What should be the default value of a missing metadataVersion key in an new database?

Using FoundationDB
1

When a new cluster is created, it is possible that the \xFF/metadataVersion system key is not yet present.

I’m adding a bunch of API to return the value of that field as a versionstamp, and was wondering what should be the default value if the key is not present?

Previously, I was returning the bytes as-is, so the value was “null”. I’m considering returning the empty versionstamp (10 zero-bytes) instead. Is this a good default value?

Bonus question: is it ok to expect the value of the metadata version key to always to a valid versionstamp (10 and only 10 bytes) or can it be something else?

2

You couldn’t really write anything else by accident: this is guarded in the client. See https://github.com/apple/foundationdb/blob/d94a1dfef251cc0fd4bf94c4bb80416796d48e6a/fdbclient/ReadYourWrites.actor.cpp#L1530 and https://github.com/apple/foundationdb/blob/d94a1dfef251cc0fd4bf94c4bb80416796d48e6a/fdbclient/ReadYourWrites.actor.cpp#L1604. Maybe if someone were using an old client library without that check and enabled the access system keys option in their transaction for some reason?

I think 10 zero-bytes is a fine default for the intended use, since it’s different from any possible version stamp set in the normal way.

3

Yes but the code does not seem to check the offset of the versionstamp? Could I write a 100-byte value with a versionstamp at offset 42 with a SetVersionstampedValue and still succeed the check?

I’m unsure if I can safely change the signature of accessing the metadataVersion in my binding to return a VersionStamp, instead of returning a byte buffer and only rely on binary ordering to compare it to the previously cached value.

By the way, if SetVersionstampedValue with offset non-zero is allowed for this key, won’t this break cache layers which rely on ordering, since the leading bytes could be whatever ?

4

SetVersionstampedValue with a non-zero offset is not allowed for this key. Only this exact value is allowed: https://github.com/apple/foundationdb/blob/dbfa3dc217f09757ba996d20bdbe1c215d8fc45f/fdbclient/SystemData.cpp#L683, and that value specifies offset zero.

1 Like
5

I now see the check for the operand value in https://github.com/apple/foundationdb/blob/07331ab5fd0687ef790d291c30034e7ef675cbcd/fdbclient/ReadYourWrites.actor.cpp#L1531

This means I have to change some things in my binding because the spot occupied by a versionstamp could be any bytes, while here I have to force it to all-zeroes :slight_smile:

So the good news is that I can safely change the API to expose the metadata version as a parsed versionstamp (with value “0” if missing), instead of a byte buffer.