Metadata

Datastore provides programmatic access to some of its metadata to support metaprogramming, implementing backend administrative functions, simplify consistent caching, and similar purposes; you can use it, for instance, to build a custom Datastore viewer for your application. The metadata available includes information about the entity groups, namespaces, entity kinds, and properties your application uses, as well as the property representations for each property.

The Datastore Dashboard in the Google Cloud console also provides some metadata about your application, but the data displayed there differs in some important respects from that returned by these functions.

• Freshness. Reading metadata using the API gets current data, whereas data in the dashboard is updated only once daily.
• Contents. Some metadata in the dashboard is not available via the APIs; the reverse is also true.
• Speed. Metadata gets and queries are billed in the same way as Datastore gets and queries. Metadata queries that fetch information on namespaces, kinds, and properties are generally slow to execute. As a rule of thumb, expect a metadata query that returns N entities to take about the same time as N ordinary queries each returning a single entity. Furthermore, property representation queries (non-keys-only property queries) are slower than keys-only property queries. Metadata gets of entity group metadata are somewhat faster than getting a regular entity.

Entity group metadata

Cloud Datastore provides access to the "version" of an entity group, a strictly positive number that is guaranteed to increase on every change to the entity group.

Entity group versions are obtained by calling get() on a special pseudo-entity that contains a strictly positive __version__ property. The pseudo-entity's key can be created using the Entities.createEntityGroupKey() method:

privatestaticlonggetEntityGroupVersion(DatastoreServiceds,Transactiontx,KeyentityKey){
try{
returnEntities.getVersionProperty(ds.get(tx,Entities.createEntityGroupKey(entityKey)));
}catch(EntityNotFoundExceptione){
// No entity group information, return a value strictly smaller than any
// possible version
return0;
}
}
privatestaticvoidprintEntityGroupVersions(DatastoreServiceds,PrintWriterwriter){
Entityentity1=newEntity("Simple");
Keykey1=ds.put(entity1);
KeyentityGroupKey=Entities.createEntityGroupKey(key1);
// Print entity1's entity group version
writer.println("version "+getEntityGroupVersion(ds,null,key1));
// Write to a different entity group
Entityentity2=newEntity("Simple");
ds.put(entity2);
// Will print the same version, as entity1's entity group has not changed
writer.println("version "+getEntityGroupVersion(ds,null,key1));
// Change entity1's entity group by adding a new child entity
Entityentity3=newEntity("Simple",entity1.getKey());
ds.put(entity3);
// Will print a higher version, as entity1's entity group has changed
writer.println("version "+getEntityGroupVersion(ds,null,key1));
}

Legacy behavior

In the legacy entity group version behavior, the entity group version increases only on changes to the entity group. The legacy entity group metadata behavior could be used, for example, to keep a consistent cache of a complex ancestor query on an entity group.

This example caches query results (a count of matching results) and uses the legacy behavior of entity group versions to use the cached value if it's current:

// A simple class for tracking consistent entity group counts.
privatestaticclass EntityGroupCountimplementsSerializable{
longversion;// Version of the entity group whose count we are tracking
intcount;
EntityGroupCount(longversion,intcount){
this.version=version;
this.count=count;
}
// Display count of entities in an entity group, with consistent caching
voidshowEntityGroupCount(
DatastoreServiceds,MemcacheServicecache,PrintWriterwriter,KeyentityGroupKey){
EntityGroupCountegCount=(EntityGroupCount)cache.get(entityGroupKey);
// Reuses getEntityGroupVersion method from the previous example.
if(egCount!=null && egCount.version==getEntityGroupVersion(ds,null,entityGroupKey)){
// Cached value matched current entity group version, use that
writer.println(egCount.count+" entities (cached)");
}else{
// Need to actually count entities. Using a transaction to get a consistent count
// and entity group version.
Transactiontx=ds.beginTransaction();
PreparedQuerypq=ds.prepare(tx,newQuery(entityGroupKey));
intcount=pq.countEntities(FetchOptions.Builder.withLimit(5000));
cache.put(
entityGroupKey,
newEntityGroupCount(getEntityGroupVersion(ds,tx,entityGroupKey),count));
tx.rollback();
writer.println(count+" entities");
}
}
}

__entity_group__ entities may not exist for entity groups which have never been written to.

Metadata queries

The Java class Entities, defined in the com.google.appengine.api.datastore package, provides three special entity kinds that are reserved for metadata queries. They are denoted by static constants of the Entities class:

These kinds will not conflict with others of the same names that may already exist in your application. By querying on these special kinds, you can retrieve entities containing the desired metadata.

