////
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this work for additional information regarding copyright ownership.
The ASF licenses this file to You under the Apache License, Version 2.0
(the "License"); you may not use this file except in compliance with
the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
////

= TinkerPop 3.4.0

image::https://raw.githubusercontent.com/apache/tinkerpop/master/docs/static/images/avant-gremlin.png[width=225]

*Avant-Gremlin Construction #3 for Theremin and Flowers*

== TinkerPop 3.4.8

*Release Date: NOT OFFICIALLY RELEASED YET*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.9/CHANGELOG.asciidoc#release-3-4-9[changelog] for a
complete list of all the modifications that are part of this release.



*Release Date: August 3, 2020*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.8/CHANGELOG.asciidoc#release-3-4-8[changelog] for a
complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Gremlin.NET: Automatic Reconnect

The Gremlin.NET driver now automatically tries to reconnect to a server when no open connection is available to submit
a request. This should enable the driver to handle cases where the server is only temporarily unavailable or where the
server has closed connections which some graph providers do when no requests were sent for some time.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2288[TINKERPOP-2288]

== TinkerPop 3.4.7

*Release Date: June 1, 2020*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.7/CHANGELOG.asciidoc#release-3-4-7[changelog] for a
complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Clear Screen Command

Gremlin Console now has the `:cls` command to clear the screen. This feature acts as an alternative to platform
specific clear operations and provides a common way to perform that function.

link:https://issues.apache.org/jira/browse/TINKERPOP-2357[TINKERPOP-2357]

== TinkerPop 3.4.6

*Release Date: February 20, 2020*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.6/CHANGELOG.asciidoc#release-3-4-6[changelog] for a
complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== drop() Properties

In 3.4.5 the equality of the `Property` object changed to allow language features like `dedup()` to work more
consistently. An unnoticed side-effect of that change was that calling `drop()` on properties that had the same values
would not properly remove all their instances. This problem affected edge and meta property instances but not the
properties of vertices as their equality definitions had not changed.

This issue is now resolved with the side-effect being that the inclusion of `drop()` will disable `LazyBarrierStrategy`
which prevents automatic bulking. In most common cases, the impact of that optimization loss should be minimal and
could be added back manually with `barrier()` steps in the appropriate places.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2338[TINKERPOP-2338]

== TinkerPop 3.4.5

*Release Date: February 3, 2020*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.5/CHANGELOG.asciidoc#release-3-4-5[changelog] for a
complete list of all the modifications that are part of this release.

WARNING: A link:https://issues.apache.org/jira/browse/TINKERPOP-2338[bug] was noted in 3.4.5 soon after release and
was quickly patched. Users and providers should avoid version 3.4.5 and should instead prefer usage of 3.4.6.

=== Upgrading for Users

==== by(String) Modulator

It is quite common to use the `by(String)` modulator when doing some for of selection operation where the `String` is
the key to the value of the current `Traverser`, demonstrated as follows:

[source,text]
----
gremlin> g.V().project('name').by('name')
==>[name:marko]
==>[name:vadas]
==>[name:lop]
==>[name:josh]
==>[name:ripple]
==>[name:peter]
gremlin> g.V().order().by('name').values('name')
==>josh
==>lop
==>marko
==>peter
==>ripple
==>vadas
----

Of course, this approach usually only works when the current `Traverser` is an `Element`. If it is not an element, the
error is swift and severe:

[source,text]
----
gremlin> g.V().valueMap().project('x').by('name')
java.util.LinkedHashMap cannot be cast to org.apache.tinkerpop.gremlin.structure.Element
Type ':help' or ':h' for help.
Display stack trace? [yN]n
----

and while it is perhaps straightforward to see the problem in the above example, it is not always clear exactly where
the mistake is. The above example is the typical misuse of `by(String)` and comes when one tries to treat a `Map` the
same way as an `Element` (which is quite reasonable).

In 3.4.5, the issue of using `by(String)` on a `Map` and the error messaging have been resolved as follows:

[source,text]
----
gremlin> g.V().valueMap().project('x').by('name')
==>[x:[marko]]
==>[x:[vadas]]
==>[x:[lop]]
==>[x:[josh]]
==>[x:[ripple]]
==>[x:[peter]]
gremlin> g.V().elementMap().order().by('name')
==>[id:4,label:person,name:josh,age:32]
==>[id:3,label:software,name:lop,lang:java]
==>[id:1,label:person,name:marko,age:29]
==>[id:6,label:person,name:peter,age:35]
==>[id:5,label:software,name:ripple,lang:java]
==>[id:2,label:person,name:vadas,age:27]
gremlin> g.V().values('name').project('x').by('name')
The by("name") modulator can only be applied to a traverser that is an Element or a Map - it is being applied to [marko] a String class instead
Type ':help' or ':h' for help.
Display stack trace? [yN]n
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2314[TINKERPOP-2314]

==== hasKey() Step and hasValue() Step

Previously, `hasKey()`-step and `hasValue()`-step only applied to vertex properties. In order to support more
generalized scenarios, the behavior of these steps were modified to allow them to be applied to both edge properties
and meta-properties.

The original behavior is demonstrated as follows:

[source,groovy]
----
gremlin> graph = TinkerFactory.createTheCrew()
==>tinkergraph[vertices:6 edges:14]
gremlin> g = graph.traversal()
==>graphtraversalsource[tinkergraph[vertices:6 edges:14], standard]
gremlin> g.E().properties().hasKey('since')
==>TinkerProperty cannot be cast to Element
gremlin> g.V().properties("location").properties().hasKey("startTime")
==>TinkerProperty cannot be cast to Element
gremlin> g.E().properties().hasValue(2010)
==>TinkerProperty cannot be cast to Element
gremlin> g.V().properties("location").properties().hasValue(2005)
==>TinkerProperty cannot be cast to Element
----

The new behavior of `hasKey()` with edge property can be seen as:

[source,groovy]
----
gremlin> g.E().properties().hasKey('since')
==>p[since->2009]
==>p[since->2010]
==>p[since->2010]
==>p[since->2011]
==>p[since->2012]
----

Similar behavior of for `hasKey()` can be seen with meta-properties:

[source,groovy]
----
gremlin> g.V().properties().hasKey('location').properties().hasKey('startTime')
==>p[startTime->1997]
==>p[startTime->2001]
==>p[startTime->2004]
==>p[startTime->2004]
==>p[startTime->2005]
==>p[startTime->2005]
==>p[startTime->1990]
==>p[startTime->2000]
==>p[startTime->2006]
==>p[startTime->2007]
==>p[startTime->2011]
==>p[startTime->2014]
==>p[startTime->1982]
==>p[startTime->2009]
----

The new behavior for `hasValue()` with edge property is as follows:

[source,groovy]
----
gremlin> g.E().properties().hasValue(2010)
==>p[since->2010]
==>p[since->2010]
----

and similarly with `hasValue()` for meta-properties:

[source,groovy]
----
gremlin> g.V().properties().hasKey('location').properties().hasValue(2005)
==>p[endTime->2005]
==>p[endTime->2005]
==>p[startTime->2005]
==>p[startTime->2005]
----

link:https://issues.apache.org/jira/browse/TINKERPOP-1733[TINKERPOP-1733]

==== Properties Equality

There was some inconsistency in terms of `Property` equality in earlier versions of Gremlin. Equality is now
defined as follows: two properties are equal only if their key and value are equal, even if their parent elements are
not equal. It makes sense when comparing properties regardless of parent elements to just focus on the property itself
as it yields more uniform and concise results to reason about. The benefit of this change is that the behavior of
property comparison and `dedup()`-step are predictable, and it will not affect the result if the property is detached
from the parent element.

