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我正在使用 JanusGraph 和 Cassandra 后端，但我有一些疑问。

我有一个 Cassandra 服务器在我的机器上运行（使用 Docker），在我的 API 中我有这个代码：

 GraphTraversalSource g = JanusGraphFactory.build()
        .set("storage.backend", "cql")
        .set("storage.hostname", "localhost")
        .open()
        .traversal();

然后，通过我的 API，我使用 Gremlin 保存和获取数据。它工作正常，我看到数据保存在 Cassandra 数据库中。

当我重新启动 API 并尝试获取数据时，问题就出现了。数据仍存储在 Cassandra 中，但 JanusGraph 查询返回空。为什么？

我是否需要将后端存储数据加载到内存或类似的东西中？我试图了解它是如何工作的。

编辑

这是我添加项目的方式：

 Vertex vertex = g.addV("User")
          .property("username", username)
          .property("email", email)
          .next();

并获取所有内容：

List<Vertex> all = g.V().toList()
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1 回答 1

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提交您的交易

您现在正在使用 JanusGraph作为库中嵌入的应用程序，它使您可以访问 JanusGraph 的完整 API。这意味着您必须自己管理事务，其中还包括提交事务以将您的修改持久化到图表的必要性。

你可以简单地通过调用来做到这一点：

g.tx().commit();

在您使用修改迭代遍历之后（addV()在您的情况下是遍历）。

如果没有提交，则更改仅在您的事务中本地可用。当您重新启动 Docker 容器时，所有数据都将丢失，因为您尚未提交它。

推荐的方法：通过远程连接

If you don't have a good reason to embed JanusGraph as a library in your JVM application, then it's recommended to deploy it independently as JanusGraph Server to which you can send your traversals for execution. This has the benefit that you can scale JanusGraph independently of your application and also that you can use it from non-JVM languages.

JanusGraph Server then also manages transactions for you transparently by executing each traversal in its own transaction. If the traversal succeeds, then the results are committed and they are also rolled back automatically if an exception occurs.

The JanusGraph docs contain a section about how to connect to JanusGraph Server from Java but the important part is this code to create a graph traversal source g connected to your JanusGraph Server(s):

Graph graph = EmptyGraph.instance();
GraphTraversalSource g = graph.traversal().withRemote("conf/remote-graph.properties");

You can start JanusGraph Server of course also as a Docker container:

docker run --rm janusgraph/janusgraph:latest

More information about the JanusGraph Docker image and how it can be configured to connect to your Cassandra backend can be found here.

The part below is not directly relevant for this question any more given the comments to my first version of the answer. I am still leaving it here in case that others have a similar problem where this could actually be the cause.

Persistent Storage with Docker Containers

JanusGraph stores the data in your storage backend which is Cassandra in your case. That means that you have to ensure that Cassandra persists the data. If you start Cassandra in a Docker container, then you have to mount a volume where Cassandra stores the data to persist it beyond restarts of the container. Otherwise, the data will be lost once you stop the Cassandra container.

To do this, you can start the Cassandra container for example like this:

docker run -v /my/own/datadir:/var/lib/cassandra -d cassandra

where /my/own/datadir is the directory of your host system where you want the Cassandra data to be stored. This is explained in the docs of the official Cassandra Docker image under Caveats > Where to Store Data.

于 2020-03-10T14:58:33.973 回答