What are “LedgerEntry objects” and “Asset pairs” in relation to Stellar Core Configuration?

Question

The following is mentioned in the example config but I am wondering what a LedgerEntry object and Asset pair actually are, in this context.

# Data layer cache configuration
# - ENTRY_CACHE_SIZE controls the maximum number of LedgerEntry objects
#   that will be stored in the cache (default 4096)
# - BEST_OFFERS_CACHE_SIZE controls the maximum number of Asset pairs that
#   will be stored in the cache, although many LedgerEntry objects may be
#   associated with a single Asset pair (default 64)

Thanks for any help.

Answer 1

From https://www.stellar.org/developers/guides/concepts/ledger.html:

The ledger is a collection of entries. Currently there are 4 types of ledger entries. They’re specified in src/xdr/Stellar-ledger-entries.x.

• Account entry
  This entry represents an account. In Stellar, everything is built around accounts: transactions are performed by accounts, and accounts control the access rights to balances.
• Trustline entry
  Trustlines are lines of credit the account has given a particular issuer in a specific currency.
• Offer entry
  Offers are entries that an account creates in the orderbook.
• Data entry
  Data entries are key value pairs attached to an account. They allow account controllers to attach arbitrary data to their account.

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BEST_OFFERS_CACHE_SIZE in theory should control an in-memory cache size for trading pairs, like XLM <> BTC-GAHK..0P2N. As far as I can tell from the source code, best offers cache is used by "in-memory transaction-in-progress" internally to cache offers consumed by current transaction.