Buildbot stores most of its state in a database. This section describes the database connector classes, which allow other parts of Buildbot to access the database. It also describes how to modify the database schema and the connector classes themselves.
3.7.1. Database Overview
All access to the Buildbot database is mediated by database connector classes. These classes provide a functional, asynchronous interface to other parts of Buildbot, and encapsulate the database-specific details in a single location in the codebase.
The connector API, defined below, is a stable API in Buildbot, and can be
called from any other component. Given a master
master, the root of the
database connectors is available at
master.db, so, for example, the state
getState method is
All the connectors use SQLAlchemy Core to achieve (almost) database-independent operation. Note that the SQLAlchemy ORM is not used in Buildbot. Database queries are carried out in threads, and report their results back to the main thread via Twisted Deferreds.
Changes to the schema are accomplished through migration scripts, supported by Alembic.
The schema itself is considered an implementation detail, and may change significantly from version to version. Users should rely on the API (below), rather than performing queries against the database itself.
Restrictions on many string fields in the database are referred to as the Identifier concept.
An “identifier” is a nonempty unicode string of limited length, containing only UTF-8 alphanumeric characters along with
- (dash) and
_ (underscore), and not beginning with a digit.
Wherever an identifier is used, the documentation will give the maximum length in characters.
buildbot.util.identifiers.isIdentifier is useful to verify a well-formed identifier.
3.7.4. Writing Database Connector Methods
The information above is intended for developers working on the rest of Buildbot, and treating the database layer as an abstraction. The remainder of this section describes the internals of the database implementation, and is intended for developers modifying the schema or adding new methods to the database layer.
It’s difficult to change the database schema, especially after it has been released. Changing the database API is disruptive to users. Consider very carefully the future-proofing of any changes here!
184.108.40.206. The DB Connector and Components
- class buildbot.db.connector.DBConnector
The root of the database connectors,
master.db, is a
DBConnectorinstance. Its main purpose is to hold a reference to each of the connector components, but it also handles timed cleanup tasks.
If you are adding a new connector component, import its module and create an instance of it in this class’s constructor.
- class buildbot.db.base.DBConnectorComponent
This is the base class for connector components.
There should be no need to override the constructor defined by this base class.
A reference to the
DBConnector, so that connector components can use e.g.,
self.db.model. In the unusual case that a connector component needs access to the master, the easiest path is
- checkLength(col, value)
For use by subclasses to check that ‘value’ will fit in ‘col’, where ‘col’ is a table column from the model. Ignore this check for database engines that either provide this error themselves (postgres) or that do not enforce maximum-length restrictions (sqlite).
- findSomethingId(self, tbl, whereclause, insert_values, _race_hook=None, autoCreate=True)
whereclause) or add (using
insert_values) a row to
table, and return the resulting ID. If
autoCreate== False, we will not automatically insert the row.
Hash the given values in a consistent manner: None is represented as xf5, an invalid unicode byte; strings are converted to utf8; and integers are represented by their decimal expansion. The values are then joined by ‘0’ and hashed with sha1.
- doBatch(batch, batch_n=500)
returns an Iterator that batches stuff in order to not push to many things in a single request. Especially sqlite has 999 limit that it can take in a request.
220.127.116.11. Direct Database Access
The connectors all use SQLAlchemy Core as a wrapper around database
client drivers. Unfortunately, SQLAlchemy is a synchronous library, so some
extra work is required to use it in an asynchronous context, like in Buildbot.
This is accomplished by deferring all database operations to threads, and
returning a Deferred. The
Pool class takes care of
A connector method should look like this:
def myMethod(self, arg1, arg2): def thd(conn): q = ... # construct a query for row in conn.execute(q): ... # do something with the results return ... # return an interesting value return self.db.pool.do(thd)
Picking that apart, the body of the method defines a function named
taking one argument, a
Connection object. It then calls
self.db.pool.do, passing the
thd function. This function is called in
a thread, and can make blocking calls to SQLAlchemy as desired. The
method will return a Deferred that will fire with the return value of
or with a failure representing any exception raised by
The return value of
thd must not be an SQLAlchemy object - in particular,
objects must be parsed into lists or other data structures before they are
As the name
thd indicates, the function runs in a thread. It should
not interact with any other part of Buildbot, nor with any of the Twisted
components that expect to be accessed from the main thread – the reactor,
Queries can be constructed using any of the SQLAlchemy core methods, using
Model, and executed with the connection
SQLAlchemy requires the use of a syntax that is forbidden by pep8. If in where clauses you need to select rows where a value is NULL, you need to write (tbl.c.value == None). This form is forbidden by pep8 which requires the use of is None instead of == None. As sqlalchemy is using operator overloading to implement pythonic SQL statements, and the is operator is not overloadable, we need to keep the == operators. In order to solve this issue, Buildbot uses buildbot.db.NULL constant, which is None. So instead of writing tbl.c.value == None, please write tbl.c.value == NULL).
