Equinox/p2/Query Language for p2

Background

As p2 gets more widely used, we predict that repositories will grow larger and larger over time. The Galileo repository alone contains 3866 IU's (in one single service release). Starting with Helios, we plan to keep everything over time which will put the IU count close to 10.000. And that's just one year of Eclipse IP approved material. OSGi is gaining in popularity and p2 is getting increasingly popular. Some companies plan to map the central maven repository as p2. We can soon expect to see p2 repositories with over a 100.000 IU's in them.

The problem

p2 has a query mechanism today that makes it hard to create a repository implementation that is based on a database. It is also diffiult to create an efficient client/server solution. The reason for this is that most of the queries are written as implementations of the IQuery interface, either as ContextQuery derivates that need access to all rows in the database, or as MatchQuery derivates with a java method to which all rows need to be passed.

There is no uniform way to translate these queries into an expression that can be understood by a database or by something running in another process.

Some attempts has been made to rectify this and a repository implementation could potentially make some intelligent decisions based on information derived from looking at the query (what class it is, and in some cases, what properties that are exposed). While this is an improvement, the general problem remains; A large amount of the queries are still more or less completely black-boxed and it's enough with one single such query to force a full selection of everything from the database/remote server.

The p2QL expression language discussed here is an attempt to rectify this problem. It can be used in two ways:

• Coexisting with current IQuery / IQueryable

  Using the ExpressionQuery (a ContextQuery subclass), or the PredicateQuery (a MatchQuery subclass), it can coexist with todays IQuery/IQueryable. The queries are created based on an expression and an array of parameters. See section #IQuery examples.

• With more explicit separation of concerns

  It can also be used in an alternative solution where an expression returns an Iterator. This usage scenario is particularly useful when implementing remote repositories or implementations on top of a database. It is also a very efficient way of querying for things that you don't want to collect in memory.

Bugzilla

We discussed the use of a query language at length on the p2 meeting on November 9 and November 16. The issue is captured in bugzilla Create a QueryLanguage for p2.

Language Design

Predicate Queries

Queries can be written as simple predicates such as "id == $0". When executed, the predicate will be evaluated once for each row in the queried material. This form is used for the IMatchQuery implementation. The current value is available in a predefined variable named item. Normally, item does not need to be specified so that:

id == $0

is actually the same as:

item.id == $0

Context Queries

A query can also be declared as something that acts on "everything". This is particularly useful when things like search for latest or doing requirements traversal. These queries make use of a predefined variable named everything. When accessed, that variable returns an iterator over all queried material. As with item in a predicate query, everything is something that you don't normally need to specify, i.e.:

select(x | x.id == $0)

is the same as:

everything.select(x | x.id = $0)

A predicate query can always be written as a context query. These two queries are fully equivalent:

Predicate: id == $0
Context:   select(x | x.id = $0)

Operators with special meaning

• Matches operator
  

  A large amount of queries involve versions, version ranges, and capability matching. So managing that is important. Another thing that is important is to be able to support filtering based on ID's. All of this is now also built in to the language through the matches operator ~=. It can be though of as the 'satisfies' method on an installable unit or provided capability, as 'is included in' when used with a version and version range, as a plain matches when used with a string and a pattern, or as 'instanceof' when comparing with a class.

  The following things can be matched using the operator

  LHS	RHS	Implemented as
  IProvidedCapability	IRequiredCapability	lhs.satisfies(rhs)
  IInstallableUnit	IRequiredCapability	lhs.satisfies(rhs)
  Version	VersionRange	rhs.isIncluded(lhs)
  IInstallableUnit	Filter	rhs.matches(lhs.properties)
  Map	Filter	rhs.match(lhs)
  String	Pattern	rhs.matcher(lhs).matches()
  <any>	Class	rhs.isInstance(lhs)
  Class	Class	rhs.isAssignableFrom(lhs)

Collection functions

A number of functions was added to enable various manipulations of collections. A common way of writing collection functions is:

elements.someFunction(element | <a predicate expression>)

Here are the functions that are available in the p2QL language:

• Select
  The select function will evaluate its expression, once for each element, and return a new collection containing all elements for which the evaluation result was true. Example:

  select(x | x.id == 'org.eclipse.equinox.p2.ql')

• Reject
  The reject function is similar to select but instead, returns the elements for which the evaluation result was false.
• Exists
  The exists function will evaluate an expression for each element in the collection until an evaluation yields true. If that happens, the result of the whole evaluation yields true. If no such element is found, the whole evaluation yields false. Example:

  providedCapabilities.exists(p | p ~= $0)

• All
  The all function will evaluate an expression for each element until an evaluation yields false. If that happens, the whole evaluation yields false. If no such element is found, the whole evaluation yields true. Example:

  $0.all(rc | item ~= rc)

  Assuming $0 is a list of required capabilities, this query asks for items that fulfill all requirements.
• Latest
  Some queries must make sure that the result only contains the latest version of each found IU. The special function latest to makes that possible. The function will require that the collection elements implement the IVersionedId interface. Here is an example of how to use it:

  select(x | x.id = $0).latest()

  As a convenience, it is also possible to write:

  latest(x | x.id = $0)

• Limit
  It is sometimes desirable to limit the number of rows returned by a query. The function 'limit' can be used for that:

  select(...).limit(100)

• Unique
  This function will ensure that the resulting collection contains unique elements. The function can operate in two modes, with or without a cache argument. I.e.

  x.unique()

  or

  x.unique(cache)

  The latter expects the argument to be a java.util.Set that it can use to enforce the uniqueness of the element. This enables the result to be unique in a larger scope then the collection itself.
• Traverse
  A common scenario in p2 is that you want to start with a set of roots and then find all items that fulfill the root requirements. Those items in turn introduce new requirements so you want to find them too. The process continues until no more requirements can be satisfied. This type of query can be performed using the traverse function. The function will evaluate an expression, once for each element, collect elements for which the evaluation returned true, then then re-evaluate using the collected result as source of elements. No element is evaluated twice. This continues until no more elements are found:

  $0.traverse(parent | select(child | parent.requiredCapabilities.exists(rc | child ~= rc)))

  This is of course a fairly naive slicing mechanism. See #Currying below for more advanced examples.

