HTTP Remoting Utilities

This repository provides an opinionated set of libraries for defining and creating RESTish/RPC servers and clients based on Feign or Retrofit as a client and Dropwizard/Jersey with JAX-RS service definitions as a server. Refer to the API Contract section for details on the contract between clients and servers. This library requires Java 8.

Core libraries:

• jaxrs-clients: Clients for JAX-RS-defined service interfaces
• retrofit2-clients: Clients for Retrofit-defined service interfaces
• jersey-servers: Configuration library for Dropwizard/Jersey servers

Usage

Maven artifacts are published to JCenter. Example Gradle dependency configuration:

repositories {
  jcenter()
}

dependencies {
  compile "com.palantir.remoting2:jaxrs-clients:$version"
  compile "com.palantir.remoting2:retrofit2-clients:$version"
  compile "com.palantir.remoting2:jersey-servers:$version"
  // optional support for PEM key store type using Bouncy Castle libraries:
  //     compile "com.palantir.remoting2:pkcs1-reader-bouncy-castle:$version"
  // optional support for PEM key store type using Sun libraries:
  //     compile "com.palantir.remoting2:pkcs1-reader-sun:$version"
}

jaxrs-clients

Provides the JaxRsClient factory for creating clients for JAX-RS services. Example:

MyService service = JaxRsClient.builder()
    .build(MyService.class, "my user agent", "https://my-server/");

The client is implemented using Feign; however, the Feign dependency is hidden away from both the Java API and the classpath (via shadowing).

The JaxRsClient#create factory comes in two flavours: one for creating immutable clients given a fixed ServiceConfiguration, and one for creating mutable clients whose configuration (e.g., server URLs, timeouts, SSL configuration, etc.) changes when the underlying ServiceConfiguration changes.

retrofit2-clients

Similar to jaxrs-clients, but generates clients using the Retrofit library. Example:

MyService service = Retrofit2Client.builder()
    .build(MyService.class, "my user agent", "https://my-server/");

Similar to JaxRsClient, the Retrofit2Client#create factory can create mutable and immutable clients.

jersey-servers

Provides Dropwizard/Jersey exception mappers for translating common JAX-RS exceptions to appropriate HTTP error codes. A Dropwizard server is configured for http-remoting as follows:

public class MyServer extends Application<Configuration> {
    @Override
    public final void run(Configuration config, final Environment env) throws Exception {
        env.jersey().register(HttpRemotingJerseyFeature.DEFAULT);
        env.jersey().register(new MyResource());
    }
}

tracing

Provides Zipkin-style call tracing libraries. All JaxRsClient and Retrofit2Client instances are instrumented by default. Jersey server instrumentation is enabled via the HttpRemotingJerseyFeature (see above).

By default, the instrumentation forwards trace and span information through HTTP headers, but does not emit completed spans to a log file or to Zipkin. Span observers are static (similar to SLF4J appenders) and can be configured as follows:

// Emit all completed spans to a default SLF4J logger:
Tracer.subscribe("SLF4J" /* user-defined name */, AsyncSlf4jSpanObserver.of(executor));

// No longer emit span events to SLF4J:
Tracer.unsubscribe("SLF4J");

Note that span observers are static; a server typically subscribes span observers in its initialization phase. Libraries should never register span observers (since they can trample observers registered by consumers of the library whose themselves register observers).

In addition to cross-service call tracing, the Tracer library supports intra-thread tracing, for example:

// Record tracing information for expensive doSomeComputation() call:
try {
    Tracer.startSpan("doSomeComputation");
    doSomeComputation();  // may itself invoke cross-service or local traced calls
} finally {
    Tracer.completeSpan(); // triggers all span observers
}

The tracing library can be used independently of jaxrs-clients or retrofit2-clients:

// build.gradle
dependencies {
  compile "com.palantir.remoting2:tracing:$version"
}

Tracer.subscribe("SLF4J", AsyncSlf4jSpanObserver.of(executor));
try {
    Tracer.startSpan("doSomeComputation");
    doSomeComputation();
} finally {
    Tracer.completeSpan();
}

service-config

Provides utilities for setting up service clients from file-based configuration. Example:

# config.yml
services:
  myService:  # the key used in `config.getServices().get("myService")` below
    uris:
      - https://my-server/
    security:
      trustStorePath: path/to/trustStore.jks

ServiceDiscoveryConfiguration config = readFromYaml("path/to/config.yml");
MyService service = JaxRsClient.create(
    MyService.class, "my user agent", config.getServices().get("myService"));

ssl-config

Provides utilities for interacting with Java trust stores and key stores and acquiring SSLSocketFactory instances using those stores, as well as a configuration class for use in server configuration files.

The SslConfiguration class specifies the configuration that should be used for a particular SSLContext. The configuration is required to include information for creating a trust store and can optionally be provided with information for creating a key store (for client authentication).

The configuration consists of the following properties:

• trustStorePath: path to a file that contains the trust store information. The format of the file is specified by the trustStoreType property.
• trustStoreType: the type of the trust store. See section below for details. The default value is JKS.
• (optional) keyStorePath: path to a file that contains the key store information. If unspecified, no key store will be associated with this configuration.
• (optional) keyStorePassword: password for the key store. Will be used to read the keystore provided by keyStorePath (if relevant for the format), and will also be used as the password for the in-memory key store created by this configuration. Required if keyStorePath is specified.
• (optional) keyStoreType: the type of the key store. See section below for details. The default value is JKS.
• (optional) keyStoreAlias: specifies the alias of the key that should be read from the key store (relevant for file formats that contain multiple keys). If unspecified, the first key returned by the store is used.