NOTE: The "property" here refer to edge properties and meta-properties, thus excluding vertex property.

The old behavior can be shown using "The Crew" toy graph as follows:

[source,groovy]
----
gremlin> g.E().properties().count()
==>13
gremlin> g.E().properties()
==>p[since->2009]
==>p[since->2010]
==>p[skill->4]
==>p[skill->5]
==>p[since->2010]
==>p[since->2011]
==>p[skill->5]
==>p[skill->4]
==>p[since->2012]
==>p[skill->3]
==>p[skill->3]
==>p[skill->5]
==>p[skill->3]
gremlin> g.E().properties().dedup().count()
==>13
gremlin> g.E().dedup().properties()
==>p[since->2009]
==>p[since->2010]
==>p[skill->4]
==>p[skill->5]
==>p[since->2010]
==>p[since->2011]
==>p[skill->5]
==>p[skill->4]
==>p[since->2012]
==>p[skill->3]
==>p[skill->3]
==>p[skill->5]
==>p[skill->3]
----

The new more consistent behavior is demonstrated below:

[source,groovy]
----
gremlin> g.E().properties().count()
==>13
gremlin> g.E().properties()
==>p[since->2009]
==>p[since->2010]
==>p[skill->4]
==>p[skill->5]
==>p[since->2010]
==>p[since->2011]
==>p[skill->5]
==>p[skill->4]
==>p[since->2012]
==>p[skill->3]
==>p[skill->3]
==>p[skill->5]
==>p[skill->3]
gremlin> g.E().properties().dedup().count()
==>7
gremlin> g.E().properties().dedup()
==>p[since->2009]
==>p[since->2010]
==>p[skill->4]
==>p[skill->5]
==>p[since->2011]
==>p[since->2012]
==>p[skill->3]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2318[TINKERPOP-2318]

=== Upgrading for Providers

==== Graph Driver Providers

===== GraphBinary API Change

In GraphBinary serialization, Java `GraphBinaryReader` and `GraphBinaryWriter`, along with `TypeSerializer<T>`
interface now take a `Buffer` instance instead of Netty's `ByteBuf`, that way we avoid exposing Netty's API in our own
public API.

Using our own `Buffer` interface, wrapping Netty's buffer API, allowed us to move `TypeSerializer<T>` implementations,
the reader and the writer to `org.apache.tinkerpop.gremlin.structure.io.binary` package in `gremlin-core`.

Additionally, `GraphBinaryReader` and `GraphBinaryWriter` methods now throw an java's `IOException`, instead of our
own `SerializationException`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2305[TINKERPOP-2305]

== TinkerPop 3.4.4

*Release Date: October 14, 2019*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.4/CHANGELOG.asciidoc#release-3-4-4[changelog] for a complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Python GraphBinary

There is now support for GraphBinary in Python. As with Java, it remains a working but experimental format that is
still under evaluation. This new serializer can be used by first ensuring that it is available on the server and then
configuring the connection as follows:

[source,python]
----
from gremlin_python.driver.serializer import GraphBinarySerializersV1
gremlin_server_url = "ws://172.17.0.2:45940/gremlin"
remote_conn = DriverRemoteConnection(gremlin_server_url, 'g',
                                     message_serializer=GraphBinarySerializersV1())
g = Graph().traversal().withRemote(remote_conn)
----

link:https://issues.apache.org/jira/browse/TINKERPOP-2279[TINKERPOP-2279]

==== elementMap() Step

Since graph elements (i.e. `Vertex`, `Edge`, and `VertexProperty`) are returned from remote sources as references
(i.e. without properties), one of the more common needs for Gremlin users is the ability to easily return a `Map`
representation of the elements that they are querying. Typically, such transformations are handled by `valueMap()`:

[source,text]
----
gremlin> g.V().has('person','name','marko').valueMap(true)
==>[id:1,label:person,name:[marko],age:[29]]
gremlin> g.V().has('person','name','marko').valueMap().by(unfold())
==>[name:marko,age:29]
----

or by way of `project()`:

[source,text]
----
gremlin> g.V().has('person','name','marko').
......1>   project('name','age','vid','vlabel').
......2>     by('name').
......3>     by('age').
......4>     by(id).
......5>     by(label)
==>[name:marko,age:29,vid:1,vlabel:person]
----

While `valueMap()` works reasonably well for `Vertex` and `VertexProperty` transformations it does less well for `Edge`
as it fails to include incident vertices:

[source,text]
----
gremlin> g.E(11).valueMap(true)
==>[id:11,label:created,weight:0.4]
----

This limitation forces a fairly verbose use of `project()` for what is a reasonably common requirement:

[source,text]
----
gremlin> g.E(12).union(valueMap(true),
......1>               project('inV','outV','inVLabel','outVLabel').
......2>                 by(inV().id()).
......3>                 by(outV().id()).
......4>                 by(inV().label()).
......5>                 by(outV().label())).unfold().
......6>               group().
......7>                 by(keys).
......8>                 by(select(values))
==>[inV:3,id:12,inVLabel:software,weight:0.2,outVLabel:person,label:created,outV:6]
----

By introducing `elementMap()`-step, there is now a single step that covers the most common transformation requirements
for all three graph elements:

[source,text]
----
gremlin> g.V().has('person','name','marko').elementMap()
==>[id:1,label:person,name:marko,age:29]
gremlin> g.V().has('person','name','marko').elementMap('name')
==>[id:1,label:person,name:marko]
gremlin> g.V().has('person','name','marko').properties('name').elementMap()
==>[id:0,key:name,value:marko]
gremlin> g.E(11).elementMap()
==>[id:11,label:created,IN:[id:3,label:software],OUT:[id:4,label:person],weight:0.4]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2284[TINKERPOP-2284],
link:https://tinkerpop.apache.org/docs/3.4.4/reference/#elementmap-step[Reference Documentation]

== TinkerPop 3.4.3

*Release Date: August 5, 2019*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.3/CHANGELOG.asciidoc#release-3-4-3[changelog] for a complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Deprecated store()

The `store()`-step and `aggregate()`-step do the same thing in different ways, where the former is lazy and the latter
is eager in the side-effect collection of objects from the traversal. The different behaviors can be thought of as
differing applications of `Scope` where `global` is eager and `local` is lazy. As a result, there is no need for both
steps when one will do.

