Modeling complex Java classes into Protocol Buffers

Question

What's the best way to represent this in Protocol Buffer (.proto)?

public class EntityId {
  private String type;
  private String id;  
}

public class EntityBlob {
  private String blobName;
  private byte[] blobBytes; 
}

public class Entity {
  private String dir;
  private String entityType;
  private String entityId;
  private Set<EntityBlob> blobs; 
  private Map<String,EntityProperty> properties;
  private Multimap<String, EntityId> relatedEntities;
}

public abstract class EntityProperty<T> {
  // ...
}

// Example concrete EntityProperty:
public class EntityStringProperty extends EntityProperty<String> {
  public EntityStringProperty(String value) {
    super(value);
  }
}

Where the field properties can only accept the following, EntityStringProperty, EntityBooleanProperty, EntityDoubleProperty, etc.

With some special classes:

public class EntityArrayProperty extends EntityProperty<List<EntityProperty>> {
  public EntityArrayProperty(List<EntityProperty> value) {
    super(value);
  }
}

public class EntityObjectProperty extends EntityProperty<Map<String, EntityProperty>> {
  public EntityObjectProperty(Map<String, EntityProperty> value) {
    super(value);
  }
}

How can this complex classes can be modeled with Protocol Buffers? Specifically the Map<String,EntityProperty> properties?

Louis Wasserman · Accepted Answer · 2021-01-03 18:30:40Z

2

The Map property isn't too bad: maps are supported in protobuf. For the property types, you'll use a message wrapping a oneof. So it'll go something like

message Entity {
  string dir = 1; 
  string entity_type = 2;
  string entity_id = 3;
  repeated EntityBlob blobs = 4;
  map<string, EntityProperty> properties = 5;
  map<string, EntityIdList> related_entities = 6;
}
message EntityProperty {
  oneof property_value {
    string string_value = 1;
    EntityArrayProperty array_value = 2;
    EntityObjectProperty object_value = 3;
    bool bool_value = 4;
    double double_value = 5;
  }
}
message EntityArrayProperty {
  repeated EntityProperty values = 1;
}
message EntityObjectProperty {
  map<string, EntityProperty> property_map = 1;
}
message EntityIdList {
  repeated EntityId ids = 1;
}
message EntityBlob {
  string blob_name = 1;
  bytes blob_bytes = 2;
}
message EntityId {
  string type = 1;
  string id = 2;
}

Alternately, it looks like EntityProperty might be equivalent to google.protobuf.Value, so you may not have to write it yourself but may be able to use the pre-defined message type.

edited Jan 3, 2021 at 18:30

answered Jan 3, 2021 at 18:26

Louis Wasserman

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1 Comment

quarks Over a year ago

How about the Multimap<String, EntityId> relatedEntities it's not a map or list per se but a multi map, where the it can have the same key

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