A minimal implementation for a Dgraph client for Java 1.8 and above, using grpc.
This client follows the Dgraph Go client closely.
Before using this client, we highly recommend that you go through docs.dgraph.io, and understand how to run and work with Dgraph.
grab via Maven:
<dependency>
<groupId>io.dgraph</groupId>
<artifactId>dgraph4j</artifactId>
<version>1.7.1</version>
</dependency>or Gradle:
compile 'io.dgraph:dgraph4j:1.7.1'Build and run the DgraphJavaSample project in the samples folder, which
contains an end-to-end example of using the Dgraph Java client. Follow the
instructions in the README of that project.
This library supports two styles of clients, the synchronous client DgraphClient and
the async client DgraphAsyncClient.
A DgraphClient or DgraphAsyncClient can be initialised by passing it
a list of DgraphBlockingStub clients. The anyClient() API can randomly pick a stub, which can
then be used for GRPC operations. In the next section, we will explain how to create a
synchronous client and use it to mutate or query dgraph. For the async client, more details can
be found in the Using the Asynchronous Client section.
The following code snippet shows how to create a synchronous client using just one connection.
ManagedChannel channel =
ManagedChannelBuilder.forAddress("localhost", 9080).usePlaintext(true).build();
DgraphStub stub = DgraphGrpc.newStub(channel);
DgraphClient dgraphClient = new DgraphClient(Collections.singletonList(stub));Alternatively, you can specify a deadline (in seconds) after which the client will time out when making requests to the server.
DgraphClient dgraphClient = new DgraphClient(stub);To set the schema, create an Operation object, set the schema and pass it to
DgraphClient#alter method.
String schema = "name: string @index(exact) .";
Operation op = Operation.newBuilder().setSchema(schema).build();
dgraphClient.alter(op);Operation contains other fields as well, including drop predicate and
drop all. Drop all is useful if you wish to discard all the data, and start from
a clean slate, without bringing the instance down.
// Drop all data including schema from the dgraph instance. This is useful
// for small examples such as this, since it puts dgraph into a clean
// state.
dgraphClient.alter(Operation.newBuilder().setDropAll(true).build());There are two types of transactions in dgraph, i.e. the read-only transactions that only include
queries and the transactions that change data in dgraph with mutate operations. Both the
synchronous client DgraphClient and the async client DgraphAsyncClient support the two types
of transactions by providing the newTransaction and the newReadOnlyTransaction APIs. Creating
a transaction is a local operation and incurs no network overhead.
In most of the cases, the normal read-write transactions should be used, which can have any number of query, or mutate operations. However, if a transaction only has queries, you might benefit from a read-only transaction, which can share the same read timestamp across multiple such read-only transactions, thus, potentially providing better latency.
For normal read-write transactions, it is a good practise to call Transaction#discard() in a
finally block after running the transaction. Calling Transaction#discard() after
Transaction#commit() is a no-op and you can call discard() multiple times with no additional
side-effects.
Transaction txn = dgraphClient.newTransaction();
try {
// Do something here
// ...
} finally {
txn.discard();
}For read-only transactions, there is no need to call Transaction.discard, which is equivalent
to a no-op.
Transaction#mutate runs a mutation. It takes in a Mutation object,
which provides two main ways to set data: JSON and RDF N-Quad. You can choose
whichever way is convenient.
We're going to use JSON. First we define a Person class to represent a person.
This data will be seralized into JSON.
class Person {
String name
Person() {}
}Next, we initialise a Person object, serialize it and use it in Mutation object.
// Create data
Person p = new Person();
p.name = "Alice";
// Serialize it
Gson gson = new Gson();
String json = gson.toJson(p);
// Run mutation
Mutation mu =
Mutation.newBuilder()
.setSetJson(ByteString.copyFromUtf8(json.toString()))
.build();
txn.mutate(mu);Sometimes, you only want to commit mutation, without querying anything further.
In such cases, you can use a CommitNow field in Mutation object to
indicate that the mutation must be immediately committed.
The IgnoreIndexConflict flag can be set to true on the Mutation object
to not run conflict detection over the index, which would decrease the number
of transaction conflicts and aborts. However, this would come at the cost of
potentially inconsistent upsert operations.
A transaction can be committed using the Transaction#commit() method. If your transaction
consisted solely of calls to Transaction#query(), and no calls to Transaction#mutate(),
then calling Transaction#commit() is not necessary.
An error will be returned if other transactions running concurrently modify the same data that was modified in this transaction. It is up to the user to retry transactions when they fail.
Transaction txn = dgraphClient.newTransaction();
try {
// …
// Perform any number of queries and mutations
//…
// and finally…
txn.commit()
} catch (TxnConflictException ex) {
// Retry or handle exception.
} finally {
// Clean up. Calling this after txn.commit() is a no-op
// and hence safe.
txn.discard();
}You can run a query by calling Transaction#query(). You will need to pass in a GraphQL+-
query string, and a map (optional, could be empty) of any variables that you might want to
set in the query.
The response would contain a JSON field, which has the JSON encoded result. You will need
to decode it before you can do anything useful with it.
