Read data from cache

An item written to cache by cachelib is associated with a key. To read the item from cache, call the find() method (defined in allocator/CacheAllocator.h) with the key to look up the item:

ReadHandle find(Key key);

For example:

auto handle = cache->find("key1");

The find() method returns a ReadHandle that provides the read-only view of an item. You can use ReadHandle to get the read-only memory location of the data:

const void* pdata = handle->getMemory();

where handle is of type ReadHandle.

The getMemory() method via a ReadHandle returns a const void* pointer. Use reinterpret_cast<const T*> to cast it to a pointer of a specific type const T:

auto data = reinterpret_cast<const T*>(pdata);

For example:

auto handle = cache->find("key1");
if (handle) {
  auto data = folly::StringPiece{reinterpret_cast<const char*>(handle->getMemory()), handle->getSize()};
  std::cout << data << '\n';
}

You can also use iterators to read all the items written to the cache. See Visit data in cache.

To read data from chained items, start from the parent item handle, for example:

auto chainedAllocs = cache->viewAsChainedAllocs(parentItemHandle);
for (auto& c : chainedAllocs.getChain()) {
  auto data = folly::StringPiece{reinterpret_cast<const char*>(c.getMemory()), c.getSize()};
  std::cout << data << '\n';
}

Refer to Chained items to see how chained items are ordered in cache.

To get the nth item in the chain, call the getNthInChain() method via CacheChainedAllocs. The returned nth item will be read-only:

auto chainedAllocs = cache->viewAsChainedAllocs(parentItemHandle);
auto item = chainedAllocs.getNthInChain(1);
if (item) {
  std::cout << folly::StringPiece{reinterpret_cast<const char*>(item->getMemory()), item->getSize()}; << '\n';
}

Note that the first item has index 0, second item has index 1, and so on.