User Tools

Site Tools


GSoC-2013 Status page for project "Non-Blocking Database Access"

Student: Nazar Gerasymchuk (email: nazar.gerasymchuk at, IRC: tr0)

Mentor: Daniel Schürmann (email: daschuer at, IRC: daschuer)

My blogs

You can find me there, and contact me through blogs comments:

My fork of Mixxx at GitHub:

Most interesting for now

  1. Code review of pause feature in library scanner –
  2. Article on new database access scheme – Lambda scheme

Main goals for final term

  1. Pausable Library scanner. We are on this way.
  2. No writes access to database from the GUI thread.
  3. ThreadDAO – let's do and test on just one query, and then generalize to common case – do queue.
  4. Clever locking – to avoid the race condition with duplicated data from database and models/member variables and to discard double user database commands.
  5. Some kind of widget in Mixxx, like status-bar, QProgressIndicator or a Mouse cursor.
  6. Fix some linked bugs.

Weeks schedule / Tasks

In general, I see two milestones:

  1. Before midterm: prepare separate working prototype for db access.
  2. Before finalterm: merge prototype into Mixxx.
  1. Week (17.06 – 23.06)
    1. Write down this schedule.
    2. Write article “Overview of all what we have in Mixxx (Qt, SQLite, threads, …). What to do with it?”:
    3. Fork Mixxx on GitHub.
  2. Week (24.06 – 30.06)
    1. Dig into Mixxx and find all usage of db access, analyze it, write brief description of different types of DB usage in Mixxx and think on what to aware of when designing solution.
    2. Find all related bugs.
    3. (some of items was moved forward to next week)
  3. Week (01.07 – 07.07)
    1. Dig into Sqlite and define or find some suitable test cases.
    2. Dig into Mixxx and find all usage of db access, analyze it, write brief description of different types of DB usage in Mixxx and think on what to aware of when designing solution.
    3. Prepare UML class (and probably, sequence) dias for nowadays Mixxx DB accesss. For this purposes I use Dia (, ArgoUML ( or Umrello (
    4. Begin writing documentation.
    5. Writing the specs for a possible solution.
    6. Prepare UML class (and probably, sequence) dias for new DB accesss in Mixxx.
    7. Create separate repo on GitHub for prototype.
    8. Start implementing of prototype database access class with separate test project.
    9. (Try to) Fix some bugs.
  4. Week (08.07 – 14.07)
    1. Start Open discussion to involve other developers to review prototype.
    2. Implement prototype database access class with separate test project.
    3. (Try to) Fix some bugs.
  5. Week (15.07 – 21.07)
    1. Get acquainted with “Google Test” framework.
    2. Write tests using “Google Test” framework.
    3. (Try to) Fix some bugs.
  6. Week (22.07 – 28.07)
    1. Add and test all possible ways of database usage present in Mixxx handled by DAO.
    2. (Try to) Fix some bugs.
  7. Week (29.07 – 04.08)
    1. Finish prototype that is able to handle asynchronous db access, …, flexible and extendable.
    • Important date 29.07 – Begins midterm deadline
    • Important date 20.08 – Midterm deadline
    • For midterm I'll prepare separate prototype project on separate repo.

  1. Week (05.08 – 11.08)
    1. Plan (re-plan) schedule of work for finalterm.
    2. Discussion possible race conditions.
    3. Discuss further work on (pausable library scanner and this branchs place in further library refactoring).
    4. Work on to introduce separate thread for DB access `ThreadDAO` class.
    5. Discuss migration to C++11.
    6. Do re-planning for further work (if needed).
  2. Week (12.08 – 18.08)
    1. Disscuss better way to do locking of UI – “Clever locking”.
    2. Implement locking of UI.
    3. Do re-planning for further work (if needed).
  3. Week (19.08 – 25.08)
    1. Implementing of queue (or not queue but one-shot if it would be better) queries inside `ThreadDAO`.
    2. Test implemented solution.
    3. (Try to) Fix some bugs.
    4. Do re-planning for further work (if needed).
  4. Week (26.08 – 01.09)
    1. Implementing & testing.
    2. Fixing bugs using implemented scheme. Here is blueprint, and there are linked bugs, which could be solved using new API.
    3. Do re-planning for further work (if needed).
  5. Week (02.09 – 08.09)
    1. Write specs and docs on solution.
    2. Prepare general API for applying queries.
    3. Fixing bugs using new API.
    4. Do re-planning for further work (if needed).
  6. Week (09.09 – 15.09)
    1. Iron out remaining bugs.
    2. Merge my branch with master.
  7. Week (16.09 – 22.09)
    • Important date 16.09 – Pencils down begins
  8. Week (23.09 – 29.09) :
    • Important date 27.09 – Finalterm deadline
  9. 01.10 : end

Current status

Currently I'm working on:

  1. SQLite tests (including using WAL, see:
  2. Reading and by the way cleaning code of database access.
  3. Preparing prototype of access to database.
  4. Working on implementing “pause” feature in library scanner (See LibraryRefactoring branch here –



Original task: Non-Blocking Database Access

Currently some database transactions are stalling the GUI. This is because some database queries are preformed from the GUI thread.

This should be solved during the project by something like a standard non blocking interface to sqlite. All Database queries should be issued though this new interface.

This project would make Mixxx more reliable by this new concept how to deal with database actions.

Your proposal should include a draft proposal how do you will achieve the goal. You should have already have experience in parallel processing and sqlite.

My proposal

Let’s use RPC-style pure callbacks. I can propose to create class DBManager. DBManager will be a singleton and it will be instance in separate thread (inherited from QThread). This class will have own event loop. Also it will have queue of queries and pointers to QFuture objects to control (in own event loop) what queries have been executed.

We need one thread (DBManager) for management database access (and if needed – solve probably conflicts), other (there can be more than one, so we can use QtConcurrent::run and pointers to QFuture objects to handle states and results) is for executing queries to database.

The task of processing a short queue of queries to database is a probably long time procedure, so we need to have separate thread for it. By the way we don’t miss possibility to execute multiple queries in the same time (if it is possible).

Executing queries from GUI thread is not good idea – we’ll avoid it. After DBManager will be applied, we’ll got only one unified entry point to access database.

As this class will be singleton, it can be reachable from every module. All queries will be applied through DBManager this way:

  • On module side (from concrete class):
    • prepare query,
    • set query parameters (such as type of needed DAO, priority or transaction recommendations),
    • prepare callback function (function to apply after successfully executing query),
    • call DBManager’s function to apply query with own callback functions(s) as parameter(s).
  • On DBManager side:
    • got new query into own queue of queries,
    • scan current active queries,
    • solve conflicts on how to place this query into queue of queries (to avoid blocking and other moments – must discuss it),
    • create new thread using QtConcurrent::run and run query from queue (if possible) using respective DAO-object in it and control it using QFuture,
    • check what queries executed and execute respective callbacks.

Of course, there are lot of moments to discuss on.

More information will be here soon.

Translations of this page:
nonblockingdb_status.txt · Last modified: 2013/09/03 17:48 by troyane