Transaction in GPSS

A transaction is a dynamic entity in GPSS that moves through the block diagram representing temporary objects flowing through the system (such as customers, jobs, messages, etc.).

• Transactions are created by the GENERATE block and destroyed by the TERMINATE block
• They represent the units of traffic that flow through the model
• Each transaction has attributes called parameters that carry information
• Transactions move from block to block, competing for system resources like facilities and storages
• They are placed on chains (Current Events Chain and Future Events Chain) to manage scheduling

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Facility in GPSS

A facility is a system resource in GPSS that can be used by only one transaction at a time, representing single-server entities like a barber, a machine, or a counter.

• A facility has two states: busy or idle
• A transaction captures a facility using the SEIZE block
• A transaction releases a facility using the RELEASE block
• If a transaction tries to SEIZE a busy facility, it is placed in a delay chain (queue)
• GPSS automatically collects statistics for facilities: utilization, average time per transaction, number of entries

Key Statistics Collected:

• Utilization — fraction of time the facility is busy
• Entries — number of transactions that used the facility
• Average time/transaction — mean time a transaction held the facility

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Barbershop Problem — GPSS Model

Problem Statement:

• Customers arrive every 15±3 minutes (uniform distribution)
• Haircut takes 18±2 minutes
• Simulate for 9 hours = 540 minutes

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GPSS Block Diagram (Described)

GENERATE 15,3         → Customers arrive every 15±3 min
      ↓
QUEUE BARBERQ         → Join the waiting line
      ↓
SEIZE JOEY            → Capture the barber (facility)
      ↓
DEPART BARBERQ        → Leave the waiting line
      ↓
ADVANCE 18,2          → Get haircut (18±2 min)
      ↓
RELEASE JOEY          → Free the barber
      ↓
TERMINATE 1           → Customer leaves the system

Timer Segment:

GENERATE 540          → Generate one transaction after 540 min
      ↓
TERMINATE 1           → Stop simulation

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GPSS Code

* Joey Barbershop Simulation
* Simulation time: 9 hours = 540 minutes

        SIMULATE

* Customer Segment
        GENERATE    15,3          ;Customers arrive every 15±3 min
        QUEUE       BARBERQ       ;Enter the queue
        SEIZE       JOEY          ;Capture barber facility
        DEPART      BARBERQ       ;Leave the queue
        ADVANCE     18,2          ;Haircut takes 18±2 min
        RELEASE     JOEY          ;Release the barber
        TERMINATE   0             ;Customer exits system

* Timer Segment
        GENERATE    540           ;Timer transaction at 540 min
        TERMINATE   1             ;Decrement TG1 counter

* Control Statement
        START       1             ;Run until 1 termination from timer
        END

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Explanation of Key Blocks

Block	Purpose
GENERATE 15,3	Creates transactions (customers) at mean 15 min, spread ±3 min
QUEUE BARBERQ	Collects queue statistics (waiting time, queue length)
SEIZE JOEY	Transaction captures the single-server facility "JOEY"
DEPART BARBERQ	Removes transaction from queue statistics collection
ADVANCE 18,2	Holds transaction for service time (18±2 min)
RELEASE JOEY	Frees the facility for the next transaction
TERMINATE	Removes transaction from the model
START 1	Runs simulation until termination counter reaches 1

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Conclusion

This model uses one facility (JOEY) representing the single barber. Since service time (18 min avg) exceeds inter-arrival time (15 min avg), we expect high utilization and queue buildup. The simulation runs for exactly 540 minutes (9 hours), after which GPSS automatically generates statistics on facility utilization, queue length, and waiting times.