[Feature Request] Enable WriteSerializable Isolation Level

[koertkuipers]

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Feature request

Overview

The codebase for delta shows 2 isolation levels: Serializable and WriteSerializable However in OSS Delta the default isolation level is Serializable, and this cannot be changed. I would like to make this configurable and support WriteSerializable isolation level.

Motivation

Of the two isolation levels i would say generally WriteSerializable is the mode useful. Serializable is too restrictive. For example with Serializable it it not safe to write to 2 unrelated partitions in parallel.

In Databricks platform on aws and azure the default level is WriteSerializable.

After WriteSerializable has been enabled in OSS Delta we should probably make it also the default in a future pullreq.

Further details

It is not clear to me why the isolation level is not exposed in OSS Delta? Perhaps there is a good reason for it? Can someone explain?

Willingness to contribute

We already made the changes in-house to open this up and i can contribute this. However we might have missed a good reason why it is not safe to expose this? Waiting for comments on this first.