MVStore: support concurrent transactions (PostgreSQL read-committed)

h2/src/docsrc/html/mvstore.html

@@ -24,10 +24,21 @@ MVStore
     Store Builder</a><br />
 <a href="#r_tree">
     R-Tree</a><br />
-
-
 <a href="#features">
     Features</a><br />
+    - <a href="#maps">Maps</a><br />
+    - <a href="#versions">Versions</a><br />
+    - <a href="#transactions">Transactions</a><br />
+    - <a href="#inMemory">In-Memory Performance and Usage</a><br />
+    - <a href="#dataTypes">Pluggable Data Types</a><br />
+    - <a href="#blob">BLOB Support</a><br />
+    - <a href="#pluggableMap">R-Tree and Pluggable Map Implementations</a><br />
+    - <a href="#caching">Concurrent Operations and Caching</a><br />
+    - <a href="#logStructured">Log Structured Storage</a><br />
+    - <a href="#fileSystem">File System Abstraction, File Locking and Online Backup</a><br />
+    - <a href="#encryption">Encrypted Files</a><br />
+    - <a href="#tools">Tools</a><br />
+    - <a href="#exceptionHandling">Exception Handling</a><br />
 <a href="#differences">
     Similar Projects and Differences to Other Storage Engines</a><br />
 <a href="#current_state">
@@ -45,8 +56,7 @@ But it can be also directly within an application, without using JDBC or SQL.
 </li><li>Both file-based persistence and in-memory operation are supported.
 </li><li>It is intended to be fast, simple to use, and small.
 </li><li>Old versions of the data can be read concurrently with all other operations.
-</li><li>Transaction are supported (currently only one transaction at a time).
-</li><li>Transactions (even if they are persisted) can be rolled back.
+</li><li>Transaction are supported.
 </li><li>The tool is very modular. It supports pluggable data types / serialization,
 pluggable map implementations (B-tree, R-tree, concurrent B-tree currently), BLOB storage,
 and a file system abstraction to support encrypted files and zip files.
@@ -166,7 +176,7 @@ The minimum number of dimensions is 1, the maximum is 255.
 
 <h2 id="features">Features</h2>
 
-<h3>Maps</h3>
+<h3 id="maps">Maps</h3>
 <p>
 Each store supports a set of named maps.
 A map is sorted by key, and supports the common lookup operations,
@@ -186,13 +196,13 @@ of the index, and the value of the map is the primary key of the table (for non-
 the key of the map must also contain the primary key).
 </p>
 
-<h3>Versions / Transactions</h3>
+<h3 id="versions">Versions</h3>
 <p>
 Multiple versions are supported.
 A version is a snapshot of all the data of all maps at a given point in time.
 A transaction is a number of actions between two versions.
 </p><p>
-Versions / transactions are not immediately persisted; instead, only the version counter is incremented.
+Versions are not immediately persisted; instead, only the version counter is incremented.
 If there is a change after switching to a new version, a snapshot of the old version is kept in memory,
 so that it can still be read.
 </p><p>
@@ -203,7 +213,23 @@ This behavior is also called COW (copy on write).
 Rollback is supported (rollback to any old in-memory version or an old persisted version).
 </p>
 
-<h3>In-Memory Performance and Usage</h3>
+<h3 id="transactions">Transactions</h3>
+<p>
+The multi-version support is the basis for the transaction support.
+In the simple case, when only one transaction is open at a time,
+rolling back the transaction only requires to revert to an old version.
+</p><p>
+To support multiple concurrent open transactions, a transaction utility is included,
+the <code>TransactionStore</code>.
+This utility stores the changed entries in a separate map, similar to a transaction log
+(except that only the key of a changed row is stored,
+and the entries of a transaction are removed when the transaction is committed).
+The storage overhead of this utility is very small compared to the overhead of a regular transaction log.
+The tool supports PostgreSQL style "read committed" transaction isolation.
+There is no limit on the size of a transaction (the log is not kept in memory).
+</p>
+
+<h3 id="inMemory">In-Memory Performance and Usage</h3>
 <p>
 Performance of in-memory operations is comparable with <code>java.util.TreeMap</code>
 (many operations are actually faster), but usually slower than <code>java.util.HashMap</code>.
@@ -220,7 +246,7 @@ If a file name is specified, all operations occur in memory (with the same
 performance characteristics) until data is persisted.
 </p>
 
-<h3>Pluggable Data Types</h3>
+<h3 id="dataTypes">Pluggable Data Types</h3>
 <p>
 Serialization is pluggable. The default serialization currently supports many common data types,
 and uses Java serialization for other objects. The following classes are currently directly supported:
@@ -236,7 +262,7 @@ Also, there is no inherent limit to the number of maps and chunks.
 Due to using a log structured storage, there is no special case handling for large keys or pages.
 </p>
 
-<h3>BLOB Support</h3>
+<h3 id="blob">BLOB Support</h3>
 <p>
 There is a mechanism that stores large binary objects by splitting them into smaller blocks.
 This allows to store objects that don't fit in memory.
@@ -244,7 +270,7 @@ Streaming as well as random access reads on such objects are supported.
 This tool is written on top of the store (only using the map interface).
 </p>
 
