在Gremlin中，仅当version属性与数字匹配时，如何修改顶点的属性？

Question

在Gremlin/Tinkerpop中，我想在顶点上执行版本化的插入。如果存在现有顶点，我只希望在版本与版本号属性匹配的情况下对其进行变异。

下面是试图使用gremlinjs进行此操作的代码。它无法创建顶点，随后的查询将无法找到它。

(以前的版本出现了编译错误，但这是javscriptism的一个无文档语法问题)

更新见评论，以回答什么是错误的。https://gist.github.com/pszabop/3b07fa7caadf1dbd86953a713ed96ce0的工作版本

//
// o.id o.__version, and o.__lastUpdate have special meaning or are reserved
//
graphdb.upsertVertexVersioned = async function(type, o) {
  const g = traversal().withRemote(this.connection);
  let oldVersion;

  if (!o.id) {
    o.id = uuidv4();
  }  
  if (!o.__version) {
    o.__version = 0;
    oldVersion = 0;
  } else {
    oldVersion = o.__version;
    o.__version++;
  }  
  o.__lastUpdate = Date.now();

  // @see http://tinkerpop.apache.org/docs/current/recipes/#element-existence
  // The pattern we are using is keys get copied into properties that can be used
  // by the graph database for its work, and then the
  // entire object is JSON serialized into a generic `obj` property.
  // XXX TBD use graphson?
  const v1 = await g.V().has(type, 'id', o.id) 
  .fold()  
  .coalesce(__.unfold(),
    __.addV(type).property('id', o.id)                       
    .property('version', o.__version)                        
  ).choose(__.values('version').is(oldVersion),
    __.in_() 
    .property('lastUpdate', o.__lastUpdate)                  // updated properties go here
    .property('version', o.__version)                        // updated properties go here
    .property('obj', JSON.stringify(o)),                     // updated properties go here
    __.out()
  ).next();

  return o;
};

参考文献：

• Understanding different methods for adding edges with Gremlin-Python
• http://tinkerpop.apache.org/docs/current/recipes/

版本

• janusgraph/janusgraph:latest
• gremlinjs 3.4.4

Answer 1

我试着用“现代”玩具图来改变你的代码，你的代码被证明是对的。我相信以下是你所做的事情的精神所在：

gremlin> g = TinkerFactory.createModern().traversal()
==>graphtraversalsource[tinkergraph[vertices:6 edges:6], standard]
gremlin> g.V().property('version',1).iterate()
gremlin> name = 'marko'
==>marko
gremlin> oldVersion = 1
==>1
gremlin> version = 2
==>2
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>edited
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>same
gremlin> name = 'stephen'
==>stephen
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>same
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>same
gremlin> oldVersion = 2
==>2
gremlin> version = 3
==>3
gremlin> 
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>edited
gremlin> g.V().has('person','name',name).
......1>   fold().
......2>   coalesce(unfold(), 
......3>            addV('person').property('name',name).property('version',version)).
......4>   choose(values('version').is(oldVersion),
......5>          property('version', version).constant('edited'),
......6>          constant('same'))
==>same
gremlin> g.V().has('person','name','stephen').elementMap()
==>[id:19,label:person,name:stephen,version:3]

考虑到你对这个问题的描述，我建议尽量简化一下。你说问题是：

无法创建顶点，稍后的查询将无法找到它。

如果删除choose()逻辑，它是否正常工作？换句话说，你能让基本的重新插入操作开始工作吗？如果不是，那么这个问题似乎孤立于查询的这一部分，尽管您似乎遵循了推荐的实践，所以我不确定可能出了什么问题。

Answer 2

下面的gremlinjs代码只在版本号属性匹配的情况下更新顶点。这允许使用乐观并发对顶点进行安全的读-修改-写入(例如，冲突应该很少见，或者您应该做其他的事情)。

代码可用作gist：https://gist.github.com/pszabop/3b07fa7caadf1dbd86953a713ed96ce0

//
// o.id o.__version, and o.__lastUpdate have special meaning or are reserved
//
graphdb.upsertVertexVersioned = async function(type, o) {
  const g = traversal().withRemote(this.connection);
  let oldVersion;

  // don't modify original in case of exceptions
  // return the new object and let user decide to reassign or not
  o = Object.assign({}, o);

  if (!o.id) {
    o.id = uuidv4();
  }
  if (!Number.isInteger(o.__version)) {
    o.__version = 0;
    oldVersion = 0;
  } else {
    oldVersion = o.__version;
    o.__version++;
  }
  o.__lastUpdate = Date.now();

  // @see http://tinkerpop.apache.org/docs/current/recipes/#element-existence
  // @see https://stackoverflow.com/questions/58513680/in-gremlin-how-do-i-modify-a-vertexs-properties-only-if-a-version-property-mat
  // The pattern we are using is keys get copied into properties that can be used
  // by the graph database for its work, and then the
  // entire object is JSON serialized into a generic `obj` property.
  // XXX TBD use graphson?
  const v1 = await g.V().has(type, 'id', o.id)
  .fold()
  .coalesce(__.unfold(),
    __.addV(type).property('id', o.id)
    .property('version', o.__version)
  ).choose(__.values('version').is(oldVersion),
    __.property('lastUpdate', o.__lastUpdate)                  // updated properties go here
    .property('version', o.__version)
    .property('obj', JSON.stringify(o)).constant('edited'),
    __.constant('unchanged')
  ).next();

  if (v1.value === 'unchanged') {
    throw new Error('version mismatch, vertex not updated');
  }

  return o;
};

test('test vertex versioned upsert and get', async function(t) {

  graphdb.open();

  // initial write and verify
  const o = { randomText: uuidv4(), foo: 'bar'}
  const osent1 = await graphdb.upsertVertexVersioned('testtype', o);
  t.ok(osent1.id, 'a random ID was assigned');
  const oget1 = await graphdb.getVertex('testtype', osent1.id);
  t.equal(oget1.randomText, o.randomText, 'random text was as written');
  t.equal(oget1.id, osent1.id, 'ID was as assigned');
  t.equal(oget1.foo, 'bar', 'field foo is "bar"');

  // make sure version gets updated when field foo is modified
  oget1.foo = 'beyond all repair';
  const osent2 = await graphdb.upsertVertexVersioned('testtype', oget1);
  t.equal(osent2.__version, 1, 'version was changed from 0 to 1');
  const oget2 = await graphdb.getVertex('testtype', oget1.id);
  t.equal(oget2.randomText, o.randomText, 'random text was as written and was unchanged on second write');
  t.equal(oget2.id, osent1.id, 'ID was as assigned');
  t.equal(oget2.foo, 'beyond all repair', 'field foo was changed to "beyond all repair"');

  // if we are using a stale copy of the object an update should not happen
  osent1.foo = 'illegal update';
  try {
    const osent3 = await graphdb.upsertVertexVersioned('testtype', osent1);
    t.fail('should never returned from an incorrect version update');
  } catch (err) {
    t.ok(err.toString().includes('not updated'), 'error message is correct on illegal version update attempt');
  }
  const oget3 = await graphdb.getVertex('testtype', oget1.id);
  t.equal(oget3.randomText, o.randomText, 'random text was as written and was unchanged on second write');
  t.equal(oget3.id, osent1.id, 'ID was as assigned');
  t.equal(oget3.foo, 'beyond all repair', 'field foo was unchanged after failed update');
  graphdb.close();

});