A persistent multi-version map: support store() in a background thread

h2/src/docsrc/html/mvstore.html

@@ -243,10 +243,14 @@ Caching is done on the page level.
 The page cache is a concurrent LIRS cache,
 which should be resistant against scan operations.
 </p><p>
-Concurrent modification operations on the maps are currently not supported,
+Concurrent modification operations on a map are currently not supported
+(the same as <code>HashMap</code> and <code>TreeMap</code>),
 however it is planned to support an additional map implementation
 that supports concurrent writes
 (at the cost of speed if used in a single thread, same as <code>ConcurrentHashMap</code>).
+</p><p>
+Storing changes can occur concurrently to modifying the data,
+as <code>store()</code> operates on a snapshot.
 </p>
 
 <h3>Log Structured Storage</h3>

h2/src/main/org/h2/mvstore/MVMap.java

@@ -682,9 +682,11 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      * Set the position of the root page.
      *
      * @param rootPos the position, 0 for empty
+     * @param version the version of the root
      */
-    void setRootPos(long rootPos) {
+    void setRootPos(long rootPos, long version) {
         root = rootPos == 0 ? Page.createEmpty(this, -1) : readPage(rootPos);
+        root.setVersion(version);
     }
 
     /**
@@ -899,7 +901,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         Page newest = null;
         // need to copy because it can change
         Page r = root;
-        if (r.getVersion() == version) {
+        if (r.getVersion() <= version && r.getVersion() >= 0) {
             newest = r;
         } else {
             // find the newest page that has a getVersion() <= version

h2/src/main/org/h2/mvstore/MVStore.java

@@ -40,10 +40,9 @@ header:
 H:3,...
 
 TODO:
-- build script
 - test concurrent storing in a background thread
 - store store creation in file header, and seconds since creation
--- in chunk header (plus a counter)
+-- in chunk header (plus a counter) - ensure time never goes backwards
 - recovery: keep some old chunks; don't overwritten
 -- for 5 minutes (configurable)
 - allocate memory with Utils.newBytes and so on
@@ -91,7 +90,8 @@ TODO:
 - and maps without keys (counted b-tree)
 - use a small object cache (StringCache)
 - dump values
-- tool to import / manipulate CSV files
+- tool to import / manipulate CSV files (maybe concurrently)
+- map split / merge (fast if no overlap)
 
 */
 
@@ -216,7 +216,7 @@ public class MVStore {
         String r = oldMeta.get("root." + template.getId());
         long rootPos = r == null ? 0 : Long.parseLong(r);
         MVMap<?, ?> m = template.openReadOnly();
-        m.setRootPos(rootPos);
+        m.setRootPos(rootPos, version);
         return (T) m;
     }
 
@@ -286,7 +286,7 @@ public class MVStore {
             root = r == null ? 0 : Long.parseLong(r);
         }
         m.open(this, c);
-        m.setRootPos(root);
+        m.setRootPos(root, -1);
         maps.put(name, m);
         return (T) m;
     }
@@ -317,7 +317,7 @@ public class MVStore {
         }
         c = readChunkHeader(c.start);
         MVMap<String, String> oldMeta = meta.openReadOnly();
-        oldMeta.setRootPos(c.metaRootPos);
+        oldMeta.setRootPos(c.metaRootPos, version);
         return oldMeta;
     }
 
@@ -414,7 +414,7 @@ public class MVStore {
         Chunk header = readChunkHeader(rootChunkStart);
         lastChunkId = header.id;
         chunks.put(header.id, header);
-        meta.setRootPos(header.metaRootPos);
+        meta.setRootPos(header.metaRootPos, -1);
         Iterator<String> it = meta.keyIterator("chunk.");
         while (it.hasNext()) {
             String s = it.next();
@@ -553,6 +553,10 @@ public class MVStore {
             return currentVersion;
         }
 
+        int currentUnsavedPageCount = unsavedPageCount;
+        long storeVersion = currentVersion;
+        long version = incrementVersion();
+
         // the last chunk was not completely correct in the last store()
         // this needs to be updated now (it's better not to update right after
         // storing, because that would modify the meta map again)
@@ -565,7 +569,7 @@ public class MVStore {
         c.maxLengthLive = Long.MAX_VALUE;
         c.start = Long.MAX_VALUE;
         c.length = Integer.MAX_VALUE;
-        c.version = currentVersion + 1;
+        c.version = version;
         chunks.put(c.id, c);
         meta.put("chunk." + c.id, c.asString());
 
