GEPS 010: Relational Backend

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Revision as of 17:58, 26 March 2009 by AaronS (talk | contribs) (heading cleanup)
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This page is for the discussion of a proposed implementation of a SQL backend for GRAMPS.

SQL stands for "Structured Query Language" and is pronounced "sequel" (it is a joke: as it came after QUEL, it is its sequel).

Reasons for making the switch

Currently, GRAMPS uses a BSD database as its internal file format. While this is considerably better than, say, an XML format, the choice of the BSD-DB has a considerable number of drawbacks. This proposal explores the use of SQL as an alternative backend. This should allow easy, single db file implementations (eg, SQLite) to more complex and sophisticated client/server (eg, MySQL).

First, there are a number of facts related to this proposal:

  1. BSDDB is being removed from the standard distribution of Python (as of Python 2.6)
  2. SQLITE is being added to the standard Python distribution
  3. BSDDB is not a relational database, but a hierarchical one
  4. BSDDB databases do not have schema or data-definitions. BSDDB requires all of the database structure logic to reside in code
  5. BSDDB is a programmer's API
  6. SQL is a declarative, independent abstraction layer
  7. SQL can optimize queries (in low-level C) whereas BSDDB is done in Python
  8. SQLite tables of a database reside in a single file

Next, are a number of claims that need to be tested:

  1. An SQLite version of a GRAMPS BSDDB may be 4 times smaller
  2. An SQLite version of a GRAMPS BSDDB may be faster
    1. The files may be smaller
    2. The smaller files may allow more into memory
    3. More code would reside in C, rather than in Python
    4. SQL Engines can perform query optimizations
  3. Enterprise SQL versions of GRAMPS would allow people to create and manage much larger trees
  4. An SQLite version of GRAMPS might allow people to create larger trees
    1. Because we move all of the DB logic into SQL, we can focus on making GRAMPS stable with large databases
  5. SQL code is simpler than the equivalent BSDDB code, because SQL is declarative/abstract and BSDDB is a low-level API

Further implications:

  1. A fullscale MySQL backend would be a trivial step from SQLite (although maybe harder to setup and maintain; although see Django)
  2. Easy to allow multiple users in a SQLite database (uses file-locking)
  3. There is a lot of code that we have written dealing with BSDDB. It would have to all be rewritten in SQL (on the other hand, a lot of code can be deleted, which will make GRAMPS easier to maintain and adapt)
  4. We will have to develop SQL experts
It's good to see this discussion on gramps and is actually why I'm thinking of giving
it another try depending on how hard it is to implement this. Yes I know it will be hard
but probably much easier and productive than starting my own project. I'm  a developer my
self and when it came time to evaluate gramps the lack of a relational db backend was one
of the main reasons I decided to keep looking.

Don't discount MySQL over SQLite. While I haven't tried it out yet there is an embeddable
 version of MySQL which might overcome some of sqlites advantages. If a database abstraction
 layer is used both could be
 easily supported. They both have their advantages and disadvantages.



Personally I think SQLite makes more sense for genealogical software. but mysqls
tools and the fact that it's a "real" enterprise level relational db are serious advantages.
-- AaronS

Discussions of BSDDB in Python

BSDDB has had a hard time in Python. Python Developers have been wrestling with trying to keep it stable. Guido finally decided to separate BSDDB from the standard Python Distribution. See discussions:

PEP 3108 marks BSDDB to be removed: http://www.python.org/dev/peps/pep-3108/ Development is not death however, it will only be out of sync of the python cycle. The home of pybsdb offering the bsddb3 package is hereL http://www.jcea.es/programacion/pybsddb.htm


Research

Relational database comparison

Database abstraction layers comparison




Additional Issues

If we use a well-known SQL backend, we should consider the ability for other languages to be able to natively access the database. For example, a PHP program should be able to use the same database. Does using a Python-based ORM tie the data to Python? Or can the database still be used natively from other systems?

Using a Python based ORM wont tie the data just to python. any language should be able to access the db just fine. However, they wouldn't have access to pythons orm layer. Since I haven't used a true orm before I'm not certain exactly how it will effect our table relationships but I don't believe they wont make some sense in a relational way. Not that I'm saying we should use it but a quick google search started to bring up things like this php python package. so there may be some hope for even using the orm layer but how complex would we really want to make it! And of course there is always the option of just using an orm and building similar objects in the new language. --AaronS 03:30, 26 March 2009 (UTC)



Power vs Dependencies

Do we want to have an additional layer over the Database Abstraction Layer (eg, an ORM)?

PROS:

  1. Makes GRAMPS code more abstract

CONS:

  1. Makes it harder for other languages to use the native GRAMPS db (but they can use the native db)
  2. Adds a dependency

Given that GRAMPS's developers have, in the past, written their own db transactions, and their own HTML abstractions, does it make sense to add such a dependency?

Is the ORM available for all platforms?


Recomendations

Let me preface this by restating that I've never actually used any of these abstraction layers and I'm not yet familiar with the gramps code and developers strengths. Other people with more knowledge should be the ones making the decision. Also any decisions need to be revistable after we actually start coding in case they just don't work.

