SSF API reference manual
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1. argument to mapto e Finally a particular event transmission may be given an additional per write delay on each write to the channel specified using the optional second argument to write These three sources of delay are independent additive immutable and equal to zero by default The event will be delivered at the time it was sent plus the minimal channel delay if any plus the minimal mapping delay if any plus the per write delay if any SSF API Reference Manual 7 8 3 Receiving Events The event that is delivered is guaranteed to be equal to but not storage identical with the event that was written For these purposes equal is defined by the modeler in the form of a mandatory copy constructor which must appear in every Event derived class See section 5 1 for a detailed discussion of event storage management A process may receive events from an inChannel if and only if the owner of the receiving process and the inChannel are identical or coaligned To receive events from an inChannel J a process may issue an activeEvents statement Event E_set Event E_set I activeEvents Java gt activeEvents C In C this set of events will be null terminated In Java the built in array size operator returns the received event count In either case the storage for both the array and the delivered events themselves belongs to the framework and may be reclaimed by the framework after last eve2. called more than once on the same event the framework may not release the event s storage until a matching number of release calls have been called on the event Save returns a pointer to the saved event this event is guaranteed to be value identical to the original event as defined by the modeler s copy constructor but may occupy different storage SSF API Reference Manual 4 5 2 Event Aliasing An entity s aliased method shall return true if it may be accessible to processes of multiple noncoaligned entities This aliasing condition may arise for example when an event has been written between a noncoaligned sender and receiver or written to multiple noncoaligned recipients The results of modifying the state of an event whose aliased method returns true is undefined 6 Entities An Entity is an Object that can own processes and channel endpoints and can be aligned with other Entities 6 1 Ownership Each entity is said to own zero or more processes inChannels and outChannels The base class constructors for the process inChannel and outChannel classes each take one argument of type Entity specifying the owner of the process inChannel or outChannel instance This ownership is immutable The lists of processes inChannels and outChannels owned by an entity are returned by the processes inChannels and outChannels methods of Entity 6 2 Alignment Fach entity is aligned to some implementation dependent obje
3. process Entity theowner int simple 0 process Entity theowner void Entity body process int simple 0 virtual void action virtual void init virtual boolean isSimple Entity owner void waitOn inChannel waitchannels void waitOn inChannel waitchannel j void waitForever void waitFor ltime_t waitinterval SSF API Reference Manual 14 boolean waitOnFor inChannel waitchannels ltime_t timeout class outChannel public Entity owner inChannel mappedto null terminated void write Event evt ltime_t delay 0 ltime_t mapto inChannel tgt ltime_t mapping_delay 0 ltime_t unmap inChannel tgt class inChannel public Entity owner Event activeEvents null terminated outChannel mappedto null terminated hy SSF API Reference Manual 15 Appendix C Sequential and Simple Sequential SSF The Scalable Simulation Framework API was designed with high performance implementation in mind that is scalability to very large problem sizes by utilization of parallel processing resources At the same time the SSF directives that control model concurrency alignment alignto makeInde pendent can be treated as pure annotations in a single processor environment That is an SSF imple mentation can ignore these statements altogether while maintaining compliance Such an implementation is said to implement Sequential SSF T
4. 11 method allows the simulation to be paused gracefully after some implementation dependent delay A call to pauseA11 blocks until the simulation has paused and then returns the current simulation time After pauseAl11 acall to resumeA11 resumes the simulation s forward progress At any point a call to joinAl1 e g from main will block without possibility of pause or resumption until simulation execution is complete 4 6 Framework concurrency Each entity is said to be aligned to some object cf 6 2 This alignment target whose exact type is implementation dependent represents a common scheduling authority in parallel discrete event simulation parlance a logical timeline for the processes of coaligned entities each set of coaligned entities is said to constitute an alignment group Processes that are eligible to execute at each instant of simulation time will be scheduled by the frame work sequentially within an alignment group but in implementation dependent order and concurrently across alignment groups Put another way the framework transparently manages any required synchroniza tion to support in order event exchange among the entities aligned to a set of potentially concurrent logical timelines 4 7 Simulation time The behavior of an SSF model is defined by the collective actions of its processes on its state Every process resumption takes place at a particular simulation time and each modification of mode
