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*Proposal 4*

== TinkerGraph Transaction Support

=== Introduction

Now, if you need to use transactions in TinkerPop, the only solution is to use the Neo4j plugin. Unfortunately, this plugin has not been updated for a long time and is only compatible with Neo4j version 3.4, which reached end of life in March 2020. Gremlin cannot be used as a replacement for other graphs that support transactions.

=== Motivation
* Users will be able to use transactions without the outdated Neo4j plugin.
* Test framework simplification.
* A more robust Gremlin Server setup out-of-the-box.
* A better learning experience for new users.
* A less complicated transaction story for new users.
* A better test graph that is more readily swappable with other graphs.

=== Assumptions

* Need to reuse existing code as much as possible.
* Users should be able to work both with and without transactions.
* Transaction implementation should not have additional external dependencies.
* Existing drivers should work without changes.

=== Specifications
==== Design Overview
The main changes relate to TinkerGraph, need to make a similar solution but with transaction support. Also TinkerGraph by default will have 2 configurations on Gremlin Server and users themselves will choose what they need, TinkerGraph or TinkerTransactionGraph.
Locks will be write-only to reduce the impact of transactions on performance. In this case the isolation level will be `read committed`, the same as now with the Neo4j plugin. 
Only ThreadLocalTransaction is planned, therefore embedded graph transactions may not be fully supported.

==== Overview of changes to code base
===== ElementContainer
Storage for current Element value and ThreadLocal updated value with `isDeleted` flag. Similar for `Vertices` and `Edges`.
[code]
----
class ElementContainer {
    Vertex current;
    ThreadLocal<Vertex> transactionUpdatedValue;
    ThreadLocal<Boolean> isDeleted;
}
----

===== TinkerTransactionGraph
Vertices and Edges will store with version number and dirty transaction values
`Map<Object, ElementContainer> vertices` where `Object` is Vertex Id.
Shared code with TinkerGraph will be moved to AbstractTinkerGraph.

===== TinkerTransactionElement
Add `currentVersion` to all TinkerElements. This is the number of the transaction that last updated it.

===== TinkerThreadLocalTransaction
Extension of AbstractThreadLocalTransaction. Add a unique transaction number. Can use `AtomicLong.incrementAndGet`

===== Graph Feature files
Add TinkerTransactionGraphFeatures, TinkerTransactionGraphVertexFeatures, TinkerTransactionGraphEdgeFeatures, TinkerTransactionGraphGraphFeatures, TinkerTransactionGraphVertexPropertyFeatures.

==== CRUD operations without transaction
1. Wrap into transaction
2. Commit

==== CRUD operations with transaction
===== Create
Add new ElementContainer to `vertices` and `edges` in TinkerTransactionGraph.

==== Read
Read values from `vertices` ElementContainer, `transactionUpdatedValue` if present, otherwise `current`.
if `isDeleted` flag is set, then the element is deleted in transaction and should be skipped.

===== Update
Update corresponding ElementContainer  in `vertices`.

===== Delete
Set `isDeleted` flag in ElementContainer  in `vertices`.

==== Transaction flows
===== Commit flow
To reduce lock time double-checked locking used.
1. Make list of all affected elements sorted by transaction#. The reason is to reduce the lock time.
2. If any has a newer version, then fail.
3. Try to lock all Vertices/Edges changed in transaction. For vertex/edge delete operation also need to lock adjacent edges/vertices. Lock is for write operations only. If some Vertex/Edge is already locked then fail.
4. Check versions again, fail if some element is updated. Code: `Vertices.get(id).current()).currentVersion() !=Vertices.get(id).transactionUpdatedValue().currentVersion()`
5. For all Elements replace current version with value updated in transaction (or remove Element on Delete operation). Cleanup `transactionUpdatedValue`.
6. Change version of all updated Elements.
7. Unlock.
8. Update indexes if needed.

===== Rollback
Cleanup `transactionUpdatedValue` in `vertices`

===== Error
On any error, including transaction conflict:
1. Rollback
2. Throw exception

===== Timeout
Add transactionWatchdog which will rollback locked/expired transactions. Similar to `Session.touch`.
1. If the transaction was open longer than some preconfigured (default 10-20 minutes?) time.
2. If the transaction start commit, but not finish (default 1-2 seconds?).

===== Examples of conflict resolution
* tx1 adds a property to v1, tx2 deletes v1 (same for edges)
transaction that executed first will set the lock on v1, second transaction will fail on one of steps 2-4.
* tx1 adds an edge from v1 to v2, tx2 deletes v1 or v2
If tx1 try to commit first: tx1 add edge, tx2 delete v1 AND all edges including newly added.
If tx2 try to commit first: tx2 deletes v1 or v2, tx1 will fail on step 1.
* tx1 changes a Cardinality.single property to x, tx2 changes the same Cardinality.single property to y
Last transaction will fail on step 2 or 4.
* tx1 adds a new vertex v1, tx2 adds the same vertex (same for edges)
Last transaction will fail on step 2 or 4 or 5.
* tx1 removes vertex v1, tx2 removes the same vertex (same for edges)
Both transactions will be successful. 
* tx1 adds vertex v1, tx2 removes vertex v1 (same for edges)
If v1 exists before and tx1 commit first: tx1 fail on step 5
If v1 exists before and tx2 commit first: tx1 fail on step 2 or 4
If v1 not exists before and tx1 commit first: tx1 ok; tx2 depends on commit time, ok when tx1 finished and fail on step 2 or 4 otherwise.
If v1 not exists before and tx2 commit first: tx2 ok, tx1 ok


==== Additional changes
Split TinkerFactory into AbstractTinkerFactory and TinkerFactory, add TinkerTransactionGraphFactory.
Transfer responsibility of graph update operations from static TinkerHelper to TinkerGraph/TinkerTransactionGraph.
