It's not great to keep lifting functionality higher and higher up the
crate hierarchy, but we really do want to intern while we parse.
Eventually, I expect that we will split the `edn` crate into `types`
and `parsing`, and the `types` crate can depend on a more efficient
interning dependency.
* Part 3: Parameterize Entity by value type.
This isn't quite right, because after parsing, we shouldn't care
about` `edn::ValueAndSpan`, we should care only about edn::Value.
However, I think we can drop `ValueAndSpan` entirely if we just use
`rust-peg` (and its simpler error messages) rather than a mix of
`rust-peg` and `combine`.
In any case, this paves the way to transacting `Entity<TypedValue>`,
which is a nice step towards building general entities.
* Part 1: Add AttributePlace.
* Part 2: Name other places EntityPlace and ValuePlace.
Now we're consistent and closer to self-documenting. Both matter more
as we expose `Entity` as the thing to build for programmatic usage.
* Part 4: Allow Ident and TempId in ValuePlace.
The parser will never produce these, since determining whether an
integer/keyword or string is an ident or a tempid, respectively, in
the value place requires the schema.
But a builder that produces `Entity` instances directly will want to
produce these.
This should address #663, by re-inserting type checking in the
transactor stack after the entry point used by the term builder.
Before this commit, we were using an SQLite UNIQUE index to assert
that no `[e a]` pair, with `a` a cardinality one attribute, was
asserted more than once. However, that's not in line with Datomic,
which treats transaction inputs as a set and allows a single datom
like `[e a v]` to appear multiple times. It's both awkward and not
particularly efficient to look for _distinct_ repetitions in SQL, so
we accept some runtime cost in order to check for repetitions in the
transactor. This will allow us to address #532, which is really about
whether we treat inputs as sets. A side benefit is that we can
provide more helpful error messages when the transactor does detect
that the input truly violates the cardinality constraints of the
schema.
This commit builds a trie while error checking and collecting final
terms, which should be fairly efficient. It also allows a simpler
expression of input-provided :db/txInstant datoms, which in turn
uncovered a small issue with the transaction watcher, where-by the
watcher would not see non-input-provided :db/txInstant datoms.
This transition to Datomic-like input-as-set semantics allows us to
address #532. Previously, two tempids that upserted to the same entid
would produce duplicate datoms, and that would have been rejected by
the transactor -- correctly, since we did not allow duplicate datoms
under the input-as-list semantics. With input-as-set semantics,
duplicate datoms are allowed; and that means that we must allow
tempids to be equivalent, i.e., to resolve to the same tempid.
To achieve this, we:
- index the set of tempids
- identify tempid indices that share an upsert
- map tempids to a dense set of contiguous integer labels
We use the well-known union-find algorithm, as implemented by
petgraph, to efficiently manage the set of equivalent tempids.
Along the way, I've fixed and added tests for two small errors in the
transactor. First, don't drop datoms resolved by upsert (#679).
Second, ensure that complex upserts are allocated.
I don't know quite what happened here. The Clojure implementation
correctly kept complex upserts that hadn't resolved as complex
upserts (see
9a9dfb502a/src/common/datomish/transact.cljc (L436))
and then allocated complex upserts if they didn't resolve (see
9a9dfb502a/src/common/datomish/transact.cljc (L509)).
Based on the code comments, I think the Rust implementation must have
incorrectly tried to optimize by handling all complex upserts in at
most a single generation of evolution, and that's just not correct.
We're effectively implementing a topological sort, using very specific
domain knowledge, and its not true that a node in a topological sort
can be considered only once!
* Make properties on NamespacedKeyword/NamespacedSymbol private
* Use only a single String for NamespacedKeyword/NamespacedSymbol
* Review comments.
* Remove unsafe code in namespaced_name.
Benchmarking shows approximately zero change.
* Allow the types of ns and name to differ when constructing a NamespacedName.
* Make symbol namespaces optional.
* Normalize names of keyword/symbol constructors.
This will make the subsequent refactor much less painful.
* Use expect not unwrap.
* Merge Keyword and NamespacedKeyword.
There are few reasons to do this:
- it's difficult to add symbol interning to combine-based parsers like
tx-parser -- literally every type changes to reflect the interner,
and that means every convenience macro we've built needs to chagne.
It's trivial to add interning to rust-peg-based parsers.
