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[v3] Re-opening an existing RemoteStore V3 group causes the first child to be deleted #2359
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I understand what is going on here. I believe this is the intended behavior for our new stores but am open to the idea that we should change it! In Zarr-Python 2, stores were just a mapping interface on top of a storage backend. No access mode was applied at the store level - except for stores that required an access mode ( In Zarr-Python 3, we moved the access mode handling to the store so that we could have a single place where we could enforce access. The simplest way to think of this is that the store is kind of like a file object, opening it in In your example, opened the store in Questions:
Edit: here's a short example using a memory store with what I would consider expected behavior. Note that the only place where data = {}
store = zarr.storage.MemoryStore(data, mode='w')
# create a group with two children
group = zarr.open_group(store=store, zarr_format=3)
print(group.members())
# ()
group.create("A", shape=1)
print(group.members())
# (('A', <Array memory://5393639360/A shape=(1,) dtype=float64>),)
group.create("B", shape=1)
print(group.members())
# (('B', <Array memory://5393639360/B shape=(1,) dtype=float64>), ('A', <Array memory://5393639360/A shape=(1,) dtype=float64>))
# re-open the group in a mode
store2 = zarr.storage.MemoryStore(data, mode='a')
group = zarr.open_group(store=store2, zarr_format=3)
print(group.members())
(('B', <Array memory://5393696256/B shape=(1,) dtype=float64>), ('A', <Array memory://5393696256/A shape=(1,) dtype=float64>))
# re-open the group in w mode
store2 = zarr.storage.MemoryStore(data, mode='w')
group = zarr.open_group(store=store2, zarr_format=3)
print(group.members())
# () |
I think this is a really bad default behavior and will lead to catastrophic scenarios of data loss. Simply opening a store in 'w' mode should absolutely not delete the entire contents of the store. IMO, simply opening or initializing a store should have no side effects regardless of the mode, other than perhaps checking for the existence of the store. On the other hand, the various Array and Group open / create APIs also have a |
I think this is right. @jhamman could you restate (or link to) the original reasoning for moving the |
I don't want to come across like I have a particular stake in the outcome here, but I'll try to describe the argument for how things are today. We can definitely change the semantics and/or order of operations for when actions occure. Zarr-Python 2
Zarr-Python 3
Some additional notes:
|
I spent some time over the weekend thinking through this situation and I've come up with a new proposal.
@zarr-developers/python-core-devs - what do you think about this plan? |
I like it. Especially that the operations are local to the prefix being updated, and are delayed until the operation is invoked. |
I like having less modes and side effects! |
This is a weird and unexpected behavior
Array
A
has disappeared! That's bad!This might have something to do with #2338. Really not sure. I haven't gotten to the bottom of it, but I thought I would post a report anyway.
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