op-node,specs: Clarify channel bank pruning

op-node/rollup/derive/channel_bank.go

@@ -78,9 +78,6 @@ func (cb *ChannelBank) IngestData(data []byte) {
 	origin := cb.Origin()
 	cb.log.Debug("channel bank got new data", "origin", origin, "data_len", len(data))
 
-	// TODO: Why is the prune here?
-	cb.prune()
-
 	frames, err := ParseFrames(data)
 	if err != nil {
 		cb.log.Warn("malformed frame", "err", err)
@@ -108,6 +105,10 @@ func (cb *ChannelBank) IngestData(data []byte) {
 			cb.log.Warn("failed to ingest frame into channel", "channel", f.ID, "frame_number", f.FrameNumber, "err", err)
 			continue
 		}
+
+		// Prune after the frame is loaded.
+		// TODO: Ingest a single frame at a time (must enable reads before ingesting the data / after this prune.)
+		cb.prune()
 	}
 }
 

specs/derivation.md

@@ -521,6 +521,14 @@ As currently implemented, each step in this stage performs the following actions
           frame are discarded.
   - Concatenate the data of the *contiguous frame sequence* (in sequential order) and push it to the next stage.
 
+The ordering of these actions is very important to be consistent across nodes & pipeline resets. The rollup node
+must attempt to do the following in order to maintain a consistent channel bank even in the presence of pruning.
+
+1. Attempt to read as many channels as possible from the channel bank.
+2. Load in a single frame
+3. Check if channel bank needs to be pruned & do so if needed.
+4. Go to step 1 once the channel bank is under it's size limit.
+
 > **TODO** Instead of waiting on the first seen channel (which might not contain the oldest batches, meaning buffering
 > further down the pipeline), we could process any channel in the queue that is ready. We could do this by checking for
 > channel readiness upon writing into the bank, and moving ready channel to the front of the queue.