As of 3.4.3, `store(String)` is now deprecated in favor of `aggregate(Scope, String)` where the `Scope` should be set
to `local` to ensure the same functionality as `store()`. Note that `aggregate('x')` is the same as
`aggregate(global,'x')`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1553[TINKERPOP-1553]

==== Deprecate Gryo in Gremlin Server

Gryo is now deprecated as a serialization format for Gremlin Server, however, it is still configured as a default
option in the sample configuration files packaged with the server. The preferred serialization option should now be
GraphBinary. Note that Gremlin Console is now configured to use GraphBinary by default.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2250[TINKERPOP-2250]

=== Upgrading for Providers

==== Graph Driver Providers

===== Gremlin Server Test Configuration

Gremlin Server has a test configuration built into its Maven build process which all integration tests and Gremlin
Language Variants use to validate their operations. While this approach has worked really well for test automation
within Maven, there are often times where it would be helpful to simply have Gremlin Server running with that
configuration. This need is especially true when developing Gremlin Language Variants which is something that is done
outside of the JVM.

This release introduces a Docker script that will start Gremlin Server with this test configuration. It can be started
with:

[source,text]
docker/gremlin-server.sh

Once started, it is then possible to run GLV tests directly from a debugger against this instance which should
hopefully reduce development friction.

see: link:https://tinkerpop.apache.org/docs/3.4.3/dev/developer/#docker-integration[Developer Documentation]

== TinkerPop 3.4.2

*Release Date: May 28, 2019*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.2/CHANGELOG.asciidoc#release-3-4-2[changelog] for a complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Per Request Options

In 3.4.0, the notion of `RequestOptions` were added so that users could have an easier way to configure settings on
individual requests made through the Java driver. While that change provided a way to set those configurations for
script based requests, it didn't include options to make those settings in a `Traversal` submitted via `Bytecode`. In
this release those settings become available via `with()` step on the `TraversalSource` as follows:

[source,java]
----
GraphTraversalSource g = traversal().withRemote(conf);
List<Vertex> vertices = g.with(Tokens.ARGS_SCRIPT_EVAL_TIMEOUT, 500L).V().out("knows").toList()
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2211[TINKERPOP-2211]

==== Gremlin Console Timeout

The Gremlin Console timeout that is set by `:remote config timeout x` was client-side only in prior versions, which
meant that if the console timeout was less than the server timeout the client would timeout but the server might still
be processing the request. Similarly, a longer timeout on the console would not change the server and the timeout
would occur sooner than expected. These discrepancies often led to confusion.

As of 3.4.0, the Java Driver API allowed for timeout settings to be more easily passed per request, so the console
was modified for this current version to pass the console timeout for each remote submission thus yielding more
consistent and intuitive behavior.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2203[TINKERPOP-2203]

=== Upgrading for Providers

==== Graph System Providers

===== Warnings

It is now possible to pass warnings over the Gremlin Server protocol using a `warnings` status attribute. The warnings
are expected to be a string value or a `List` of string values which can be consumed by the user or tools that check
for that status attribute. Note that Gremlin Console is one such tool that will respond to this status attribute - it
will print the messages to the console as they are detected when doing remote script submissions.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2216[TINKERPOP-2216]

==== Graph Driver Providers

===== GraphBinary API Change

In GraphBinary serialization, Java `write()` and `writeValue()` from `TypeSerializer<T>` interface now take a Netty's
`ByteBuf` instance instead of an `ByteBufAllocator`, that way the same buffer instance gets reused during the write
of a message. Additionally, we took the opportunity to remove the unused parameter from `ResponseMessageSerializer`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2161[TINKERPOP-2161]

== TinkerPop 3.4.1

*Release Date: March 18, 2019*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.1/CHANGELOG.asciidoc#release-3-4-1[changelog] for a complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== Mix SPARQL and Gremlin

In the initial release of `sparql-gremlin` it was only possible to execute a SPARQL query and have it translate to
Gremlin. Therefore, it was only possible to write a query like this:

[source,text]
----
gremlin> g.sparql("SELECT ?name ?age WHERE { ?person v:name ?name . ?person v:age ?age }")
==>[name:marko,age:29]
==>[name:vadas,age:27]
==>[name:josh,age:32]
==>[name:peter,age:35]
gremlin> g.sparql("SELECT * WHERE { }")
==>v[1]
==>v[2]
==>v[3]
==>v[4]
==>v[5]
==>v[6]
----

In this release, however, it is now possible to further process that result with Gremlin steps:

[source,text]
----
gremlin> g.sparql("SELECT ?name ?age WHERE { ?person v:name ?name . ?person v:age ?age }").select("name")
==>marko
==>vadas
==>josh
==>peter
gremlin> g.sparql("SELECT * WHERE { }").out("knows").values("name")
==>vadas
==>josh
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2171[TINKERPOP-2171],
link:https://tinkerpop.apache.org/docs/3.4.1/reference/#sparql-with-gremlin[Reference Documentation]

=== Upgrading for Providers

==== Graph Database Providers

===== GraphBinary Serializer Changes

In GraphBinary serialization, Java `write()` and `writeValue()` from `TypeSerializer<T>` interface now take a Netty's
`ByteBuf` instance instead of an `ByteBufAllocator`, that way the same buffer instance gets reused during the write
of a message. Additionally, we took the opportunity to remove the unused parameter from `ResponseMessageSerializer`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2161[TINKERPOP-2161]

== TinkerPop 3.4.0

*Release Date: January 2, 2019*

Please see the link:https://github.com/apache/tinkerpop/blob/3.4.0/CHANGELOG.asciidoc#release-3-4-0[changelog] for a complete list of all the modifications that are part of this release.

=== Upgrading for Users

==== sparql-gremlin

The `sparql-gremlin` module is a link:https://en.wikipedia.org/wiki/SPARQL[SPARQL] to Gremlin compiler, which allows
SPARQL to be executed over any TinkerPop-enabled graph system.

[source,groovy]
----
graph = TinkerFactory.createModern()
g = graph.traversal(SparqlTraversalSource)
g.sparql("""SELECT ?name ?age
            WHERE { ?person v:name ?name . ?person v:age ?age }
            ORDER BY ASC(?age)""")
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1878[TINKERPOP-1878],
link:https://tinkerpop.apache.org/docs/3.4.0/reference/#sparql-gremlin[Reference Documentation]

==== Gremlin.NET Driver Improvements

The Gremlin.NET driver now uses request pipelining. This allows connections to be reused for different requests in
parallel which should lead to better utilization of connections. The `ConnectionPool` now also has a fixed size
whereas it could previously create an unlimited number of connections. Each `Connection` can handle up to
`MaxInProcessPerConnection` requests in parallel. If this limit is reached for all connections, then a
`NoConnectionAvailableException` is thrown which makes this a breaking change.

