Seminar on Component-based Software Engineering

1. Seminar on Component-based Software Engineering Component Coordination Nadia Ramjiawan LIACS - Leiden University Fall 2005

2. Component • Every component instance contains one or more active entities (thread of control). • The active entities inside a component C can communicate with the active entities in other components only through I/O operations performed on the channel ends connected to C.

3. Channels • The only primitive medium of communication between two components. • A channel has exactly two ends. • A channel end is either: • Source: accepts data into its channel. • Sink: dispenses data out of its channel. • Example: ab represents the channel with source end a and sink end b. • Although every channel has exactly two ends, these ends can be of the same or different types (two sources, two sinks, or one source and one sink). • Various kinds of channel are possible, offering different synchronisation, buffering, lossy and even directionality policies. • Channel I/O Operations • Read/write data items from /to their channel ends.

4. Abstract Behavior Type (ABT) Model • An ABT defines an abstract behavior as a relation among a set of timed-data-streams, without specifying any detail about: • the operations that may be used to implement such behavior • or the data types it may manipulate for its realization. • A timed data stream is atwin pair of streams consisting of a data stream and a time stream with the interpretation that ∀i≥0, the data item appears at its corresponding time moment • Used to model the data flow through channel ends.

5. ABT Examples • Basic channels • Synchronous channel : • Asynchronous unbounded FIFO channel : • Asynchronous FIFO channel with the bounded capacity of 1:

6. ABT Examples (II) • Asynchronous FIFO channel with the bounded capacity of 1 and initially contain the data item D : • Synchronous drain channel: • Two source ends, cannot produce any data items. • Every data item is lost

7. Merge and Replicate • The merger ABT is defined as: • The replicator ABT is defined as:

8. Reo • The ABT model is too low level to provide non-trivial coordination, an effective exogenous coordination model is needed. • Reo is a channel-based exogenous coordination model wherein complex coordinators, called connectors are compositionally built of simpler ones. • The simplest connectors in Reo are channels. • A connector is a set of channel ends and their connecting channels organized in a graph of nodes and edges(channels) such that: • Zero or more channel ends coincide on every node. • Every channel end coincides on exactly one node. • There is an edge between two nodes if and only if there is a channel one end of which coincids on each of those nodes. • Powerful mechanism for connector construction: channel composition.

9. Nodes • Reo defines three types of nodes • Source nodes: Src(N) ≠ ø ∧ Snk(N) = ø • Src(N) and Snk(N): sets of source and sink channel ends coincide on N, respectively. • A source node replicates every data item written to it as soon as all of its coincident source channel ends can consume that data item • Sink nodes: Src(N) = ø ∧ Snk(N) ≠ ø • non-deterministically selects one of the data items available through its coincident sink channel ends • Mixed nodes: Src(N) ≠ ø ∧ Snk(N) ≠ ø • Combines the behavior of a sink node and a source node. • Every time a data item is available through its coincident sink channel ends it non-deterministically selects an appropriate data item and replicates that data item into all of its coincident source channel ends.

10. Nodes (II) • Node I/O operations: • read and take can be done only on sink nodes • write can be done on source nodes • They can not be done on mixed nodes • A component can write to a source node and read from a sink node only if it is connected to that node • Components don’t read from or write to mixed nodes.

11. Examples of connectors in Reo • Write-Cue Regulator • Channels ab and cd are of type Sync and ef is of type Syncdrain • Barrier Synchronizer • The four channels ab, cd, gh, ij are all of type Sync and ef is of type Syncdrain • Can be extended trivially

12. Example of connectors in Reo (II) • Ordering • The channels ab and ac are Syncdrain and Sync, respectively and bc is of type FIFO1. • Coordination pattern c = (ab)* • Sequencer • The enclosing box represents the fact that the details of this connector are abstracted away: the topology inside the box is immutable • Inside the connector, four Sync, a FIFO1(o), and three FIFO1 channels are connected together.

13. Utility of the Sequencer • Two-node sequencer • Coordination pattern c = (ab)* • Three-node sequencer • Coordination pattern c = (aab)*

14. Questions ?