diff --git a/ApolloDeveloperKit.xcodeproj/project.pbxproj b/ApolloDeveloperKit.xcodeproj/project.pbxproj
index 5765e215..89643f20 100644
--- a/ApolloDeveloperKit.xcodeproj/project.pbxproj
+++ b/ApolloDeveloperKit.xcodeproj/project.pbxproj
@@ -7,13 +7,13 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		5B00A17D266D0132009A4DCD /* HTTPConnectionTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B00A17C266D0132009A4DCD /* HTTPConnectionTests.swift */; };
 		5B05890824EB53220071DB57 /* ConsoleDidWriteNotification.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B05890724EB53220071DB57 /* ConsoleDidWriteNotification.swift */; };
 		5B05890A24EB5DC60071DB57 /* HTTPRequestMessage.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B05890924EB5DC60071DB57 /* HTTPRequestMessage.swift */; };
 		5B05890C24EB5DCF0071DB57 /* HTTPResponseMessage.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B05890B24EB5DCF0071DB57 /* HTTPResponseMessage.swift */; };
 		5B0EC95E22B6D728003D7933 /* DebuggableRequestChainNetworkTransportTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B0EC95D22B6D728003D7933 /* DebuggableRequestChainNetworkTransportTests.swift */; };
 		5B15067424F149960081B1E8 /* Schema+JSONEncodable.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B15067324F149960081B1E8 /* Schema+JSONEncodable.swift */; };
 		5B15067624F14B520081B1E8 /* ErrorLike+Error.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B15067524F14B520081B1E8 /* ErrorLike+Error.swift */; };
-		5B16DECD23F45C5300EFEA16 /* MockNetworkTransport.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B16DECC23F45C5300EFEA16 /* MockNetworkTransport.swift */; };
 		5B1DFA0E2526377E00594E80 /* AddressInfoErrorTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B1DFA0D2526377E00594E80 /* AddressInfoErrorTests.swift */; };
 		5B207730236E098700817E45 /* HTTPChunkedResponse.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B20772F236E098700817E45 /* HTTPChunkedResponse.swift */; };
 		5B207732236E0FD800817E45 /* HTTPChunkedResponseTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B207731236E0FD800817E45 /* HTTPChunkedResponseTests.swift */; };
@@ -173,6 +173,7 @@
 /* End PBXCopyFilesBuildPhase section */
 
