Summary Span: Add encoder/decoder for Server Stats. (#1272)

* Summary Span: Add encoder/decoder for Server Stats.

* Fixed build errors reported by Kokoro/Travis.

* Fixed review comments.
- Added missing javadoc annotation.
- included version in encoder/decoder.
- renamed get methods for ServerStats.

* Change version from 0.15 to 0.16
- also fixed CURRENT_VERSION for encoder/decoder and added test for it.

* Make ServerStatsEncoding public.

* Add Test ServerStatsFieldEnum.Size
replace traceOption() wiht getTraceOption()

7 files changed, 704 insertions, 0 deletions

diff --git a/api/src/main/java/io/opencensus/common/ServerStats.java b/api/src/main/java/io/opencensus/common/ServerStats.java
new file mode 100644
index 00000000..42efa1f2
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ServerStats.java
@@ -0,0 +1,86 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import com.google.auto.value.AutoValue;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A representation of stats measured on the server side.
+ *
+ * @since 0.16
+ */
+@Immutable
+@AutoValue
+public abstract class ServerStats {
+
+  ServerStats() {}
+
+  /**
+   * Returns Load Balancer latency, a latency observed at Load Balancer.
+   *
+   * @return Load Balancer latency in nanoseconds.
+   * @since 0.16
+   */
+  public abstract long getLbLatencyNs();
+
+  /**
+   * Returns Service latency, a latency observed at Server.
+   *
+   * @return Service latency in nanoseconds.
+   * @since 0.16
+   */
+  public abstract long getServiceLatencyNs();
+
+  /**
+   * Returns Trace options, a set of bits indicating properties of trace.
+   *
+   * @return Trace options a set of bits indicating properties of trace.
+   * @since 0.16
+   */
+  public abstract byte getTraceOption();
+
+  /**
+   * Creates new {@link ServerStats} from specified parameters.
+   *
+   * @param lbLatencyNs Represents request processing latency observed on Load Balancer. It is
+   *     measured in nanoseconds. Must not be less than 0. Value of 0 represents that the latency is
+   *     not measured.
+   * @param serviceLatencyNs Represents request processing latency observed on Server. It is
+   *     measured in nanoseconds. Must not be less than 0. Value of 0 represents that the latency is
+   *     not measured.
+   * @param traceOption Represents set of bits to indicate properties of trace. Currently it used
+   *     only the least signification bit to represent sampling of the request on the server side.
+   *     Other bits are ignored.
+   * @return new {@code ServerStats} with specified fields.
+   * @throws IllegalArgumentException if the arguments are out of range.
+   * @since 0.16
+   */
+  public static ServerStats create(long lbLatencyNs, long serviceLatencyNs, byte traceOption) {
+
+    if (lbLatencyNs < 0) {
+      throw new IllegalArgumentException("'getLbLatencyNs' is less than zero: " + lbLatencyNs);
+    }
+
+    if (serviceLatencyNs < 0) {
+      throw new IllegalArgumentException(
+          "'getServiceLatencyNs' is less than zero: " + serviceLatencyNs);
+    }
+
+    return new AutoValue_ServerStats(lbLatencyNs, serviceLatencyNs, traceOption);
+  }
+}
diff --git a/api/src/main/java/io/opencensus/common/ServerStatsDeserializationException.java b/api/src/main/java/io/opencensus/common/ServerStatsDeserializationException.java
new file mode 100644
index 00000000..2332733c
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ServerStatsDeserializationException.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+/**
+ * Exception thrown when a {@link ServerStats} cannot be parsed.
+ *
+ * @since 0.16
+ */
+public final class ServerStatsDeserializationException extends Exception {
+  private static final long serialVersionUID = 0L;
+
+  /**
+   * Constructs a new {@code ServerStatsDeserializationException} with the given message.
+   *
+   * @param message a message describing the error.
+   * @since 0.16
+   */
+  public ServerStatsDeserializationException(String message) {
+    super(message);
+  }
+
+  /**
+   * Constructs a new {@code ServerStatsDeserializationException} with the given message and cause.
+   *
+   * @param message a message describing the error.
+   * @param cause the cause of the error.
+   * @since 0.16
+   */
+  public ServerStatsDeserializationException(String message, Throwable cause) {
+    super(message, cause);
+  }
+}
diff --git a/api/src/main/java/io/opencensus/common/ServerStatsEncoding.java b/api/src/main/java/io/opencensus/common/ServerStatsEncoding.java
new file mode 100644
index 00000000..024a93f8
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ServerStatsEncoding.java
@@ -0,0 +1,125 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+/**
+ * A service class to encode/decode {@link ServerStats} as defined by the spec.