- class buildbot.db.pool.DBThreadPool
- do(callable, ...)
callablein a thread, with a
Connectionobject as first argument. Returns a deferred that will fire with the results of the callable, or with a failure representing any exception raised during its execution.
Any additional positional or keyword arguments are passed to
18.104.22.168. Database Schema
Database connector methods access the database through SQLAlchemy, which requires access to Python objects representing the database tables. That is handled through the model.
- class buildbot.db.model.Model
This class contains the canonical description of the Buildbot schema. It is represented in the form of SQLAlchemy
Tableinstances, as class variables. At runtime, the model is available at
master.db.model. So, for example, the
buildrequeststable can be referred to as
master.db.model.buildrequests, and columns are available in its
The source file, master/buildbot/db/model.py, contains comments describing each table; that information is not replicated in this documentation.
Note that the model is not used for new installations or upgrades of the Buildbot database. See Modifying the Database Schema for more information.
The model object also has a
metadataattribute containing a
MetaDatainstance. Connector methods should not need to access this object. The metadata is not bound to an engine.
Modelclass also defines some migration-related methods:
boolean via Deferred
Returns true if the current database’s version is current.
Upgrades the database to the most recent schema version.
Connector component methods that get an object based on an ID are good
candidates for caching. The
makes this automatic:
cache_name – name of the cache to use
A decorator for “getter” functions that fetch an object from the database based on a single key. The wrapped method will only be called if the named cache does not contain the key.
The wrapped function must take one argument (the key); the wrapper will take a key plus an optional
no_cacheargument which, if true, will cause it to invoke the underlying method even if the key is in the cache.
The resulting method will have a
cacheattribute which can be used to access the underlying cache.
In most cases, getter methods return a well-defined dictionary. Unfortunately,
Python does not handle weak references to bare dictionaries, so components must
instantiate a subclass of
dict. The whole assembly looks something like
class ThDict(dict): pass class ThingConnectorComponent(base.DBConnectorComponent): @base.cached('thdicts') def getThing(self, thid): def thd(conn): ... thdict = ThDict(thid=thid, attr=row.attr, ...) return thdict return self.db.pool.do(thd)
It goes without saying that any new connector methods must be fully tested!
You will also want to add an in-memory implementation of the methods to the
fake classes in
master/buildbot/test/fake/fakedb.py. Non-DB Buildbot code
is tested using these fake implementations in order to isolate that code from
the database code, and to speed-up tests.
The keys and types used in the return value from a connector’s
get methods are described in master/buildbot/test/util/validation.py, via the
dbdict module-level value.
This is a dictionary of
DictValidator objects, one for each return value.
These values are used within test methods like this:
rv = yield self.db.masters.getMaster(7) validation.verifyDbDict(self, 'masterdict', rv)
3.7.5. Modifying the Database Schema
Changes to the schema are accomplished through migration scripts, supported by Alembic.
The schema is tracked by a revision number, stored in the
It can be anything, but by convention Buildbot uses revision numbers that are numbers incremented by one for each revision.
The master will refuse to run with an outdated database.
To make a change to the schema, first consider how to handle any existing data. When adding new columns, this may not be necessary, but table refactorings can be complex and require caution so as not to lose information.
Refer to the documentation of Alembic for details of how database migration scripts should be written.
The database schema itself is stored in master/buildbot/db/model.py which should be updated to represent the new schema. Buildbot’s automated tests perform a rudimentary comparison of an upgraded database with the model, but it is important to check the details - key length, nullability, and so on can sometimes be missed by the checks. If the schema and the upgrade scripts get out of sync, bizarre behavior can result.
Changes to database schema should be reflected in corresponding fake database table definitions in master/buildbot/test/fakedb
The upgrade scripts should have unit tests.
The classes in master/buildbot/test/util/migration.py make this straightforward.