 /* Begin PBXFileReference section */
+		5B00A17C266D0132009A4DCD /* HTTPConnectionTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = HTTPConnectionTests.swift; sourceTree = "<group>"; };
 		5B05890724EB53220071DB57 /* ConsoleDidWriteNotification.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ConsoleDidWriteNotification.swift; sourceTree = "<group>"; };
 		5B05890924EB5DC60071DB57 /* HTTPRequestMessage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = HTTPRequestMessage.swift; sourceTree = "<group>"; };
 		5B05890B24EB5DCF0071DB57 /* HTTPResponseMessage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = HTTPResponseMessage.swift; sourceTree = "<group>"; };
@@ -522,6 +523,7 @@
 			children = (
 				5B1DFA0D2526377E00594E80 /* AddressInfoErrorTests.swift */,
 				5B207731236E0FD800817E45 /* HTTPChunkedResponseTests.swift */,
+				5B00A17C266D0132009A4DCD /* HTTPConnectionTests.swift */,
 				5B3A83AC22C93361002B4FFB /* HTTPServerTests.swift */,
 				5B71522122DA00CB00002BA6 /* HTTPServerErrorTests.swift */,
 				5B2E1B2C230CC93C003C85CB /* InterfaceAddressTests.swift */,
@@ -910,6 +912,7 @@
 				5BF1E71226682BB80071E99A /* GraphQLResult+AnyGraphQLOperation.swift in Sources */,
 				5B7AE3942375F38A00EF6D43 /* ApolloDebugServerLoadTests.swift in Sources */,
 				5B3A837322C6721D002B4FFB /* AnyGraphQLSelectionSetTests.swift in Sources */,
+				5B00A17D266D0132009A4DCD /* HTTPConnectionTests.swift in Sources */,
 				5B3A83AD22C93361002B4FFB /* HTTPServerTests.swift in Sources */,
 				5B2E1B38230CD1D0003C85CB /* sockaddr+Factory.swift in Sources */,
 				5B2E1B36230CD1BA003C85CB /* ifaddrs+Factory.swift in Sources */,
@@ -925,7 +928,6 @@
 				5BA76BC2240ABAB600501193 /* BackgroundTaskTests.swift in Sources */,
 				5B2E1B29230C2CE0003C85CB /* InterfaceAddressIteratorTests.swift in Sources */,
 				5B0EC95E22B6D728003D7933 /* DebuggableRequestChainNetworkTransportTests.swift in Sources */,
-				5B16DECD23F45C5300EFEA16 /* MockNetworkTransport.swift in Sources */,
 				5B3A839322C72197002B4FFB /* Reference+JSONEncodableTests.swift in Sources */,
 				5B53B15524FBFD0D00A946D4 /* ApolloDeveloperKitTests.swift in Sources */,
 				5BCE500E24F4C06400310E26 /* MockFileDescriptorDuplicator.swift in Sources */,
diff --git a/Sources/ApolloDeveloperKit/WebServer/HTTPConnection.swift b/Sources/ApolloDeveloperKit/WebServer/HTTPConnection.swift
index 6e11cc3a..914ec0ad 100644
--- a/Sources/ApolloDeveloperKit/WebServer/HTTPConnection.swift
+++ b/Sources/ApolloDeveloperKit/WebServer/HTTPConnection.swift
@@ -27,11 +27,13 @@ final class HTTPConnection {
     weak var delegate: HTTPConnectionDelegate?
     private let incomingRequest = HTTPRequestMessage()
     private let socket: Socket
-    private var eventQueue = ArraySlice<Event>()
-    private let eventQueueLock = NSLock()
+    private let operationQueue = OperationQueue()
 
-    init(httpVersion: String, nativeHandle: CFSocketNativeHandle) throws {
+    init(httpVersion: String, nativeHandle: CFSocketNativeHandle, queue: DispatchQueue) throws {
         self.httpVersion = httpVersion
+        self.operationQueue.maxConcurrentOperationCount = 1
+        self.operationQueue.qualityOfService = .userInitiated
+        self.operationQueue.underlyingQueue = queue
         let socket = try Socket(nativeHandle: nativeHandle, callbackTypes: [.dataCallBack, .writeCallBack])
         self.socket = socket
         socket.isNonBlocking = true
@@ -48,10 +50,9 @@ final class HTTPConnection {
 
 extension HTTPConnection: HTTPOutputStream {
     func write(data: Data) {
-        eventQueueLock.lock()
-        eventQueue.append(.write(data))
-        eventQueueLock.unlock()
-        tryFlush()
+        operationQueue.addOperation { [weak self] in
+            self?.sendOrSuspend(data: data, timeout: 0)
+        }
     }
 
     func writeAndClose(contentsOf url: URL) throws {
@@ -61,13 +62,13 @@ extension HTTPConnection: HTTPOutputStream {
     }
 
     func close() {
-        eventQueueLock.lock()
-        eventQueue.append(.close)
-        eventQueueLock.unlock()
-        tryFlush()
+        operationQueue.addOperation { [weak self] in
+            self?.closeImmediately()
+        }
     }
 
     func closeImmediately() {
+        operationQueue.cancelAllOperations()
         delegate?.httpConnectionWillClose(self)
         socket.invalidate()
     }
@@ -84,31 +85,16 @@ extension HTTPConnection: HTTPOutputStream {
         close()
     }
 
