feat(db_api): add an ability to set ReadOnly/ReadWrite connection mode

[IlyaFaer]

Proposal for the user's question: googleapis/python-spanner-sqlalchemy#97
Will require small changes in SQLAlchemy dialect as well.

[larkee]

Read-only transaction in the Python client are represented by the Snapshot class.

Please do not modify transaction.py. Use Snapshot instead.

[IlyaFaer]

@larkee, hm, okay. If we want to use the Snapshot() class, we probably could re-use the existing method for single-use reads. Let me know if you think it's better to use a snapshot as a transaction for several sequential reads.

[larkee]

@larkee, hm, okay. If we want to use the Snapshot() class, we probably could re-use the existing method for single-use reads. Let me know if you think it's better to use a snapshot as a transaction for several sequential reads.

By default, a snapshot is a single-use read-only transaction so it would only be valid for one read operation. However, there is an option make a multi-use snapshot in which all reads are from the same snapshot of data i.e. data is consistent.

Multi-use snapshot example: https://cloud.google.com/spanner/docs/transactions#ro_transaction_example
Single-use snapshot example: https://cloud.google.com/spanner/docs/reads#single_read_methods

With this in mind, I think using multi_use snapshots should only be used when read_only=True and AUTOCOMMIT=False. WDYT?

[olavloite]

With this in mind, I think using multi_use snapshots should only be used when read_only=True and AUTOCOMMIT=False. WDYT?

Yes, that is the intention here. So the implementation should roughly be:

1. read_only=True and auto_commit=False: Use multi_use snapshots and only allow queries/reads. Calling commit() or rollback() should close the current multi-use snapshot, and the next statement that is executed should create a new multi-use snapshot.
2. auto_commit=True (and regardless of the state of read_only): Always use single-use snapshots for queries/reads.

[skuruppu]

I would be a bit worried about doing the extra performance cost of keeping track of checksums for read-write transactions though. I wouldn't be comfortable releasing that. Is it hard to fix @IlyaFaer?

[IlyaFaer]

@skuruppu, of course, it's not. I don't want to sound boring, but I can't see why you both think it's that performance influencing. We don't calculate checksums in read_only - the ifs are already added (8332d49). As for retrying read_only, well, read_only transactions don't fail with Aborted, so they're not gonna be retried anyway. Thus, I don't see why these changes should be urgent and delay other fixes...

Anyway it's pushed.

[olavloite]

... but I can't see why you both think it's that performance influencing. We don't calculate checksums in read_only - the ifs are already added (8332d49).

Sorry, that was my fault. I didn't realize that you had already created that change, as the comment that you referenced above didn't say anything about that.

... Thus, I don't see why these changes should be urgent and delay other fixes...

In my opinion, the current implementation would have been slightly confusing for a potential new developer. The retry mechanism is not needed for read-only transactions. I think that it is better that it is completely clear in the code to prevent anyone from accidentally re-enabling it in the future. (This last commit for example has also removed an unnecessary test case that tested the retry mechanism for a query in autocommit mode; a situation that could not happen, but still was part of the code base.)

Anyways, I'm happy enough with this as-is, but we should add a system test ASAP to actually show that it is working as intended. If @skuruppu and/or @larkee are OK with that, then we could do that in a separate PR.

[skuruppu]

Ultimately @larkee needs to approve this as the code owner so please do add any tests that they ask for before merging.

[larkee]

@IlyaFaer I second what @olavloite said. The refactor is less about performance (since it should be roughly equivalent) but is more about readability and reducing confusion for new developers.

However, if you create a tracking issue for the refactor work, I will approve and merge this PR 👍

[IlyaFaer]

I think all comments are already processed.

I've pushed changes with adding if statements into fetch methods to avoid retrying read_only transactions, etc. (see MaxxleLLC@ac8c4b2). So, I don't think any refactoring left here.

I also returned back that deleted-by-merge-of-another-commit system test. It checks that UPDATE operation will cause an error, while SELECT will work fine.

I think it's worth adding a unit test, which'll check that read_only transactions are not retried on Aborted exception. Otherwise, I think we don't have anything for a separate issue.

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[IlyaFaer]

Looking at automerge label set by googler, I suppose it's not a problem if I'll merge it by myself (while tests are green and there are no changes in the main branch).