cipd: instance cache

client/cipd/internal/messages/messages.proto

Index: client/cipd/internal/messages/messages.proto
diff --git a/client/cipd/internal/messages/messages.proto b/client/cipd/internal/messages/messages.proto
index 25b248ce8180ef96bbaa2730dd604b8b24d00a69..503953b09870b2fe7b4345a560952278b12a0b96 100644
--- a/client/cipd/internal/messages/messages.proto
+++ b/client/cipd/internal/messages/messages.proto
@@ -4,6 +4,8 @@
 
 syntax = "proto3";
 
+import "google/protobuf/timestamp.proto";
+
 package messages;
 
 // BlobWithSHA1 is a wrapper around a binary blob with SHA1 hash to verify
@@ -25,3 +27,20 @@ message TagCache {
   // Capped list of entries, most recently resolved is last.
   repeated Entry entries = 1;
 }
+
+// InstanceCache stores a list of instances in cache
+// and their last access time.
+message InstanceCache {
+  // Entry stores info about an instance.
+  message Entry {
+    // LastAccess is last time this instance was retrieved from or put to the
+    // cache.
+    google.protobuf.Timestamp last_access = 2;
+  }
+
+  // Entries is a map of {instance id -> information about instance}.
+  map<string, Entry> entries = 1;
+  // LastSynced is timestamp when we synchronized Entries with actual
+  // instance files.
+  google.protobuf.Timestamp last_synced = 2;
+}