- combine has rolled forward to 3.2, and I spent a similar amount of
time investigating how to upgrade tx-parser (to take advantage of
the new parser! macros in combine that I think are necessary for
adapting to changing types) as I did just converting to rust-peg.
- it's easy to improve the error messages in rust-peg, where-as I have
tried twice to improve the nested error messages in combine and am
stumped.
- it's roughly 4x faster to parse strings directly as opposed to
edn::ValueAndSpan, and it'll be even better when we intern directly.
This innocuous looking change (upserts_ev -> upserts_e -> resolved in
all situations, rather than upserts_ev -> resolved in some situations)
is a significant change in semantics and assumptions in the
transactor. Witness the large comment being removed about the same
tempid resolving in different generations!
To support this change, we provide more holistic errors for
conflicting upserts, which entails collecting some (relatively
expensive) diagnostic data.
I left in some debug logging, simply since it shouldn't hurt in
general, and will likely be useful for the next bug we see in the
transactor.
:db/tx (and Datomic's version, :datomic/tx) suffer from the same
ambiguities that [a v] lookup references do -- determining the type of
the result is context sensitive. (In this case, is :db/tx a reference
to the current transaction ID, or is it a valid keyword?) This commit
addresses the ambiguity by introducing a notion of a transaction
functions, and provides a little scaffolding for adding more (should
the need arise). I left the scaffolding in place rather than handling
just (transaction-tx) because I started trying to
implement (transaction-instant) as well, which is more difficult --
see the comments.
It's worth noting that this approach generalizes more or less directly
to ?input variables, since those can be eagerly bound like the
implemented transaction function (transaction-tx).
* Pre: eliminate some occurrences of Rc, largely through the magic of Into.
* Pre: introduce FromRc to convert between refcounted types.
* Introduce ValueRc as an abstraction over Rc/Arc choice.
* Move Cloned to core.
* Move CString-creation methods to TypedValue.
* Finish transition.
* Use the cache to make constant queries super fast.
* Fix translate tests to match: we no longer generate SQL for many of them!
* Accumulate additions and removals into the cache.
* Make attribute cache clone-on-write; store it in Metadata.
* Allow caching of fulltext attributes, interning strings.
This puts caching in mentat_db, adds a reverse lookup capability for
unique attributes, and populates bidirectional caches with a single
SQL cursor walk.
Differentiate between begin_read and begin_uncached_read.
Note that we still allow toggling within InProgress, because there might be
transient local state that makes starting a new transaction impossible.
Also move `now` into core, implement microsecond truncation.
This is so we don't return a more granular -- and thus subtly different --
timestamp in a `TxReport` than we put into the store.
Pre: export AttributeBuilder from mentat_db.
Pre: fix module-level comment for tx/src/entities.rs.
Pre: rename some `to_` conversions to `into_`.
Pre: make AttributeBuilder::unique less verbose.
Pre: split out a HasSchema trait to abstract over Schema.
Pre: rename SchemaMap/schema_map to AttributeMap/attribute_map.
Pre: TypedValue/NamespacedKeyword conversions.
Pre: turn Unique and ValueType into TypedValue::Keyword.
Pre: export IntoResult.
Pre: export NamespacedKeyword from mentat_core.
Pre: use intern_set in tx.
Pre: add InternSet::len.
Pre: comment gardening.
Pre: remove inaccurate TODO from TxReport comment.
* Pre: rename begin_transaction to begin_tx_application.
* Take an EXCLUSIVE transaction when bootstrapping, and an IMMEDIATE transaction when writing.
This avoids the remote possibility of another write sneaking in the door
while we're preparing to write, avoids us needing to upgrade locks, etc.
After a BEGIN IMMEDIATE, no other database connection will be able to write
to the database or do a BEGIN IMMEDIATE or BEGIN EXCLUSIVE. Other processes
can continue to read from the database, however.
An exclusive transaction causes EXCLUSIVE locks to be acquired on all
databases. After a BEGIN EXCLUSIVE, no other database connection except for
read_uncommitted connections will be able to read the database and no other
connection without exception will be able to write the database until the
transaction is complete.
* Hacky implementation of atomic multi-tx.
* Hold the last report, returning the InProgress from each operation.
* Rewrite transact in terms of InProgress.
* Test rollback.
* Remove unused imports.
* Don't use Rc for transaction reports.
* Pre: break out USER0 as a part boundary constant.
* Export TX0 and USER0 from mentat_db. This is for testing.
