diff --git a/crates/libanalysis/src/lib.rs b/crates/libanalysis/src/lib.rs
index d01144627f..fee0d10d6c 100644
--- a/crates/libanalysis/src/lib.rs
+++ b/crates/libanalysis/src/lib.rs
@@ -17,7 +17,6 @@ use rayon::prelude::*;
 
 use std::{
     fmt,
-    mem,
     path::{Path},
     sync::{
         Arc,
@@ -36,18 +35,16 @@ use libeditor::{LineIndex, FileSymbol, find_node};
 
 use self::{
     symbol_index::FileSymbols,
-    module_map::ModuleMap,
+    module_map::{ModuleMap, ChangeKind},
 };
 pub use self::symbol_index::Query;
 
 pub type Result<T> = ::std::result::Result<T, ::failure::Error>;
-const INDEXING_THRESHOLD: usize = 128;
 
 pub type FileResolver = dyn Fn(FileId, &Path) -> Option<FileId> + Send + Sync;
 
 #[derive(Debug)]
 pub struct WorldState {
-    updates: Vec<FileId>,
     data: Arc<WorldData>
 }
 
@@ -79,32 +76,16 @@ pub struct FileId(pub u32);
 impl WorldState {
     pub fn new() -> WorldState {
         WorldState {
-            updates: Vec::new(),
             data: Arc::new(WorldData::default()),
         }
     }
 
     pub fn snapshot(
-        &mut self,
+        &self,
         file_resolver: impl Fn(FileId, &Path) -> Option<FileId> + 'static + Send + Sync,
     ) -> World {
-        let needs_reindex = self.updates.len() >= INDEXING_THRESHOLD;
-        if !self.updates.is_empty() {
-            let updates = mem::replace(&mut self.updates, Vec::new());
-            let data = self.data_mut();
-            for file_id in updates {
-                let syntax = data.file_map
-                    .get(&file_id)
-                    .map(|it| it.syntax());
-                data.module_map.update_file(
-                    file_id,
-                    syntax,
-                    &file_resolver,
-                );
-            }
-        }
         World {
-            needs_reindex: AtomicBool::new(needs_reindex),
+            needs_reindex: AtomicBool::new(false),
             file_resolver: Arc::new(file_resolver),
             data: self.data.clone()
         }
@@ -115,21 +96,26 @@ impl WorldState {
     }
 
     pub fn change_files(&mut self, changes: impl Iterator<Item=(FileId, Option<String>)>) {
-        let mut updates = Vec::new();
-        {
-            let data = self.data_mut();
-            for (file_id, text) in changes {
-                data.file_map.remove(&file_id);
-                if let Some(text) = text {
-                    let file_data = FileData::new(text);
-                    data.file_map.insert(file_id, Arc::new(file_data));
+        let data = self.data_mut();
+        for (file_id, text) in changes {
+            let change_kind = if data.file_map.remove(&file_id).is_some() {
+                if text.is_some() {
+                    ChangeKind::Update
                 } else {
-                    data.file_map.remove(&file_id);
+                    ChangeKind::Delete
                 }
-                updates.push(file_id);
+            } else {
+                ChangeKind::Insert
+            };
+            data.module_map.update_file(file_id, change_kind);
+            data.file_map.remove(&file_id);
+            if let Some(text) = text {
+                let file_data = FileData::new(text);
+                data.file_map.insert(file_id, Arc::new(file_data));
+            } else {
+                data.file_map.remove(&file_id);
             }
         }
-        self.updates.extend(updates)
     }
 
