diff --git a/pdb-ui/src/main/java/org/lucares/pdbui/PdbController.java b/pdb-ui/src/main/java/org/lucares/pdbui/PdbController.java
index 52101a7..328753f 100644
--- a/pdb-ui/src/main/java/org/lucares/pdbui/PdbController.java
+++ b/pdb-ui/src/main/java/org/lucares/pdbui/PdbController.java
@@ -10,6 +10,7 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.SortedSet;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.lucares.pdb.datastore.Proposal;
 import org.lucares.pdb.plot.api.PlotSettings;
@@ -50,6 +51,8 @@ public class PdbController implements HardcodedValues {
 
 	private final Plotter plotter;
 	private final PerformanceDb db;
+	
+	private final ReentrantLock plotterLock = new ReentrantLock();
 
 	public PdbController(final PerformanceDb db, final Plotter plotter) {
 		this.db = db;
@@ -71,20 +74,32 @@ public class PdbController implements HardcodedValues {
 			produces = MediaType.APPLICATION_JSON_UTF8_VALUE //
 	)
 	@ResponseBody
-	PlotResponse createPlot(@RequestBody final PlotRequest request) throws InternalPlottingException {
+	PlotResponse createPlot(@RequestBody final PlotRequest request)
+			throws InternalPlottingException, InterruptedException {
 
-		final PlotSettings plotSettings = PlotSettingsTransformer.toSettings(request);
+		final PlotSettings plotSettings = PlotSettingsTransformer
+				.toSettings(request);
 
-		try {
-			final PlotResult result = plotter.plot(plotSettings);
+		// TODO the UI should cancel requests that are in flight before sending a plot request
+		if (plotterLock.tryLock()) {
+			try {
+				final PlotResult result = plotter.plot(plotSettings);
 
-			final String imageUrl = WEB_IMAGE_OUTPUT_PATH + "/" + result.getImageName();
-			LOGGER.trace("image url:  {}", imageUrl);
-			System.gc();
+				final String imageUrl = WEB_IMAGE_OUTPUT_PATH + "/"
+						+ result.getImageName();
+				LOGGER.trace("image url:  {}", imageUrl);
+				System.gc();
 
-			return new PlotResponse(DataSeries.toMap(result.getDataSeries()), imageUrl);
-		} catch (final NoDataPointsException e) {
-			throw new NotFoundException(e);
+				return new PlotResponse(
+						DataSeries.toMap(result.getDataSeries()), imageUrl);
+			} catch (final NoDataPointsException e) {
+				throw new NotFoundException(e);
+			} finally {
+				plotterLock.unlock();
+			}
+
+		} else {
+			throw new ServiceUnavailableException("Too many parallel requests!");
 		}
 	}
 
diff --git a/pdb-ui/src/main/java/org/lucares/pdbui/ServiceUnavailableException.java b/pdb-ui/src/main/java/org/lucares/pdbui/ServiceUnavailableException.java
new file mode 100644
index 0000000..d8d67f0
--- /dev/null
+++ b/pdb-ui/src/main/java/org/lucares/pdbui/ServiceUnavailableException.java
@@ -0,0 +1,14 @@
+package org.lucares.pdbui;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value = HttpStatus.SERVICE_UNAVAILABLE, reason = "Service Unavailable")
+public class ServiceUnavailableException extends RuntimeException {
+
+	private static final long serialVersionUID = -4512668277873760500L;
+
+	public ServiceUnavailableException(String message) {
+		super(message);
+	}
+}
diff --git a/performanceDb/src/main/java/org/lucares/performance/db/TagsToFile.java b/performanceDb/src/main/java/org/lucares/performance/db/TagsToFile.java
index 2c4e400..dc1cd1c 100644
--- a/performanceDb/src/main/java/org/lucares/performance/db/TagsToFile.java
+++ b/performanceDb/src/main/java/org/lucares/performance/db/TagsToFile.java
@@ -80,7 +80,7 @@ public class TagsToFile implements AutoCloseable {
 		final List<PdbFile> result = new ArrayList<>();
 		
 		final List<Doc> searchResult = db.search(query);
-		if (searchResult.size() > 100_000){
+		if (searchResult.size() > 500_000){
 			throw new IllegalStateException("Too many results.");
 		}