* Review comments: commenting.
* Test tempid allocation and rollback.
* Update some dependencies.
* Update rusqlite to 0.12.
* Update error-chain to a forked version that implements Sync.
* Fix some compiler warnings.
* Remove unused imports in tests.
* Parse errors no longer naturally print with the expected symbol.
This commit adds a check to the partition map that a provided entity ID
has been mentioned (i.e., is present in the start:index range of one of
our partitions).
We introduce a newtype for known entity IDs, using this internally in
the tx expander to track user-provided entids that have passed the above
check (and IDs that we allocate as part of tempid processing). This
newtype is stripped prior to tx assertion.
In order that DB tests can continue to write
[:db/add 111 :foo/bar 222]
we add an additional fake partition to our test connections, ranging
from 100 to 1000.
This is an optimization that trades rejecting inputs earlier at the
cost of expressive error messages. It should be possible to recover
the error messages, however.
This will reject input like `[:db/{add,retract} v :attribute/_reversed NOT-AN-ENTITY]`.
There are two broad approaches:
1) Handle reverse attribute notation dynamically, in the style that
Datomic does. This is the most flexible, but it's not a good fit
given that we produce strongly typed output from the parser.
Strongly typed input to the transactor has had many benefits, so I
don't want to roll it back for a relatively unimportant feature
like reverse notation -- especially not since Mentat does not
require :db.install/_attribute to modify schema attributes.
2) Handle reverse attribute in the parser itself, so that we can
produce strongly typed parser output while restricting the input.
I implemented this first and discovered that it's very difficult to
give sensible error messages in common cases.
In any case, the bulk of the code is the same between the two
approaches, and I wrote the tests for the dynamic version (with error
output), so that's what I'm rolling with.
This patch preserves the existing indentation, to highlight the
differences. The next patch will indent.
This is a big commit, but it breaks into two conceptual pieces. The
first is to "parse without copying". We replace a stream of an owned
collection of edn::ValueAndSpan and instead have a stream of a
borrowed collection of &edn::ValueAndSpan references. (Generally,
this is represented as an iterator over a slice, but it can be over
other things too.) Cloning such iterators is constant time, which
improves on cloning an owned collection of edn::ValueAndSpan, which is
linear time in the length of the collection and additional time
depending on the complexity of the EDN values.
The second conceptual piece is to parse keyword maps using a special
parser and a macro to build the parser implementations. Before, we
created a new edn::ValueAndSpan::Map to represent a keyword map in
vector form; since we're working with &edn::ValueAndSpan references
now, we can't create an &edn::ValueAndSpan reference with an
appropriate lifetime. Therefore we generalize the concept of
iteration slightly and turn keyword maps in map form into linear
iterators by flattening the value maps. This is a potentially
obscuring transformation, so we have to take care to protect against
some failure cases. (See the comments and the tests in the code.)
After these changes, parsing using `combine` is linear time (and
reasonably fast).
* Pre: unused import in translate.rs.
* Part 2: take a dependency on rusqlite for query arguments.
* Part 1: flatten V2 schema into V1. Add UUID and URI.
Bump expected ident and bootstrap datom count in tests.
* Part 5: parse edn::Value::Uuid.
* Part 3: extend ValueType and TypedValue to include Uuid.
* Part 4: add Uuid to query arguments.
* Part 6: extend db to support Uuid.
* Part 8: add a tx-parser test for #f NaN and #uuid.
* Part 7: parse and algebrize UUIDs in queries.
* Part 1: parse #inst in EDN and throughout query engine.
* Part 3: handle instants in db.
* Part 2: instants never matches integers in queries.
* Part 4: use DateTime for tx_instants.
* Add a test for adding and querying UUIDs and instants.
* Review comments.
* Pre: Expose more in edn.
* Pre: Make it easier to work with ValueAndSpan.
with_spans() is a temporary hack, needed only because I don't care to
parse the bootstrap assertions from text right now.
* Part 1a: Add `value_and_span` for parsing nested `edn::ValueAndSpan` instances.
I wasn't able to abstract over `edn::Value` and `edn::ValueAndSpan`;
there are multiple obstacles. I chose to roll with
`edn::ValueAndSpan` since it exposes the additional span information
that we will want to form good error messages in the future.
* Part 1b: Add keyword_map() parsing an `edn::Value::Vector` into an `edn::Value::map`.
* Part 1c: Add `Log`/`.log(...)` for logging parser progress.
