `Flag.IGNORE_RETURN_VALUE`,
`Flag.SKIP_REMOTE_LOOKUP`, or `Flag.SKIP_CACHE_LOAD`.
2. When the cache is configured with `unreliable-return-values="true"`.
3. When using asynchronous communications.
4. When there are multiple concurrent writes to the same key, and the cache topology
changes.
The topology change will make {brandname} retry the write operations, and a retried
operation's return value is not reliable.

Transactional caches return the correct previous value in cases 3 and 4.
However, transactional caches also have a gotcha: in distributed mode, the
read-committed isolation level is implemented as repeatable-read.
That means this example of "double-checked locking" won't work:

[source,java]
----
include::code_examples/TransactionDoubleCheckedLocking.java[]
----

The correct way to implement this is to use
`cache.getAdvancedCache().withFlags(Flag.FORCE_WRITE_LOCK).get(k)`.

In caches with optimistic locking, writes can also return stale previous values. Write skew checks can avoid stale previous values.