+ *
+ * <p>See <a
+ * href="https://github.com/census-instrumentation/opencensus-specs/blob/master/encodings/CensusServerStatsEncoding.md">opencensus-server-stats-specs</a>
+ * for encoding {@code ServerStats}
+ *
+ * <p>Use {@code ServerStatsEncoding.toBytes(ServerStats stats)} to encode.
+ *
+ * <p>Use {@code ServerStatsEncoding.parseBytes(byte[] serialized)} to decode.
+ *
+ * @since 0.16
+ */
+public final class ServerStatsEncoding {
+
+  private ServerStatsEncoding() {}
+
+  /**
+   * The current encoding version. The value is {@value #CURRENT_VERSION}
+   *
+   * @since 0.16
+   */
+  public static final byte CURRENT_VERSION = (byte) 0;
+
+  /**
+   * Encodes the {@link ServerStats} as per the Opencensus Summary Span specification.
+   *
+   * @param stats {@code ServerStats} to encode.
+   * @return encoded byte array.
+   * @since 0.16
+   */
+  public static byte[] toBytes(ServerStats stats) {
+    // Should this be optimized to not include invalid values?
+
+    ByteBuffer bb = ByteBuffer.allocate(ServerStatsFieldEnums.getTotalSize() + 1);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+
+    // put version
+    bb.put(CURRENT_VERSION);
+
+    bb.put((byte) ServerStatsFieldEnums.Id.SERVER_STATS_LB_LATENCY_ID.value());
+    bb.putLong(stats.getLbLatencyNs());
+
+    bb.put((byte) ServerStatsFieldEnums.Id.SERVER_STATS_SERVICE_LATENCY_ID.value());
+    bb.putLong(stats.getServiceLatencyNs());
+
+    bb.put((byte) ServerStatsFieldEnums.Id.SERVER_STATS_TRACE_OPTION_ID.value());
+    bb.put(stats.getTraceOption());
+    return bb.array();
+  }
+
+  /**
+   * Decodes serialized byte array to create {@link ServerStats} as per Opencensus Summary Span
+   * specification.
+   *
+   * @param serialized encoded {@code ServerStats} in byte array.
+   * @return decoded {@code ServerStats}. null if decoding fails.
+   * @since 0.16
+   */
+  public static ServerStats parseBytes(byte[] serialized)
+      throws ServerStatsDeserializationException {
+    final ByteBuffer bb = ByteBuffer.wrap(serialized);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    long serviceLatencyNs = 0L;
+    long lbLatencyNs = 0L;
+    byte traceOption = (byte) 0;
+
+    // Check the version first.
+    if (!bb.hasRemaining()) {
+      throw new ServerStatsDeserializationException("Serialized ServerStats buffer is empty");
+    }
+    byte version = bb.get();
+
+    if (version > CURRENT_VERSION || version < 0) {
+      throw new ServerStatsDeserializationException("Invalid ServerStats version: " + version);
+    }
+
+    while (bb.hasRemaining()) {
+      ServerStatsFieldEnums.Id id = ServerStatsFieldEnums.Id.valueOf((int) bb.get() & 0xFF);
+      if (id == null) {
+        // Skip remaining;
+        bb.position(bb.limit());
+      } else {
+        switch (id) {
+          case SERVER_STATS_LB_LATENCY_ID:
+            lbLatencyNs = bb.getLong();
+            break;
+          case SERVER_STATS_SERVICE_LATENCY_ID:
+            serviceLatencyNs = bb.getLong();
+            break;
+          case SERVER_STATS_TRACE_OPTION_ID:
+            traceOption = bb.get();
+            break;
+        }
+      }
+    }
+    try {
+      return ServerStats.create(lbLatencyNs, serviceLatencyNs, traceOption);
+    } catch (IllegalArgumentException e) {
+      throw new ServerStatsDeserializationException(
+          "Serialized ServiceStats contains invalid values: " + e.getMessage());
+    }
+  }
+}
diff --git a/api/src/main/java/io/opencensus/common/ServerStatsFieldEnums.java b/api/src/main/java/io/opencensus/common/ServerStatsFieldEnums.java
new file mode 100644
index 00000000..92194d96
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/ServerStatsFieldEnums.java
@@ -0,0 +1,157 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import java.util.TreeMap;
+
+/**
+ * A Enum representation for Ids and Size for attributes of {@code ServerStats}.
