beam-migrate- SQL DDL support and migrations support library for Beam

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Data types and functions to discover sequences of DDL commands to go from one database state to another. Used for migration generation.

For our purposes, a database state is fully specified by the set of predicates that apply to that database.

Migration generation is approached as a graph search problem over the infinite graph of databases G. The nodes of G are database states, which (as said above) are simply sets of predicates (see DatabaseState for the realization of this concept in code). For two vertices S1 and S2 in G, there is an edge between the two if and only if there is a DDL command that can take a database at S1 to S2.

We generate migrations by exploring this graph, starting at the source state and ending at the destination state. By default we use an optimizing solver that weights each edge by the complexity of the particular command, and we attempt to find the shortest path using Dijkstra's algorithm, although a user may override this behavior and provide a custom edge selection mechanism (or even defer this choice to the user).

In order to conduct the breadth-first search, we must know which edges lead out of whichever vertex we're currently visiting. The solving algorithm thus takes a set of ActionProviders, which are means of discovering edges that are incident to the current database state.

Conceptually, an ActionProvider is a function of type ActionProviderFn, which takes the current database state and produces a list of edges in the form of PotentialAction objects. For optimization purposes, ActionProviders also take in the desired destination state, which it can use to select only edges that make sense. This does not affect the result, just the amount of time it may take to get there.

Note that because the graph of database states is infinite, a breadth-first search may easily end up continuing to explore when there is no chance of reaching our goal. This would result in non-termination and is highly undesirable. In order to prevent this, we limit ourselves to only exploring edges that take us closer to the destination state. Here, we measure distance between two states as the number of elements in the symmetric difference of two database states. Thus, every action we take must either remove a predicate that doesn't exist in the destination state, or add a predicate that does. If a potential action only adds predicates that do not exist in the final state or removes predicates that do not exist in the first, then we never explore that edge.

A note on speed

There are some issues with this approach. Namely, if there is no solution, we can end up exploring the entire action space, which may be quite a lot. While beam-migrate can solve all databases that can be made up of predicates in this module, other beam backends may not make such strict guarantees (although in practice, all do). Nevertheless, if you're hacking on this module and notice what seems like an infinite loop, you may have accidentally removed code that exposed the edge that leads to a solution to the migration.


Database state

data DatabaseStateSource Source #

Used to indicate whether a particular predicate is from the initial database state, or due to a sequence of actions we've committed too. Used to prevent runaway action generation based off of derived states.



Predicate is from the original set given by the user


Predicate is from an action we've committed to in this action chain


Bounded DatabaseStateSource Source # 
Enum DatabaseStateSource Source # 
Eq DatabaseStateSource Source # 
Ord DatabaseStateSource Source # 
Show DatabaseStateSource Source # 
Generic DatabaseStateSource Source # 
NFData DatabaseStateSource Source # 


rnf :: DatabaseStateSource -> () #

type Rep DatabaseStateSource Source # 
type Rep DatabaseStateSource = D1 * (MetaData "DatabaseStateSource" "Database.Beam.Migrate.Actions" "beam-migrate-" False) ((:+:) * (C1 * (MetaCons "DatabaseStateSourceOriginal" PrefixI False) (U1 *)) (C1 * (MetaCons "DatabaseStateSourceDerived" PrefixI False) (U1 *)))

data DatabaseState cmd Source #

Represents the state of a database as a migration is being generated





Show cmd => Show (DatabaseState cmd) Source # 
NFData (DatabaseState cmd) Source # 


rnf :: DatabaseState cmd -> () #

Action generation

data PotentialAction cmd Source #

Represents an edge (or a path) in the database graph.

Given a particular starting point, the destination database is the database where each predicate in actionPreConditions has been removed and each predicate in actionPostConditions has been added.





Monoid (PotentialAction cmd) Source #

PotentialActions can represent edges or paths. Monadically combining two PotentialActions results in the path between the source of the first and the destination of the second. mempty here returns the action that does nothing (i.e., the edge going back to the same database state)

newtype ActionProvider cmd Source #

Edge discovery mechanism. A newtype wrapper over ActionProviderFn.

An ActionProviderFn takes two arguments. The first is the set of predicates that exist in the current database.

The function should a set of edges from the database specified in the first argument to possible destination databases. For optimization purposes, the second argument is the set of predicates that ought to exist in the destination database. This can be used to eliminate edges that will not lead to a solution.

This second argument is just an optimization and doesn't change the final result, although it can significantly impact the time it takes to get there.