These settings can be set as properties on the `ConnectionPoolSettings` instance that can be passed to the `GremlinClient`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1774[TINKERPOP-1774],
link:https://issues.apache.org/jira/browse/TINKERPOP-1775[TINKERPOP-1775],
link:https://tinkerpop.apache.org/docs/3.4.0/reference/#_connection_pool[Reference Documentation]

==== Indexing of Collections

TinkerPop 3.4.0 adds a new `index()`-step, which allows users to transform simple collections into index collections or maps.

```
gremlin> g.V().hasLabel("software").values("name").fold().
......1>   order(local).
......2>   index().unfold()
==>[lop,0]
==>[ripple,1]
gremlin> g.V().hasLabel("person").values("name").fold().
......1>   order(local).by(decr).
......2>   index().
......3>     with(WithOptions.indexer, WithOptions.map)
==>[0:vadas,1:peter,2:marko,3:josh]
```

==== Modulation of valueMap()

The `valueMap()` step now supports `by` and `with` modulation, which also led to the deprecation of `valueMap(true)` overloads.

===== by() Modulation

With the help of the `by()` modulator `valueMap()` result values can now be adjusted, which is particularly useful to turn multi-/list-values into single values.

```
gremlin> g.V().hasLabel("person").valueMap()
==>[name:[marko],age:[29]]
==>[name:[vadas],age:[27]]
==>[name:[josh],age:[32]]
==>[name:[peter],age:[35]]
gremlin> g.V().hasLabel("person").valueMap().by(unfold())
==>[name:marko,age:29]
==>[name:vadas,age:27]
==>[name:josh,age:32]
==>[name:peter,age:35]
```
===== with() Modulation

The `with()` modulator can be used to include certain tokens (`id`, `label`, `key` and/or `value`).

The old way (still valid, but deprecated):

```
gremlin> g.V().hasLabel("software").valueMap(true)
==>[id:10,label:software,name:[gremlin]]
==>[id:11,label:software,name:[tinkergraph]]
gremlin> g.V().has("person","name","marko").properties("location").valueMap(true)
==>[id:6,key:location,value:san diego,startTime:1997,endTime:2001]
==>[id:7,key:location,value:santa cruz,startTime:2001,endTime:2004]
==>[id:8,key:location,value:brussels,startTime:2004,endTime:2005]
==>[id:9,key:location,value:santa fe,startTime:2005]
```

The new way:

```
gremlin> g.V().hasLabel("software").valueMap().with(WithOptions.tokens)
==>[id:10,label:software,name:[gremlin]]
==>[id:11,label:software,name:[tinkergraph]]
gremlin> g.V().has("person","name","marko").properties("location").valueMap().with(WithOptions.tokens)
==>[id:6,key:location,value:san diego,startTime:1997,endTime:2001]
==>[id:7,key:location,value:santa cruz,startTime:2001,endTime:2004]
==>[id:8,key:location,value:brussels,startTime:2004,endTime:2005]
==>[id:9,key:location,value:santa fe,startTime:2005]
```

Furthermore, now there's a finer control over which of the tokens should be included:

```
gremlin> g.V().hasLabel("software").valueMap().with(WithOptions.tokens, WithOptions.labels)
==>[label:software,name:[gremlin]]
==>[label:software,name:[tinkergraph]]
gremlin> g.V().has("person","name","marko").properties("location").valueMap().with(WithOptions.tokens, WithOptions.values)
==>[value:san diego,startTime:1997,endTime:2001]
==>[value:santa cruz,startTime:2001,endTime:2004]
==>[value:brussels,startTime:2004,endTime:2005]
==>[value:santa fe,startTime:2005]
```

As shown above, the support of the `with()` modulator for `valueMap()` makes the `valueMap(boolean)` overload
superfluous, hence this overload is now deprecated. This is a breaking API change, since `valueMap()` will now always
yield instances of type `Map<Object, Object>`. Prior this change only the `valueMap(boolean)` overload yielded
`Map<Object, Object>` objects, `valueMap()` without the boolean parameter used to yield instances of type
`Map<String, Object>`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2059[TINKERPOP-2059]

==== Predicate Number Comparison

In previous versions `within()` and `without()` performed strict number comparisons; that means these predicates did
not only compare number values, but also the type. This was inconsistent with how other predicates (like `eq`, `gt`,
etc.) work. All predicates will now ignore the number type and instead compare numbers only based on their value.

Old behavior:

```
gremlin> g.V().has("age", eq(32L))
==>v[4]
gremlin> g.V().has("age", within(32L, 35L))
gremlin>
```

New behavior:

```
gremlin> g.V().has("age", eq(32L))
==>v[4]
gremlin> g.V().has("age", within(32L, 35L))
==>v[4]
==>v[6]
```

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2058[TINKERPOP-2058]

==== ReferenceElementStrategy

Gremlin Server has had some inconsistent behavior in the serialization of the results it returns. Remote traversals
based on Gremlin bytecode always detach returned graph elements to "reference" (i.e. removes properties and only
include the `id` and `label`), but scripts would detach graph elements and include the properties. For 3.4.0,
TinkerPop introduces the `ReferenceElementStrategy` which can be configured on a `GraphTraversalSource` to always
detach to "reference".

[source,text]
----
gremlin> graph = TinkerFactory.createModern()
==>tinkergraph[vertices:6 edges:6]
gremlin> g = graph.traversal().withStrategies(ReferenceElementStrategy.instance())
==>graphtraversalsource[tinkergraph[vertices:6 edges:6], standard]
gremlin> v = g.V().has('person','name','marko').next()
==>v[1]
gremlin> v.class
==>class org.apache.tinkerpop.gremlin.structure.util.reference.ReferenceVertex
gremlin> v.properties()
gremlin>
----

The packaged initialization scripts that come with Gremlin Server now pre-configure the sample graphs with this
strategy to ensure that both scripts and bytecode based requests over any protocol (HTTP, websocket, etc) and
serialization format all return a "reference". To revert to the old form, simply remove the strategy in the
initialization script.

It is recommended that users choose to configure their `GraphTraversalSource` instances with `ReferenceElementStrategy`
as working with "references" only is the recommended method for developing applications with TinkerPop. In the future,
it is possible that `ReferenceElementStrategy` will be configured by default for all graphs on or off Gremlin Server,
so it would be best to start utilizing it now and grooming existing Gremlin and related application code to account
for it.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2075[TINKERPOP-2075]

==== Text Predicates

Gremlin now supports simple text predicates on top of the existing `P` predicates. Both, the new `TextP` text
predicates and the old `P` predicates, can be chained using `and()` and `or()`.

