What are the best possible solution to enable auto save within the Appian Interface?

Not possible without a user interaction via fields or buttons! You can Write or update record within saveInto of UI fields - might seem to user as auto-save. But caution should be placed so that not all fields write to records, especially text, paragraph etc free text fields. Choose some fields like dropdown, radio buttons etc if you have and save your data within saveInto of such a component - then again if user doesn’t interact with this field data wont be saved. 

Generate a row with the primary key before the User Input Task, and use WriteRecords() in each input task to store the data in the database.

As a quick suggestion, you could also consider having a separate audit table. A Before Trigger might help keep track of the changes and maintain a proper audit trail.

Just add a writeRecords() to the saveInto of each and every input component.

But, what do you try to achieve? I mean, which problem does this solve?

​@harshas2775’s solution is the only workaround and it has its caveats. I would recommend opening a support ticket to request the feature.

As others have noted, there is no “AUTOSAVE”.  Mathieu has registered it previously as a feature request, but it would be so complex to implement (and then complex to manage) that I doubt we’ll see much in the way of this all that soon.

My preferred solution?  Add a “SAVE AND CONTINUE” button prominently on your form which you encourage the user to click occasionally; it would submit the form, then in the process model, loop instantly BACK TO the form, persisting any user data in the process model while they don’t know much of a difference otherwise. Easy to set up, easy to maintain, requires SOME (but not much) user intervention to save their data.

If you were DESPERATE for a 100% automated solution (and i would suggest that you selectively implement this only in critical or high-value places), then springboarding off Harsha and Stefan’s suggestions, you would place a database write call in the SaveInto of every user data entry field - it occurs to me that if you architect something carefully, you could, for instance, save everything to a “temporary” table that stores data keyed by username and by field name, to be reloaded if a form is reopened after being closed prior to (normal) submission, and then of course this data would be cleared out and/or deactivated when the user actually submits the form in question.  This would sidestep the normal issue of worrying about writing data to the “real” tables “too early” or “without validation”, as your temporary storage table could be engineered to hold essentially any value (given that it’s intentionally temporary), and the user would still be subject to data validation (etc) when they go to “properly submit”.

Note that the above is a workaround and would require manual labor to install on every field of every form you intend to use it on, and if you have tons of users abandoning forms midway through (and in some cases never returning), you’d potentially end up with junk rows in the temporary table.