Neo4j合并和原子事务 [英] Neo4j Merge and Atomic Transaction

问题描述

我正在10个并行线程中从客户端应用程序对我的Neo4j服务器运行以下MERGE查询，newFoo和id参数在所有10次运行中都相同:

I am running the below MERGE query against my Neo4j server from a client application in 10 parallel threads, the newFoo and id parameters are the same on all 10 runs:

MERGE (foo:Foo { id: {id} })
ON MATCH
SET foo = {newFoo}

运行此命令后，我运行以下查询以期望1，但我却得到了10:

After running this, I run the below query to expect 1 but I instead get 10:

match (f:Foo)
return count(f)

我认为MERGE在原子事务中运行，但显然不是.我在这里做错什么了吗?

I thought that MERGE runs in an atomic transaction but apparently not. Am I doing something wrong here?

下面是我用来重现该问题的代码:

Below is the code that I used to reproduce the issue:

public static async Task RunInParallel()
{
    var client = new GraphClient(new Uri("http://localhost:7474/db/data"), "neo4j", "1234567890")
    {
        JsonContractResolver = new CamelCasePropertyNamesContractResolver()
    };

    client.Connect();

    var foo = new Foo
    {
        Id = "1",
        Name = "Foo",
        Slug = "foo-bar-foo"
    };

    List<Task> tasks = new List<Task>();
    for (int i = 0; i < 10; i++)
    {
        var task = client.Cypher
            .Merge("(foo:Foo { id: {id} })")
            .OnMatch()
            .Set("foo = {newFoo}")
            .WithParams(new
            {
                Id = foo.Id,
                NewFoo = foo
            })
            .ExecuteWithoutResultsAsync();

        tasks.Add(task);
    }

    await Task.WhenAll(tasks.ToArray());
}

推荐答案

MERGE(本身)不能保证唯一性.在唯一属性上使用MERGE时，应始终创建唯一性约束指定的属性:

MERGE (by itself) does not guarantee uniqueness. When using MERGE (on a unique property) you should always create a uniqueness constraint for the specified property:

CREATE CONSTRAINT ON (f:Foo) ASSERT f.id IS UNIQUE

这将确保您不会创建任何重复项.

This will ensure you aren't creating any duplicates.

修改

MERGE不是线程安全的.添加唯一性约束可确保在写入之前保持索引锁，从而使操作线程安全.

MERGE without a uniqueness constraint is not thread safe. Adding a uniqueness constraint ensures an index lock is held before writing, making the operation thread safe.

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