[source,groovy]
----
gremlin> g.V().has("person","name", containing("o")).valueMap()
==>[name:[marko],age:[29]]
==>[name:[josh],age:[32]]
gremlin> g.V().has("person","name", containing("o").and(gte("j").and(endingWith("ko")))).valueMap()
==>[name:[marko],age:[29]]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2041[TINKERPOP-2041]

==== Changed Infix Behavior

The infix notation of `and()` and `or()` now supports an arbitrary number of traversals and `ConnectiveStrategy`
produces a traversal with proper AND and OR semantics.

```
Input: a.or.b.and.c.or.d.and.e.or.f.and.g.and.h.or.i

*BEFORE*
Output: or(a, or(and(b, c), or(and(d, e), or(and(and(f, g), h), i))))

*NOW*
Output: or(a, and(b, c), and(d, e), and(f, g, h), i)
```

Furthermore, previous versions failed to apply 3 or more `and()` steps using the infix notation, this is now fixed.

[source,groovy]
----
gremlin> g.V().has("name","marko").and().has("age", lt(30)).or().has("name","josh").and().has("age", gt(30)).and().out("created")
==>v[1]
==>v[4]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2029[TINKERPOP-2029]

==== GraphBinary

GraphBinary is a new language agnostic, network serialization format designed to replace Gryo and GraphSON. At this
time it is only available on the JVM, but support will be added for other languages in upcoming releases. The
serializer has been configured in Gremlin Server's packaged configuration files. The serializer can be configured
using the Java driver as follows:

[source,java]
----
Cluster cluster = Cluster.build("localhost").port(8182).
                          serializer(Serializers.GRAPHBINARY_V1D0).create();
Client client = cluster.connect();
List<Result> r = client.submit("g.V().has('person','name','marko')").all().join();
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1942[TINKERPOP-1942],
link:https://tinkerpop.apache.org/docs/3.4.0/dev/io/#graphbinary[IO Documentation]

==== Status Attributes

The Gremlin Server protocol allows for status attributes to be returned in responses. These attributes were typically
for internal use, but were designed with extensibility in mind so that providers could place return their own
attributes to calling clients. Unfortunately, unless the client was being used with protocol level requests (which
wasn't convenient) those attributes were essentially hidden from view. As of this version however, status attributes
are fully retrievable for both successful requests and exceptions.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1913[TINKERPOP-1913]

==== with() Step

This version of TinkerPop introduces the `with()`-step to Gremlin. It isn't really a step but is instead a step
modulator. This modulator allows the step it is modifying to accept configurations that can be used to alter the
behavior of the step itself. A good example of its usage is shown with the revised syntax of the `pageRank()`-step
which now uses `with()` to replace the old `by()` options:

[source,groovy]
----
g.V().hasLabel('person').
  pageRank().
    with(PageRank.edges, __.outE('knows')).
    with(PageRank.propertyName, 'friendRank').
  order().
    by('friendRank',desc).
  valueMap('name','friendRank')
----

A similar change was made for `peerPressure()`-step:

[source,groovy]
----
g.V().hasLabel('person').
  peerPressure().
    with(PeerPressure.propertyName, 'cluster').
  group().
    by('cluster').
    by('name')
----

Note that the `by()` modulators still work, but should be considered deprecated and open for removal in a future
release where breaking changes are allowed.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1975[TINKERPOP-1975],
link:https://tinkerpop.apache.org/docs/3.4.0/reference/#with-step[Reference Documentation]

==== shortestPath() Step

Calculating the link:https://en.wikipedia.org/wiki/Shortest_path_problem[shortest path] between vertices is a common
graph use case. While the traversal to determine a shortest path can be expressed in Gremlin, this particular problem
is common enough that the feature has been encapsulated into its own step, demonstrated as follows:

[source,text]
----
gremlin> g.withComputer().V().has('name','marko').
......1>   shortestPath().with(ShortestPath.target, has('name','peter'))
==>[v[1],v[3],v[6]]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1990[TINKERPOP-1990],
link:https://tinkerpop.apache.org/docs/3.4.0/reference/#shortestpath-step[Reference Documentation]

==== connectedComponent() Step

In prior version of TinkerPop, it was recommended that the identification of
link:https://en.wikipedia.org/wiki/Connected_component_(graph_theory)[Connected Component] instances in a graph be
computed by way of a reasonably complex bit of Gremlin that looked something like this:

[source,groovy]
----
g.V().emit(cyclicPath().or().not(both())).repeat(both()).until(cyclicPath()).
  path().aggregate("p").
  unfold().dedup().
  map(__.as("v").select("p").unfold().
         filter(unfold().where(eq("v"))).
         unfold().dedup().order().by(id).fold()).
  dedup()
----

The above approach had a number of drawbacks that included a large execution cost as well as incompatibilities in OLAP.
To simplify usage of this commonly use graph algorithm, TinkerPop 3.4.0 introduces the `connectedComponent()` step
which reduces the above operation to:

[source,groovy]
----
g.withComputer().V().connectedComponent()
----

It is important to note that this step does require the use of a `GraphComputer` to work, as it utilizes a
`VertexProgram` behind the scenes.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1967[TINKERPOP-1967],
link:https://tinkerpop.apache.org/docs/x.y.z/reference/#connectedcomponent-step[Reference Documentation]

==== io() Step

There have been some important changes to IO operations for reading and writing graph data. The use of `Graph.io()`
has been deprecated to further remove dependence on the Graph (Structure) API for users and to extend these basic
operations to GLV users by making these features available as part of the Gremlin language.

It is now possible to simply use Gremlin:

[source,groovy]
----
graph = ...
g = graph.traversal()
g.io(someInputFile).read().iterate()
g.io(someOutputFile).write().iterate()
----

While `io()`-step is still single-threaded for OLTP style loading, it can be utilized in conjunction with OLAP which
internally uses `CloneVertexProgram` and therefore any graph `InputFormat` or `OutputFormat` can be configured in
conjunction with this step for parallel loads of large datasets.

It is also worth noting that the `io()`-step may be overridden by graph providers to utilize their native bulk-loading
features, so consult the documentation of the implementation being used to determine if there are any improved
efficiencies there.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1996[TINKERPOP-1996],
link:https://tinkerpop.apache.org/docs/3.4.0/reference/#io-step[Reference Documentation]

==== Per Request Options

The Java driver now allows for various options to be set on a per-request basis via new overloads to `submit()` that
accept `RequestOption` instances. A good use-case for this feature is to set a per-request override to the
`scriptEvaluationTimeout` so that it only applies to the current request.

[source,java]
----
Cluster cluster = Cluster.open();
Client client = cluster.connect();
RequestOptions options = RequestOptions.build().timeout(500).create();
List<Result> result = client.submit("g.V()", options).all().get();
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1342[TINKERPOP-1342]

==== min() max() and Comparable

Previously `min()` and `max()` were only working for numeric values. This has been changed and these steps can now
operate over any `Comparable` value. The common workaround was the combination of `order().by()` and `limit()` as
shown here:

[source,groovy]
----
gremlin> g.V().values('name').order().by().limit(1)      // workaround for min()
==>josh
gremlin> g.V().values('name').order().by(decr).limit(1)  // workaround for max()
==>vadas
----

Any attempt to use `min()` or `max()` on non-numeric values lead to an exception:

[source,groovy]
----
gremlin> g.V().values('name').min()
java.lang.String cannot be cast to java.lang.Number
Type ':help' or ':h' for help.
Display stack trace? [yN]
----

With the changes in this release these kind of queries became a lot easier:

[source,groovy]
----
gremlin> g.V().values('name').min()
==>josh
gremlin> g.V().values('name').max()
==>vadas
----

==== Nested Loop Support

Traversals now support nesting of `repeat()` loops.

