void UpdateContact(string identifier, Action<Contact> updateMethod)
{
  var contactRepository = (ContactRepositoryBase)Factory.CreateObject("contactRepository", true);
  var sharedSessionManager = (SharedSessionStateManager)Factory.CreateObject("tracking/sharedSessionState/manager", true)

  if (Tracker.Current.Contact != null && identifier == Tracker.Current.Contact.Identifiers.Identifier)
  {
    // The current contact is the one we need to update.
    updateMethod(Tracker.Current.Contact);
  }
  else
  {
    // Find out if a contact with the given identifier exists in the database.
    Contact databaseContact = contactRepository.LoadContactReadOnly(identifier);

    if (databaseContact != null)
    {
      Guid contactId = databaseContact.ContactId;

      // Try to load the contact from the Shared Session.
      Contact sessionContact = sharedSessionManager.LockAndLoadContact(contactId);

      if (sessionContact != null)
      {
        // Update this contact and release it back to the Shared Session.
        updateMethod(sessionContact);

        sharedSessionManager.SaveAndReleaseContact(sessionContact);
      }
      else
      {
        // In this branch, we know that the contact we need exists in the database, but it's not locked by our cluster.
        // We'll try to lock it and update its data.
        LeaseOwner leaseOwner = new LeaseOwner("SOME_UNIQUE_WORKER_NAME", LeaseOwnerType.OutOfRequestWorker);
        LockAttemptResult<Contact> lockResult = contactRepository.TryLoadContact(identifier, leaseOwner, TimeSpan.FromMinutes(1));

        if (lockResult.Status == LockAttemptStatus.Success)
        {
          // Update the contact and release it to the Collection DB.
          updateMethod(lockResult.Object);

          var options = new ContactSaveOptions(release: true, owner: leaseOwner);
          contactRepository.SaveContact(lockResult.Object, options);
        }
        else
        {
          Log.Warn("Cannot modify the contact with identifier '" + identifier + "' because it is locked by another process.", this);

          // Decide what else you want to do in that case.
          // One option is to schedule a Sitecore Job to update the contact later:
          // https://briancaos.wordpress.com/2012/02/06/using-sitecore-jobs/
        }
      }
    }
    else
    {
      // In this branch, we know that the contact with the provided identifier does not exist yet.
      // You will have to decide what to do in that case. Below is an example where you identify the current contact.

      Tracker.Current.Session.Identify(identifier);
      updateMethod(Tracker.Current.Contact);
    }
  }
}