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To fence a ledger from the client API, you can simply open it using BookKeeper#asyncOpenLedger or BookKeeper#openLedger. If the ledger you are opening is currently being written to by another ledger, it will automatically be fenced, and the writing client will receive a BKWriteException for any subsequent calls to LedgerHandle#addEntry/LedgerHandle#asyncAddEntry.

However, there are cases where you do not want to fence a ledger when you open it. For example, if you have a standby node tailing the WAL to maintain a up to date state for warm failover, then you do not want to impede the writer. In this case, you can open the ledger with BookKeeper#asyncOpenLedgerNoRecovery or BookKeeper#openLedgerNoRecovery. These methods do not guarantee that you will read the entire ledger, as if there is a concurrent writer, you will only be able to read the prefix of entries which had been confirmed at the time the ledger was opened. In these cases, you must open the ledger again with BookKeeper#asyncOpenLedger or BookKeeper#openLedger before failing over to your warm backup.

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