-    private func tryFlush() {
-        eventQueueLock.lock()
-        switch eventQueue.first {
-        case .write(let data)?:
-            if sendOrClose(data: data, timeout: 0) {
-                eventQueue = eventQueue[eventQueue.startIndex.advanced(by: 1)..<eventQueue.endIndex]
-                eventQueueLock.unlock()
-                tryFlush()
-            } else {
-                eventQueueLock.unlock()
-            }
-        case .close?:
-            closeImmediately()
-            eventQueueLock.unlock()
-        case nil:
-            eventQueueLock.unlock()
-        }
-    }
-
-    private func sendOrClose(data: Data, timeout: TimeInterval) -> Bool {
+    private func sendOrSuspend(data: Data, timeout: TimeInterval) {
         do {
-            return try socket.send(data: data, timeout: timeout)
+            if try !socket.send(data: data, timeout: timeout) {
+                operationQueue.isSuspended = true
+                operationQueue.addOperation { [weak self] in
+                    self?.sendOrSuspend(data: data, timeout: timeout)
+                }
+            }
         } catch {
             closeImmediately()
-            return false
         }
     }
 }
@@ -154,6 +140,6 @@ extension HTTPConnection: SocketDelegate {
     }
 
     func socketDidBecomeWritable(_ socket: Socket) {
-        tryFlush()
+        operationQueue.isSuspended = false
     }
 }
diff --git a/Sources/ApolloDeveloperKit/WebServer/HTTPServer.swift b/Sources/ApolloDeveloperKit/WebServer/HTTPServer.swift
index 33ae22f7..8cf5e13c 100644
--- a/Sources/ApolloDeveloperKit/WebServer/HTTPServer.swift
+++ b/Sources/ApolloDeveloperKit/WebServer/HTTPServer.swift
@@ -50,6 +50,7 @@ protocol HTTPServerDelegate: class {
  */
 final class HTTPServer {
     weak var delegate: HTTPServerDelegate?
+    private let connectionQueue = DispatchQueue(label: "com.github.manicmaniac.ApolloDeveloperKit.HTTPConnection")
     private var socket: Socket?
     private var connections = Set<HTTPConnection>()
 
