feat: add support for BatchWriteAtLeastOnce

[rajatbhatta]

No description provided.

[olavloite]

When should I create multiple MutationGroups instead of just one large mutation group? What's the tradeoff between a large number of small MutationGroups vs a few large MutationGroups? Is there any limits on the number of mutations that I may have in a MutationGroup?

[rajatbhatta]

When should I create multiple MutationGroups instead of just one large mutation group?

Dependent mutations (to be committed together) should be part of a single MutationGroup.

For example, the following should be part of same MutationGroup:

• Inserting rows into both parent and child tables. If they're part of different MutationGroups, it will result in a failure when a commit on the child table’s row is attempted first.
• Inserting rows into tables T1, T2; T2 has a foreign key pointing to T1. If they're part of different MutationGroups, it will result in a failure when a commit on T2 is attempted first.

If there's a need to create just one large MutationGroup, it's better to use the existing Commit RPC for it.

What's the tradeoff between a large number of small MutationGroups vs a few large MutationGroups?

Depends on the use case. The bottomline is we should only put mutations meant to be committed together in a group, otherwise, should prefer a different group.

Is there any limits on the number of mutations that I may have in a MutationGroup?

Considering that in the base case, each mutation group could be committed into its own transaction, which has a limit of 40k mutations, the same limit would apply to the MutationGroup also.

[olavloite]

LGTM, with a couple of nits on the documentation. This is a non-trivial feature, so making sure that we make the documentation as clear as possible will help our users make the most out of this.