Inside The Room- DCBln21 revised edition

Transcript

1. Inside the room Effie Barak (@CodingChick)
3. Talk Outline: 1. Intro 2. Threading in Room 3. Triggers 4. Transactions 5. Large queries
5. Intro to SQLite
6. Embedded SQL database engine
12. Intro to Android and Android x APIs
13. Android package android.database.sqlite SQLiteStatement SQLiteCursor SQLiteDatabase SQLiteOpenHelper SQLiteQuery
14. Android X androidx.sqlite.db SupportSQLiteDatabase SupportSQLiteOpenHelper SupportSQLiteQuery
15. /** * A database abstraction which removes the framework dependency and allows swapping underlying * sql versions. It mimics the behavior of {@link android.database.sqlite.SQLiteDatabase} */ public interface SupportSQLiteDatabase extends Closeable {
16. Android X androidx.sqlite.db.framework FrameworkSQLiteDatabase FrameworkSQLiteOpenHelper FrameworkSQLiteStatement
17. /** * Delegates all calls to an implementation of {@link SQLiteDatabase}. */ class FrameworkSQLiteDatabase implements SupportSQLiteDatabase {
18. private final SQLiteDatabase mDelegate; @Override public void beginTransaction() { mDelegate.beginTransaction(); } @Override public void beginTransactionNonExclusive() { mDelegate.beginTransactionNonExclusive(); }
19. Intro to Room Architecture
20. Isolated modules room-common room-compiler room-runtime room-ktx room-rxjava2
21. Threads, processes etc
22. Locking in SQLite
24. Types of locks
25. Types of locks Unlocked
26. Types of locks Unlocked Shared
27. Types of locks Unlocked Shared Reserved
28. Types of locks Unlocked Shared Reserved Pending
29. Types of locks Unlocked Shared Reserved Pending Exclusive
30. Reading from the database
31. Writing to the database 1. Get share lock 2. Get reserved lock 3. Write to memory 4. Write to rollback journal
32. The rollback journal
33. Writing to the database 5. Obtain a PENDING lock 6. Obtain an EXCLUSIVE lock 7. Wait for SHARED locks to be done 8. Write to database
34. Write-Ahead Log (WAL)
35. WAL continued
36. Writes with WAL 1. Append to WAL 2. COMMIT 3. Continue writing 4. Checkpoint- write to file
37. Reads with WAL 1. Check WAL 2. Check DB
38. Database connections
40. SQLite connections
41. Android APIs
42. Only one write connection and multiple reads
43. Get a connection to SQLite
44. package android.database.sqlite; public abstract class SQLiteOpenHelper { public SQLiteDatabase getWritableDatabase() { synchronized (this) { return getDatabaseLocked(true); } } public SQLiteDatabase getReadableDatabase() { synchronized (this) { return getDatabaseLocked(false); } } }
45. public final class SQLiteDatabase extends SQLiteClosable { // The connection pool for the database, null when closed. // The pool itself is thread-safe, but the reference to it can only be acquired // when the lock is held. // INVARIANT: Guarded by mLock. private SQLiteConnectionPool mConnectionPoolLocked; }
46. Where Room sets up WAL public abstract class RoomDatabase { public void init(...) { boolean wal = false; if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) { wal = configuration.journalMode == JournalMode.WRITE_AHEAD_LOGGING; mOpenHelper.setWriteAheadLoggingEnabled(wal); } ... } }
47. Change SQLite settings public final class SQLiteDatabase extends SQLiteClosable { public boolean enableWriteAheadLogging() { ... mConfigurationLocked.openFlags |= ENABLE_WRITE_AHEAD_LOGGING; try { mConnectionPoolLocked.reconfigure(mConfigurationLocked); } catch (RuntimeException ex) { mConfigurationLocked.openFlags &= ~ENABLE_WRITE_AHEAD_LOGGING; throw ex; } } }
48. public final class SQLiteConnection ... { private void setWalModeFromConfiguration() { ... setJournalMode("WAL"); ... } } }
49. private void setJournalMode(String newValue) { String value = executeForString("PRAGMA journal_mode", null, null); if (!value.equalsIgnoreCase(newValue)) { try { String result = executeForString("PRAGMA journal_mode=" + newValue, null, null); if (result.equalsIgnoreCase(newValue)) { return; } ... }
50. Room with threads
51. Queries (reads) in Room
52. RxJava public static Flowable<Object> createFlowable(final RoomDatabase database, final String... tableNames) { return Flowable.create(new FlowableOnSubscribe<Object>() { @Override public void subscribe(final FlowableEmitter<Object> emitter) throws Exception { ...