Query parameters

The query must be parameterised so that expression parsing can be done once and then executed multiple times. A parameter is expressed as $n where n is either the parameter index, originating from 0, or an identifier. When identifiers are used, the parameters should be passed in a Map.

Java API

The expression tree created by the parser must be well documented and easy to use so that queries can be created programmatically. Since all expressions are immutable and without context, they can be combined freely. Hence, code like this is fully possible:

// Create some expressions. Note the use of identifiers instead of
// indexes for the parameters

Variable item = new Variable("item");
Expression cmp1 = new Equals(new Member(item, "id"), new KeyedParameter("id"), false);
Expression cmp2 = new Equals(new At(new Member(item, "properties"), new KeyedParameter("propKey")), new KeyedParameter("propValue"), false);

Variable everything = new Variable("everything");
LambdaExpression lambda = new LambdaExpression(new And(new Expression[] {cmp1, cmp2}), new Expression[0], new Variable[] {item});
Expression latest = new Latest(new Select(everything, lambda));
ContextExpression e3 = new ContextExpression(everything, latest);

// Put the parameters in a map
Map args = new HashMap();
args.put("id", "test.bundle");
args.put("propKey", "org.eclipse.equinox.p2.type.group");
args.put("propValue", "true");

// Create the query
IQuery query = new ExpressionQuery(e3, args);

IQuery examples

Here are some examples of query expressions:

Query for all IU's that has an id:

IQuery query = new PredicateQuery("id = $0", id);

Query for the latest IU of some specific id:

IQuery query = new ExpressionQuery("latest(x | x.id = $0)", id);

Query an artifact repository for all keys with a specific classifier:

IQuery query = new PredicateQuery(IArtifactKey.class, "classifier = $0", classifier);

Query for the latest IU that matches a specific version range. Since the second parameter is a VersionRange, the ~= operator is interpreted as isIncluded:

IQuery query = new ExpressionQuery("latest(x | x.id = $0 && x.version ~= $1)", id, range);

Query for an IU that has a specific property set:

IQuery query = new PredicateQuery("properties[$0] = $1", key, value);

The same query, but this time for multiple possible values:

IQuery query = new PredicateQuery("$1.exists(v | properties[$0] = v)", key, new Object[] { v1, v2, v3 });

Query for all categories found in the repository:

IQuery query = new PredicateQuery("properties[$0] = 'true'", IInstallableUnit.PROP_TYPE_CATEGORY);

Query for all IU's that fulfil at least one of the requirements from another IU. Since the first parameter is a list of IRequiredCapabilites, the ~= applied to each combination of provided / required capability is intepreted as satisfies.

IQuery query = new PredicateQuery("$0.exists(rc | providedCapabilities.exists(pc ~= rc))", iu.getRequiredCapabilities());
OR
IQuery query = new PredicateQuery("$0.exists(rc | item ~= rc)", iu.getRequiredCapabilities());

Query for the latest version of all patches:

IQuery query = new ExpressionQuery("latest(x | x ~= $0)", IInstallableUnitPatch.class);

Query for all IU's affected by a patch:

IQuery query = new PredicateQuery("$0.exists(rcs | rcs.all(rc | item ~= rc)))", patch.getApplicabilityScope());

The BNF

collectionExpression
	: memberExpression ( '.' collectionFunction )*
	;

memberExpression : constructor ( ( '.' ID ) | ( '[' memberExpression ']' ) )* ;

collectionFunction
	: ( select | reject | exists | all | traverse ) '(' lambdaDefinition ')'
	| limit '(' INT | parameter ')'
	| latest '(' ')'
	| unique '(' STRING? ')'
	;

constructor
	: ( filter | version | range | class ) '(' unaryExpression ')'
	| set '(' ( collectionExpression ( ',' collectionExpression )* )? ')'
	| unaryExpression
	;

lambdaDefinition
	: initializer ( ',' initializer )* ( ',' '{' lambda '}' )?
	| '{' lambda '}'
	| lambda
	;

initializer
	: '_'
	| collectionExpression
	;

lambda
	: ( ID ( ',' ID )* )? '|' collectionExpression
	;

predicate : andExpression ( '||' andExpression )* ;

andExpression : notExpression ( '&&' notExpression )* ;

notExpression
	: '!' notExpression
	| binaryExpression
	;

op : '=' | '!=' | '>' | '>=' | '<' | '<=' | '~=' ;

binaryExpression : collectionExpression ( op collectionExpression )?;

unaryExpression
	: '(' predicate ')'
	| '[' collectionExpression ( ',' collectionExpression )* ']' // #array construct
	| '/' regexpPattern '/'
	| STRING
	| INT
	| parameter
	| 'null'
	| 'true'
	| 'false'
	| ID
	;

parameter
	: '$' INT | ID
	;

The expression tree model