Sequence Diagram Generator for UML Message Flows

Core elements: lifelines, messages, and fragments

• Actors and participants: an actor (a stick figure) is a user or external system, while a participant is an object, service, or component drawn as a labeled box at the top of a lifeline.
• Lifelines: the dashed vertical line under each participant representing its existence through time.
• Synchronous messages: a solid line with a filled arrowhead — the caller waits for a response (a normal method call or blocking request).
• Return messages: a dashed line with an open arrowhead carrying the reply back to the caller.
• Asynchronous messages: a solid line with an open arrowhead — the sender does not wait (events, queued messages, fire-and-forget calls).
• Activation (execution) bars: the thin rectangles on a lifeline showing when a participant is actively processing a message.
• Combined fragments: boxed regions for control flow — loop for repetition, alt for either/or branches, and opt for an optional step that may be skipped.

Sequence diagram vs flowchart

A flowchart shows the steps of a single process or algorithm — decisions, actions, and the path through them — usually from one actor’s point of view. A sequence diagram instead shows how several participants collaborate: the emphasis is on the messages exchanged between lifelines and their exact ordering over time. Reach for a flowchart when you are modeling logic inside one component, and a sequence diagram when the interesting part is the conversation between components — which service calls which, what comes back, and what happens in parallel. The two are complementary, and many designs use a flowchart for internal logic and a sequence diagram for the cross-service call flow.

Common uses: API, auth, and system interactions

• API call flows: trace a request from client through gateway, services, and database, with each response drawn as a return message.
• Login and authentication flows: model password checks, two-factor OTP, OAuth 2.0 authorization-code handshakes, and JWT issuance step by step.
• Microservice interactions: show synchronous service-to-service calls alongside asynchronous events published to a broker like Kafka.
• Distributed and real-time systems: capture WebSocket delivery, pub/sub fan-out, retries, and fallback paths for offline or failed cases.
• Documentation and review: a sequence diagram makes a design reviewable — reviewers can see ordering and error branches at a glance.

Free sequence diagram tool: how to start

You do not need to learn a diagram language to create a sequence diagram online. Start free with 2 image credits, valid for 7 days, no card required, then describe your flow in the box above. Because the input is plain English rather than Mermaid or PlantUML syntax, you can paste a paragraph from a design doc or ticket and get a first draft to review. Generated diagrams are images, so check every participant name and message order against your design before you share it.

API sequence diagram example, step by step

• List the participants in call order: Client, API Gateway, Auth Service, Database.
• Write each request as "A sends X to B" so the arrow direction is explicit.
• Say what comes back: "returns a token" becomes a dashed return arrow.
• Name conditions in words: "if the payment is declined, return 402" becomes an alt fragment.
• Mark fire-and-forget work as asynchronous: "publish an OrderPaid event without waiting" becomes an open-arrowhead message.

How to generate a sequence diagram from text

Describe the interaction in plain language: name the participants (actors, services, objects), list the messages they exchange and the order they happen in, and call out any conditional paths, loops, or async calls. The generator turns that into a sequence diagram with proper lifelines, solid arrows for synchronous calls, dashed arrows for returns, open arrowheads for async messages, activation bars, and loop/alt/opt fragments where you describe them. The more specific you are — for example, "if the payment is declined, show an alt fragment that returns an error" — the more accurate the result. Generate a few options, pick the clearest, and download it.

Modeling loops, alternatives, and async messages

Real interactions are rarely a straight line, and combined fragments are how a sequence diagram handles that. Use a loop fragment for steps that repeat, such as polling a job until it completes or processing each item in a batch. Use an alt fragment for mutually exclusive branches — success versus failure, authorized versus rejected — and an opt fragment for a step that only sometimes runs, like sending a notification only when a user has opted in. Asynchronous messages (open-arrowhead arrows) model work that does not block the caller, such as emitting an event or enqueuing a task, so the timeline keeps moving. Describe these conditions in your prompt and the generator draws the right fragments and arrow styles for each.