Rules & Automation

The Rule Builder lets you define how FIXSIM automatically responds to incoming messages — ideal for repeatable regression scenarios, certification rehearsals, and stress testing. A rule has two parts: a trigger (which messages it matches) and one or more actions (what FIXSIM does in response).

Rules are managed under Setup → Rules in the portal sidebar.

Step 1 — Create a rule and define its trigger

Click Create Rule and define what the rule listens for:

• Target Session - The FIX session this rule applies to.
• Rule Name - A descriptive name, e.g. "Partial Fill IBM".
• Target Msg Type - The FIX message type (tag 35) that triggers the rule, e.g. D for New Order Single.
• Condition 1 - An optional tag condition: a Target Tag (e.g. 55, Symbol), a condition (e.g. Equal To), and a value (e.g. IBM).
• Logic - And/Or + Condition 2 - Optionally combine a second condition with And/Or logic. A condition can also test that a tag is not present on the message.

The example above triggers on every New Order Single (35=D) received on session TESTSELL1 where tag 55 equals IBM.

Step 2 — Review the rule's message pipeline

After creating the rule, its detail page shows the message pipeline — a left-to-right view of how a message flows through the rule:

INCOMING (MsgType D) → SESSION (TESTSELL1) → WHEN (Tag 55 = IBM) → EXECUTE (your actions)

The Rule Actions table below the pipeline lists the actions attached to the rule. A new rule starts with no actions — click Create New to add one.

Step 3 — Add actions

An action defines what FIXSIM does when the trigger matches:

• Rule Action Name - A descriptive name, e.g. "Partial Fill Order".
• Order Action - Act on the triggering order — for example PartialExecute to send a partial fill. The same order actions available manually in the blotters (fills, cancels, rejects) can be automated here.
• Exec Mode - How the execution is styled. Session Default follows the ExecMode configured in the session's App Settings; or choose a specific mode for this action.
• Pause Time (milliseconds) - Delay before the action runs, to simulate counterparty latency. E.g. 1000 responds after one second.
• Raw Message(s) - Optionally send one or more raw FIX messages as the response instead of (or in addition to) an order action.
• Additional Tags - Add specific tags/values to the messages this action generates, on top of the session's pre-configured tags.

A rule can have multiple actions, so a single trigger can produce a realistic multi-message response.

Putting it together

The worked example above — "Partial Fill IBM" — partially fills any IBM order received on the session, one second after it arrives. The same pattern covers most simulation needs: reject orders matching a condition, delay fills to test timeout handling, respond to specific symbols with specific executions, or fire raw message sequences in response to a trigger.

Combine multiple rules to simulate venue behavior, risk checks, partial fills, and latency scenarios. For scripted control from CI/CD instead of rules, see the REST API.