Hazelcast与MongoDB集成：loadAll方法参数传递疑问

Let’s walk through exactly how the Collection of keys gets passed to MongoMapStore.loadAll() and how those keys are determined in the sample code you’re examining.

How the keys Parameter is Passed #

When you call supplements.loadAll() (the no-argument version on the IMap), Hazelcast handles the heavy lifting to populate the Collection parameter for MongoMapStore.loadAll():

1. First, Hazelcast invokes the loadAllKeys() method implemented in MongoMapStore. This method’s job is to fetch all the primary keys from the corresponding MongoDB collection.
2. The collection of keys returned by loadAllKeys() is then passed directly as the argument to MongoMapStore.loadAll(Collection keys).

If you instead called supplements.loadAll(Collection specificKeys) (passing a set of keys explicitly), Hazelcast would skip the loadAllKeys() step and directly pass your specificKeys collection to loadAll().

How the Sample Determines These Keys #

In the sample’s MongoMapStore implementation:

• The loadAllKeys() method queries MongoDB to retrieve all document IDs (typically the _id field) from the target collection. It uses a projection that only fetches the _id field to keep the operation efficient.
• A simplified version of this code might look like:

  @Override
  public Iterable<String> loadAllKeys() {
      List<String> keys = new ArrayList<>();
      try (MongoCursor<Document> cursor = collection.find()
              .projection(Projections.include("_id"))
              .iterator()) {
          while (cursor.hasNext()) {
              Document doc = cursor.next();
              keys.add(doc.getString("_id"));
          }
      }
      return keys;
  }
  

• These fetched _id values become the Collection of keys passed to loadAll(). When loadAll() runs, it uses these keys to query MongoDB for the full documents corresponding to each key, then returns them as a Map to Hazelcast for caching.

In short:

• No-argument IMap.loadAll() = keys come from MongoMapStore.loadAllKeys() (all MongoDB collection IDs)
• Argument-based IMap.loadAll(Collection) = keys are the ones you explicitly pass in

内容的提问来源于stack exchange，提问作者arrhhh