The entities returned by metadata queries are generated dynamically, based on the current state of Datastore. While you can create local Entity objects of kinds __namespace__, __kind__, or __property__, any attempt to store them in Datastore will fail with an IllegalArgumentException .

The easiest way to issue metadata queries is with the low-level Datastore API. The following example prints the names of all namespaces in an application:

voidprintAllNamespaces(DatastoreServiceds,PrintWriterwriter){
Queryq=newQuery(Entities.NAMESPACE_METADATA_KIND);
for(Entitye:ds.prepare(q).asIterable()){
// A nonzero numeric id denotes the default namespace;
// see <a href="#Namespace_Queries">Namespace Queries</a>, below
if(e.getKey().getId()!=0){
writer.println("<default>");
}else{
writer.println(e.getKey().getName());
}
}
}

Namespace queries

If your application uses the Namespaces API, you can use a namespace query to find all namespaces used in the application's entities. This allows you to perform activities such as administrative functions across multiple namespaces.

Namespace queries return entities of the special kind __namespace__ whose key name is the name of a namespace. (An exception is the default namespace designated by the empty string "": since the empty string is not a valid key name, this namespace is keyed with the numeric ID 1 instead.) Queries of this type support filtering only for ranges over the special pseudoproperty __key__, whose value is the entity's key. The results can be sorted by ascending (but not descending) __key__ value. Because __namespace__ entities have no properties, both keys-only and non-keys-only queries return the same information.

The following example returns a list of an application's namespaces in the range between two specified names, start and end:

List<String>getNamespaces(DatastoreServiceds,Stringstart,Stringend){
// Start with unrestricted namespace query
Queryq=newQuery(Entities.NAMESPACE_METADATA_KIND);
List<Filter>subFilters=newArrayList();
// Limit to specified range, if any
if(start!=null){
subFilters.add(
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
FilterOperator.GREATER_THAN_OR_EQUAL,
Entities.createNamespaceKey(start)));
}
if(end!=null){
subFilters.add(
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
FilterOperator.LESS_THAN_OR_EQUAL,
Entities.createNamespaceKey(end)));
}
q.setFilter(CompositeFilterOperator.and(subFilters));
// Initialize result list
List<String>results=newArrayList<String>();
// Build list of query results
for(Entitye:ds.prepare(q).asIterable()){
results.add(Entities.getNamespaceFromNamespaceKey(e.getKey()));
}
// Return result list
returnresults;
}

Kind queries

Kind queries return entities of kind __kind__ whose key name is the name of an entity kind. Queries of this type are implicitly restricted to the current namespace and support filtering only for ranges over the __key__ pseudoproperty. The results can be sorted by ascending (but not descending) __key__ value. Because __kind__ entities have no properties, both keys-only and non-keys-only queries return the same information.

The following example prints all kinds whose names start with a lowercase letter:

voidprintLowercaseKinds(DatastoreServiceds,PrintWriterwriter){
// Start with unrestricted kind query
Queryq=newQuery(Entities.KIND_METADATA_KIND);
List<Filter>subFils=newArrayList();
// Limit to lowercase initial letters
subFils.add(
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
FilterOperator.GREATER_THAN_OR_EQUAL,
Entities.createKindKey("a")));
StringendChar=Character.toString((char)('z'+1));// Character after 'z'
subFils.add(
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
FilterOperator.LESS_THAN,
Entities.createKindKey(endChar)));
q.setFilter(CompositeFilterOperator.and(subFils));
// Print heading
writer.println("Lowercase kinds:");
// Print query results
for(Entitye:ds.prepare(q).asIterable()){
writer.println(" "+e.getKey().getName());
}
}

Property queries

Property queries return entities of kind __property__ denoting the properties associated with an entity kind. The entity representing property P of kind K is built as follows:

• The entity's key has kind __property__ and key name P.
• The parent entity's key has kind __kind__ and key name K.

The behavior of a property query depends on whether it is a keys-only or a non-keys-only (property representation) query, as detailed in the subsections below.