+ *
+ * <p>See <a
+ * href="https://github.com/census-instrumentation/opencensus-specs/blob/master/encodings/CensusServerStatsEncoding.md">opencensus-server-stats-specs</a>
+ * for the field ids and their length defined for Server Stats
+ *
+ * @since 0.16
+ */
+public final class ServerStatsFieldEnums {
+
+  /**
+   * Available Ids for {@code ServerStats} attributes.
+   *
+   * @since 0.16
+   */
+  public enum Id {
+    /**
+     * Id for Latency observed at Load Balancer.
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_LB_LATENCY_ID(0),
+    /**
+     * Id for Latency observed at Server.
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_SERVICE_LATENCY_ID(1),
+    /**
+     * Id for Trace options.
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_TRACE_OPTION_ID(2);
+
+    private final int value;
+
+    private Id(int value) {
+      this.value = value;
+    }
+
+    /**
+     * Returns the numerical value of the {@link Id}.
+     *
+     * @return the numerical value of the {@code Id}.
+     * @since 0.16
+     */
+    public int value() {
+      return value;
+    }
+
+    private static final TreeMap<Integer, Id> map = new TreeMap<Integer, Id>();
+
+    static {
+      for (Id id : Id.values()) {
+        map.put(id.value, id);
+      }
+    }
+
+    /**
+     * Returns the {@link Id} representing the value value of the id.
+     *
+     * @param value integer value for which {@code Id} is being requested.
+     * @return the numerical value of the id. null if the id is not valid
+     * @since 0.16
+     */
+    public static Id valueOf(int value) {
+      return (Id) map.get(value);
+    }
+  }
+
+  /**
+   * Size for each attributes in {@code ServerStats}.
+   *
+   * @since 0.16
+   */
+  public enum Size {
+    /**
+     * Number of bytes used to represent latency observed at Load Balancer
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_LB_LATENCY_SIZE(8),
+    /**
+     * Number of bytes used to represent latency observed at Server
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_SERVICE_LATENCY_SIZE(8),
+    /**
+     * Number of bytes used to represent Trace option
+     *
+     * @since 0.16
+     */
+    SERVER_STATS_TRACE_OPTION_SIZE(1);
+
+    private final int value;
+
+    private Size(int value) {
+      this.value = value;
+    }
+
+    /**
+     * Returns the numerical value of the {@link Size}.
+     *
+     * @return the numerical value of the {@code Size}.
+     * @since 0.16
+     */
+    public int value() {
+      return value;
+    }
+  }
+
+  private static final int TOTALSIZE = computeTotalSize();
+
+  private ServerStatsFieldEnums() {}
+
+  private static int computeTotalSize() {
+    int sum = 0;
+    for (Size sizeValue : Size.values()) {
+      sum += sizeValue.value();
+      sum += 1; // For Id
+    }
+    return sum;
+  }
+
+  /**
+   * Returns the total size required to encode the {@code ServerStats}
+   *
+   * @return the total size required to encode all fields in {@code ServerStats}.
+   * @since 0.16
+   */
+  public static int getTotalSize() {
+    return TOTALSIZE;
+  }
+}
diff --git a/api/src/test/java/io/opencensus/common/ServerStatsEncodingTest.java b/api/src/test/java/io/opencensus/common/ServerStatsEncodingTest.java
new file mode 100644
index 00000000..6db14a79
--- /dev/null
+++ b/api/src/test/java/io/opencensus/common/ServerStatsEncodingTest.java
@@ -0,0 +1,155 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ServerStatsEncoding}. */
+@RunWith(JUnit4.class)
+public class ServerStatsEncodingTest {
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void encodeDecodeTest() throws ServerStatsDeserializationException {
+    ServerStats serverStatsToBeEncoded = null;
+    ServerStats serverStatsDecoded = null;
+    byte[] serialized = null;
+
+    serverStatsToBeEncoded = ServerStats.create(31, 22, (byte) 1);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+    serverStatsDecoded = ServerStatsEncoding.parseBytes(serialized);
+    assertThat(serverStatsDecoded).isEqualTo(serverStatsToBeEncoded);
+
+    serverStatsToBeEncoded = ServerStats.create(0, 22, (byte) 0);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+    serverStatsDecoded = ServerStatsEncoding.parseBytes(serialized);
+    assertThat(serverStatsDecoded).isEqualTo(serverStatsToBeEncoded);
+
+    serverStatsToBeEncoded = ServerStats.create(450, 0, (byte) 0);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+    serverStatsDecoded = ServerStatsEncoding.parseBytes(serialized);
+    assertThat(serverStatsDecoded).isEqualTo(serverStatsToBeEncoded);
+  }
+
+  @Test
+  public void skipUnknownFieldTest() throws ServerStatsDeserializationException {
+    ServerStats serverStatsToBeEncoded = null;
+    ServerStats serverStatsDecoded = null;
+    byte[] serialized = null;
+
+    serverStatsToBeEncoded = ServerStats.create(31, 22, (byte) 1);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+
+    // Add new field at the end.