5. SSF API Reference Manual 1 SCALABLE SIMULATION FRAMEWORK API REFERENCE MANUAL VERSION 1 0 DOCUMENTATION DRAFT REVISION MARCH 25 1999 James H Cowie Editor 1 Introduction This reference manual specifies the detailed syntax and semantics of the Java and C bindings of version 1 0 of the Scalable Simulation Framework Application Programming Interface SSF API and is intended as a resource for SSF implementors and expert modelers Modelers seeking an introduction to the use of SSF for constructing discrete event simulations may prefer to start with the companion document the SSF User Manual SSF provides a single unified interface for discrete event simulation the SSF API Object oriented models that utilize and extend the framework can be portable across SSF compliant simulation environ ments This maximizes the potential for direct reuse of model code while minimizing dependencies on a particular simulator kernel implementation In addition to its concrete modeling applications the API also functions as an abstract target for compilation of models specified in higher level modeling languages or graphical modeling environments The framework s primary design goal was to support high performance simulation SSF makes it pos sible to build models that are efficient and predictable in their use of space able to transparently utilize parallel processor resources and scalable to very large collections of simulated entities 2 F
6. ch calls only simple functions By default a process s isSimple method returns false It may be overridden by the modeler in derived classes to return true if and only if the act ion method of the process is a simple function SSF API Reference Manual 6 7 4 Process Scheduling Processes owned by entities belonging to different alignment groups may execute concurrently Within an alignment group the framework shall implement appropriate process scheduling policies and write buffering strategies to guarantee fairness Fairness is defined by the requirement that no process may execute twice at simulation time T or affect any other process by means of an event written with zero delay before all other eligible processes have executed once The process scheduling policy shall further ensure that if multiple processes are eligible to run at simulation time T all processes that suspended via waitFor shall execute before any process that suspended via waitOn or waitOnFor 8 inChannels outChannels Like a process both inChannel and outChannel take a reference to an owner entity as their first constructor argument 8 1 Mapping Channels Associated with each outChannel is a list of zero or more inChannels to which the outChannel is currently mapped Associated with each inChannel is a list of zero or more outChannels that are currently mapped to the inChannel The maximum size of these mapping lists is implementation dependent Execu
7. ct that represents an align ment group the default alignment of an entity for which no alignment is specified is implementation dependent as is the maximum number of distinct alignment groups that can be created 6 3 Realignment An entity receives an overriding alignment when its alignto ormakeIndepen dent methods are called e The alignto method takes one argument a reference to the entity whose alignment serves as the new alignment target and realigns this entity to the argument entity effective at the return value timestamp e The makeIndependent method takes no arguments If the creation of a new alignment group would exceed the maximum number of distinct alignment groups supported by the framework makeIn dependent has no effect and returns now otherwise it realigns this entity to a new unique alignment group effective at the return value timestamp Both forms of realignment are subject to the following rules e Realignments may not be issued during entity or process construction e Realignments issued during the initialization via init of a process or entity are always immedi ately effective and shall return the simulation start time e Realignments issued during the execution of a process via action or code called by action shall return a timestamp greater than or equal to now the exact value of the timestamp returned is implementation dependent The modeler must wait until the specifi
8. ed time has arrived to take any action whose correctness depends on the new alignment Finally it is an error to realign an entity if the realignment would result in the creation of channel mappings with zero minimal delay and zero mapping delay between noncoaligned entities SSF API Reference Manual 5 7 Processes 7 1 Construction and Initialization A process may not be created by another process unless both pro cesses are owned by coaligned entities The process constructor takes as an argument a reference to the owner entity the framework will invoke the process s init method for post construction initialization at some point after the owner entity s init method has been called and before the new process has run for the first time The new process becomes eligible to run at the current simulation time Each process specifies a callback method action that the framework executes one or more times during the simulation This code consists of arbitrary computation that takes up zero elapsed simulation time interspersed with one or more wait statements that take up non negative amounts of simulation time In the C API a process may borrow a callback method from its owner entity specified as the second constructor argument and the default action will be to call this method passing it a pointer to the process Such a process may not override action 7 2 Wait Statements A process s wait methods may not be ca