Property queries: keys-only

Keys-only property queries return a key for each indexed property of a specified entity kind. (Unindexed properties are not included.) The following example prints the names of all of an application's entity kinds and the properties associated with each:

voidprintProperties(DatastoreServiceds,PrintWriterwriter){
// Create unrestricted keys-only property query
Queryq=newQuery(Entities.PROPERTY_METADATA_KIND).setKeysOnly();
// Print query results
for(Entitye:ds.prepare(q).asIterable()){
writer.println(e.getKey().getParent().getName()+": "+e.getKey().getName());
}
}

Queries of this type are implicitly restricted to the current namespace and support filtering only for ranges over the pseudoproperty __key__, where the keys denote either __kind__ or __property__ entities. The results can be sorted by ascending (but not descending) __key__ value. Filtering is applied to kind-property pairs, ordered first by kind and second by property: for instance, suppose you have an entity with these properties:

• kind Account with properties
  • balance
  • company
• kind Employee with properties
  • name
  • ssn
• kind Invoice with properties
  • date
  • amount
• kind Manager with properties
  • name
  • title
• kind Product with properties
  • description
  • price

The query to return the property data would look like this:

voidprintPropertyRange(DatastoreServiceds,PrintWriterwriter){
// Start with unrestricted keys-only property query
Queryq=newQuery(Entities.PROPERTY_METADATA_KIND).setKeysOnly();
// Limit range
q.setFilter(
CompositeFilterOperator.and(
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
Query.FilterOperator.GREATER_THAN_OR_EQUAL,
Entities.createPropertyKey("Employee","salary")),
newFilterPredicate(
Entity.KEY_RESERVED_PROPERTY,
Query.FilterOperator.LESS_THAN_OR_EQUAL,
Entities.createPropertyKey("Manager","salary"))));
q.addSort(Entity.KEY_RESERVED_PROPERTY,SortDirection.ASCENDING);
// Print query results
for(Entitye:ds.prepare(q).asIterable()){
writer.println(e.getKey().getParent().getName()+": "+e.getKey().getName());
}
}

The above query would return the following:

Employee:ssn
Invoice:date
Invoice:amount
Manager:name

Notice that the results do not include the name property of kind Employee and the title property of kind Manager, nor any properties of kinds Account and Product, because they fall outside the range specified for the query.

Property queries also support ancestor filtering on a __kind__ or __property__ key, to limit the query results to a single kind or property. You can use this, for instance, to get the properties associated with a given entity kind, as in the following example:

List<String>propertiesOfKind(DatastoreServiceds,Stringkind){
// Start with unrestricted keys-only property query
Queryq=newQuery(Entities.PROPERTY_METADATA_KIND).setKeysOnly();
// Limit to specified kind
q.setAncestor(Entities.createKindKey(kind));
// Initialize result list
ArrayList<String>results=newArrayList<String>();
//Build list of query results
for(Entitye:ds.prepare(q).asIterable()){
results.add(e.getKey().getName());
}
// Return result list
returnresults;
}

Property queries: non-keys-only (property representation)

Non-keys-only property queries, known as property representation queries, return additional information on the representations used by each kind-property pair. (Unindexed properties are not included.) The entity returned for property P of kind K has the same key as for a corresponding keys-only query, along with an additional property_representation property returning the property's representations. The value of this property is an instance of class java.util.Collection<String> containing one string for each representation of property P found in any entity of kind K.

Note that representations are not the same as property classes; multiple property classes can map to the same representation. (For example, java.lang.String and com.google.appengine.api.datastore.PhoneNumber both use the STRING representation.)

The following table maps from property classes to their representations:

The following example finds all representations of a specified property for a given entity kind:

Collection<String>representationsOfProperty(DatastoreServiceds,Stringkind,Stringproperty){
// Start with unrestricted non-keys-only property query
Queryq=newQuery(Entities.PROPERTY_METADATA_KIND);
// Limit to specified kind and property
q.setFilter(
newFilterPredicate(
"__key__",Query.FilterOperator.EQUAL,Entities.createPropertyKey(kind,property)));
// Get query result
EntitypropInfo=ds.prepare(q).asSingleEntity();
// Return collection of property representations
return(Collection<String>)propInfo.getProperty("property_representation");
}