Figure out and document how to concurrently backfill and append to a symbol

[poodlewars]

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From @jamesmunro

Implementation question. I think this could be a common use case. I've got a process that appends new rows to a symbol whenever new bars/ticks arrive.

while True:
    df = read_ticks(asset_id)
    lib.append(asset_id, df)
I want to separately change, replace or backfill the history.  The rows I'm changing are before those of the last append.  How can I do that without the risk of losing the rows in one of those appends, or losing the update?
# manual backfill of history from separate process
df = build_backfill(asset_id, endofdayyesterday)
lib.update(asset_id, df)

One idea is to use staged.

Appending process:

while True:
    df = read_ticks(asset_id)
    lib.append(asset_id, df, staged=True)
    lib.finalize_staged_data(asset_id)

Meddling process:

df = build_backfill(asset_id, endofdayyesterday)
lib.update(asset_id, df, staged=True)

Are we think that finalize_staged_data works simultaneous with other staged writes? Is any synchronisation necessary?