Troubleshooting#
Symptoms, likely causes and fixes. If your problem is not here, check Assumptions and simplifications: many surprising results are the model doing what it says.
The run does not start#
**The status says The code contains errors.** Open the Code tab. The list under the editor shows each error with its line; click it to jump there. The most common ones are listed under Error messages.
The status mentions an input line. A line in External inputs does not follow TIME NODE Event | arguments, names an event that the main algorithm does not accept, or has the wrong number of arguments. The event must be a request of the main algorithm's interface.
The status mentions a distribution or a duration. A field in the Timing tab is invalid; it is outlined in red. Durations need a unit or are read as milliseconds: 50ms, 2s. Distributions look like uniform(5ms, 30ms).
**The status says Emulated rounds need known DELTA and PHI.** With synchronous rounds emulated on clocks, the round length is DELTA + PHI, so both must have a value.
**The status says No algorithm implements … or Circular dependency.** A uses clause names an interface that no algorithm in the code implements, or two algorithms use each other. Add the missing module (Add module may have it) or break the cycle.
The run stops early#
**The status says Runtime error.** The line is highlighted in the editor and listed under it. Scrub to the end of the run and open State to see the values that led to it.
**The status says Limit of 150000 events reached.** Something produces a lot of events: a timer that fires too often, retransmissions to a crashed process, a gossip with a large fanout. Shorten the simulated duration in the Scenario tab, lengthen the timer, or reduce the parameters.
**The status says Timing violation (policy: halt).** The policy in the Timing tab is Stop the simulation. That is intended: the run stops at the first broken promise.
**The status says Assertion failed.** Stop at the first failed assertion is checked and an assert failed. The log says which one.
Nothing happens#
No input. Most algorithms do nothing until the application asks. Add a line to External inputs, such as 0ms 1 Start.
The event has no handler. The log shows a warning such as event ⟨app, Start⟩ has no handler. Check the spelling of the event and of the instance name in the handler.
The wrong main algorithm. The Main algorithm menu decides which algorithm receives the inputs. With several algorithms in the code, make sure the right one is selected.
The run is over. Playback stops where the action ends. Press ⏮ and play again, or check that Autoplay is on.
The speed is too high. With Auto speed on a long run, short events flash by. Use Event by event.
Messages disappear#
Filter the log by Dropped messages, or hover the arrow on the diagram to read the status.
| Status | Cause | Fix |
|---|---|---|
| no link | the recipient is not a neighbor | add a link, or use a complete graph |
| lost by the network | Loss is above zero | use perfect links from the library, or set loss to 0 |
| dropped by a link failure or a partition | a fault in the Scenario tab | check the fault list |
| discarded because late | a violation with the Discard policy | raise DELTA, change the policy, or change the actual delays |
| recipient crashed | the recipient was down | check the fault list |
| still in transit | the run ended first | lengthen the simulated duration |
Library broadcast modules send to every process in Π. On a topology that is not complete, the messages to non-neighbors end up as no link.
The algorithm misbehaves#
A handler never runs. Handlers for the same event are tried in order, and only the first match runs. An earlier, more general handler may catch everything. Move the specific one up or add where clauses.
A tag is treated as a variable. Atoms need at least two capital letters. A is a variable name; AB is an atom. A state variable written in capitals hides the atom with the same name.
A division gives a whole number. Division between two whole numbers truncates. Use mean for averages, or multiply first.
The algorithm works only with one seed. Try others with the dice button, and random order for simultaneous events in the Timing tab. If it still works only sometimes, the algorithm probably depends on a timing or ordering assumption.
The algorithm depends on set order. forall, choose and upon exists follow sorted order. Replace choose with pick to check whether the order matters.
A timer never fires. It was cancelled, restarted, or the process crashed. After a recovery all timers are cleared; start them again in the Init or Recovery handler.
State is lost after a recovery. Only variables declared stable survive. That is intended.
A crash in the middle of a broadcast has no effect. A step is never interrupted. Add message loss so that some sends need retransmission, and crash the sender before it retransmits.
FloodSet never disagrees under realistic timing. Disagreement needs a particular combination of late messages and is rare. Try several seeds; the FloodSet example uses one where it happens.
The display#
The graph is too small or off-center. Press Fit.
Packets have no labels. With more than 120 packets in flight, labels are hidden automatically. Zoom into time with a slower speed, or use a smaller scenario.
The diagram is empty on the right. The Action view fits the part of the run where things happen. Press All to see the whole run.
The diagram does not follow the cursor. Dragging the diagram turns Follow off. Tick it again.
Colors are hard to distinguish. Turn off Layers; colors then only indicate the status of messages. The playground follows the system's light or dark theme.
On a phone, the page does not scroll over the graph. Start the swipe on an empty part of the graph. A swipe that starts on a process moves the process, and with the Node, Link or Delete tool active the graph captures all touches.
Part of the interface is still in English. With the Italian language, code, checker diagnostics, examples inside the quick reference and the documentation are not translated. That is intended.
Presentation mode does not go full screen. Some browsers and embedded viewers do not allow it. The mode works anyway; press F to try again, or use the browser's own full-screen command.
The image export does not download. In embedded viewers, downloads may need your confirmation or may be blocked. Open the playground from GitHub Pages or from a local copy.
Saving and sharing#
My work is gone. The scenario is saved in the browser where you edited it, per site. A different browser, a private window, or clearing site data starts fresh. Export important scenarios.
The link does not open my scenario. The link must be copied whole: the scenario is in the part after #. Some chat applications cut long links; share the exported JSON file instead.
Save file does nothing. Some embedded viewers block downloads. Use Copy JSON and paste it into a file.
**Import says not a scenario.** The JSON must contain at least nodes and code. Export a scenario to see the full format.
Error messages#
| Message | Meaning |
|---|---|
Expected "⟩" (or ">"), found … | an event is not closed, often because of a > comparison inside an ASCII trigger; wrap it in parentheses |
Expected ":=" (statements start with a keyword or an assignment) | a line starts with a name that is not assigned, often a missing trigger or call |
Variable "x" is not declared | declare it in state or params, assign it first, or check the spelling |
"DELTA" is not available | the assumed model does not know DELTA; set it in the Timing tab or avoid it |
"Rounds" is only available when the assumed model is "synchronous rounds" | switch the synchrony model, or use Net |
"X" is not an indication of I (only indications can be emitted upwards) | trigger ⟨myAlias, X⟩ must use an indication of your own interface |
"X" is not a request of I (only requests can be sent downwards) | trigger ⟨usedAlias, X⟩ must use a request of the used interface |
Unknown instance "x" in this algorithm | the instance name does not match implements … as or uses … as |
"return" can only be used inside a function | handlers do not return; restructure with if |
"call" is for your own functions | built-in functions are used in expressions: x := size(S) |
Expected a boolean, found … | a condition or where clause is not true or false |
Variable "x" is not defined (while running) | a local variable was read on a path where it was never assigned; assign it before every use |
while loop exceeded 100000 iterations | the loop condition never becomes false |
A guard stays true forever | an upon condition or upon exists handler must make its condition false |
Function calls nested deeper than 200 | a recursive function never stops |
Too many statements in a single step | a loop does not terminate |
min of an empty set (and similar) | check that the collection is not empty first |