-<h3>R-Tree and Pluggable Map Implementations</h3>
+<h3 id="pluggableMap">R-Tree and Pluggable Map Implementations</h3>
 <p>
 The map implementation is pluggable.
 In addition to the default <code>MVMap</code> (multi-version map),
@@ -252,7 +278,7 @@ there is a multi-version R-tree map implementation
 for spatial operations (contain and intersection; nearest neighbor is not yet implemented).
 </p>
 
-<h3>Concurrent Operations and Caching</h3>
+<h3 id="caching">Concurrent Operations and Caching</h3>
 <p>
 The default map implementation supports concurrent reads on old versions of the data.
 All such read operations can occur in parallel. Concurrent reads from the page cache,
@@ -281,7 +307,7 @@ the map could be split into multiple maps in different stores ('sharding').
 The plan is to add such a mechanism later when needed.
 </p>
 
-<h3>Log Structured Storage</h3>
+<h3 id="logStructured">Log Structured Storage</h3>
 <p>
 Changes are buffered in memory, and once enough changes have accumulated,
 they are written in one continuous disk write operation.
@@ -327,7 +353,7 @@ But temporarily, disk space usage might actually be a bit higher than for a regu
 as disk space is not immediately re-used (there are no in-place updates).
 </p>
 
-<h3>File System Abstraction, File Locking and Online Backup</h3>
+<h3 id="fileSystem">File System Abstraction, File Locking and Online Backup</h3>
 <p>
 The file system is pluggable (the same file system abstraction is used as H2 uses).
 The file can be encrypted using an encrypting file system.
@@ -347,7 +373,7 @@ new data is always appended at the end of the file.
 Then, the file can be copied (the file handle is available to the application).
 </p>
 
-<h3>Encrypted Files</h3>
+<h3 id="encryption">Encrypted Files</h3>
 <p>
 File encryption ensures the data can only be read with the correct password.
 Data can be encrypted as follows:
@@ -378,12 +404,12 @@ The following algorithms and settings are used:
     Only little more than one AES-128 round per block is needed.
 </li></ul>
 
-<h3>Tools</h3>
+<h3 id="tools">Tools</h3>
 <p>
 There is a tool (<code>MVStoreTool</code>) to dump the contents of a file.
 </p>
 
-<h3>Exception Handling</h3>
+<h3 id="exceptionHandling">Exception Handling</h3>
 <p>
 This tool does not throw checked exceptions.
 Instead, unchecked exceptions are thrown if needed.

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

@@ -1011,9 +1011,9 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         Page newest = null;
         // need to copy because it can change
         Page r = root;
-        if (version >= r.getVersion() && 
-                (r.getVersion() >= 0 || 
-                version <= createVersion || 
+        if (version >= r.getVersion() &&
+                (r.getVersion() >= 0 ||
+                version <= createVersion ||
                 store.getFile() == null)) {
             newest = r;
         } else {

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

@@ -95,6 +95,7 @@ TODO:
 - to save space when persisting very small transactions,
 -- use a transaction log where only the deltas are stored
 - serialization for lists, sets, sets, sorted sets, maps, sorted maps
+- maybe rename 'rollback' to 'revert'
 
 */
 

h2/src/test/org/h2/test/TestAll.java

@@ -114,7 +114,7 @@ import org.h2.test.store.TestMVTableEngine;
 import org.h2.test.store.TestObjectDataType;
 import org.h2.test.store.TestSpinLock;
 import org.h2.test.store.TestStreamStore;
-import org.h2.test.store.TestTransactionMap;
+import org.h2.test.store.TestTransactionStore;
 import org.h2.test.synth.TestBtreeIndex;
 import org.h2.test.synth.TestCrashAPI;
 import org.h2.test.synth.TestDiskFull;
@@ -689,7 +689,7 @@ kill -9 `jps -l | grep "org.h2.test." | cut -d " " -f 1`
         new TestObjectDataType().runTest(this);
         new TestSpinLock().runTest(this);
         new TestStreamStore().runTest(this);
-        new TestTransactionMap().runTest(this);
+        new TestTransactionStore().runTest(this);
 
         // unit
         new TestAutoReconnect().runTest(this);

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

@@ -733,7 +733,7 @@ public class TestMVStore extends TestBase {
         assertEquals("[10, 11, 12, 13, 14, 50, 100, 90, 91, 92]", list.toString());
         s.close();
     }
-    
+
     private void testOldVersion() {
         MVStore s;
         for (int op = 0; op <= 1; op++) {

h2/src/tools/org/h2/build/doc/dictionary.txt

@@ -723,4 +723,4 @@ versioning sector survives goes ssd ambiguity sizing perspective jumps
 incompressible distinguished factories throughput vectors tripodi cracking
 brown tweak pbkdf sharding ieee galois otterstrom sharded hruda argaul gaul
 simo unpredictable overtakes conditionally decreases warned coupled spin
-unsynchronized reality cores effort slice addleman koskela ville
\ No newline at end of file
+unsynchronized reality cores effort slice addleman koskela ville blocking seen
\ No newline at end of file