Let’s run the following query:
query all($a: string) {
all(func: eq(name, $a)) {
name
}
}
First we must create a People class that will help us deserialize the JSON result:
class People {
List<Person> all;
People() {}
}Then we run the query, deserialize the result and print it out:
// Query
String query =
"query all($a: string){\n" +
" all(func: eq(name, $a)) {\n" +
" name\n" +
" }\n" +
"}\n";
Map<String, String> vars = Collections.singletonMap("$a", "Alice");
Response res = dgraphClient.newReadOnlyTransaction().queryWithVars(query, vars);
// Deserialize
People ppl = gson.fromJson(res.getJson().toStringUtf8(), People.class);
// Print results
System.out.printf("people found: %d\n", ppl.all.size());
ppl.all.forEach(person -> System.out.println(person.name));This should print:
people found: 1
Alice
It is recommended that you always set a deadline for each client call, after which the client terminaltes. This is in line with the recommendation for any gRPC client. Read this forum post for more details.
channel = ManagedChannelBuilder.forAddress("localhost", 9080).usePlaintext(true).build();
DgraphGrpc.DgraphStub stub = DgraphGrpc.newStub(channel);
ClientInterceptor timeoutInterceptor = new ClientInterceptor(){
@Override
public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
MethodDescriptor<ReqT, RespT> method, CallOptions callOptions, Channel next) {
return next.newCall(method, callOptions.withDeadlineAfter(500, TimeUnit.MILLISECONDS));
}
};
stub.withInterceptors(timeoutInterceptor);
DgraphClient dgraphClient = new DgraphClient(stub);Certain headers such as authentication tokens need to be set globally for all subsequent calls. Below is an example of setting a header with the name "auth-token":
// create the stub first
ManagedChannel channel =
ManagedChannelBuilder.forAddress(TEST_HOSTNAME, TEST_PORT).usePlaintext(true).build();
DgraphStub stub = DgraphGrpc.newStub(channel);
// use MetadataUtils to augment the stub with headers
Metadata metadata = new Metadata();
metadata.put(
Metadata.Key.of("auth-token", Metadata.ASCII_STRING_MARSHALLER), "the-auth-token-value");
stub = MetadataUtils.attachHeaders(stub, metadata);
// create the DgraphClient wrapper around the stub
DgraphClient dgraphClient = new DgraphClient(stub);
// trigger a RPC call using the DgraphClient
dgraphClient.alter(Operation.newBuilder().setDropAll(true).build());The example below uses the helper method Helpers#deleteEdges to delete
multiple edges corresponding to predicates on a node with the given uid.
The helper method takes an existing mutation, and returns a new mutation
with the deletions applied.
Mutation mu = Mutation.newBuilder().build()
mu = Helpers.deleteEdges(mu, uid, "friends", "loc");
dgraphClient.newTransaction().mutate(mu);Dgraph Client for Java also bundles an asynchronous API, which can be used by
instantiating the DgraphAsyncClient class. The usage is almost exactly the
same as the DgraphClient (show in previous section) class. The main
differences is that the DgraphAsyncClient#newTransacation() returns an
AsyncTransaction class. The API for AsyncTransaction is exactly
Transcation. The only difference is that instead of returning the results
directly, it returns immediately with a corresponding CompletableFuture<T>
object. This object represents the computation which runs asynchronously to
yield the result in the future. Read more about CompletableFuture<T> in the
Java 8 documentation.
Here is the asynchronous version of the code above, which runs a query.
// Query
String query =
"query all($a: string){\n" +
" all(func: eq(name, $a)) {\n" +
" name\n" +
}\n" +
"}\n";
Map<String, String> vars = Collections.singletonMap("$a", "Alice");
AsyncTransaction txn = dgraphAsyncClient.newTransaction();
txn.query(query).thenAccept(response -> {
// Deserialize
People ppl = gson.fromJson(res.getJson().toStringUtf8(), People.class);
// Print results
System.out.printf("people found: %d\n", ppl.all.size());
ppl.all.forEach(person -> System.out.println(person.name));
});If you would like to see the latency for either a mutation or query request, the latency field in the returned result can be helpful. Here is an example to log the latency of a query request:
Response resp = txn.query(query);
Latency latency = resp.getLatency();
logger.info("parsing latency:" + latency.getParsingNs());
logger.info("processing latency:" + latency.getProcessingNs());
logger.info("encoding latency:" + latency.getEncodingNs());Similarly you can get the latency of a mutation request:
Assigned assignedIds = dgraphClient.newTransaction().mutate(mu);
Latency latency = assignedIds.getLatency();Warning: The gradle build runs integration tests on a locally running Dgraph server. The tests will remove all data from your Dgraph instance. So make sure that you don't have any important data on your Dgraph instance.
./gradlew build
If you have made changes to the task.proto file, this step will also regenerate the source files
generated by Protocol Buffer tools.
We use google-java-format to format the source code. If you run ./gradlew build, you will be
warned if there is code that is not conformant. You can run ./gradlew goJF to format the source
code, before committing it.
Warning: This command will runs integration tests on a locally running Dgraph server. The tests will remove all data from your Dgraph instance. So make sure that you don't have any important data on your Dgraph instance.
Make sure you have a Dgraph server running on localhost before you run this task.
./gradlew test