These can now be used to repeat another traversal while in a looped context, either inside the body of a `repeat()` or
in its step modifiers (`until()` or `emit()`).

[source,groovy]
----
gremlin> g.V().repeat(__.in('traverses').repeat(__.in('develops')).emit()).emit().values('name')
==>stephen
==>matthias
==>marko
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-967[TINKERPOP-967]

==== EventStrategy API

There were some minor modifications to how `EventStrategy` is constructed and what can be expected from events raised
from the addition of new properties.

With respect to the change in terms of `EventStrategy` construction, the `detach()` builder method formerly took a
`Class` as an argument and that `Class` was meant to be one of the various "detachment factories" or `null`. That
approach was a bit confusing, so that signature has changed to `detach(EventStrategy.Detachment)` where the argument
is a more handy enum of detachment options.

As for the changes related to events themselves, it is first worth noting that the previously deprecated
`vertexPropertyChanged(Vertex, Property, Object, Object...)` on `MutationListener` has been removed for what should
have originally been the correct signature of `vertexPropertyChanged(Vertex, VertexProperty, Object, Object...)`. In
prior versions when this method and its related `edgePropertyChanged()` and `vertexPropertyPropertyChanged()` were
triggered by way of the addition of a new property a "fake" property was included with a `null` value for the
"oldValue" argument to these methods (as it did not exist prior to this event). That was a bit awkward to reason about
when dealing with that event. To make this easier, the event now raises with a `KeyedVertexProperty` or
`KeyedProperty` instance, which only contains a property key and no value in them.

link:https://issues.apache.org/jira/browse/TINKERPOP-1831[TINKERPOP-1831]

==== Reducing Barrier Steps

The behavior of `min()`, `max()`, `mean()` and `sum()` has been modified to return no result if there's no input.
Previously these steps yielded the internal seed value:

[source,groovy]
----
gremlin> g.V().values('foo').min()
==>NaN
gremlin> g.V().values('foo').max()
==>NaN
gremlin> g.V().values('foo').mean()
==>NaN
gremlin> g.V().values('foo').sum()
==>0
----

These traversals will no longer emit a result. Note, that this also affects more complex scenarios, e.g. if these
steps are used in `by()` modulators:

[source,groovy]
----
gremlin> g.V().group().
......1>   by(label).
......2>   by(outE().values("weight").sum())
==>[software:0,person:3.5]
----

Since software vertices have no outgoing edges and thus no weight values to sum, `software` will no longer show up in
the result. In order to get the same result as before, one would have to add a `coalesce()`-step:

[source,groovy]
----
gremlin> g.V().group().
......1>   by(label).
......2>   by(outE().values("weight").sum())
==>[person:3.5]
gremlin> g.V().group().
......1>   by(label).
......2>   by(coalesce(outE().values("weight"), constant(0)).sum())
==>[software:0,person:3.5]
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1777[TINKERPOP-1777]

==== Order of select() Scopes

The order of select scopes has been changed to: maps, side-effects, paths. Previously the order was: side-effects,
maps, paths - which made it almost impossible to select a specific map entry if a side-effect with the same name
existed.

The following snippets illustrate the changed behavior:

[source,groovy]
----
gremlin> g.V(1).
......1>   group("a").
......2>     by(__.constant("a")).
......3>     by(__.values("name")).
......4>   select("a")
==>[a:marko]
gremlin> g.V(1).
......1>   group("a").
......2>     by(__.constant("a")).
......3>     by(__.values("name")).
......4>   select("a").select("a")
==>[a:marko]
----

Above is the old behavior; the second `select("a")` has no effect, it selects the side-effect `a` again, although one
would expect to get the map entry `a`. What follows is the new behavior:

[source,groovy]
----
gremlin> g.V(1).
......1>   group("a").
......2>     by(__.constant("a")).
......3>     by(__.values("name")).
......4>   select("a")
==>[a:marko]
gremlin> g.V(1).
......1>   group("a").
......2>     by(__.constant("a")).
......3>     by(__.values("name")).
......4>   select("a").select("a")
==>marko
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1522[TINKERPOP-1522]

==== GraphSON BulkSet

In earlier versions of TinkerPop, `BulkSet` was coerced to a `List` for GraphSON which was convenient in that it
didn't add a new data type to support, but inconvenient in that it meant that certain process tests were not consistent
in terms of how they ran and the benefits of the `BulkSet` were "lost" in that the "bulk" was being resolved server
side. With the addition of `BulkSet` as a GraphSON type the "bulk" is now resolved on the client side by the language
variant. How that resolution occurs depends upon the language variant. For Java, there is a `BulkSet` object which
maintains that structure sent from the server. For the other variants, the `BulkSet` is deserialized to a `List` form
which results in a much larger memory footprint than what is contained the `BulkSet`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2111[TINKERPOP-2111]

==== Python Bindings

Bindings were formerly created using a Python 2-tuple as a bit of syntactic sugar, but all other language variants
used an explicit `Bindings` object which `gremlin-python` already had in place. To make all work variants behave
consistently, the 2-tuple syntax has been removed in favor of the explicit `Bindings.of()` option.

[source,python]
----
g.V(Bindings.of('id',1)).out('created').map(lambda: ("it.get().value('name').length()", "gremlin-groovy")).sum()
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2116[TINKERPOP-2116]

==== Deprecation and Removal

This section describes newly deprecated classes, methods, components and patterns of usage as well as which previously
deprecated features have been officially removed or repurposed.

