This commit extends compatibility with the bbolt kvdb implementation,
which returns ErrIncompatibleValue in case of a bucket/value key
collision. Furthermore the commit also adds an extra precondition to the
transaction when a key doesn't exist. This is needed as we fix reads to
a snapshot revision and other writers may commit the key otherwise.
This commit removes the lock set which was used to only add bucket keys
to the tx predicate while also bumping their mod version.
This was useful to reduce the size of the compare set but wasn't useful
to reduce contention as top level buckets were always in the lock set.
This commit changes the key derivation algo we use to emulate buckets
similar to bbolt. The issue with prefixing keys with either a bucket or
a value prefix is that the cursor couldn't effectively iterate trough
all keys in a bucket, as it skipped the bucket keys.
While there are multiple ways to fix that issue (eg. two pointers,
iterating value keys then bucket keys, etc), the cleanest is to instead
of prefixes in keys we use a postfix indicating whether a key is a
bucket or a value. This also simplifies all operations where we
(recursively) iterate a bucket and is equivalent with the prefixing key
derivation with the addition that bucket and value keys are now
continous.
This commit extends etcd db with namespaces without additional storage
space requirements. This is simply done by instead of using an all zero
root bucket id, we use the sha256 hash of the name space as our root
bucket id.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
This commit reduces the compare set size the STM will submit in
transactions by adding only the bucket keys along the bucket path to a
specific lock set. This lock set then used to filter the read set,
effectively removing all read only keys from the transaction predicate
that are not bucket keys.
By tracking if a read-write tx actually changes something, we can also
"bump" the mod revision of the bucket keys.
With this trick we essentially implement a read-write lock for our
bucket structure greatly reducing transaction processing time.
