Problem: data is not available to `SELECT` from new connection to SQLite DB with WAL mode (Write Ahead Log) on if previous connection hasn't been closed properly. Main question is: why data is lost and is there some way to get lost transactions?
I'm trying to store data in a SQLite table with `WAL mode` on. I'll describe 3 cases: case A results in transaction loss, cases B and C - don't.
Case A (quit app, don't close connection):
-------
0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. `INSERT` (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. `SELECT` (Connection 1) // All data is present.
6. `SELECT` (Connection 2) // All data is present.
7. **Close application (without closing Connection 1)**
8. Open application, open Connection 3
9. `SELECT` (Connection 3) // **Some data is lost** - select could return 2 out of 5 inserted transactions, for example. Or 0 out of 5 transactions. Totally random.
Case B (close connection before quit app):
-------
0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. `INSERT` (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. `SELECT` (Connection 1) // All data is present.
6. `SELECT` (Connection 2) // All data is present.
7. **Close Connection 1**
8. Close application
9. Open application, open Connection 3
10. `SELECT` (Connection 3) // **All data is present.**
Case C (perform checkpoint before close app + don't close connection):
----------------------------------------------------------------------
0. Open application, open Connection 1, open Connection 2 (on the same DB)
1. Begin transaction (Connection 1)
2. `INSERT` (Connection 1)
3. End transaction (Connection 1)
4. Repeat 5 times steps 1-3
5. `SELECT` (Connection 1) // All data is present.
6. `SELECT` (Connection 2) // All data is present.
7. **Perform WAL checkpoint**
8. Close application (without closing Connection 1)
9. Open application, open Connection 3
10. `SELECT` (Connection 3) // **All data is present.**
To sum up, when i `SELECT` from the table before closing app, all data is present, but after incorrect closing of the application (for example if app crashed) some of the data i inserted is missing. However if i perform checkpoint **before** closing the app (or close connection before closing app)- all the data is available.
Extra info:
- If i perform checkpoint **after** re-opening app (Case A), transactions don't appear (don't proceed from journal to main db file).
- WAL hook: i've registered callback using `sqlite3_wal_hook` to check whether transactions are actually commited to WAL-journal file and it shows that pages being successfully written to the log file.
- WAL file: i've tried to look into `-wal` file using Device File Explorer in Android Studio or by copying it to external storage from internal storage (/data/data/com.package.my/files) and most of the time it is either empty or doesn't exist.
- Threadsafe: i've tried to turn on SERIALIZED threadsafe mode by using SQLITE_OPEN_FULLMUTEX flag when opening DB:
`sqlite3_open_v2(db_name.c_str(), &handle, SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_FULLMUTEX, nullptr);`
It didn't make any difference. However, it lead to problems with reading from 2nd connection, so i use `sqlite3_open` without `SQLITE_OPEN_FULLMUTEX`.
- Stack: android 7 - JNI - c++ 11 - sqlite 3.27.2
Code:
```
// DB init (executed once on app start)
void initDB()
int32 rc = sqlite3_open(db_name.c_str(), &handle); // rc = 0
// check threadsafe mode
int stResult = sqlite3_threadsafe(); // stResult = 1
// register WAL hook
char* pointerContext = new char[4]();
pointerContext[0] = 't';
pointerContext[1] = 'e';
pointerContext[2] = 's';
pointerContext[3] = 't';
sqlite3_wal_hook(handle, wal_hook, pointerContext);
// turn WAL mode on
int32 rcWAL = sqlite3_exec(handle, "PRAGMA journal_mode=WAL;", processResults, &result, &errMsg); // rcWAL = 0
}
// close connection
int32 close() {
return sqlite3_close(handle);
}
// WAL checkpoint
sqlite3_exec(handle, "pragma wal_checkpoint;", processResults, &result, &errMsg);
// Insert
EventPtr persist(EventPtr obj) {
vector<DBData*> args;
int beginResult = sqlite3_exec(_connector->handle, "BEGIN TRANSACTION;", NULL, NULL, NULL);
try {
args.push_back(&obj->eventId);
// ... push more args
string query = "insert into events values(?1,?2,?3,?4,?5,?6,?7,?8,?9,?10,?11,?12,?13,?14);";
int32_t rc = _connector->executePreparedWOresult(query.c_str(),&args);
if(rc == SQLITE_DONE) {
int endResult = sqlite3_exec(_connector->handle, "END TRANSACTION;", NULL, NULL, NULL);
return obj;
}
} catch(...){ }
}
// SELECT
vector<EventPtr> readAll()
{
string query = "select * from Events;";
ResultSetPtr result= _connector->executePrepared(query.c_str(), NULL);
vector<EventPtr> vec;
for(int32_t i = 0; i < result->size(); i ++){
EventPtr event(new Event);
// init event
vec.push_back(EventPtr(event));
}
return vec;
}
// executePreparedWOresult
int32 executePreparedWOresult(const string& query, vector<DBData*> *args){
sqlite3_stmt *stmt;
cout << query ;
sqlite3_prepare_v2(handle, query.c_str(), query.size(), &stmt, NULL);
for(uint32 i = 0;args && i < args->size(); i ++){
switch(args->at(i)->getType()){
// statement bindings (sqlite3_bind_text, etc.)
}
}
int32 rc = sqlite3_step(stmt);
sqlite3_finalize(stmt);
return rc;
}
// executePrepared
ResultSetPtr executePrepared(const char *query, const vector<DBData*> *args){
ResultSetPtr res = ResultSetPtr(new ResultSet);
sqlite3_stmt *stmt;
int32_t rs = sqlite3_prepare_v2(handle, query, strlen(query), &stmt, NULL);
for(uint32 i = 0;args && i < args->size(); i ++){
switch(args->at(i)->getType()){
// statement bindings (sqlite3_bind_text, etc.)
}
}
int32 count = sqlite3_column_count(stmt);
int32 rc;
ResultRow row;
int32 rows = 0;
while((rc = sqlite3_step(stmt)) == SQLITE_ROW){
rows ++;
for(int32 i = 0; i < count; i++){
// multiple row.push_back-s for all columns
}
res->push_back(row);
row.clear();
}
sqlite3_finalize(stmt);
return res;
}
```