@@ -242,7 +243,7 @@ extension HTTPServer: HTTPConnectionDelegate {
 
 extension HTTPServer: SocketDelegate {
     func socket(_ socket: Socket, didAccept nativeHandle: CFSocketNativeHandle, address: Data) {
-        guard let connection = try? HTTPConnection(httpVersion: kCFHTTPVersion1_1 as String, nativeHandle: nativeHandle) else {
+        guard let connection = try? HTTPConnection(httpVersion: kCFHTTPVersion1_1 as String, nativeHandle: nativeHandle, queue: connectionQueue) else {
             return
         }
         connection.delegate = self
diff --git a/Tests/ApolloDeveloperKitTests/Store/OperationStoreControllerTests.swift b/Tests/ApolloDeveloperKitTests/Store/OperationStoreControllerTests.swift
index 2b202ddf..dee49566 100644
--- a/Tests/ApolloDeveloperKitTests/Store/OperationStoreControllerTests.swift
+++ b/Tests/ApolloDeveloperKitTests/Store/OperationStoreControllerTests.swift
@@ -140,3 +140,15 @@ private class MockOperationStore: OperationStore {
         return State(mutations: [], queries: [])
     }
 }
+
+private class MockNetworkTransport: NetworkTransport {
+    func send<Operation>(operation: Operation, cachePolicy: CachePolicy, contextIdentifier: UUID?, callbackQueue: DispatchQueue, completionHandler: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) -> Cancellable where Operation : GraphQLOperation {
+        return MockCancellable()
+    }
+}
+
+private class MockCancellable: Cancellable {
+    func cancel() {
+        // Do nothing.
+    }
+}
diff --git a/Tests/ApolloDeveloperKitTests/TestHelpers/MockNetworkTransport.swift b/Tests/ApolloDeveloperKitTests/TestHelpers/MockNetworkTransport.swift
deleted file mode 100644
index 503f4871..00000000
--- a/Tests/ApolloDeveloperKitTests/TestHelpers/MockNetworkTransport.swift
+++ /dev/null
@@ -1,50 +0,0 @@
-//
-//  MockNetworkTransport.swift
-//  ApolloDeveloperKitTests
-//
-//  Created by Ryosuke Ito on 2/13/20.
-//  Copyright © 2020 Ryosuke Ito. All rights reserved.
-//
-
-import Apollo
-import Foundation
-
-class MockNetworkTransport: NetworkTransport {
-    var clientName = "clientName"
-    var clientVersion = "clientVersion"
-
-    private var results = ArraySlice<Result<Any, Error>>()
-
-    var isResultsEmpty: Bool {
-        return results.isEmpty
-    }
-
-    func append<Data>(response: GraphQLResponse<Data>) where Data: GraphQLSelectionSet {
-        results.append(.success(response))
-    }
-
-    func append(error: Error) {
-        results.append(.failure(error))
-    }
-
-    func send<Operation>(operation: Operation, cachePolicy: CachePolicy, contextIdentifier: UUID?, callbackQueue: DispatchQueue, completionHandler: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) -> Cancellable where Operation : GraphQLOperation {
-        let result = results.popFirst()
-        switch result {
-        case .success(let graphQLResult as GraphQLResult<Operation.Data>)?:
-            completionHandler(.success(graphQLResult))
-        case .failure(let error)?:
-            completionHandler(.failure(error))
-        case .success:
-            fatalError("The type of the next response doesn't match the expected type.")
-        case nil:
-            fatalError("The number of invocation of send(operation:completionHandler:) exceeds the number of results.")
-        }
-        return MockCancellable()
-    }
-}
-
-class MockCancellable: Cancellable {
-    func cancel() {
-        // do nothing
-    }
-}
diff --git a/Tests/ApolloDeveloperKitTests/WebServer/HTTPConnectionTests.swift b/Tests/ApolloDeveloperKitTests/WebServer/HTTPConnectionTests.swift
new file mode 100644
index 00000000..bad32af7
--- /dev/null
+++ b/Tests/ApolloDeveloperKitTests/WebServer/HTTPConnectionTests.swift
@@ -0,0 +1,144 @@
+//
+//  HTTPConnectionTests.swift
+//  ApolloDeveloperKitTests
+//
+//  Created by Ryosuke Ito on 6/6/21.
+//  Copyright © 2021 Ryosuke Ito. All rights reserved.
+//
+
+import XCTest
+@testable import ApolloDeveloperKit
+
+class HTTPConnectionTests: XCTestCase {
+    private let httpVersion = kCFHTTPVersion1_1 as String
+    private var queue: DispatchQueue!
+    private var writerFileDescriptor: Int32!
+    private var readerFileDescriptor: Int32!
+
+    override func setUpWithError() throws {
+        var fileDescriptors = [Int32](repeating: 0, count: 2)
+        errno = 0
+        guard socketpair(AF_UNIX, SOCK_STREAM, 0, &fileDescriptors) == 0 else {
+            throw POSIXError(POSIXErrorCode(rawValue: errno)!)
+        }
+        writerFileDescriptor = fileDescriptors[0]
+        readerFileDescriptor = fileDescriptors[1]
+    }
+
+    override func setUp() {
+        queue = DispatchQueue(label: "com.github.manicmaniac.ApolloDeveloperKitTests.HTTPConnectionTests")
+    }
+
+    override func tearDownWithError() throws {
+        for case let handle? in [writerFileDescriptor, readerFileDescriptor] {