+    byte[] serializedExpanded = new byte[serialized.length + 9];
+    System.arraycopy(serialized, 0, serializedExpanded, 0, serialized.length);
+    final ByteBuffer bb = ByteBuffer.wrap(serializedExpanded);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    bb.position(serialized.length);
+    bb.put((byte) 255);
+    bb.putLong(0L);
+    byte[] newSerialized = bb.array();
+
+    serverStatsDecoded = ServerStatsEncoding.parseBytes(newSerialized);
+    assertThat(serverStatsDecoded).isEqualTo(serverStatsToBeEncoded);
+  }
+
+  @Test
+  public void negativeLbLatencyValueTest() throws ServerStatsDeserializationException {
+    ServerStats serverStatsToBeEncoded = null;
+    ServerStats serverStatsDecoded = null;
+    byte[] serialized = null;
+
+    serverStatsToBeEncoded = ServerStats.create(31, 22, (byte) 1);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+
+    // update serialized byte[] with negative value for lbLatency.
+    final ByteBuffer bb = ByteBuffer.wrap(serialized);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    bb.position(2);
+    bb.putLong(-100L);
+
+    byte[] newSerialized = bb.array();
+    thrown.expect(ServerStatsDeserializationException.class);
+    thrown.expectMessage("Serialized ServiceStats contains invalid values");
+    ServerStatsEncoding.parseBytes(newSerialized);
+  }
+
+  @Test
+  public void negativeServerLatencyValueTest() throws ServerStatsDeserializationException {
+    ServerStats serverStatsToBeEncoded = null;
+    ServerStats serverStatsDecoded = null;
+    byte[] serialized = null;
+
+    serverStatsToBeEncoded = ServerStats.create(31, 22, (byte) 1);
+    serialized = ServerStatsEncoding.toBytes(serverStatsToBeEncoded);
+
+    // update serialized byte[] with negative value for serviceLatency.
+    final ByteBuffer bb = ByteBuffer.wrap(serialized);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    bb.position(11);
+    bb.putLong(-101L);
+
+    byte[] newSerialized = bb.array();
+    thrown.expect(ServerStatsDeserializationException.class);
+    thrown.expectMessage("Serialized ServiceStats contains invalid values");
+    ServerStatsEncoding.parseBytes(newSerialized);
+  }
+
+  @Test
+  public void emptySerializedBuffer() throws ServerStatsDeserializationException {
+    final ByteBuffer bb = ByteBuffer.allocate(0);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+
+    byte[] newSerialized = bb.array();
+    thrown.expect(ServerStatsDeserializationException.class);
+    thrown.expectMessage("Serialized ServerStats buffer is empty");
+    ServerStatsEncoding.parseBytes(newSerialized);
+  }
+
+  @Test
+  public void invalidNegativeVersion() throws ServerStatsDeserializationException {
+    final ByteBuffer bb = ByteBuffer.allocate(10);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    bb.put((byte) -1);
+    byte[] newSerialized = bb.array();
+    thrown.expect(ServerStatsDeserializationException.class);
+    thrown.expectMessage("Invalid ServerStats version: -1");
+    ServerStatsEncoding.parseBytes(newSerialized);
+  }
+
+  @Test
+  public void invalidCompatibleVersion() throws ServerStatsDeserializationException {
+    final ByteBuffer bb = ByteBuffer.allocate(10);
+    bb.order(ByteOrder.LITTLE_ENDIAN);
+    bb.put((byte) (ServerStatsEncoding.CURRENT_VERSION + 1));
+    byte[] newSerialized = bb.array();
+    thrown.expect(ServerStatsDeserializationException.class);
+    thrown.expectMessage(
+        "Invalid ServerStats version: " + (ServerStatsEncoding.CURRENT_VERSION + 1));
+    ServerStatsEncoding.parseBytes(newSerialized);
+  }
+}
