Major Description
NameNodeRpcServer#getEditsFromTxid currently decides which transactions are able to be served, i.e. which transactions are durable, using the following logic:
long syncTxid = log.getSyncTxId();
// If we haven't synced anything yet, we can only read finalized
// segments since we can't reliably determine which txns in in-progress
// segments have actually been committed (e.g. written to a quorum of JNs).
// If we have synced txns, we can definitely read up to syncTxid since
// syncTxid is only updated after a transaction is committed to all
// journals. (In-progress segments written by old writers are already
// discarded for us, so if we read any in-progress segments they are
// guaranteed to have been written by this NameNode.)
boolean readInProgress = syncTxid > 0;
This assumes that the NameNode serving this request is the current writer/active NameNode, which may not be true in the ObserverNode situation. Since selectInputStreams now has a onlyDurableTxns flag, which, if enabled, will only return durable/committed transactions, we can instead leverage this to provide the same functionality. We should utilize this to avoid consistency issues when serving this request from the ObserverNode.