diff --git a/flow/ITrace.h b/flow/ITrace.h
index 3af788ca85..667f352426 100644
--- a/flow/ITrace.h
+++ b/flow/ITrace.h
@@ -40,10 +40,10 @@ struct ITraceLogWriter {
 class TraceEventFields;
 
 struct ITraceLogFormatter {
-	virtual const char* getExtension() = 0;
-	virtual const char* getHeader() = 0; // Called when starting a new file
-	virtual const char* getFooter() = 0; // Called when ending a file
-	virtual std::string formatEvent(const TraceEventFields&) = 0; // Called for each event
+	virtual const char* getExtension() const = 0;
+	virtual const char* getHeader() const = 0; // Called when starting a new file
+	virtual const char* getFooter() const = 0; // Called when ending a file
+	virtual std::string formatEvent(const TraceEventFields&) const = 0; // Called for each event
 
 	virtual void addref() = 0;
 	virtual void delref() = 0;
diff --git a/flow/JsonTraceLogFormatter.cpp b/flow/JsonTraceLogFormatter.cpp
index 8574f35a80..2323d3d272 100644
--- a/flow/JsonTraceLogFormatter.cpp
+++ b/flow/JsonTraceLogFormatter.cpp
@@ -31,55 +31,55 @@ void JsonTraceLogFormatter::delref() {
 	ReferenceCounted<JsonTraceLogFormatter>::delref();
 }
 
-const char* JsonTraceLogFormatter::getExtension() {
+const char* JsonTraceLogFormatter::getExtension() const {
 	return "json";
 }
 
-const char* JsonTraceLogFormatter::getHeader() {
+const char* JsonTraceLogFormatter::getHeader() const {
 	return "";
 }
 
-const char* JsonTraceLogFormatter::getFooter() {
+const char* JsonTraceLogFormatter::getFooter() const {
 	return "";
 }
 
 namespace {
 
-void escapeString(std::stringstream& ss, const std::string& source) {
+void escapeString(std::ostringstream& oss, const std::string& source) {
 	for (auto c : source) {
 		if (c == '"') {
-			ss << "\\\"";
+			oss << "\\\"";
 		} else if (c == '\\') {
-			ss << "\\\\";
+			oss << "\\\\";
 		} else if (c == '\n') {
-			ss << "\\n";
+			oss << "\\n";
 		} else if (c == '\r') {
-			ss << "\\r";
+			oss << "\\r";
 		} else if (isprint(c)) {
-			ss << c;
+			oss << c;
 		} else {
 			constexpr char hex[] = "0123456789abcdef";
 			int x = int{ static_cast<uint8_t>(c) };
-			ss << "\\x" << hex[x / 16] << hex[x % 16];
+			oss << "\\x" << hex[x / 16] << hex[x % 16];
 		}
 	}
 }
 
 } // namespace
 
-std::string JsonTraceLogFormatter::formatEvent(const TraceEventFields& fields) {
-	std::stringstream ss;
-	ss << "{  ";
+std::string JsonTraceLogFormatter::formatEvent(const TraceEventFields& fields) const {
+	std::ostringstream oss;
+	oss << "{  ";
 	for (auto iter = fields.begin(); iter != fields.end(); ++iter) {
 		if (iter != fields.begin()) {
-			ss << ", ";
+			oss << ", ";
 		}
-		ss << "\"";
-		escapeString(ss, iter->first);
-		ss << "\": \"";
-		escapeString(ss, iter->second);
-		ss << "\"";
+		oss << "\"";
+		escapeString(oss, iter->first);
+		oss << "\": \"";
+		escapeString(oss, iter->second);
+		oss << "\"";
 	}
-	ss << " }\r\n";
-	return ss.str();
+	oss << " }\r\n";
+	return std::move(oss).str();
 }
diff --git a/flow/JsonTraceLogFormatter.h b/flow/JsonTraceLogFormatter.h
index 246ba0f455..38e4e4f8f0 100644
--- a/flow/JsonTraceLogFormatter.h
+++ b/flow/JsonTraceLogFormatter.h
@@ -22,10 +22,10 @@
 #include "flow/Trace.h"
 
 struct JsonTraceLogFormatter final : public ITraceLogFormatter, ReferenceCounted<JsonTraceLogFormatter> {
-	const char* getExtension() override;
-	const char* getHeader() override; // Called when starting a new file
-	const char* getFooter() override; // Called when ending a file
-	std::string formatEvent(const TraceEventFields&) override; // Called for each event
+	const char* getExtension() const override;
+	const char* getHeader() const override; // Called when starting a new file
+	const char* getFooter() const override; // Called when ending a file
+	std::string formatEvent(const TraceEventFields&) const override; // Called for each event
 