     fn data_mut(&mut self) -> &mut WorldData {
@@ -171,13 +157,17 @@ impl World {
         let module_map = &self.data.module_map;
         let id = module_map.file2module(id);
         module_map
-            .parent_modules(id)
+            .parent_modules(
+                id,
+                &*self.file_resolver,
+                &|file_id| self.file_syntax(file_id).unwrap(),
+            )
             .into_iter()
-            .map(|(id, m)| {
+            .map(|(id, name, node)| {
                 let id = module_map.module2file(id);
                 let sym = FileSymbol {
-                    name: m.name().unwrap().text(),
-                    node_range: m.syntax().range(),
+                    name,
+                    node_range: node.range(),
                     kind: MODULE,
                 };
                 (id, sym)
@@ -235,7 +225,11 @@ impl World {
         let module_map = &self.data.module_map;
         let id = module_map.file2module(id);
         module_map
-            .child_module_by_name(id, name.as_str())
+            .child_module_by_name(
+                id, name.as_str(),
+                &*self.file_resolver,
+                &|file_id| self.file_syntax(file_id).unwrap(),
+            )
             .into_iter()
             .map(|id| module_map.module2file(id))
             .collect()
diff --git a/crates/libanalysis/src/module_map.rs b/crates/libanalysis/src/module_map.rs
index 9b4c778b61..83e6e57f71 100644
--- a/crates/libanalysis/src/module_map.rs
+++ b/crates/libanalysis/src/module_map.rs
@@ -2,17 +2,38 @@ use std::{
     path::{PathBuf},
 };
 
+use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 use libsyntax2::{
-    ast::{self, AstNode, NameOwner},
-    SyntaxNode, ParsedFile, SmolStr,
+    ast::{self, AstNode, NameOwner, ParsedFile},
+    SyntaxNode, SmolStr,
 };
 use {FileId, FileResolver};
 
+type SyntaxProvider<'a> = dyn Fn(FileId) -> ParsedFile + 'a;
+
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
 pub struct ModuleId(FileId);
 
-#[derive(Clone, Debug, Default)]
+#[derive(Debug, Default)]
 pub struct ModuleMap {
+    state: RwLock<State>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ChangeKind {
+    Delete, Insert, Update
+}
+
+impl Clone for ModuleMap {
+    fn clone(&self) -> ModuleMap {
+        let state = self.state.read().clone();
+        ModuleMap { state: RwLock::new(state) }
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+struct State {
+    changes: Vec<(FileId, ChangeKind)>,
     links: Vec<Link>,
 }
 
@@ -24,31 +45,8 @@ struct Link {
 }
 
 impl ModuleMap {
-    pub fn update_file(
-        &mut self,
-        file_id: FileId,
-        syntax: Option<&ParsedFile>,
-        file_resolver: &FileResolver,
-    ) {
-        let mod_id = ModuleId(file_id);
-        self.links.retain(|link| link.owner != mod_id);
-        match syntax {
-            None => {
-                for link in self.links.iter_mut() {
-                    link.points_to.retain(|&x| x != mod_id);
-                }
-            }
-            Some(syntax) => {
-                self.links.extend(
-                    syntax.ast().modules().filter_map(|it| {
-                        Link::new(mod_id, it)
-                    })
-                )
-            }
-        }
-        self.links.iter_mut().for_each(|link| {
-            link.resolve(file_resolver)
-        })
+    pub fn update_file(&mut self, file: FileId, change_kind: ChangeKind) {
+        self.state.get_mut().changes.push((file, change_kind));
     }
 
     pub fn module2file(&self, m: ModuleId) -> FileId {
@@ -59,8 +57,15 @@ impl ModuleMap {
         ModuleId(file_id)
     }
 
-    pub fn child_module_by_name(&self, parent_mod: ModuleId, child_mod: &str) -> Vec<ModuleId> {
-        self.links
+    pub fn child_module_by_name<'a>(
+        &self,
+        parent_mod: ModuleId,
+        child_mod: &str,
+        file_resolver: &FileResolver,
+        syntax_provider: &SyntaxProvider,
+    ) -> Vec<ModuleId> {
+        self.links(file_resolver, syntax_provider)
+            .links
             .iter()
             .filter(|link| link.owner == parent_mod)
             .filter(|link| link.name() == child_mod)
@@ -69,13 +74,90 @@ impl ModuleMap {
             .collect()
     }
 