This is a terrible hack, but it sure helps to debug complicated nested
parsers. I don't even know what a principled approach would look
like; since our parser combinators are so frequently expressed in
code, it's hard to imagine a data-driven interpreter that can help
debug things.
* Part 2: Use `value_and_span` apparatus in tx-parser/.
I break an abstraction boundary by returning a value column
`edn::ValueAndSpan` rather than just an `edn::Value`. That is, the
transaction processor shouldn't care where the `edn::Value` it is
processing arose -- even we care to track that information we should
bake it into the `Entity` type. We do this because we need to
dynamically parse the value column to support nested maps, and parsing
requires a full `edn::ValueAndSpan`. Alternately, we could cheat and
fake the spans when parsing nested maps, but that's potentially
expensive.
* Part 3: Use `value_and_span` apparatus in query-parser/.
* Part 4: Use `value_and_span` apparatus in root crate.
* Review comment: Make Span and SpanPosition Copy.
* Review comment: nits.
* Review comment: Make `or` be `or_exactly`.
I baked the eof checking directly into the parser, rather than using
the skip and eof parsers. I also took the time to restore some tests
that were mistakenly commented out.
* Review comment: Extract and use def_matches_* macros.
* Review comment: .map() as late as possible.
Part 1, core: use Rc for String and Keyword.
Part 2, query: use Rc for Variable.
Part 3, sql: use Rc for args in SQLiteQueryBuilder.
Part 4, query-algebrizer: use Rc.
Part 5, db: use Rc.
Part 6, query-parser: use Rc.
Part 7, query-projector: use Rc.
Part 8, query-translator: use Rc.
Part 9, top level: use Rc.
Part 10: intern Ident and IdentOrKeyword.
* Pre: Fix error in parser macros.
* Pre: Make test unwrapping more verbose.
* Pre: Make lookup refs be (lookup-ref a v) in the entity position.
This has the advantage of being explicit in all situations and
unambiguous at parse-time. This choice agrees with the Clojure
implementation but not with Datomic. Datomic treats [a v] as a lookup
ref, is ambiguous at parse-time, and is disambiguated in ways I do not
understand at transaction time. We mooted making lookup refs [[a v]]
and outlawing nested value vectors in transactions, but after
implementing that approach I decided it was better to handle lookup
refs at parse time and therefore outlawing nested value vectors is not
necessary.
* Handle lookup refs in the entity and value columns. Fixes#183.
* Pre 0a: Use a stack instead of into_iter.
* Pre 0b: Dedent.
* Pre 0c: Handle `e` after `v`.
This allows to use the original `e` while handling `v`.
* Explode value lists for :db.cardinality/many attributes. Fixes#284.
* Parse and accept map notation. Fixes#180.
* Pre: Modernize add() and retract() into one add_or_retract().
* Pre: Add is_collection and is_atom to edn::Value.
* Pre: Differentiate atoms from lookup-refs in value position.
Initially, I expected to accept arbitrary edn::Value instances in the
value position, and to differentiate in the transactor. However, the
implementation quickly became a two-stage parser, since we always
wanted to parse the resulting value position into some other known
thing using the tx-parser. To save calls into the parser and to allow
the parser to move forward with a smaller API surface, I push as much
of this parsing as possible into the initial parse.
* Pre: Modernize entities().
* Pre: Quote edn::Value::Text in Display.
* Review comment: Add and use edn::Value::into_atom.
* Review comment: Use skip(eof()) throughout.
* Review comment: VecDeque instead of Vec.
* Review comment: Part 0: Rename TempId to TempIdHandle.
* Review comment: Part 1: Differentiate internal and external tempids.
This breaks an abstraction boundary by pushing the Internal/External
split up to the Entity level in tx/ and tx-parser/. This just makes
it easier to explode Entity map notation instances into Entity
instances, taking an existing External tempid :db/id or generating a
new Internal tempid as appropriate. To do this without breaking the
abstraction boundary would require adding flexibility to the
transaction processor: we'd need to be able to turn Entity instances
into some internal enum and handle the two cases independently. It
wouldn't be too hard, but this reduces the combinatorial type
explosion.
* Pre: Don't retract :db/ident in test.
Datomic (and eventually Mentat) don't allow to retract :db/ident in
this way, so this runs afoul of future work to support mutating
metadata.
* Pre: s/VALUETYPE/VALUE_TYPE/.