@@ -574,7 +578,7 @@ public class MVStore {
             if (m == meta || !m.hasUnsavedChanges()) {
                 continue;
             }
-            Page p = m.getRoot();
+            Page p = m.openVersion(storeVersion).getRoot();
             if (p.getTotalCount() == 0) {
                 meta.put("root." + m.getId(), "0");
             } else {
@@ -606,7 +610,7 @@ public class MVStore {
             if (m == meta || !m.hasUnsavedChanges()) {
                 continue;
             }
-            Page p = m.getRoot();
+            Page p = m.openVersion(storeVersion).getRoot();
             if (p.getTotalCount() > 0) {
                 long root = p.writeUnsavedRecursive(c, buff);
                 meta.put("root." + m.getId(), "" + root);
@@ -654,11 +658,11 @@ public class MVStore {
         rootChunkStart = filePos;
         revertTemp();
 
-        long version = incrementVersion();
         // write the new version (after the commit)
         writeFileHeader();
         shrinkFileIfPossible(1);
-        unsavedPageCount = 0;
+        // some pages might have been changed in the meantime (in the newest version)
+        unsavedPageCount = Math.max(0, unsavedPageCount - currentUnsavedPageCount);
         return version;
     }
 
@@ -756,7 +760,7 @@ public class MVStore {
             return false;
         }
         for (MVMap<?, ?> m : mapsChanged.values()) {
-            if (m.hasUnsavedChanges()) {
+            if (m == meta || m.hasUnsavedChanges()) {
                 return true;
             }
         }
@@ -867,6 +871,9 @@ public class MVStore {
         }
         Chunk.fromHeader(buff, chunk.start);
         int chunkLength = chunk.length;
+        // mark a change, even if it doesn't look like there was a change
+        // as changes in the metadata alone are not detected
+        markChanged(meta);
         while (buff.position() < chunkLength) {
             int start = buff.position();
             int pageLength = buff.getInt();
@@ -1161,14 +1168,11 @@ public class MVStore {
         if (last != null) {
             if (last.version >= version) {
                 revertTemp();
-            }
-            if (last.version > version) {
                 loadFromFile = true;
-                while (last != null && last.version > version) {
-                    chunks.remove(lastChunkId);
+                do {
+                    last = chunks.remove(lastChunkId);
                     lastChunkId--;
-                    last = chunks.get(lastChunkId);
-                }
+                } while (last.version > version && chunks.size() > 0);
                 rootChunkStart = last.start;
                 writeFileHeader();
                 readFileHeader();
@@ -1183,7 +1187,7 @@ public class MVStore {
                 if (loadFromFile) {
                     String r = meta.get("root." + m.getId());
                     long root = r == null ? 0 : Long.parseLong(r);
-                    m.setRootPos(root);
+                    m.setRootPos(root, version);
                 }
             }
         }

h2/src/main/org/h2/mvstore/Page.java

@@ -37,7 +37,7 @@ public class Page {
     private static final Object[] EMPTY_OBJECT_ARRAY = new Object[0];
 
     private final MVMap<?, ?> map;
-    private final long version;
+    private long version;
     private long pos;
     private long totalCount;
     private int keyCount;
@@ -886,4 +886,8 @@ public class Page {
         return mem;
     }
 
+    void setVersion(long version) {
+        this.version = version;
+    }
+
 }

h2/src/main/org/h2/mvstore/type/ObjectType.java

@@ -77,7 +77,7 @@ public class ObjectType implements DataType {
     static final int TAG_BYTE_ARRAY_0_15 = 104;
 
