Add database link to span context(#732)

apm-agent-core/src/main/java/co/elastic/apm/agent/impl/transaction/Db.java

@@ -81,6 +81,12 @@ public void recycle(CharBuffer object) {
      */
     @Nullable
     private String user;
+
+    /**
+     * DB Link for connections between 2 databases
+     */
+    @Nullable
+    private String dbLink;
 
     /**
      * Database instance name
@@ -176,13 +182,31 @@ public Db withUser(@Nullable String user) {
         this.user = user;
         return this;
     }
+
+
+    /**
+     * DB Link for connections between 2 databases
+     */
+    @Nullable
+    public String getDbLink() {
+        return dbLink;
+    }
+
+    /**
+     * DB Link for connections between 2 databases
+     */
+    public Db withDbLink(@Nullable String dbLink) {
+        this.dbLink = dbLink;
+        return this;
+    }
 
     @Override
     public void resetState() {
         instance = null;
         statement = null;
         type = null;
         user = null;
+        dbLink = null;
         if (statementBuffer != null) {
             charBufferPool.recycle(statementBuffer);
         }
@@ -194,6 +218,7 @@ public boolean hasContent() {
             statement != null ||
             type != null ||
             user != null ||
+            dbLink != null ||
             statementBuffer != null;
     }
 
@@ -202,5 +227,6 @@ public void copyFrom(Db other) {
         statement = other.statement;
         type = other.type;
         user = other.user;
+        dbLink = other.dbLink;
     }
 }

apm-agent-core/src/main/java/co/elastic/apm/agent/report/serialize/DslJsonSerializer.java

@@ -730,6 +730,7 @@ private void serializeDbContext(final Db db) {
                 }
             }
             writeField("type", db.getType());
+            writeField("link", db.getDbLink());
             writeLastField("user", db.getUser());
             jw.writeByte(OBJECT_END);
             jw.writeByte(COMMA);

apm-agent-core/src/main/resources/schema/transactions/span.json

@@ -55,6 +55,10 @@
                         "user": {
                            "type": ["string", "null"],
                            "description": "Username for accessing database"
+                        },
+                        "link": {
+                           "type": ["string", "null"],
+                           "description": "DB Link for connections between 2 databases"
                         }
                     }
                 }

apm-agent-core/src/test/java/co/elastic/apm/agent/TransactionUtils.java

@@ -93,7 +93,8 @@ public static List<Span> getSpans(Transaction t) {
             .withInstance("customers")
             .withStatement("SELECT * FROM product_types WHERE user_id=?")
             .withType("sql")
-            .withUser("readonly_user");
+            .withUser("readonly_user")
+            .withDbLink("DB_LINK");
         span.addLabel("monitored_by", "ACME");
         span.addLabel("framework", "some-framework");
         spans.add(span);

apm-agent-core/src/test/java/co/elastic/apm/agent/impl/payload/TransactionPayloadJsonSchemaTest.java

@@ -251,6 +251,7 @@ private void validateDbSpanSchema(JsonNode serializedSpans, boolean shouldContai
                 assertThat(db.get("statement").textValue()).isEqualTo("SELECT * FROM product_types WHERE user_id=?");
                 assertThat(db.get("type").textValue()).isEqualTo("sql");
                 assertThat(db.get("user").textValue()).isEqualTo("readonly_user");
+                assertThat(db.get("link").textValue()).isEqualTo("DB_LINK");
                 JsonNode tags = context.get("tags");
                 if (shouldContainTags) {
                     assertThat(tags).isNotNull();