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cachingtransactionsguavatinkerpopjanusgraph

Janusgraph Cache

I was reading about Janusgraph Cache in Janusgraph documentation. I have few doubts Regarding the transaction cache. I'm using an embedded janusgrah server in my application.

  1. If I'm only doing a read query for eg. - g. V().has("name","ABC") using gremlin HTTP endpoint, so will this value be cached in transaction cache or database level cache? because here I'm not opening any transaction.
  2. If it is stored in the transaction cache, how updated values will be fetched for this vertex if I have multi-node deployment?
Solution

  • Regarding question 1:
    If not created explicitly, transactions are created automatically. From the JanusGraph reference docs:

    Every graph operation in JanusGraph occurs within the context of a transaction. According to the TinkerPop’s transactional specification, each thread opens its own transaction against the graph database with the first operation (i.e. retrieval or mutation) on the graph.

    A vertex retrieved during a transaction is stored in both the transaction cache and database cache. After closing the transaction the vertex is still in the database cache (but note that since janusgraph-0.5.x the database cache is disabled by default).

    Regarding question 2:
    Indeed, a JanusGraph instance cannot know about modifications to vertices in the transaction caches of other instances. Only after these transactions have been closed and persisted to the storage and index backends, other instances can read modified vertices from the the backends. This also means that caches in other JanusGraph instances can be out of date, so if you want to be sure that you have the latest data from the backends, you should start a new transaction and disable the database cache (default setting).

    Additional comments (added Sept 12):

    The vertex caches are private members of JanusGraph and nowhere exposed to the user (not even in the debug logging). Cache hits in a traversal are only visible from a fast (sub-millisecond) return time.

    If data consistency between transactions or janusgraph instances matters to you, you can take a look at: