# Detailed behavior See also: * [XForms Error Handling](https://doc.orbeon.com/xforms/core/error-handling) ## Philosophy of error handling * Upon initial page load: * It is acceptable if not desirable to immediately show a static error, as the user hasn't yet interacted with the page. * For dynamic XForms, it is better not to show an error immediately (case of Form Builder and `xxf:dynamic`). * Upon subsequent interactions: * The XForms engine attempts to recover from errors occurring with XPath expressions, bindings, and actions. ## How the XForms engine recovers from errors 1. For control bindings, model bindings, values, variables * Upon XPath errors, a default value is chosen: * the empty sequence for bindings and variable values * the empty string for string values * Upon binding errors with the `bind` attribute * the binding resolves to the empty sequence * Upon binding errors with the `model` attribute * the model doesn't change, as if the `model` attribute was missing 2. For XPath MIP values * Upon XPath errors for `calculate` and `xxforms:default` * the destination value is set to blank * if `xxforms:expose-xpath-types="true"` and there is an error accessing a typed value * the error is logged at debug level * else if `xxforms:expose-xpath-types="false"` or there is any other dynamic error * `xxforms-xpath-error` is dispatched to the model * Upon XPath errors for other MIPs * the MIP is not modified, as if the attribute specifying the property was missing * `xxforms-xpath-error` is dispatched to the model * Upon binding errors with complex or readonly content (`calculate` or `xxforms:default` only) * the instance value is not modified * `xxforms-binding-error` is dispatched to the model 3. For the submission `instance` and `xxforms:instance` attributes * Upon incorrect instance id * `xforms-submit-error` is dispatched (as in the case of a target error with the `targetref` attribute) * Upon binding errors with complex or readonly content * `xforms-submit-error` is dispatched 4. For actions * Any error taking place during action processing stops the outermost action handler, including: * XPath errors * binding errors with the `bind` or `model` attribute * binding errors with complex or readonly content * missing attributes or unsupported attribute values on action elements * `xxforms-action-error` event is dispatched to observer of the action * *NOTE: Some actions silently ignore some error conditions, including:* * `` pointing to an empty sequence or to an atomic item (such as a string) instead of a node * `` with an empty sequence or an empty overridden context * `` with an empty or non-element insert context, an empty overridden context, or an empty origin * actions with AVTs evaluating to the empty sequence * ``, ``, ``, ``, `` when the target element is not found In all cases except typed value access XPath errors on MIPs: * errors are logged at `debug` level \[`warning` level until Orbeon Forms 4.8] * errors are added to a list of errors to send to the client * the client shows an error dialog which the user can discard (when enabled via properties) In the case of typed value access XPath errors on MIPs: * errors are logged at `debug` level ## Events dispatched The `xxforms-xpath-error` event is dispatched: * to the model, upon encountering an XPath error during processing of an XPath model item property (MIP) The `xxforms-binding-error` event is dispatched: * to the model, if a `calculate` or `xxforms:default` MIP points to complex or readonly content * to the control, if an attempt to store an external value (control value, filename, metadata or size) on a control to a node with complex or readonly content takes place *NOTE: This should ideally not occur as the value control binding should not point to complex content, and readonly is disallowed.* The `xxforms-action-error` event is dispatched: * to the *observer* of the action, upon encountering an error during processing of an action * this includes: controls, model, instance, and submission *NOTE: The fatal, non-cancelable XForms* `xforms-compute-exception` *and* `xforms-binding-exception` *are no longer dispatched by Orbeon Forms.* ## Reference: kinds of errors that can occur * XForms errors * static XPath errors * some are detected during the static analysis phase (PE) * some expressions are not analyzed during static analysis of the page, and so can occur at runtime * dynamic XPath errors * binding errors with the `bind` or `model` attributes, e.g.: `bind="foobar"` where id `foobar` doesn't exist * some are detected during the static analysis phase * some are not analyzed during static analysis of the page, and so can occur at runtime * binding errors with complex or readonly content * XForms disallows setting the value of an element with complex content * XForms disallows setting the value of a readonly element or attribute * errors outside of XForms, for example: * in the XPL (XML pipeline) engine * in the controller * other server errors: * major errors such as the server running out of resources * server bugs! --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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