53. Coroutines @JvmStatic public fun <R> createFlow( db: RoomDatabase, inTransaction: Boolean, tableNames: Array<String>, callable: Callable<R> ): Flow<@JvmSuppressWildcards R> = flow { coroutineScope { val resultChannel = Channel<R>() launch(queryContext) { ... emitAll(resultChannel)
54. Writes in Room?
55. Transactions
56. Transactions in SQLite
57. SQLite BEGIN TRANSACTION
58. SQLite COMMIT
59. Transaction on journal vs WAL
60. Transactions / writes with Android API
61. Room wraps a lot of things in transactions
62. Multiple write statements - same transaction val preparedQuery = prepareQuery("INSERT...") beginTransaction input.forEach { bind(preparedQuery, it) } endTransaction
63. Triggers
64. Triggers in SQLite
65. Triggers for RxJava and LiveData (& coroutines flows!) public static Flowable<Object> createFlowable(final RoomDatabase database, final String... tableNames) { return Flowable.create(new FlowableOnSubscribe<Object>() { @Override public void subscribe(final FlowableEmitter<Object> emitter) throws Exception { ... };
66. public static Flowable<Object> createFlowable(final RoomDatabase database, final String... tableNames) { return Flowable.create(new FlowableOnSubscribe<Object>() { @Override public void subscribe(final FlowableEmitter<Object> emitter) throws Exception { final InvalidationTracker.Observer observer = new InvalidationTracker.Observer( tableNames) { @Override public void onInvalidated(@androidx.annotation.NonNull Set<String> tables) { if (!emitter.isCancelled()) { emitter.onNext(NOTHING); } } };
67. static class WeakObserver extends Observer { final InvalidationTracker mTracker; final WeakReference<Observer> mDelegateRef; WeakObserver(InvalidationTracker tracker, Observer delegate) { super(delegate.mTables); mTracker = tracker; mDelegateRef = new WeakReference<>(delegate); } @Override public void onInvalidated(@NonNull Set<String> tables) { final Observer observer = mDelegateRef.get(); if (observer == null) { mTracker.removeObserver(this); } else { observer.onInvalidated(tables); } } }
68. Invalidation Tracking public class InvalidationTracker { static class ObservedTableTracker { static final int NO_OP = 0; // don't change trigger state for this table static final int ADD = 1; // add triggers for this table static final int REMOVE = 2; // remove triggers for this table
69. public class InvalidationTracker { void syncTriggers(SupportSQLiteDatabase database) { try { for (int tableId = 0; tableId < limit; tableId++) { switch (tablesToSync[tableId]) { case ObservedTableTracker.ADD: startTrackingTable(database, tableId); break; case ObservedTableTracker.REMOVE: stopTrackingTable(database, tableId); break; } } }
70. private void startTrackingTable(SupportSQLiteDatabase writableDb, int tableId) { final String tableName = mTableNames[tableId]; StringBuilder stringBuilder = new StringBuilder(); for (String trigger : TRIGGERS) { stringBuilder.setLength(0); stringBuilder.append("CREATE TEMP TRIGGER IF NOT EXISTS "); appendTriggerName(stringBuilder, tableName, trigger); stringBuilder.append(" AFTER ") .append(trigger) .append(" ON `") .append(tableName) .append("` BEGIN INSERT OR REPLACE INTO ") .append(UPDATE_TABLE_NAME) .append(" VALUES(null, ") .append(tableId) .append("); END"); writableDb.execSQL(stringBuilder.toString()); } }
71. private Set<Integer> checkUpdatedTable() { HashSet<Integer> invalidatedTableIds = new HashSet<>(); Cursor cursor = mDatabase.query(new SimpleSQLiteQuery(SELECT_UPDATED_TABLES_SQL)); //noinspection TryFinallyCanBeTryWithResources try { while (cursor.moveToNext()) { final int tableId = cursor.getInt(0); invalidatedTableIds.add(tableId); } } finally { cursor.close(); } if (!invalidatedTableIds.isEmpty()) { mCleanupStatement.executeUpdateDelete(); } return invalidatedTableIds; }
72. public void endTransaction() { mInvalidationTracker.refreshVersionsAsync(); }