-    pub fn parent_modules<'a>(&'a self, m: ModuleId) -> impl Iterator<Item=(ModuleId, ast::Module<'a>)> + 'a {
-        self.links
+    pub fn parent_modules(
+        &self,
+        m: ModuleId,
+        file_resolver: &FileResolver,
+        syntax_provider: &SyntaxProvider,
+    ) -> Vec<(ModuleId, SmolStr, SyntaxNode)> {
+        let links = self.links(file_resolver, syntax_provider);
+        let res = links
+            .links
             .iter()
             .filter(move |link| link.points_to.iter().any(|&it| it == m))
             .map(|link| {
-                (link.owner, link.ast())
+                (link.owner, link.name().clone(), link.syntax.clone())
             })
+            .collect();
+        res
+    }
+
+    fn links(
+        &self,
+        file_resolver: &FileResolver,
+        syntax_provider: &SyntaxProvider,
+    ) -> RwLockReadGuard<State> {
+        {
+            let guard = self.state.read();
+            if guard.changes.is_empty() {
+                return guard;
+            }
+        }
+        let mut guard = self.state.write();
+        if !guard.changes.is_empty() {
+            guard.apply_changes(file_resolver, syntax_provider);
+        }
+        assert!(guard.changes.is_empty());
+        RwLockWriteGuard::downgrade(guard)
+    }
+}
+
+impl State {
+    pub fn apply_changes(
+        &mut self,
+        file_resolver: &FileResolver,
+        syntax_provider: &SyntaxProvider,
+    ) {
+        let mut reresolve = false;
+        for (file_id, kind) in self.changes.drain(..) {
+            let mod_id = ModuleId(file_id);
+            self.links.retain(|link| link.owner != mod_id);
+            match kind {
+                ChangeKind::Delete => {
+                    for link in self.links.iter_mut() {
+                        link.points_to.retain(|&x| x != mod_id);
+                    }
+                }
+                ChangeKind::Insert => {
+                    let file = syntax_provider(file_id);
+                    self.links.extend(
+                        file
+                            .ast()
+                            .modules()
+                            .filter_map(|it| Link::new(mod_id, it))
+                    );
+                    reresolve = true;
+                }
+                ChangeKind::Update => {
+                    let file = syntax_provider(file_id);
+                    self.links.extend(
+                        file
+                            .ast()
+                            .modules()
+                            .filter_map(|it| Link::new(mod_id, it))
+                            .map(|mut link| {
+                                link.resolve(file_resolver);
+                                link
+                            })
+                    );
+                }
+            }
+        }
+        if reresolve {
+            for link in self.links.iter_mut() {
+                link.resolve(file_resolver)
+            }
+        }
     }
 }
 
diff --git a/crates/server/src/main_loop/mod.rs b/crates/server/src/main_loop/mod.rs
index 12a903dac5..752d6ddb21 100644
--- a/crates/server/src/main_loop/mod.rs
+++ b/crates/server/src/main_loop/mod.rs
@@ -252,7 +252,7 @@ fn on_notification(
 fn handle_request_on_threadpool<R: req::ClientRequest>(
     req: &mut Option<RawRequest>,
     pool: &ThreadPool,
-    world: &mut ServerWorldState,
+    world: &ServerWorldState,
     sender: &Sender<Task>,
     f: fn(ServerWorld, R::Params) -> Result<R::Result>,
 ) -> Result<()>
diff --git a/crates/server/src/server_world.rs b/crates/server/src/server_world.rs
index 9850822cde..c0d2efb863 100644
--- a/crates/server/src/server_world.rs
+++ b/crates/server/src/server_world.rs
@@ -86,7 +86,7 @@ impl ServerWorldState {
         Ok(())
     }
 
-    pub fn snapshot(&mut self) -> ServerWorld {
+    pub fn snapshot(&self) -> ServerWorld {
         let pm = self.path_map.clone();
         ServerWorld {
             analysis: self.analysis.snapshot(move |id, path| {