docs/source/user_guide/manifest_cache.rst

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Manifest Cache
==============

Overview
--------

Paimon C++ caches decoded, schema-aligned manifest batches as Arrow IPC streams
in ``ObjectsFile<T>``. The cache uses the public ``Cache`` abstraction and is
enabled through ``ScanContextBuilder::WithCache()``. It covers data manifests,
manifest lists, and index manifests because they all read through ``ObjectsFile<T>``.

For repeated ``get``, ``scan``, or batch ``get/scan -f`` requests in the same
process, the same snapshot often reads the same manifest files repeatedly. On a
cache hit, the read path skips remote filesystem ``open/read`` and Avro/ORC
source-format decoding. It still reads the IPC stream, deserializes objects,
and applies scan filters. Bucket and row-range reads can select entries before
constructing their file metadata. Ordinary, bucket, and row-range reads share
the same complete, query-independent cache entry.

A cold cache load decodes the complete manifest and serializes its aligned
batches into IPC. The loading reader consumes the original batches without an
IPC round trip. Cold bucket reads may therefore decode more entries than
uncached selective reads. Without a cache, reads retain the existing
source-format reader path.

Configuration
-------------

Manifest caching is disabled by default. Embedding applications that need it can
provide a custom ``Cache`` implementation and inject it through ``WithCache``.
Manifest reads create cache keys with ``CacheKind::MANIFEST`` internally, so
callers do not need to pass the cache kind through scan or read contexts. The
same cache instance can be reused across multiple scan or read contexts when
process-local sharing is desired.

Example:

.. code-block:: cpp

   class RoutingCache : public paimon::Cache {
    public:
     RoutingCache(std::shared_ptr<paimon::Cache> default_cache,
                  std::shared_ptr<paimon::Cache> manifest_cache)
         : default_cache_(std::move(default_cache)),
           manifest_cache_(std::move(manifest_cache)) {}

     paimon::Result<std::shared_ptr<paimon::CacheValue>> Get(
         const std::shared_ptr<paimon::CacheKey>& key,
         std::function<paimon::Result<std::shared_ptr<paimon::CacheValue>>(
             const std::shared_ptr<paimon::CacheKey>&)> supplier) override {
       return Select(key)->Get(key, std::move(supplier));
     }

     // Put(), Invalidate(), InvalidateAll(), and Size() route in the same way.

    private:
     std::shared_ptr<paimon::Cache> Select(
         const std::shared_ptr<paimon::CacheKey>& key) const {
       return key && key->GetKind() == paimon::CacheKind::MANIFEST
                  ? manifest_cache_
                  : default_cache_;
     }

     std::shared_ptr<paimon::Cache> default_cache_;
     std::shared_ptr<paimon::Cache> manifest_cache_;
   };

   auto cache = std::make_shared<RoutingCache>(
       std::make_shared<MyDefaultCache>(),
       std::make_shared<MyManifestCache>());

   paimon::ScanContextBuilder scan_builder(table_path);
   scan_builder.WithCache(cache);

Passing ``nullptr`` or omitting ``ScanContextBuilder::WithCache()`` leaves
manifest caching disabled.

Cache Implementation Responsibilities
-------------------------------------

Embedding applications can implement hit/miss and eviction statistics in their
``Cache`` implementation. Coordination of concurrent loads for the same key
also belongs to that implementation; ``ObjectsFile<T>`` does not deduplicate
concurrent cache misses.

Cache implementations should use ``CacheValue::GetMemoryUsage()`` for admission
and eviction accounting. For manifest IPC, this reports the retained Arrow
buffer capacity, including unused space from growth. ``GetSegment().Size()``
continues to describe the valid IPC byte length. The built-in ``LruCache`` uses
the memory usage value; existing cache values without an explicit allocation
size continue to charge their segment length.