===== Moving of RemoteGraph

`RemoteGraph` was long ago deprecated in favor of `withRemote()`. It became even less useful with the introduction of
the `AnonymousTraversalSource` concept in 3.3.5. It's only real use was for testing remote bytecode based traversals
in the test suite as the test suite requires an actual `Graph` object to function properly. As such, `RemoteGraph` has
been moved to `gremlin-test`. It should no longer be used in any capacity besides that.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2079[TINKERPOP-2079]

===== Removal of Giraph Support

Support for Giraph has been removed as of this version. There were a number of reasons for this decision which were
discussed in the community prior to taking this step. Users should switch to Spark for their OLAP based graph-computing
needs.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1930[TINKERPOP-1930]

===== Removal of Rebindings Options

The "rebindings" option is no longer supported for clients. It was deprecated long ago at 3.1.0. The server will not
respond to them on any channel - websockets, nio or HTTP. Use the "aliases" option instead.

link:https://issues.apache.org/jira/browse/TINKERPOP-1705[TINKERPOP-1705]

===== gremlin-server.sh -i Removal

The `-i` option for installing dependencies in Gremlin Server was long ago deprecated and has now been removed. Please
use `install` as its replacement going forward.

link:https://issues.apache.org/jira/browse/TINKERPOP-2031[TINKERPOP-2031]

===== Deprecation Removal

The following deprecated classes, methods or fields have been removed in this version:

* `gremlin-core`
** `org.apache.tinkerpop.gremlin.jsr223.ImportCustomizer#GREMLIN_CORE`
** `org.apache.tinkerpop.gremlin.process.remote.RemoteGraph` - moved to `gremlin-test`
** `org.apache.tinkerpop.gremlin.process.remote.RemoteConnection.submit(Traversal)`
** `org.apache.tinkerpop.gremlin.process.remote.RemoteConnection.submit(Bytecode)`
** `org.apache.tinkerpop.gremlin.process.remote.traversal.strategy.decoration.RemoteStrategy#identity()`
** `org.apache.tinkerpop.gremlin.process.traversal.TraversalEngine`
** `org.apache.tinkerpop.gremlin.process.traversal.engine.*`
** `org.apache.tinkerpop.gremlin.process.traversal.strategy.decoration.PartitionStrategy.Builder#addReadPartition(String)`
** `org.apache.tinkerpop.gremlin.process.traversal.strategy.decoration.SubgraphStrategy.Builder#edgeCriterion(Traversal)`
** `org.apache.tinkerpop.gremlin.process.traversal.strategy.decoration.SubgraphStrategy.Builder#vertexCriterion(Traversal)`
** `org.apache.tinkerpop.gremlin.process.traversal.step.map.LambdaCollectingBarrierStep.Consumers`
** `org.apache.tinkerpop.gremlin.process.traversal.step.util.HasContainer#makeHasContainers(String, P)`
** `org.apache.tinkerpop.gremlin.process.traversal.step.util.event.MutationListener#vertexPropertyChanged(Vertex, Property, Object, Object...)`
** `org.apache.tinkerpop.gremlin.structure.Element.Exceptions#elementAlreadyRemoved(Class, Object)`
** `org.apache.tinkerpop.gremlin.structure.Graph.Exceptions#elementNotFound(Class, Object)`
** `org.apache.tinkerpop.gremlin.structure.Graph.Exceptions#elementNotFound(Class, Object, Exception)`
* `gremlin-driver`
** `org.apache.tinkerpop.gremlin.driver.Client#rebind(String)`
** `org.apache.tinkerpop.gremlin.driver.Client.ReboundClusterdClient`
** `org.apache.tinkerpop.gremlin.driver.Tokens#ARGS_REBINDINGS`
* `gremlin-groovy`
** `org.apache.tinkerpop.gremlin.groovy.jsr223.GremlinGroovyScriptEngine.close()` - no longer implements `AutoCloseable`
* `gremlin-server`
** `org.apache.tinkerpop.gremlin.server.GraphManager#getGraphs()`
** `org.apache.tinkerpop.gremlin.server.GraphManager#getTraversalSources()`
** `org.apache.tinkerpop.gremlin.server.Settings#serializedResponseTimeout`
** `org.apache.tinkerpop.gremlin.server.Settings.AuthenticationSettings#className`
** `org.apache.tinkerpop.gremlin.server.handler.OpSelectorHandler(Settings, GraphManager, GremlinExecutor, ScheduledExecutorService)`
** `org.apache.tinkerpop.gremlin.server.op.AbstractOpProcessor#makeFrame(ChannelHandlerContext, RequestMessage, MessageSerializer serializer, boolean, List, ResponseStatusCode code)`
* `hadoop-graph`
** `org.apache.tinkerpop.gremlin.hadoop.structure.HadoopConfiguration#getGraphInputFormat()`
** `org.apache.tinkerpop.gremlin.hadoop.structure.HadoopConfiguration#getGraphOutputFormat()`

Please see the javadoc deprecation notes or upgrade documentation specific to when the deprecation took place to
understand how to resolve this breaking change.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1143[TINKERPOP-1143],
link:https://issues.apache.org/jira/browse/TINKERPOP-1296[TINKERPOP-1296],
link:https://issues.apache.org/jira/browse/TINKERPOP-1705[TINKERPOP-1705],
link:https://issues.apache.org/jira/browse/TINKERPOP-1707[TINKERPOP-1707],
link:https://issues.apache.org/jira/browse/TINKERPOP-1954[TINKERPOP-1954],
link:https://issues.apache.org/jira/browse/TINKERPOP-1986[TINKERPOP-1986],
link:https://issues.apache.org/jira/browse/TINKERPOP-2079[TINKERPOP-2079],
link:https://issues.apache.org/jira/browse/TINKERPOP-2103[TINKERPOP-2103]

===== Deprecated GraphSONMessageSerializerGremlinV2d0

The `GraphSONMessageSerializerGremlinV2d0` serializer is now analogous to `GraphSONMessageSerializerV2d0` and therefore
redundant. It has technically always been equivalent in terms of functionality as both serialized to the same format
(i.e. GraphSON 2.0 with embedded types). It is no longer clear why these two classes were established this way, but
it does carry the negative effect where multiple serializer versions could not be bound to Gremlin Server's HTTP
endpoint as the MIME types conflicted on `application/json`. By simply making both message serializers support
`application/json` and `application/vnd.gremlin-v2.0+json`, it then became possible to overcome that limitation. In
short, prefer use of `GraphSONMessageSerializerV2d0` when possible.

Note that this is a breaking change in the sense that `GraphSONMessageSerializerV2d0` will no longer set the header of
requests messages to `application/json`. As a result, older versions of Gremlin Server not configured with
`GraphSONMessageSerializerGremlinV2d0` will not find a deserializer to match the request.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1984[TINKERPOP-1984]

===== Removed groovy-sql Dependency

Gremlin Console and Gremlin Server no longer include groovy-sql.  If you depend on groovy-sql,
you can install it in Gremlin Console or Gremlin Server using the plugin system.

Console:
```
:install org.codehaus.groovy groovy-sql 2.5.2
```

Server:
```
bin/gremlin-server.sh install org.codehaus.groovy groovy-sql 2.5.2
```

If your project depended on groovy-sql transitively, simply include it in your project's build file (e.g. maven: pom.xml).

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2037[TINKERPOP-2037]

=== Upgrading for Providers

==== Graph Database Providers

===== io() Step

The new `io()`-step that was introduced provides some new changes to consider. Note that `Graph.io()` has been
deprecated and users are no longer instructed to utilize that method. It is not yet decided when that method will be
removed completely, but given the public nature of it and the high chance of common usage, it should be hanging around
for some time.