Unit test scripts should be named e.g.,
The master/buildbot/test/integration/test_upgrade.py also tests
upgrades, and will confirm that the resulting database matches the model. If
you encounter implicit indexes on MySQL, that do not appear on SQLite or
Postgres, add them to
3.7.6. Foreign key checking
PostgreSQL and SQlite db backends check the foreign keys consistency. bug #2248 needs to be fixed so that we can support foreign key checking for MySQL.
To maintain consistency with real db, fakedb can check the foreign key consistency of your test data. For this, just enable it with:
self.db = fakedb.FakeDBConnector(self.master, self) self.db.checkForeignKeys = True
Note that tests that only use fakedb do not really need foreign key consistency, even if this is a good practice to enable it in new code.
3.7.7. Database Compatibility Notes
Or: “If you thought any database worked right, think again”
Because Buildbot works over a wide range of databases, it is generally limited to database features present in all supported backends. This section highlights a few things to watch out for.
In general, Buildbot should be functional on all supported database backends. If use of a backend adds minor usage restrictions, or cannot implement some kinds of error checking, that is acceptable if the restrictions are well-documented in the manual.
The metabuildbot tests Buildbot against all supported databases, so most compatibility errors will be caught before a release.
22.214.171.124. Index Length in MySQL
MySQL only supports about 330-character indexes. The actual index length is 1000 bytes, but MySQL uses 3-byte encoding for UTF8 strings. This is a longstanding bug in MySQL - see “Specified key was too long; max key length is 1000 bytes” with utf8. While this makes sense for indexes used for record lookup, it limits the ability to use unique indexes to prevent duplicate rows.
InnoDB only supports indexes up to 255 unicode characters, which is why all indexed columns are limited to 255 characters in Buildbot.
126.96.36.199. Transactions in MySQL
Unfortunately, use of the MyISAM storage engine precludes real transactions in
transaction.rollback() are essentially
no-ops: modifications to data in the database are visible to other users
immediately, and are not reverted in a rollback.
188.8.131.52. Referential Integrity in SQLite and MySQL
Neither MySQL nor SQLite enforce referential integrity based on foreign keys. Postgres does enforce it, however. If possible, test your changes on Postgres before committing, to check that tables are added and removed in the proper order.
184.108.40.206. Subqueries in MySQL
MySQL’s query planner is easily confused by subqueries. For example, a DELETE query specifying id’s that are IN a subquery will not work. The workaround is to run the subquery directly, and then execute a DELETE query for each returned id.
If this weakness has a significant performance impact, it would be acceptable to conditionalize use of the subquery on the database dialect.
220.127.116.11. Too Many Variables in SQLite
Sqlite has a limitation on the number of variables it can use.
This limitation is usually SQLITE_LIMIT_VARIABLE_NUMBER=999.
There is currently no way with pysqlite to query the value of this limit.
sqlite_limit is just not bound to the python.
When you hit this problem, you will get error like the following:
sqlalchemy.exc.OperationalError: (OperationalError) too many SQL variables u'DELETE FROM scheduler_changes WHERE scheduler_changes.changeid IN (?, ?, ?, ..., ?)
You can use the method
doBatch in order to write batching code in a consistent manner.
3.7.8. Testing migrations with real databases
By default Buildbot test suite uses SQLite database for testing database
To use other database set
BUILDBOT_TEST_DB_URL environment variable to
value in SQLAlchemy database URL specification.
For example, to run tests with file-based SQLite database you can start tests in the following way:
BUILDBOT_TEST_DB_URL=sqlite:////tmp/test_db.sqlite trial buildbot.test
18.104.22.168. Run databases in Docker
Docker allows to easily install and configure different databases locally in containers.
To run tests with PostgreSQL:
# Install psycopg pip install psycopg2 # Start container with PostgreSQL 9.5 # It will listen on port 15432 on localhost sudo docker run --name bb-test-postgres -e POSTGRES_PASSWORD=password \ -p 127.0.0.1:15432:5432 -d postgres:9.5 # Start interesting tests BUILDBOT_TEST_DB_URL=postgresql://postgres:password@localhost:15432/postgres \ trial buildbot.test
To run tests with MySQL:
# Install mysqlclient pip install mysqlclient # Start container with MySQL 5.5 # It will listen on port 13306 on localhost sudo docker run --name bb-test-mysql -e MYSQL_ROOT_PASSWORD=password \ -p 127.0.0.1:13306:3306 -d mysql:5.5 # Start interesting tests BUILDBOT_TEST_DB_URL=mysql+mysqldb://root:firstname.lastname@example.org:13306/mysql \ trial buildbot.test