I've spent the last few days trying to look at the current options for db abstraction. From what I currently know I think I'm going to recommend we use sqlalchemy with sqlite.

sqlite. no server to manage and single file db's will make them easy to share and manage multiple dbs at the same time. also make merging simpler. will allow websites to be developed that will work directly from the db. As long as gramps doesn't switch focus to be some kind of mass user website for editing large trees I think sqlite will fit the bill.

sqlalchemy. this seems to have a large following and good documentation. It should allow us to support different db back ends easier in the future. at least some people think it's the best python orm available. it seems to provide good tools for when the ORM starts to get in the way.

Reasons I don't recommend the other options include:

MySQL. probably not as user friendly and since gramps isn't a client / server sort of program I don't think it's necessary.

DB-API. with sqlite3 It sounds as if the DB-API in practice doesn't support the changing of dbs as much as might be thought. If we commit to sqlite though this might be an option.

SQLObject. this seems like a viable alternative to sqlalchemy but slqlalchemy seems to have more documentation and user acceptance. Also the ORM layer might not step out of the way very nicely. the website says it will but I wasn't quite buying it from the examples.

Storm. while this project looks promising and may be easier to use than sqlalchemy it's only a year old and as I was recently burned by picking a fringe tech for my tech stack I'm a bit skittish of anything that doesn't have wide acceptance and use.

Additional notes: I was originally advocating for database abstraction not an orm layer. I've never used a true orm and can't fully say how they work in practice. While I'm not solidly on the orm badwagon I do think an orm layer might do gramps some good. There will be situations where simply writing queries will be far easier. Our implementation model should take that into account. from the website sqlalchemy sounds like it will provide both abstraction and an orm and we'll be able to use both as the needs determine. While I don't fully agree with the severity of this article he does make some valid points. There is a reason that true object databases haven't caught on. I guess I'm advocating for something like "Developers simply accept that there is no way to efficiently and easily close the loop on the O/R mismatch, and use an O/R-M to solve 80% (or 50% or 95%, or whatever percentage seems appropriate) of the problem and make use of SQL and relational-based access ... to carry them past those areas where an O/R-M would create problems." article

--AaronS 00:52, 26 March 2009 (UTC)

What now?

Create Object model

Going over src/gen/lib/, create an object model of how GRAMPS uses and manipulates genealogy data.

For this GEP to succeed it is extremely important that the experienced developers on the devel list agree with the object model. This goes without saying. For this to succeed, the developers should agree with all of the major decisions.

Select an SQL framework

  1. finish research and pick a database abstraction layer.
  2. finish research and pick a database.

Create models/tables

  1. use the framework to set up a model of the database
  2. generate the tables
  3. create a dump of bsddb database in the sql database
  4. validate that all things present in bsddb are present in the sql database
  5. check validation rules. Eg, handle should be unique, rules must be added to ensure adding to the family table an object with handle like a person object is impossible on the database layer. These kind of rules can be done technically (a primary object table with key on handle) or with rules.
  6. best would be a framework that based on the model can generate an admin module to browse the database, see eg the admin module in django.

New db backend for GRAMPS

  1. write an implementation of src/gen/db/base.py to interface the DB abstraction layer with the rest of gramps. Gramps 3.x only has one implementation: src/gen/db/dbdir.py, but in branch22 a gedcom and a gramps xml implementation can be found (these have been deprecated).
  2. once written, this can be added as an experimental backend to GRAMPS
    1. Family Tree manager needs to list the family tree type (bsddb, sqlite), on creation of new family tree user must choose the backend.
    2. User can import .gramps/gedcom files just as this is done with bsddb backend once family tree is set up.
  3. it will be very important to use slots in src/gen/lib to make this work. Obtaining a person via get_person_from_handle, should only hit the person table. Only when the calling method needs attributes, should the attribute table be hit. This requires attributes that are not yet defined up to the moment they are accessed. It also means that the gen/lib objects for sql need to be aware of the database as it needs to know where to obtain these values... . This looks like a huge work to me, but definitely doable. Just rewriting gen/lib for an sql datamodel might be easier though, but that means rewriting the core of GRAMPS....

I don't understand the use of slots in the above. How is that idea related to db access? --Dsblank 11:14, 26 March 2009 (UTC)

Extending base.py

Once an sql backend is stable, base.py can be extended to offer extra functionality, or better optimize for SQL. Eg, in SQL one would have probably an attribute table. To know which persons have a specific attribute, SQL would select that from the attributes table, and then look up the people. In bsddb it means however to loop over all persons, and obtain the attribute sub table of a person and looking if attribute is present there.

Above clearly indicates that how one goes about in the two backends is very different. The bsddb way will work in sql though (as the get_person method works, and speed should be comparable to bsddb if above deferred obtaining of values via slots is implemented). Nevertheless, a clear mechanism to optimize for sql is needed. Continuing above example, see _HasAttributeBase.py

For sql, one would use the prepare method, obtain all people in a list, then return True or False if person is in this list. As db is passed, db can have a support_sql attribute, and code can be written depending on this setting. This does not look very ideal though.

See Also

ExportSql.py