This is consistent with the capitalization (which is "valueType") and
the other identifier.
* Pre: Remove some single quotes from error output.
* Part 1: Make materialized views be uniform [e a v value_type_tag].
This looks ahead to a time when we could support arbitrary
user-defined materialized views. For now, the "idents" materialized
view is those datoms of the form [e :db/ident :namespaced/keyword] and
the "schema" materialized view is those datoms of the form [e a v]
where a is in a particular set of attributes that will become clear in
the following commits.
This change is not backwards compatible, so I'm removing the open
current (really, v2) test. It'll be re-instated when we get to
https://github.com/mozilla/mentat/issues/194.
* Pre: Map TypedValue::Ref to TypedValue::Keyword in debug output.
* Part 3: Separate `schema_to_mutate` from the `schema` used to interpret.
This is just to keep track of the expected changes during
bootstrapping. I want bootstrap metadata mutations to flow through
the same code path as metadata mutations during regular transactions;
by differentiating the schema used for interpretation from the schema
that will be updated I expect to be able to apply bootstrap metadata
mutations to an empty schema and have things like materialized views
created (using the regular code paths).
This commit has been re-ordered for conceptual clarity, but it won't
compile because it references the metadata module. It's possible to
make it compile -- the functionality is there in the schema module --
but it's not worth the rebasing effort until after review (and
possibly not even then, since we'll squash down to a single commit to
land).
* Part 2: Maintain entids separately from idents.
In order to support historical idents, we need to distinguish the
"current" map from entid -> ident from the "complete historical" map
ident -> entid. This is what Datomic does; in Datomic, an ident is
never retracted (although it can be replaced). This approach is an
important part of allowing multiple consumers to share a schema
fragment as it migrates forward.
This fixes a limitation of the Clojure implementation, which did not
handle historical idents across knowledge base close and re-open.
The "entids" materialized view is naturally a slice of the "datoms"
table. The "idents" materialized view is a slice of the
"transactions" table. I hope that representing in this way, and
casting the problem in this light, might generalize to future
materialized views.
* Pre: Add DiffSet.
* Part 4: Collect mutations to a `Schema`.
I haven't taken your review comment about consuming AttributeBuilder
during each fluent function. If you read my response and still want
this, I'm happy to do it in review.
* Part 5: Handle :db/ident and :db.{install,alter}/attribute.
This "loops" the committed datoms out of the SQL store and back
through the metadata (schema, but in future also partition map)
processor. The metadata processor updates the schema and produces a
report of what changed; that report is then used to update the SQL
store. That update includes:
- the materialized views ("entids", "idents", and "schema");
- if needed, a subset of the datoms themselves (as flags change).
I've left a TODO for handling attribute retraction in the cases that
it makes sense. I expect that to be straight-forward.
* Review comment: Rename DiffSet to AddRetractAlterSet.
Also adds a little more commentary and a simple test.
* Review comment: Use ToIdent trait.
* Review comment: partially revert "Part 2: Maintain entids separately from idents."
This reverts commit 23a91df9c35e14398f2ddbd1ba25315821e67401.
Following our discussion, this removes the "entids" materialized
view. The next commit will remove historical idents from the "idents"
materialized view.
* Post: Use custom Either rather than std::result::Result.
This is not necessary, but it was suggested that we might be paying an
overhead creating Err instances while using error_chain. That seems
not to be the case, but this change shows that we don't actually use
any of the Result helper methods, so there's no reason to overload
Result. This change might avoid some future confusion, so I'm going
to land it anyway.
Signed-off-by: Nick Alexander <nalexander@mozilla.com>
* Review comment: Don't preserve historical idents.
* Review comment: More prepared statements when updating materialized views.
* Post: Test altering :db/cardinality and :db/unique.
These tests fail due to a Datomic limitation, namely that the marker
flag :db.alter/attribute can only be asserted once for an attribute!
That is, [:db.part/db :db.alter/attribute :attribute] will only be
transacted at most once. Since older versions of Datomic required the
:db.alter/attribute flag, I can only imagine they either never wrote
:db.alter/attribute to the store, or they handled it specially. I'll
need to remove the marker flag system from Mentat in order to address
this fundamental limitation.
* Post: Remove some more single quotes from error output.
* Post: Add assert_transact! macro to unwrap safely.