As with any step in Gremlin, it is possible to replace it with a more provider specific implementation that could be
more efficient. Developing a `TraversalStrategy` to do this is encouraged, especially for those graph providers who
might have special bulk loaders that could be abstracted by this step. Examples of this are already shown with
`HadoopGraph` which replaces the simple single-threaded loader with `CloneVertexProgram`. Graph providers are
encouraged to use the `with()` step to capture any necessary configurations required for their underlying loader to
work. Graph providers should not feel restricted to `graphson`, `gryo` and `graphml` formats either. If a graph
supports CSV or some custom graph specific format, it shouldn't be difficult to gather the configurations necessary to
make that available to users.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1996[TINKERPOP-1996]

===== Caching Graph Features

For graph implementations that have expensive creation times, it can be time consuming to run the TinkerPop test suite
as each test run requires a `Graph` instance even if the test is ultimately ignored becaue it doesn't pass the feature
checks. To possibly help alleviate this problem, the `GraphProvider` interface now includes this method:

[source,java]
----
public default Optional<Graph.Features> getStaticFeatures() {
    return Optional.empty();
}
----

This method can be implemented to return a cacheable set of features for a `Graph` generated from that `GraphProvider`.
Assuming this method is faster than the cost of creating a new `Graph` instance, the test suite should execute
significantly faster depending on how many tests end up being ignored.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1518[TINKERPOP-1518]

===== Configuring Interface

There were some changes to interfaces that were related to `Step`. A new `Configuring` interface was added that was
helpful in the implementation of the `with()`-step modulator. This new interface extends the `Parameterizing` interface
(which moved to the `org.apache.tinkerpop.gremlin.process.traversal.step` package with the other step interfaces) and
in turn is extended by the `Mutating` interface. Making this change meant that the `Mutating.addPropertyMutations()`
method could be removed in favor of the new `Configuring.configure()` method.

All of the changes above basically mean, that if the `Mutating` interface was being used in prior versions, the
`addPropertyMutations()` method simply needs to be changed to `configure()`.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1975[TINKERPOP-1975]

===== OptionsStrategy

`OptionsStrategy` is a `TraversalStrategy` that makes it possible for users to set arbitrary configurations on a
`Traversal`. These configurations can be used by graph providers to allow for traversal-level configurations to be
accessible to their custom steps. A user would write something like:

[source,java]
----
g.withStrategies(OptionsStrategy.build().with("specialLimit", 10000).create()).V();
----

The `OptionsStrategy` is really only the carrier for the configurations and while users can choose to utilize that
more verbose method for constructing it shown above, it is more elegantly constructed as follows using `with()`-step:

[source,java]
----
g.with("specialLimit", 10000)).V();
----

The graph provider could then access that value of "specialLimit" in their custom step (or elsewhere) as follows:

[source,java]
----
OptionsStrategy strategy = this.getTraversal().asAdmin().getStrategies().getStrategy(OptionsStrategy.class).get();
int specialLimit = (int) strategy.getOptions().get("specialLimit");
----

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2053[TINKERPOP-2053]

===== Removed hadoop-gremlin Test Artifact

The `hadoop-gremlin` module no longer generates a test jar that can be used as a test dependency in other modules.
Generally speaking, that approach tends to be a bad practice and can cause build problems with Maven that aren't always
obvious to troubleshoot. With the removal of `giraph-gremlin` for 3.4.0, it seemed even less useful to have this
test artifact present. All tests are still present. The follow provides a basic summary of how this refactoring
occurred:

* A new `AbstractFileGraphProvider` was created in `gremlin-test` which provided a lot of the features that
`HadoopGraphProvider` was exposing. Both `HadoopGraphProvider` and `SparkHadoopGraphProvider` extend from that class
now.
* `ToyIoRegistry` and related classes were moved to `gremlin-test`.
* The various tests that validated capabilities of `Storage` have been moved to `spark-gremlin` and are part of those
tests now. Obviously, that makes those tests specific to Spark testing now. If that location creates a problem for some
reason, that decision can be revisited at some point.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1410[TINKERPOP-1410]

===== TraversalEngine Moved

The `TraversalEngine` interface was deprecated in 3.2.0 along with all related methods that used it and classes that
implemented it. It was replaced by the `Computer` interface and provided a much nicer way to plug different
implementations of `Computer` into a traversal. `TraversalEngine` was never wholly removed however as it had some deep
dependencies in the inner workings of the test suite. That infrastructure has largely remained as is until now.

As of 3.4.0, `TraversalEngine` is no longer in `gremlin-core` and can instead be found in `gremlin-test` as it is
effectively a "test-only" component and serves no other real function. As explained in the javadocs going back to
3.2.0, providers should implement the `Computer` class and use that instead. At this point, graph providers should have
long ago moved to the `Computer` infrastructure as methods for constructing a `TraversalSource` with a
`TraversalEngine` were long ago removed.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1143[TINKERPOP-1143]

===== Upsert Graph Feature

Some `Graph` implementations may be able to offer upsert functionality for vertices and edges, which can help improve
usability and performance. To help make it clear to users that a graph operates in this fashion, the `supportsUpsert()`
feature has been added to both `Graph.VertexFeatures` and `Graph.EdgeFeatures`. By default, both of these methods will
return `false`.

Should a provider wish to support this feature, the behavior of `addV()` and/or `addE()` should change such that when
a vertex or edge with the same identifier is provided, the respective step will insert the new element if that value
is not present or update an existing element if it is found. The method by which the provider "identifies" an element
is completely up to the capabilities of that provider. In the most simple fashion, a graph could simply check the
value of the supplied `T.id`, however graphs that support some form of schema will likely have other methods for
determining whether or not an existing element is present.

The extent to which TinkerPop tests "upsert" is fairly narrow. Graph providers that choose to support this feature
should consider their own test suites carefully to ensure appropriate coverage.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-1685[TINKERPOP-1685]

===== TypeTranslator Changes

The `TypeTranslator` experienced a change in its API and `GroovyTranslator` a change in expectations.

`TypeTranslator` now implements `BiFunction` and takes the graph traversal source name as an argument along with the
object to translate. This interface is implemented by default for Groovy with `GroovyTranslator.DefaultTypeTranslator`
which encapsulates all the functionality of what `GroovyTranslator` formerly did by default. To provide customize
translation, simply extend the `DefaultTypeTranslator` and override the methods.

`GroovyTranslator` now expects that the `TypeTranslator` provide to it as part of its `of()` static method overload
is "complete" - i.e. that it provides all the functionality to translate the types passed to it. Thus, a "complete"
`TypeTranslator` is one that does everything that `DefaultTypeTranslator` does as a minimum requirement. Therefore,
the extension model described above is the easiest way to get going with a custom `TypeTranslator` approach.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2072[TINKERPOP-2072]

==== Graph Driver Providers

===== Deprecation Removal in RemoteConnection

The two deprecated synchronous `submit()` methods on the `RemoteConnection` interface have been removed, which means
that `RemoteConnection` implementations will need to implement `submitAsync(Bytecode)` if they have not already done
so.

See: link:https://issues.apache.org/jira/browse/TINKERPOP-2103[TINKERPOP-2103]