diff --git a/api/src/test/java/io/opencensus/common/ServerStatsFieldEnumsTest.java b/api/src/test/java/io/opencensus/common/ServerStatsFieldEnumsTest.java
new file mode 100644
index 00000000..ed786f6c
--- /dev/null
+++ b/api/src/test/java/io/opencensus/common/ServerStatsFieldEnumsTest.java
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.common.ServerStatsFieldEnums.Id;
+import io.opencensus.common.ServerStatsFieldEnums.Size;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ServerStatsFieldEnums}. */
+@RunWith(JUnit4.class)
+public class ServerStatsFieldEnumsTest {
+
+  @Test
+  public void enumIdValueOfTest() {
+    assertThat(Id.valueOf(0)).isEqualTo(Id.SERVER_STATS_LB_LATENCY_ID);
+    assertThat(Id.valueOf(1)).isEqualTo(Id.SERVER_STATS_SERVICE_LATENCY_ID);
+    assertThat(Id.valueOf(2)).isEqualTo(Id.SERVER_STATS_TRACE_OPTION_ID);
+  }
+
+  @Test
+  public void enumIdInvalidValueOfTest() {
+    assertThat(Id.valueOf(-1)).isNull();
+    assertThat(Id.valueOf(Id.values().length)).isNull();
+    assertThat(Id.valueOf(Id.values().length + 1)).isNull();
+  }
+
+  @Test
+  public void enumSizeValueTest() {
+    assertThat(Size.SERVER_STATS_LB_LATENCY_SIZE.value()).isEqualTo(8);
+    assertThat(Size.SERVER_STATS_SERVICE_LATENCY_SIZE.value()).isEqualTo(8);
+    assertThat(Size.SERVER_STATS_TRACE_OPTION_SIZE.value()).isEqualTo(1);
+  }
+
+  @Test
+  public void totalSizeTest() {
+    assertThat(ServerStatsFieldEnums.getTotalSize()).isEqualTo(20);
+  }
+}
diff --git a/api/src/test/java/io/opencensus/common/ServerStatsTest.java b/api/src/test/java/io/opencensus/common/ServerStatsTest.java
new file mode 100644
index 00000000..620bbb4f
--- /dev/null
+++ b/api/src/test/java/io/opencensus/common/ServerStatsTest.java
@@ -0,0 +1,78 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ServerStats}. */
+@RunWith(JUnit4.class)
+public class ServerStatsTest {
+
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void serverStatsCreate() {
+    ServerStats serverStats = null;
+
+    serverStats = ServerStats.create(31, 22, (byte) 0);
+    assertThat(serverStats.getLbLatencyNs()).isEqualTo(31);
+    assertThat(serverStats.getServiceLatencyNs()).isEqualTo(22);
+    assertThat(serverStats.getTraceOption()).isEqualTo((byte) 0);
+
+    serverStats = ServerStats.create(1000011L, 900022L, (byte) 1);
+    assertThat(serverStats.getLbLatencyNs()).isEqualTo(1000011L);
+    assertThat(serverStats.getServiceLatencyNs()).isEqualTo(900022L);
+    assertThat(serverStats.getTraceOption()).isEqualTo((byte) 1);
+
+    serverStats = ServerStats.create(0, 22, (byte) 0);
+    assertThat(serverStats.getLbLatencyNs()).isEqualTo(0);
+    assertThat(serverStats.getServiceLatencyNs()).isEqualTo(22);
+    assertThat(serverStats.getTraceOption()).isEqualTo((byte) 0);
+
+    serverStats = ServerStats.create(1010, 0, (byte) 0);
+    assertThat(serverStats.getLbLatencyNs()).isEqualTo(1010);
+    assertThat(serverStats.getServiceLatencyNs()).isEqualTo(0);
+    assertThat(serverStats.getTraceOption()).isEqualTo((byte) 0);
+  }
+
+  @Test
+  public void create_LbLatencyNegative() {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("'getLbLatencyNs' is less than zero");
+    ServerStats.create(-1L, 100, (byte) 0);
+  }
+
+  @Test
+  public void create_ServerLatencyNegative() {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("'getServiceLatencyNs' is less than zero");
+    ServerStats.create(100L, -1L, (byte) 0);
+  }
+
+  @Test
+  public void create_LbLatencyAndServerLatencyNegative() {
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("'getLbLatencyNs' is less than zero");
+    ServerStats.create(-100L, -1L, (byte) 0);
+  }
+}