 	void addref() override;
 	void delref() override;
diff --git a/flow/XmlTraceLogFormatter.cpp b/flow/XmlTraceLogFormatter.cpp
index 4229c3b0db..dff1ca15ab 100644
--- a/flow/XmlTraceLogFormatter.cpp
+++ b/flow/XmlTraceLogFormatter.cpp
@@ -30,38 +30,38 @@ void XmlTraceLogFormatter::delref() {
 	ReferenceCounted<XmlTraceLogFormatter>::delref();
 }
 
-const char* XmlTraceLogFormatter::getExtension() {
+const char* XmlTraceLogFormatter::getExtension() const {
 	return "xml";
 }
 
-const char* XmlTraceLogFormatter::getHeader() {
+const char* XmlTraceLogFormatter::getHeader() const {
 	return "<?xml version=\"1.0\"?>\r\n<Trace>\r\n";
 }
 
-const char* XmlTraceLogFormatter::getFooter() {
+const char* XmlTraceLogFormatter::getFooter() const {
 	return "</Trace>\r\n";
 }
 
-void XmlTraceLogFormatter::escape(std::stringstream& ss, std::string source) {
+void XmlTraceLogFormatter::escape(std::ostringstream& oss, std::string source) const {
 	loop {
 		int index = source.find_first_of(std::string({ '&', '"', '<', '>', '\r', '\n', '\0' }));
 		if (index == source.npos) {
 			break;
 		}
 
-		ss << source.substr(0, index);
+		oss << source.substr(0, index);
 		if (source[index] == '&') {
-			ss << "&amp;";
+			oss << "&amp;";
 		} else if (source[index] == '"') {
-			ss << "&quot;";
+			oss << "&quot;";
 		} else if (source[index] == '<') {
-			ss << "&lt;";
+			oss << "&lt;";
 		} else if (source[index] == '>') {
-			ss << "&gt;";
+			oss << "&gt;";
 		} else if (source[index] == '\n' || source[index] == '\r') {
-			ss << " ";
+			oss << " ";
 		} else if (source[index] == '\0') {
-			ss << " ";
+			oss << " ";
 			TraceEvent(SevWarnAlways, "StrippedIllegalCharacterFromTraceEvent")
 			    .detail("Source", StringRef(source).printable())
 			    .detail("Character", StringRef(source.substr(index, 1)).printable());
@@ -72,20 +72,20 @@ void XmlTraceLogFormatter::escape(std::stringstream& ss, std::string source) {
 		source = source.substr(index + 1);
 	}
 
-	ss << source;
+	oss << std::move(source);
 }
 
-std::string XmlTraceLogFormatter::formatEvent(const TraceEventFields& fields) {
-	std::stringstream ss;
-	ss << "<Event ";
+std::string XmlTraceLogFormatter::formatEvent(const TraceEventFields& fields) const {
+	std::ostringstream oss;
+	oss << "<Event ";
 
 	for (auto itr : fields) {
-		escape(ss, itr.first);
-		ss << "=\"";
-		escape(ss, itr.second);
-		ss << "\" ";
+		escape(oss, itr.first);
+		oss << "=\"";
+		escape(oss, itr.second);
+		oss << "\" ";
 	}
 
-	ss << "/>\r\n";
-	return ss.str();
+	oss << "/>\r\n";
+	return std::move(oss).str();
 }
diff --git a/flow/XmlTraceLogFormatter.h b/flow/XmlTraceLogFormatter.h
index 3d2444b583..502dcaa5e4 100644
--- a/flow/XmlTraceLogFormatter.h
+++ b/flow/XmlTraceLogFormatter.h
@@ -31,12 +31,12 @@ struct XmlTraceLogFormatter final : public ITraceLogFormatter, ReferenceCounted<
 	void addref() override;
 	void delref() override;
 
-	const char* getExtension() override;
-	const char* getHeader() override;
-	const char* getFooter() override;
+	const char* getExtension() const override;
+	const char* getHeader() const override;
+	const char* getFooter() const override;
 
-	void escape(std::stringstream& ss, std::string source);
-	std::string formatEvent(const TraceEventFields& fields) override;
+	void escape(std::ostringstream& oss, std::string source) const;
+	std::string formatEvent(const TraceEventFields& fields) const override;
 };
 
 #endif