An SslConfiguration object can be constructed using the static of() factory methods of the class, or by using the SslConfiguration.Builder builder. SslConfiguration objects can be serialized and deserialized as JSON.

Once an SslConfiguration object is obtained, it can be passed as an argument to the SslSocketFactories.createSslSocketFactory method to create an SSLSocketFactory object that can be used to configure Java SSL connections.

Store Types

The following values are supported as store types:

• JKS: a trust store or key store in JKS format. When used as a trust store, the TrustedCertificateEntry entries are used as certificates. When used as a key store, the PrivateKeyEntry specified by the keyStoreAlias parameter (or the first such entry returned if the parameter is not specifeid) is used as the private key.
• PEM: for trust stores, an X.509 certificate file in PEM or DER format, or a directory of such files. For key stores, a PEM file that contains a PKCS#1 RSA private key followed by the certificates that form the trust chain for the key in PEM format, or a directory of such files. In either case, if a directory is specified, every non-hidden file in the directory must be a file of the specified format (they will all be read).
• PKCS12: a trust store or key store in PKCS12 format. Behavior is the same as for the JKS type, but operates on stores in PKCS12 format.
• Puppet: a directory whose content conforms to the Puppet SSLdir format. For trust stores, the certificates in the certs directory are added to the trust store. For key stores, the PEM files in the private_keys directory are added as the private keys and the corresponding files in certs are used as the trust chain for the key.

Note on the PEM Key Store Type

When PEM is used as the key store type, the runtime classpath must provide a Pkcs1Reader implementation and it must be defined as a service in META-INF/services. This project provides an implementation that uses BouncyCastle libraries and another implementation that uses Sun libraries.

The pkcs1-reader-bouncy-castle library includes the Bouncy Castle PKIX/CMS/EAC/DVCS/PKCS/TSP/OPENSSL library as a dependency.

The pkcs1-reader-sun does not include any extra dependencies, but assumes the availability of the sun.security.utils package. Although this is a package in the Sun namespace, it is generally available as part of most popular JVM implementations, including the Oracle and OpenJDK JVMs for Java 7 and Java 8.

error-handling

Provides utilities for relaying Java exceptions across JVM boundaries by serializing exceptions as JSON POJOs.

API Contract

http-remoting makes the following opinionated customizations to the standard Dropwizard/Feign/Retrofit behavior.

Object serialization/deserialization

All parameters and return values of application/json endpoints are serialized/deserialized to/from JSON using a Jackson ObjectMapper with GuavaModule, ShimJdk7Module (same as Jackson’s Jdk7Module, but avoids Jackson 2.6 requirement) and Jdk8Module. Servers must not expose parameters or return values that cannot be handled by this object mapper.

Error propagation

The HttpRemotingJerseyFeature routine installs exception mappers for IllegalArgumentException, NoContentException, RuntimeException and WebApplicationException. The exception mapper sets the response media type to application/json and returns as response body a JSON representation of a SerializableError capturing the message, exception name, and optionally stacktrace of the exception. Both JaxRsClient and Retrofit2Client intercept non-successful HTTP responses and throw a RemoteException wrapping the deserialized server-side SerializableError. The error name of the RemoteException is defined by the service API and clients should switch&dispatch based on the error name. The SerializableError format is:

{
  "message": "A string explaning the error",
  "exceptionClass": "applicationSpecificErrorName",
  "stackTrace": [ {"methodName":"...","fileName":"...","lineNumber":...,"className":"...","nativeMethod":false, {...} ]
}

Note that the JSON field exceptionClass carries this name for historic and back-compatibility reasons and will be changed to errorName in a future version of this library. The optional stackTrace field contains a list of serialized Java StackTraceElement objects indicating the server-side stack trace at the time of the exception. A future version of http-remoting may replace the stack trace mechanism with a more OS-independent API for relaying stack traces.

Serialization of Optional and Nullable objects

@Nullable or Optional<?> fields in complex types are serialized using the standard Jackson mechanism:

• a present value is serialized as itself (in particular, without being wrapped in a JSON object representing the Optional object)
• an absent value is serialized as a JSON null. For example, assume a Java type of the form

public final class ComplexType {
    private final Optional<ComplexType> nested;
    private final Optional<String> string;
}

, and an instance

ComplexType value = new ComplexType(
        Optional.of(
                new ComplexType(
                        Optional.<ComplexType>absent(),
                        Optional.<String>absent(),
        Optional.of("baz"));

The JSON-serialized representation of this object is:

{"nested":{"nested":null,"string":null},"string":"baz"}

Optional return values

When a call to a service interface declaring an Optional<T> return value with media type application/json yields:

• a Optional#empty return value, then the HTTP response has error code 204 and an empty response body.
• a non-empty return value, then the HTTP response has error code 200 and the body carries the deserialized T object directly, rather than a deserialized Optional<T> object.

JaxRsClients intercept such responses, deserialize the T-typed return value and return it to the caller wrapped as an Optional<T>. No is no equivalent concept for Retrofit2Clients.

Call tracing

Clients and servers propagate call trace ids across JVM boundaries according to the Zipkin specification. In particular, clients insert X-B3-TraceId: <Trace ID> HTTP headers into all requests which get propagated by Jetty servers into subsequent client invocations.

Endpoints returning plain strings

Endpoints returning plain strings should produce media type text/plain. Return type Optional<String> is only supported for media type application/json.

Failover

JaxRsClients and Retrofit2Clients can be configured to retry requests in case of failure. Note that JaxRsClients only retry connection-level errors; HTTP responses carrying a RemoteException do typically indicate a permanent error and do not trigger a retry.

License

This repository is made available under the Apache 2.0 License.