The history junction block in Stateflow™ is a simple, powerful, and often misunderstood entity.  In this blog post, I will try to explain the use of the history junction, through a, hopefully, humorous example.

The Model

For this example, I have coded two versions of the same model, one with the history block and one without.  By doing so I can illustrate the effect of the block.

In this example, the first state is the “MortgageStatus”; it has three substates, start of loan, paying off loan and you own your home.  If all progresses normally you will be in the final state after 30 iterations of the chart.

However, there is a second top level state, “BankFraudOccured.”  For this model, I have this configured to be true every 10th cycle.  So let’s look at the results with and without the history junction.

The results

The function of the history junction is to allow users to return to the last active substate within a super state; without it, the user returns to the default substate.  If we look at the two graphs we will see the effect of history junction.

In the left-hand graph, the instance with the History Junction, the effect of the “bank fraud event” does not impact the payoff of the mortgage.  However, in the right-hand graph, the bank fraud “resets” the mortgage to the “NEW” state and keeps the lender paying off indefinitely.   This can be clearly seen by looking at the Sequence Viewer.

The red boxes her show where the model transitions to and from “BankFraud.”  With the history junction in place, you go right back to paying.  Without you follow the default junction to the starting substate.

Words of caution

So in this first example, we had a just a single state connected to our main state. Now let us take a look at a multi-state example

In this example, the History Junction would result

in the state returning to “Paying Off” without going through the “Start of Loan” state.   The history junction applies to all transitions into and out of the parent state.   To correct for this the “Refinance State” needs to be to be “Start of Loan” state

Final thoughts

The first iteration of MAAB style guidelines recommended against the use of the history junction.  With version 2.0 this recommendation was removed and it opened up new modeling patterns for our end users.  Use of the history junction enables multiple design patterns that are difficult to create without it.  Hopefully, this post sheds some light on its proper use.