9. his appendix reinterprets the general SSF specification in light of the simpler requirements of a se quential SSF implementation for the convenience of implementors and modelers C 1 Sequential SSF In sequential SSF a model contains exactly one alignment group which serves as the default alignment target for all entities The alignto and makeIndependent methods of Entity have no effect and return a timestamp equal to now Similarly the mapto and unmap operations will always take effect immediately and return a timestamp equal to now C 2 Simple Sequential SSF For highest sequential performance sequential SSF implementations may additionally mandate the exclu sive use of simple processes 7 3 Such an implementation is said to implement Simple Sequential SSF C 3 Anticipating Parallelism Any process in a sequential SSF model may freely access and modify any accessible part of the model s state However modelers must recognize that such direct access can make future parallelization significantly more difficult Sending and receiving events on channels is a higher overhead form of interaction among processes but lends itself better to subsequent parallelization of the model Avoiding the use of channels with zero inherent minimal delay is another strategy that anticipates future parallelism Finally sequential SSF models shouldn t ignore the timestamps returned by alignto makeIn dependent mapto and un
10. ic Entity owner ublic void action ublic void init ublic void waitOn inChannel waitchannels ublic void waitOn inChannel waitchannel ublic void waitForever ublic void waitFor ltime_t waitinterval ublic boolean waitOnFor inChannel waitchannels ltime_t timeout ublic boolean isSimple oO O S 09 S T S O public interface outChannel public Entity owner public inChannel mappedto public void write Event evt ltime_t delay public void write Event evt public ltime_t mapto inChannel tgt public ltime_t mapto inChannel tgt ltime_t mapping_delay public ltime_t unmap inChannel tgt SSF API Reference Manual 13 public interface inChannel public Entity owner public Event activeEvents public outChannel mappedto A 2 C Binding define Object void class Event public Event save void release boolean aliased class Entity public virtual void init 0 ltime_t now void startAll ltime_t t1 void startAll ltime_t t0 ltime_t t1 ltime_t pauseAll void resumeAll void joinAll Object alignment ltime_t alignto Entity s Entity coalignedEntities null terminated process processes null terminated inChannel inChannels null terminated outChannel outChannels null terminated class process public
11. ized they become eligible to run at the start time their actual execution is controlled by the framework which executes the process s action call back method Every time the action method returns to the caller the framework executes it again immediately as many times are are necessary to reach the end of the simulation Within the action method or within code called by action the process may elect to wait for a condition to be met it will be suspended by the framework and resumed when the condition has been met 4 4 Framework Inner Loop The SSF framework provides nonpreemptive scheduling of processes Once resumed by the framework in order to allow the simulation clock to advance processes must eventually suspend themselves by issuing waitOn or waitFor calls All computation performed during a single resumption of a given process takes place simultaneously that is at the same simulation time Within the framework simulation time advances at a possibly variable rate determined by the arrival of events on channels and the duration of waitFor statements When simulation time exceeds the end time specified in the original st artAll1 call the simulation ends See section 7 for a more extensive discussion of processes SSF API Reference Manual 3 4 5 Start Pause Resume and Join The startAll pauseAll1 resumeAll1 and joinAl11 methods may not be called from within process code The pauseA
12. l state by that resumption of the process takes place at that time The SSF framework guarantees that from the perspective of any single process event based interaction with other processes takes place in monotone nondecreasing timestamp order That is a process executing at time T will never receive an event whose scheduled delivery time is less than time 7 The SSF framework cannot make similar guarantees for direct modification of model state by processes A process may not directly access model state that may be modified by non coaligned processes that is processes belonging to entities in other alignment groups An SSF model in which a process accesses model state that may be modified by non coaligned entities has the potential for out of logical order interaction and is therefore undefined 5 Events Modelers may create arbitrary derived classes from the event base class provided by the SSF framework All derived event classes must provide their own public copy constructor 5 1 Event References Save and Release Event storage management is the responsibility of the frame work which may release the storage of an event at any point after its last recipient process has suspended unless the modeler has explicitly acquired a reference to it using the save method and not released the reference using a matching call to release The save method takes as its single argument a pointer to a written or received event Save may be
