CONNECTORS Ryan Mc Alister Introduction Integration and interaction

CONNECTORS Ryan Mc. Alister

Introduction �Integration and interaction �As important as developing functionality �More challenging decisions �Transfer control and data �Can also provide services �Persistence �Invocation �Messaging �Transactions

Introduction �Common misconception �Just calls between two components �Using large off-the-shelf components �Connectors are used to communicate �Wide range of connectors to use �Service request to named recipients �Broadcast to anyone listening �Suspend current processing �Allow component to continue processing

Introduction �Different roles connectors play �Different connector types available �Roles each one can fulfill �Variation points for each connector type �Hints and guidelines about connector’s �Applicablity �Strengths �Drawbacks

Connectors In Action �Application independent elements �How without the what �Abstraction and separation of concerns �New terminology showing up �Pipe = type of connector �Filter = component

Connectors In Action • High level view • Components A and B communicate via a Unix pipe • Doesn’t give us all the properties of the pipe • Pipe allows interaction of unformatted streams of data • Single sender single receiver • A’s task is to hand data to pipe • Actual recipient is unimportant • Send only once

Connectors In Action �Change to where B can talk back to A �Acknowledge data was received �Add a pipe from B to A �Keep trying until B receives the data �Add data buffering to pipe �Adding another component �Even more pipes �Causes substantial system downtime �Not the most effective solution

Connectors In Action �Change from unformatted byte stream to discrete, typed packets �Pipes will not work �Use an event bus connector �Similar properties �Loose component coupling �Asynchronous communication �Data buffering �Differences �Event bus is better suited for system adaptation

Connectors In Action

Connector Foundations �Building blocks of connectors �Managing the flow of control �Changing the processor program counter �Managing the flow of data �Performing memory access �Channels or ducts �Link interacting components �Ducts don’t provide additional interaction services �Simple connectors just form ducts between components �Others augment ducts

Connector Foundations �Simple connectors �Implemented in programming languages �One type of interaction service �Composite connectors �Several connectors and possibly components �Provided as libraries and frameworks �Combine many kinds of interactions

Connector Foundations Framework Explantiation � Category �Broad interaction role � Type �How interaction services are realized � Dimensions �Architecturally relevant details � Values �Set of values dimensions can take � Species �Particular connector instance

Connector Roles �Four general classes of services �Communication �Coordination �Conversion �Facilitation �Connector will provide one or more of these services �Category level of Figure 5 -3

Connector Roles Communcation �Communication services �Transmission of data among components �Examples �Pass messages �Exchange data �Communicate results

Connector Roles - Coordination �Coordination Services �Supports transfer of control among components �Interact by passing the thread of execution �Examples �Function calls �Method invocations

Connector Roles - Conversion �Conversion Services �Transform the interaction �Takes information from one and formats it to where the other can use it �Fixes mismatches caused by incompatible assumptions �Type, number, frequency and order of interactions with other components �Allow components that haven't been tailored for each other conduct interactions �Examples �Conversion of data formats �Wrappers for legacy components

Connector Roles - Facilitation �Facilitation services �Mediate and streamline component interaction �Reduces interdependences among interaction components �Examples �Load balancing �Scheduling services �Concurrency control

Selecting Appropriate Connectors �Perform these steps 1. Select the specific set of interacting components � Different sets can have different interaction needs � Focus solely on the components needed for connector 2. Determine the interaction services needed � Identify the precise characteristics of the components interaction � Study the components architectural description

Selecting Appropriate Connectors 3. Determine 8 connector types that will provide services needed � Based on identified interaction services 4. Evaluate each connector type � Study these connectors dimensions, subdimensions, and values � Eliminate any types that would be deemed suboptimal

Selecting Appropriate Connectors 5. For the remaining connector types � Set the values for the necessary dimensions and subdimensions � Identify the best or most natural connectors � � Perform a trade-off analysis Possibly choosing a composite connector.

Selecting Appropriate Connectors �Using values of dimensions from different connector types leads to a composite connector species �Creating unprecedented composite connectors is not easy �Requires deep understanding of the connectors’ complementary, orthogonal, and incompatible characteristics �Could become misguided, suboptimal or completely ineffective

Detecting Mismatches

Detecting Mismatches �Four rules for combining connector dimensions �Requires �Cautions �Restricts �Prohibits

Detecting Mismatches - Requires �Requires states that the choice of one dimension in one connector species mandates that another dimension be selected in another connector species. �For example if a distributor and an adaptor connector are composed �Distributor’s delivery requires that the adaptor support presentation conversion �Requires is a chaining rule and is used as a starting point. �An event connector the require delivery semantics also needs a notification dimension, which in turn requires cardinality, synchronicity and mode. �Mandatory dimensions are bold, optional

Detecting Mismatches - Cautions �Cautions rule indicates that certain combinations of values for two connector dimensions that are required to be used in tandem, while valid, may result in an unstable or unreliable connector. �For exam ple, a component being invoked implicitly should not have multiple entry points since an implicit invocation mechanism cannot choose among the entry points.

Detecting Mismatches – Restricts �Restricts rule indicates that the two dimensions are not require to be used together at all times, and that there are certain combinations of their values that are invalid. �For example, thread –specific data access cannot use heavy-weight concurrency

Detecting Mismatches - Prohibits �Prohibits rule is used to exclude any combination of two dimensions from being used and indicates total incompatibility of the dimensions. �For example, stream delivery cannot be built on transactional atomicity �Relatively few instances of prohibits

Detecting Mismatches �We’ve only discussed binary combinations of connector dimension, but the compatibility relations between dimensions are transitive. �We could apply the rules to determine n-ary compatibility between dimensions.