diff --git a/app/src/main/java/com/hoc081098/refreshtokensample/data/local/UserLocalSource.kt b/app/src/main/java/com/hoc081098/refreshtokensample/data/local/UserLocalSource.kt
index fd87edc..5cca6c7 100644
--- a/app/src/main/java/com/hoc081098/refreshtokensample/data/local/UserLocalSource.kt
+++ b/app/src/main/java/com/hoc081098/refreshtokensample/data/local/UserLocalSource.kt
@@ -1,10 +1,24 @@
 package com.hoc081098.refreshtokensample.data.local
 
+import java.io.IOException
 import kotlinx.coroutines.flow.Flow
 
 interface UserLocalSource {
   fun user(): Flow<UserLocal?>
 
+  /**
+   * Updates the data transactionally in an atomic read-modify-write operation. All operations
+   * are serialized, and the [transform] itself is a coroutine so it can perform heavy work
+   * such as RPCs.
+   *
+   * The coroutine completes when the data has been persisted durably to disk (after which
+   * [user] will reflect the update). If the transform or write to disk fails, the
+   * transaction is aborted and an exception is thrown.
+   *
+   * @return the snapshot returned by the transform
+   * @throws IOException when an exception is encountered when writing data to disk
+   * @throws Exception when thrown by the transform function
+   */
   suspend fun update(transform: suspend (current: UserLocal?) -> UserLocal?): UserLocal?
 }
 
diff --git a/app/src/main/java/com/hoc081098/refreshtokensample/data/remote/interceptor/AuthInterceptor.kt b/app/src/main/java/com/hoc081098/refreshtokensample/data/remote/interceptor/AuthInterceptor.kt
index 27165fd..712633c 100644
--- a/app/src/main/java/com/hoc081098/refreshtokensample/data/remote/interceptor/AuthInterceptor.kt
+++ b/app/src/main/java/com/hoc081098/refreshtokensample/data/remote/interceptor/AuthInterceptor.kt
@@ -13,8 +13,6 @@ import javax.inject.Inject
 import javax.inject.Provider
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.runBlocking
-import kotlinx.coroutines.sync.Mutex
-import kotlinx.coroutines.sync.withLock
 import okhttp3.Interceptor
 import okhttp3.Request
 import okhttp3.Response
@@ -25,70 +23,73 @@ class AuthInterceptor @Inject constructor(
   private val apiService: Provider<ApiService>,
   private val appDispatchers: AppDispatchers,
 ) : Interceptor {
-  private val mutex = Mutex()
-
   override fun intercept(chain: Interceptor.Chain): Response {
-    val req = chain.request().also { debug("[1] $it") }
+    val request = chain.request().also { debug("[1] START url=${it.url}") }
 
-    if (NO_AUTH in req.headers.values(CUSTOM_HEADER)) {
-      return chain.proceedWithToken(req, null)
+    if (NO_AUTH in request.headers.values(CUSTOM_HEADER)) {
+      return chain.proceedWithToken(request, null)
     }
 
     val token = runBlocking(appDispatchers.io) { userLocalSource.user().first() }
       ?.token
-      .also { debug("[2] $req $it") }
-    val res = chain.proceedWithToken(req, token)
+      .also { debug("[2] READ TOKEN token=${it.firstTwoCharactersAndLastTwoCharacters()}, url=${request.url}") }
+    val response = chain.proceedWithToken(request, token)
 
-    if (res.code != HTTP_UNAUTHORIZED || token == null) {
-      return res
+    if (response.code != HTTP_UNAUTHORIZED || token == null) {
+      return response
     }
 
-    debug("[3] $req")
+    val newToken = executeTokenRefreshing(request, token)
+
+    return if (newToken !== null && newToken != token) {
+      chain.proceedWithToken(request, newToken)
+    } else {
+      response
+    }
+  }
+
+  private fun executeTokenRefreshing(request: Request, token: String?): String? {
+    val requestUrl = request.url
+    debug("[3] BEFORE REFRESHING token=${token.firstTwoCharactersAndLastTwoCharacters()}, url=$requestUrl")
 
-    val newToken: String? = runBlocking(appDispatchers.io) {
-      mutex.withLock {
-        val user =
-          userLocalSource.user().first().also { debug("[4] $req $it") }
-        val maybeUpdatedToken = user?.token
+    return runBlocking(appDispatchers.io) {
+      userLocalSource.update { currentUserLocal ->
+        val maybeUpdatedToken = currentUserLocal?.token
+        debug("[4] INSIDE REFRESHING maybeUpdatedToken=${maybeUpdatedToken.firstTwoCharactersAndLastTwoCharacters()}, url=$requestUrl")
 
         when {
-          user == null || maybeUpdatedToken == null -> null.also { debug("[5-1] $req") } // already logged out!
-          maybeUpdatedToken != token -> maybeUpdatedToken.also { debug("[5-2] $req") } // refreshed by another request
+          maybeUpdatedToken == null ->
+            null
+              .also { debug("[5-1] LOGGED OUT url=$requestUrl") } // already logged out!
+          maybeUpdatedToken != token ->
+            currentUserLocal
+              .also { debug("[5-2] REFRESHED BY ANOTHER url=$requestUrl") } // refreshed by another request
           else -> {
-            debug("[5-3] $req")
+            debug("[5-3] START REFRESHING REQUEST url=$requestUrl")
 
-            val refreshTokenRes = apiService.get()
-              .refreshToken(user.toRefreshTokenBody())
-              .also { debug("[6] $req $it") }
+            val refreshTokenRes = apiService
+              .get()
+              .refreshToken(currentUserLocal.toRefreshTokenBody())
 
-            when (refreshTokenRes.code()) {
+            when (val code = refreshTokenRes.code()) {
               HTTP_OK -> {
-                debug("[7-1] $req")
-                refreshTokenRes.body()!!.token.also { updatedToken ->
-                  userLocalSource.update {
-                    (it ?: return@update null)
-                      .toBuilder()
-                      .setToken(updatedToken)
-                      .build()
-                  }
-                }
+                currentUserLocal
+                  .toBuilder()
+                  .setToken(refreshTokenRes.body()!!.token)
+                  .build()
+                  .also { debug("[6-1] REFRESH SUCCESSFULLY newToken=${it.token.firstTwoCharactersAndLastTwoCharacters()}, url=$requestUrl") }
               }
-              HTTP_UNAUTHORIZED -> {
-                debug("[7-2] $req")
-                userLocalSource.update { null }
-                null
+              HTTP_UNAUTHORIZED -> null.also {
+                debug("[6-2] REFRESH FAILED HTTP_UNAUTHORIZED url=$requestUrl")
               }
-              else -> {
-                debug("[7-3] $req")
-                null
+              else -> currentUserLocal.also {
+                debug("[6-3] REFRESH FAILED code=$code, url=$requestUrl")
               }
             }
           }
         }
-      }
+      }?.token
     }
-
-    return if (newToken !== null) chain.proceedWithToken(req, newToken) else res
   }
 
   private fun Interceptor.Chain.proceedWithToken(req: Request, token: String?): Response =
@@ -111,3 +112,9 @@ class AuthInterceptor @Inject constructor(
 }
 
 private fun UserLocal.toRefreshTokenBody() = RefreshTokenBody(refreshToken, username)
+
+@Suppress("NOTHING_TO_INLINE")
+private inline fun String?.firstTwoCharactersAndLastTwoCharacters(): String? {
+  this ?: return null
+  return if (length <= 4) this else "${take(2)}...${takeLast(2)}"
+}