13. lementations after making language specific modifica tions e g to replace Java references with C pointers Where possible modelers should avoid specific host language features that restrict cross language portability such as multiple inheritance pointer arith metic or dynamic type inspection 4 Simulation Model 4 5 Start Pause Resume and Join Calling these from within a process body would cause the simulation to deadlock These are methods for external control of a simulator e g from a tool such as a console If a simulation component wishes to signal the need for a pause e g because an error count has exceeded a significant theshold it can set a flag or execute a callback to external code to make the condition visible These are instance rather than static methods in order to allow multiple simultaneous simulations to exist within the same address space 4 7 Simulation time A process may freely modify and or access its own state and the state of the entity that owns it and the state of all entities which are known to be coaligned with that entity and the state of all processes inChannels and outChannels that are owned by those coaligned entities The modeler is solely responsible for writing processes that test coalignment and access shared state safely SSF implementations need not protect against or even detect unsafe access to shared state If in doubt a process must send and receive query events rather than accessi
14. lled from any context other than the ac tion or init methods of the process itself or in code called by those methods 7 2 1 waitFor time_tT Suspends the process until T ticks of the logical clock have elapsed The interval T must be of greater than zero duration 7 2 2 waitForever Suspends the process without the possibility of resumption 7 2 3 waitOn e waitOn inChannel I Suspends the process until an event arrives on channel I e waitOn inChannel Iset Suspends the process until an event arrives on some channel in the pro vided channel set e waitOn Suspends the process until an event arrives on some channel in the default channel set if one has been specified during initialization using waitsOn If no default channel set has been specified suspend the process without the potential for resumption If the waitsOn declaration is executed more than once per process the most recent call determines the default channel set 7 2 4 boolean waitOnFor inChannel Iset time_t T Suspend the process until an event arrives on some channel in the provided channel set or until T ticks of the logical clock have elapsed whichever comes first returning true if the wait timed out and false otherwise The interval T must be of greater than zero duration 7 3 Simple Processes A simple function is defined as one that returns control to the caller imme diately following each waitOn waitFor or waitOnFor statement and whi
15. map Correct execution of the model in the presence of future parallelism will require that the modeler wait until the effective timestamp to make use of the resultant alignment or channel mapping As a low overhead rule of thumb have model code verify that the return timestamp is equal to now the failed assertion will flag a component that was designed for sequential operation but used in a parallel context SSF API Reference Manual 16 Appendix D Summary of Implementation Dependencies 10 Results of more than one call to startAl1l1 4 1 Exact order and concurrency of entity process initialization 4 2 Timestamp returned by pauseAll 4 5 Order of resumption of simultaneously eligible processes 4 6 Default entity alignment strategy 6 2 Exact type of the object returned by alignment 6 2 Maximum number of distinct alignments 6 2 Timestamp returned by dynamic realignment 6 3 Maximum multicast channel mapping count 8 1 Fate of undelivered events on an unmapped channel 8 4
16. ng entity state directly Event exchange is the modeler s only available mechanism for coordinated interaction among non coaligned processes Simple wallclock synchronization exclusion e g via the synchronized keyword in Java is not sufficient to provide logical time synchronization and avoid logical time causality errors Note that though access to state modified by a non coaligned process is restricted non coaligned pro cesses may share access to read only data for example initialization data held in common Similarly noncoaligned processes may interact with systems outside the model altogether such as debuggers and visualizers as long as the external system is aware of the potential for out of order updates and doesn t propagate that information back into the model 5 Events 5 1 Event References Save and Release Of the following three variants of event buffering code only the last is a legal use of reference saving Event eset I gt activeEvents for n 0 eset amp amp eset n n buffer gt addElement eset n ILLEGAL Event not saved waitOn I SSF API Reference Manual 9 for n 0 eset amp amp eset n n set n gt save Oops ignored return value buffer gt addElement eset n ILLEGAL may be different storage waitoOn I for n 0 eset amp amp eset n n buffer gt addElement eset n gt save LEGAL Save a referenc waitoOn I The fac
17. nt receipt unless the modeler has specified otherwise via save release section 5 1 Event receipt is nondestructive One some or all of the processes of an entity are eligible to receive each event received on each inChannel of that entity Each process shall be given the opportunity to receive exactly once each of the events scheduled for delivery on all coaligned inChannels in the current instant of simulation time A process that explicitly advances in simulation time via waitFor forfeits the right to receive any events still pending on coaligned inChannels If no process receives an event at its time of delivery the event vanishes it is not buffered by the framework for later redelivery 8 4 Unmapping channels The unmap operation allows a channel mapping to be broken returning a simulation timestamp equal to or greater than now at which the unmapping will be effective Delivery of pending events on a newly unmapped channel is implementation dependent It is not an error to write an event to an unmapped channel or to wait on input from an unmapped channel in the first case the event simply disappears and in the second the process blocks as if the channel were mapped but no events were available SSF API Reference Manual 8 Appendix A Commentary 3 Terminology SSF compliant models should be portable between same host language implementations of SSF without modification or across different host language imp