+            errno = 0
+            guard close(handle) == 0 || errno == EBADF else {
+                throw POSIXError(POSIXErrorCode(rawValue: errno)!)
+            }
+        }
+    }
+
+    func testWrite() throws {
+        let connection = try HTTPConnection(httpVersion: httpVersion, nativeHandle: writerFileDescriptor, queue: queue)
+        connection.write(data: httpGetRequestMessage)
+        connection.close()
+        let fileHandle = FileHandle(fileDescriptor: readerFileDescriptor)
+        let data: Data?
+        if #available(macOS 10.15.4, *, iOS 13.4, *) {
+            data = try fileHandle.read(upToCount: httpGetRequestMessage.count)
+        } else {
+            data = fileHandle.readData(ofLength: httpGetRequestMessage.count)
+        }
+        XCTAssertEqual(data, httpGetRequestMessage)
+    }
+
+    func testWrite_threadSafety() throws {
+        let connection = try HTTPConnection(httpVersion: httpVersion, nativeHandle: writerFileDescriptor, queue: queue)
+        let iterations = 100
+        for _ in 0..<iterations {
+            let thread: Thread
+            if #available(macOS 10.12, *, iOS 10, *) {
+                thread = Thread { [unowned self] in
+                    self.writeData(into: connection)
+                }
+            } else {
+                thread = Thread(target: self, selector: #selector(writeData(into:)), object: connection)
+            }
+            expectation(forNotification: .NSThreadWillExit, object: thread)
+            thread.start()
+        }
+        waitForExpectations(timeout: 0.5)
+        connection.close()
+        let fileHandle = FileHandle(fileDescriptor: readerFileDescriptor)
+        let expectedBytesCount = httpGetRequestMessage.count * iterations
+        let data: Data?
+        if #available(macOS 10.15.4, *, iOS 13.4, *) {
+            data = try fileHandle.read(upToCount: expectedBytesCount)
+        } else {
+            data = fileHandle.readData(ofLength: expectedBytesCount)
+        }
+        XCTAssertEqual(data?.count, expectedBytesCount)
+    }
+
+    func testWriteAndClose() throws {
+        let fileManager = FileManager.default
+        let temporaryDirectoryURL: URL
+        if #available(macOS 10.12, *, iOS 10, *) {
+            temporaryDirectoryURL = fileManager.temporaryDirectory
+        } else {
+            temporaryDirectoryURL = URL(fileURLWithPath: NSTemporaryDirectory())
+        }
+        let itemReplacementURL = try fileManager.url(for: .itemReplacementDirectory,
+                                                     in: .userDomainMask,
+                                                     appropriateFor: temporaryDirectoryURL,
+                                                     create: true)
+        addTeardownBlock {
+            do {
+                try fileManager.removeItem(at: itemReplacementURL)
+            } catch let error {
+                XCTFail(String(describing: error))
+            }
+        }
+        let temporaryFileURL = itemReplacementURL.appendingPathComponent(ProcessInfo().globallyUniqueString)
+        try httpGetRequestMessage.write(to: temporaryFileURL)
+        let connection = try HTTPConnection(httpVersion: httpVersion, nativeHandle: writerFileDescriptor, queue: queue)
+        let fileHandle = FileHandle(fileDescriptor: readerFileDescriptor)
+        expectation(forNotification: .NSFileHandleReadToEndOfFileCompletion, object: fileHandle) { notification in
+            let data = notification.userInfo?[NSFileHandleNotificationDataItem] as? Data
+            XCTAssertEqual(data, httpGetRequestMessage)
+            return true
+        }
+        try connection.writeAndClose(contentsOf: temporaryFileURL)
+        fileHandle.readToEndOfFileInBackgroundAndNotify()
+        waitForExpectations(timeout: 0.5)
+    }
+
+    @objc private func writeData(into connection: AnyObject) {
+        let connection = connection as! HTTPConnection
+        connection.write(data: httpGetRequestMessage)
+    }
+}
+
+private class HTTPConnectionDelegateHandler: HTTPConnectionDelegate {
+    var didReceiveRequest: ((HTTPConnection, HTTPRequestMessage) -> Void)?
+    var willClose: ((HTTPConnection) -> Void)?
+    var didFailToHandleRequest: ((HTTPConnection, HTTPRequestMessage, Error) -> Void)?
+
+    func httpConnection(_ connection: HTTPConnection, didReceive request: HTTPRequestMessage) {
+        didReceiveRequest?(connection, request)
+    }
+
+    func httpConnectionWillClose(_ connection: HTTPConnection) {
+        willClose?(connection)
+    }
+
+    func httpConnection(_ connection: HTTPConnection, didFailToHandle request: HTTPRequestMessage, error: Error) {
+        didFailToHandleRequest?(connection, request, error)
+    }
+}
+
+private let httpGetRequestMessage = """
+GET / HTTP/1.1
+Host: localhost
+
+""".data(using: .utf8)!