Modeling & Notations

BPMN has four core element categories: Events — moments when something happens (start, intermediate, end); Tasks — the activities performed; Gateways — branching and merging points that route the flow; Flows — connectors that fix element order.

Common diagrams: business-process (BPMN), sequence (interaction over time), class (structure), ER model (data and relations), component (deployable parts), state machine (object states), and use case (actor goals).

The main notations are UML (structure and behaviour — class, sequence, state, use-case), BPMN (business processes — events, tasks, gateways, flows), and doc-as-code markup such as PlantUML, which keeps diagrams as text under version control.

A use case is a formalized description of how an actor interacts with the system to reach a goal. It names the actor and goal, lists the steps with the system's reactions, and structures them into a main (success) flow plus alternative flows and exceptions, with pre- and post-conditions. It captures the detailed «how» of an interaction, not just the wish.

AS-IS models the process as it works today; TO-BE models the target after the change. You draw AS-IS to agree on current reality, expose bottlenecks and hidden rules, and set a baseline that grounds the TO-BE in fact, not a guess.

Common notations are UML (structure and behavior), BPMN (business processes with flow), and DFD — the data-flow diagram for how data moves between stores. Choose by intent: BPMN for a process, UML for software structure, DFD for data movement.

A UML sequence diagram shows interaction ordered in time: participants are columns with vertical lifelines, and horizontal arrows are the messages between them, read top to bottom. It captures who calls whom, in what order, including responses and timeouts.

BPMN gateways: Exclusive (XOR) — exactly one path is taken; Parallel (AND) — all paths run together; Inclusive (OR) — one or more by condition; Event-based — the first event to occur picks the path; Complex — routing the others cannot express.

C4 has four zoom levels: Context (system in its environment), Container (deployable units — apps, services, DBs), Component (blocks inside), and Code (classes). An analyst mostly draws Context and Container; Component and Code are the developers'.

A DFD (data-flow diagram) models how data moves through a system. Its four elements are external entities, processes, data stores, and data flows. Unlike BPMN's time-ordered control flow, it shows where data originates, is stored, and is used.

A pool is one participant; lanes subdivide it into roles or systems. Message flows (dashed) cross pool boundaries; sequence flows (solid) stay in a pool. A second pool models an independent participant you do not control, like an external partner.

A user story is a brief need from the user's view — «As a <role>, I want <goal>, so that <value>» — the what and why. A job story frames it by situation. A use case is the detailed how: actor, goal, steps, alternative flows, exceptions.

By position BPMN 2.0 has start, intermediate, boundary, end events; by trigger each carries message, timer, error, or signal. An interrupting boundary event cancels the activity and routes its token to the exception flow, not a non-interrupting one.

EPC — the event-driven process chain — is a notation where functions and events strictly alternate. Branching uses AND, OR, XOR connectors. Versus BPMN it is simpler and event-centric, without pools; BPMN is a richer executable standard.

IDEF0 is a functional-modeling notation: each block is a function, and arrows attach to its four sides by role — Inputs, Controls, Outputs, Mechanisms, abbreviated ICOM. It is still mandated in Russian state and defence work via GOST standards.

Fragments wrap messages: alt (exclusive branches), opt (optional block), loop (repetition), par (parallel). An asynchronous call uses an open, thin arrowhead and does not block; a synchronous one uses a solid filled arrowhead with a return arrow.

It does not wait for a fixed count. A converging inclusive gateway inspects upstream process state and synchronizes only tokens that can still reach it — firing once none can. When the split fired two of three, the join merges exactly those two.

A token is a marker along sequence flows marking a process instance's active state; a parallel split emits one token per branch. An exclusive (XOR) join does not synchronize — it passes each token through, so the flow can complete more than once.

Draw client and service as lifelines with a synchronous call. Wrap retry-on-timeout in a loop bounded to three tries, with an alt inside separating success from timeout. Put the business error in its own alt branch that surfaces it, not looping.

Model payment as a receive task with an interrupting timer boundary event set to 15 minutes. If it fires, it cancels the task and routes the token to the cancel-and-notify path. Because it is interrupting, the two outcomes stay mutually exclusive.