I was finding it very difficult to track unwrapping errors while
making changes, due to an underlying Mac OS X symbolication issue that
makes running tests with RUST_BACKTRACE=1 so slow that they all time
out.
* Post: Don't expect or recognize :db.{install,alter}/attribute.
I had this all working... except we will never see a repeated
`[:db.part/db :db.alter/attribute :attribute]` assertion in the store!
That means my approach would let you alter an attribute at most one
time. It's not worth hacking around this; it's better to just stop
expecting (and recognizing) the marker flags. (We have all the data
to distinguish the various cases that we need without the marker
flags.)
This brings Mentat in line with the thrust of newer Datomic versions,
but isn't compatible with Datomic, because (if I understand correctly)
Datomic automatically adds :db.{install,alter}/attribute assertions to
transactions.
I haven't purged the corresponding :db/ident and schema fragments just
yet:
- we might want them back
- we might want them in order to upgrade v1 and v2 databases to the
new on-disk layout we're fleshing out (v3?).
* Post: Don't make :db/unique :db.unique/* imply :db/index true.
This patch avoids a potential bug with the "schema" materialized view.
If :db/unique :db.unique/value implies :db/index true, then what
happens when you _retract_ :db.unique/value? I think Datomic defines
this in some way, but I really want the "schema" materialized view to
be a slice of "datoms" and not have these sort of ambiguities and
persistent effects. Therefore, to ensure that we don't retract a
schema characteristic and accidentally change more than we intended
to, this patch stops having any schema characteristic imply any other
schema characteristic(s). To achieve that, I added an
Option<Unique::{Value,Identity}> type to Attribute; this helps with
this patch, and also looks ahead to when we allow to retract
:db/unique attributes.
* Post: Allow to retract :db/ident.
* Post: Include more details about invalid schema changes.
The tests use strings, so they hide the chained errors which do in
fact provide more detail.
* Review comment: Fix outdated comment.
* Review comment: s/_SET/_SQL_LIST/.
* Review comment: Use a sub-select for checking cardinality.
This might be faster in practice.
* Review comment: Put `attribute::Unique` into its own namespace.
* Test collecting tempids after upsert resolution. Fixes#299.
I just didn't finish and expose the tempid collection when I
implemented upsert resolution. Here it is!
* Review comment: Take ownership of temp_id_map; avoid contains_key().
* Pre: Order datoms deterministically in debug output.
This makes comparison much easier, and avoids a whole class of
difficult problems when introducing pattern matching with placeholder
values.
* Pre: Don't rewrite ?txN and ?msN in debug module into_edn() methods.
* Convert EDN transaction tests to Rust code. Fixes#271.
This implements
https://github.com/mozilla/mentat/issues/271#issuecomment-283125963.
I'm using the EDN pattern matching functionality
internally (extensively!), but specifically working around the tricky
edges we encountered. This should let us implement tests quickly (and
hopefully legibly) while not requiring us to encode as much behaviour
into non-standard EDN notations.
* Part 1: added limits feature to rusqlite dependencies.
* Part 2: replace references to SQLITE_MAX_VARIABLE_NUMBER with sqlite3_limit.
* Move assertion check for correct number of variables in repeat_values to before call as this is where the variable is defined.
* Part 3: add tests
* Add a failing test for EDN parsing '…'.
* Expose a SQLValueType trait to get value_type_tag values out of a ValueType.
* Add accessors to FindSpec.
* Implement querying.
* Implement rudimentary projection.
* Export mentat_db::new_connection.
* Export symbols from mentat.
* Add rudimentary end-to-end query tests.
* Add top-level `Conn`. Fixes#296.
This is a little different than the API rnewman and I originally
discussed in https://public.etherpad-mozilla.org/p/db-conn-thoughts.
A few notes:
- I was led to make a `Schema` instance the thing that is shared,
rather than a `db::DB`. It's possible that queries will want to
know the current transaction at some point (to prevent races, or to
query historical data), but that can be a future consideration.
- The generation number just allows for a cheap comparison. I don't
care to handle races to transact just yet; the long term plan might
be to make embedding applications responsible for avoiding races, or
we might handle queuing transactions and yielding report futures in
Mentat itself.
- The sharing of the partition maps is a little more subtle than
expected. Partition maps are volatile: a successful Mentat
transaction always advances the :db.part/tx partition, so it's not
worth passing references around. This means that consumers must
clone in order to maintain just a single clone per transaction.
Clean some cruft.
* Review comments.