18. oes not itself provide 4 Simulation Model The SSF host language is the language in which the user writes SSF model code for example Java or C An SSF model is a conformant program in the host language which at some point instantiates one or more entities processes and channel endpoints cf sections 6 7 8 The modeler defines the behavior of entities by associating with them processes that are to be executed on their behalf 4 1 Starting the Simulation A simulation starts when any entity s startAl1 method is called com monly from the main routine or the equivalent in the local environment The results of any subsequent calls to the startAl11 of any entity are implementation dependent The caller of startAl1 speci fies two timestamps the simulation s start time which defaults to 0 and the end time The simulation will execute over this interval of simulation time inclusive of the endpoints 4 2 Initialization After startAl1 has been called but before any process has started executing the framework calls the init routines of all processes and entities The exact order of initialization is implementation dependent and may be concurrent with the exception that no process will be initialized until the entity which owns it has completed initialization The Entity method now shall return the simulation start time for the duration of initialization 4 3 Process Execution After all processes have been initial
19. on the state of the owner entity to carry out its behavior In Java processes constructed as anonymous inner classes of entities can refer transparently to the members and methods of the enclosing class In C borrowing an Entity method as a process body gives the same effect without requiring the modeler to write owner gt foo everywhere access to the owner s state is required 7 3 Simple Processes The isSimple method is an advisory mechanism that allows the author of a process to assert that action returns control to the framework immediately after any wait significant performance improvements can result as the body of such a process may be executed on the simulator stack rather than in its own dedicated thread Not all implementations will provide this optimization Asserting simplicity incorrectly will result in undefined behavior including simulation deadlock It can be detected and flagged as an error two successive wait statements executed sequentially in the same execution of action signals the incorrect assertion but only after an arbitrarily long interval of time has elapsed This error is therefore nonrecoverable SSF should crash Users are encouraged to debug code without making this assertion and then use it selectively to improve performance SSF API Reference Manual 11 8 inChannels outChannels 8 1 Mapping Channels As with entities alignto method mappings created before the simulation starts will al
20. ramework Summary The SSF syntax comprises five base class interfaces Event Entity inChannel outChan nel and process The following sections describe the common semantics of compliant implemen tations of these classes Appendix A supplies some additional observations and interpretations that may be useful in understanding the specification Java and C class bindings are specified in Appendix B Finally Appendix C gives a summary of SSF implementation dependencies 3 Terminology e Implementation dependent means that the behavior of the framework may vary among SSF imple mentations e An SSF compliant implementation is one that provides appropriate semantics for each method of each SSF class as described in this document An SSF compliant implementation may not extend the SSF API with additional non standard methods or classes except through object oriented derivation of new classes that extend the SSF base class interfaces without modification to their semantics and without requiring particular implementation dependent semantics of those base classes Such a package of classes is said to constitute an SSF extension SSF API Reference Manual 2 e An SSF compliant model is one that extends or instantiates the framework s classes without depend ing on any particular outcome of an implementation dependent behavior e A pure SSF model is an SSF compliant model that does not depend on any SSF extensions for correct operation that it d
21. t that save may return a different pointer than the original saved event enables better storage management by the framework For example an implementation might call the copy constructor and return a pointer to the copy electing to free the storage of the original event Another implementation might do a bitwise copy of the saved event to a new location to minimize heap fragmentation and return a pointer to the new location of the same event 5 2 Event Aliasing Note that the aliased method only detects aliasing caused by the framework s implementation dependent copy strategy in the presence of events deliverable within multiple timelines It need not indeed cannot in general detect storage level aliasing within an alignment group The intent is to avoid causality violations among noncoaligned processes rather than simpler conflicts among coaligned processes read after write hazards among non coaligned entities can easily result in information travelling backwards through simulation time 6 Entities 6 1 Ownership In SSF entities rather than processes are the quantum of mobility Because ownership is immutable the processes and channel endpoints owned by the entity travel with it when it is explicitly realigned 6 2 Alignment Coalignment of entities affects model correctness in several significant ways First only coaligned entities may access or modify each others state directly rather than indirectly by means of e