     /**
-     * Contants for floating point synchronization.
+     * Constants for floating point synchronization.
      */
     static final int FLOAT_ZERO_BITS = Float.floatToIntBits(0.0f);
     static final int FLOAT_ONE_BITS = Float.floatToIntBits(1.0f);

h2/src/test/org/h2/test/store/TestMVStore.java

@@ -111,7 +111,8 @@ public class TestMVStore extends TestBase {
             map = s.openMap("test");
             for (int i = 0; i < 1024; i += 128) {
                 for (int j = 0; j < i; j++) {
-                    map.get(j);
+                    String x = map.get(j);
+                    assertEquals(10240, x.length());
                 }
             }
             assertEquals(expectedReadsForCacheSize[cacheSize],
@@ -410,11 +411,14 @@ public class TestMVStore extends TestBase {
         // concurrently with further modifications)
         // this will print Hello World
         // System.out.println(oldMap.get(1));
+        assertEquals("Hello", oldMap.get(1));
         // System.out.println(oldMap.get(2));
+        assertEquals("World", oldMap.get(2));
         oldMap.close();
 
         // print the newest version ("Hi")
         // System.out.println(map.get(1));
+        assertEquals("Hi", map.get(1));
 
         // close the store - this doesn't write to disk
         s.close();
@@ -504,12 +508,16 @@ public class TestMVStore extends TestBase {
         assertTrue(mOld.isReadOnly());
         s.getCurrentVersion();
         s.setRetainChunk(0);
-        long old2 = s.store();
+        long old3 = s.store();
 
         // the old version is still available
         assertEquals("Hello", mOld.get("1"));
         assertEquals("World", mOld.get("2"));
 
+        mOld = m.openVersion(old3);
+        assertEquals("Hallo", mOld.get("1"));
+        assertEquals("Welt", mOld.get("2"));
+
         m.put("1",  "Hi");
         assertEquals("Welt", m.remove("2"));
         s.store();
@@ -519,7 +527,8 @@ public class TestMVStore extends TestBase {
         m = s.openMap("data", String.class, String.class);
         assertEquals("Hi", m.get("1"));
         assertEquals(null, m.get("2"));
-        mOld = m.openVersion(old2);
+
+        mOld = m.openVersion(old3);
         assertEquals("Hallo", mOld.get("1"));
         assertEquals("Welt", mOld.get("2"));
         s.close();
@@ -635,23 +644,25 @@ public class TestMVStore extends TestBase {
         assertFalse(m0.isReadOnly());
         m.put("1",  "Hallo");
         s.incrementVersion();
-        assertEquals(3, s.getCurrentVersion());
+        long v3 = s.getCurrentVersion();
+        assertEquals(3, v3);
         long v4 = s.store();
         assertEquals(4, v4);
         assertEquals(4, s.getCurrentVersion());
         s.close();
 
         s = openStore(fileName);
+        assertEquals(4, s.getCurrentVersion());
         s.setRetainChunk(0);
         m = s.openMap("data", String.class, String.class);
-        m.put("1",  "Hello");
+        m.put("1",  "Hi");
         s.store();
         s.close();
 
         s = openStore(fileName);
         s.setRetainChunk(0);
         m = s.openMap("data", String.class, String.class);
-        assertEquals("Hello", m.get("1"));
+        assertEquals("Hi", m.get("1"));
         s.rollbackTo(v4);
         assertEquals("Hallo", m.get("1"));
         s.close();
@@ -715,6 +726,10 @@ public class TestMVStore extends TestBase {
         data.put("1", "Hello");
         data.put("2", "World");
         s.store();
+        assertEquals(1, s.getCurrentVersion());
+        assertTrue(m.containsKey("chunk.1"));
+        assertFalse(m.containsKey("chunk.2"));
+
         assertEquals("id:1,name:data,type:btree,createVersion:0,key:,value:",
                 m.get("map.data"));
         assertTrue(m.containsKey("chunk.1"));
@@ -725,9 +740,14 @@ public class TestMVStore extends TestBase {
         assertTrue(m.get("root.1").length() > 0);
         assertTrue(m.containsKey("chunk.1"));
         assertTrue(m.containsKey("chunk.2"));
+        assertEquals(2, s.getCurrentVersion());
+
         s.rollbackTo(1);
+        assertEquals("Hello", data.get("1"));
+        assertEquals("World", data.get("2"));
         assertTrue(m.containsKey("chunk.1"));
         assertFalse(m.containsKey("chunk.2"));
+
         s.close();
     }