73. if (invalidatedTableIds != null && !invalidatedTableIds.isEmpty()) { synchronized (mObserverMap) { for (Map.Entry<Observer, ObserverWrapper> entry : mObserverMap) { entry.getValue().notifyByTableInvalidStatus(invalidatedTableIds); } } }
74. AFTER INSERT/UPDATE/DELETE ON <table> BEGIN UPDATE SET invalidated = 1 WHERE tableId = <id> AND invalidated = 0; END
75. Transactions with Room
76. TransactionExecutor
77. class TransactionExecutor implements Executor { private final Executor mExecutor; private final ArrayDeque<Runnable> mTasks = new ArrayDeque<>(); private Runnable mActive; TransactionExecutor(@NonNull Executor executor) { mExecutor = executor; } public synchronized void execute(final Runnable command) { mTasks.offer(new Runnable() { public void run() { try { command.run(); } finally { scheduleNext(); } } }); if (mActive == null) { scheduleNext(); } } @SuppressWarnings("WeakerAccess") synchronized void scheduleNext() { if ((mActive = mTasks.poll()) != null) { mExecutor.execute(mActive); } } }
78. In RxJava public class RxRoom { private static Executor getExecutor(RoomDatabase database, boolean inTransaction) { if (inTransaction) { return database.getTransactionExecutor(); } else { return database.getQueryExecutor(); } } }
79. In Coroutines class CoroutinesRoom private constructor() { companion object { @JvmStatic suspend fun <R> execute( db: RoomDatabase, inTransaction: Boolean, callable: Callable<R> ): R { if (db.isOpen && db.inTransaction()) { return callable.call() } // Use the transaction dispatcher if we are on a transaction coroutine, otherwise // use the database dispatchers. val context = coroutineContext[TransactionElement]?.transactionDispatcher ?: if (inTransaction) db.transactionDispatcher else db.queryDispatcher return withContext(context) { callable.call() } } }
80. Transaction APIs in Room public void runInTransaction(@NonNull Runnable body) { beginTransaction(); try { body.run(); setTransactionSuccessful(); } finally { endTransaction(); } }
81. What can developers Do?
82. Conclusions
83. Large queries and paging in Room
84. Cursor - Android API package android.database.sqlite; public class SQLiteCursor extends AbstractWindowedCursor { private void fillWindow(int requiredPos) { ... } }
85. So large queries are...
86. Why is this a problem? 1. SQLiteCursor isn't in a transaction
87. Why is this a problem? 1. SQLiteCursor isn't in a transaction 2. SQLiteCursor cursorPickFillWindowStartPosition
88. public static int cursorPickFillWindowStartPosition( int cursorPosition, int cursorWindowCapacity) { return Math.max(cursorPosition - cursorWindowCapacity / 3, 0); }
89. Solution #1: @Transaction https://medium.com/androiddevelopers/7-pro-tips-for-room- fbadea4bfbd1
90. Solution #2: Write smaller queries!
91. Solution #3: Integrate with paging library package androidx.room.paging; public abstract class LimitOffsetDataSource<T> extends PositionalDataSource<T> { public List<T> loadRange(int startPosition, int loadCount) { final RoomSQLiteQuery sqLiteQuery = getSQLiteQuery(startPosition, loadCount); if (mInTransaction) { mDb.beginTransaction(); Cursor cursor = null; //noinspection TryFinallyCanBeTryWithResources try { cursor = mDb.query(sqLiteQuery); List<T> rows = convertRows(cursor); mDb.setTransactionSuccessful(); return rows; } finally { if (cursor != null) { cursor.close(); } mDb.endTransaction(); sqLiteQuery.release(); } } else { Cursor cursor = mDb.query(sqLiteQuery); //noinspection TryFinallyCanBeTryWithResources try { return convertRows(cursor); } finally { cursor.close(); sqLiteQuery.release(); } } }
92. mLimitOffsetQuery = "SELECT * FROM ( " + mSourceQuery.getSql() + " ) LIMIT ? OFFSET ?";
93. Paging + LiveData + Room support LiveData<PagedList<User>> users = new LivePagedListBuilder<>( userDao.loadUsersByAgeDesc(), /*page size*/ 20).build(); https://github.com/googlesamples/android-architecture- components/tree/master/PagingSample
94. Thanks! Effie Barak (@CodingChick)