22. ting O mapto I creates anew mapping between outChannel O and inChannel J The mapto method returns a simulation time T at which the mapping becomes effective If mapto would cause the source or target list of mappings to exceed the maximum length defined by the implementation the mapping will fail returning an effective mapping time that is greater than the end of simulation time It is an error to create a channel mapping whose minimal channel delay and minimal mapping delay are both zero between noncoaligned entities 8 2 Writing Events A process sends an event on outChannel O with optional per write delay D by issuing a write statement O write E D A process may write to any outChannel that it can access via normal program scope as long as the owner entity of the process and channel are either identical or coaligned When an event is written on a channel its receive time the time at which it will be made available to the recipient is greater than or equal to its send time the time at which it was written Receive time is greater than send time when delays are present there are three ways of specifying these delays e An outChannel may have an inherent minimal channel delay associated with it when it is constructed by supplying a nonzero delay as the second constructor argument e A particular mapping between an outChannel and an inChannel may have an additional minimal mapping delay specified using the optional second
23. vent exchange 4 7 Second processes may only wait for events to arrive from inChannels associated with coaligned entities 8 3 and may only create new processes associated with coaligned entities 7 1 Finally channel mappings whose endpoints are associated with non coaligned entities must have positive minimal delay 8 1 6 3 One common implementation of default alignment will create a pool of P timeline objects where P is the maximum sensible degree of available concurrency such as the physical processor count The default alignment for new entities may then be chosen round robin from this pool of alignments overridden by the modeler if desired In a sequential implementation there may be only one default alignment common to all entities thus making explicit alignto andmakeIndependent methods purely annotative 6 3 Realignment MakeIndependent allows the modeler to fork a single logical timeline into two allowing the modeler to partition a set of coaligned entities into two sets using make Independent followed by suitable use of alignto Note also that alignto is not dynamically transitive SSF API Reference Manual 10 A alignto B B alignto C A not necessarily aligned with C E alignto F D alignto E D is now aligned with F E alignto G D is still aligned with F Think of alignto asa tool for adjusting alignment set memberships rather than creating a durable relationship bet
24. ways return now that is they become effective immediately Dynamic mappings will usually return a future timestamp forcing the modeler to wait for that time to arrive before making use of the new mapping 8 3 Receiving Events An SSF implementation may wish to warn the modeler when events disappear for want of an interested receiving process This is sometimes done by design but is more often unintended and can be hard to debug 8 4 Unmapping Channels SSF implementations may wish to warn modelers about writes to and reads from unmapped channels Once again this is unintended more often than not and may signal a hard to diagnose error condition for example in a protocol implementation SSF API Reference Manual Appendix B SSF API A 1 Java Binding public interface Event public Event save public void release public boolean aliased public interface Entity public ltime_t now ublic void startAll ltime_t t1 ublic void startAll ltime_t t0 ltime_t t1 ublic ltime_t pauseAll ublic void resumeAll ublic void joinAll ublic ltime_t alignto Entity s ublic Object alignment ublic java util Vector coalignedEntities ublic void init ublic java util Vector processes ublic java util Vector inChannels P P P P P P P P P P P P i ublic java util Vector outChannels public interface process publ
25. ween two entities If your parents are Republicans and you choose to align yourself with your parents ideology you re a Republican If they later choose to become Communists this doesn t automatically change your ideology you re presumably still a Republican until you explicitly change party alignments There s no way to convert an entire family of registered voters to Communism without explicitly re registering each member of the family Stated another way when creating compositional entity trees modelers should take pains to align the parent container entity before constructing and then aligning its children contained entities A com mon way to facilitate this ordering is to pass a reference to the parent into the constructor of the children so that they can align themselves to it accordingly before creating their own children It s the modeler s responsibility Static alignments will always return now that is they become effective immediately Dynamic align ments may return now but more frequently will return an implementation dependent future timestamp as with mapto see section 8 1 Modelers must take this into account e g by waiting for the time to arrive before making use of the new alignment 7 Processes 7 1 Construction and Initialization Allowing a C process to borrow an Entity method for its body makes it easier to express process behavior cleanly Typically a process refers to and acts up
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