Both the current database set and the destination database set are given as polymorphic lists of predicates. When you instantiate the type, the current database predicate set is queried for predicates of that type.

For example, dropTableActionProvider provides a DROP TABLE action edge whenever it encounters a table that exists. In order to do this, it attempts to find all TableExistsPredicate that do not exist in the destination database. Its ActionProviderFn may be implemented like such:

dropTableActionProvider preConditions postConditions = do
    TableExistsPredicate srcTblNm <- preConditions
    ensuringNot_ $ $
      do TableExistsPredicate destTblNm <- postConditions
         guard (srcTblNm == destTblNm)

ensuringNot_ is a function that causes the action provider to return no results if there are any elements in the provided list. In this case, it's used to stop DROP TABLE action generation for tables which must be present in the final database.

type ActionProviderFn cmd = (forall preCondition. Typeable preCondition => [preCondition]) -> (forall postCondition. Typeable postCondition => [postCondition]) -> [PotentialAction cmd] Source #

ensuringNot_ :: Alternative m => [a] -> m () Source #

Proceeds only if no predicate matches the given pattern. See the implementation of dropTableActionProvider for an example of usage.

justOne_ :: [a] -> [a] Source #

Used to ensure that only one predicate matches the given pattern. See the implementation of createTableActionProvider for an example of usage.

createTableActionProvider :: forall cmd. (Sql92SaneDdlCommandSyntax cmd, Sql92SerializableDataTypeSyntax (Sql92DdlCommandDataTypeSyntax cmd)) => ActionProvider cmd Source #

Action provider for SQL92 CREATE TABLE actions.

addColumnProvider :: forall cmd. (Sql92SaneDdlCommandSyntax cmd, Sql92SerializableDataTypeSyntax (Sql92DdlCommandDataTypeSyntax cmd)) => ActionProvider cmd Source #

Action provider for SQL92 ALTER TABLE ... ADD COLUMN ... actions

addColumnNullProvider :: forall cmd. Sql92SaneDdlCommandSyntax cmd => ActionProvider cmd Source #

Action provider for SQL92 ALTER TABLE ... ALTER COLUMN ... SET NULL

dropColumnNullProvider :: forall cmd. Sql92SaneDdlCommandSyntax cmd => ActionProvider cmd Source #

Action provider for SQL92 ALTER TABLE ... ALTER COLUMN ... SET NOT NULL

defaultActionProvider :: (Sql92SaneDdlCommandSyntax cmd, Sql92SerializableDataTypeSyntax (Sql92DdlCommandDataTypeSyntax cmd)) => ActionProvider cmd Source #

Default action providers for any SQL92 compliant syntax.

In particular, this provides edges consisting of the following statements:



data Solver cmd where Source #

Represents current state of a database graph search.

If ProvideSolution, the destination database has been reached, and the given list of commands provides the path from the source database to the destination.

If SearchFailed, the search has failed. The provided DatabaseStates represent the closest we could make it to the destination database. By default, only the best 10 are kept around (to avoid unbounded memory growth).

If ChooseActions, we are still searching. The caller is provided with the current state as well as a list of actions, provided as an opaque type f. The getPotentialActionChoice function can be used to get the PotentialAction corresponding to any given f. The caller is free to cull the set of potential actions according however they'd like (for example, by prompting the user). The selected actions to explore should be passed to the continueSearch function.

Use of the f existential type may seem obtuse, but it prevents the caller from injecting arbitrary actions. Instead the caller is limited to choosing only valid actions as provided by the suppled ActionProvider.


ProvideSolution :: [cmd] -> Solver cmd 
SearchFailed :: [DatabaseState cmd] -> Solver cmd 
ChooseActions :: {..} -> Solver cmd 


data FinalSolution cmd Source #

Represents the final results of a search


Solved [cmd]

The search found a path from the source to the destination database, and has provided a set of commands that would work

Candidates [DatabaseState cmd]

The search failed, but provided a set of DatbaseStates it encountered that were the closest to the destination database. By default, only 10 candidates are provided.


finalSolution :: Solver cmd -> FinalSolution cmd Source #

An exhaustive solving strategy that simply continues the search, while exploring every possible action. If there is a solution, this will find it.

heuristicSolver Source #


:: ActionProvider cmd

Edge discovery function

-> [SomeDatabasePredicate]

Source database state

-> [SomeDatabasePredicate]

Destination database state

-> Solver cmd 

Conduct a breadth-first search of the database graph to find a path from the source database to the destination database, using the given ActionProvider to discovere "edges" (i.e., DDL commands) between the databases.

See the documentation on Solver for more information on how to consume the result.