This is only a start -- a brief email explanation. Better than nothing I hope.
hgsid is a key into hgcentral.sessionDb. hguid is a key into hgcentral.userDb. When we have hguid from a cookie, but don't have hgsid, we can restore settings from userDb. When we have hguid from a cookie, and do have hgsid, we use sessionDb and we save the settings into userDb as well, as a backup in case we lose hgsid. When we don't have hguid (cookies disabled/dysfunctional) but do have hgsid, settings will be updated in sessionDb and a new userDb entry will be created on each access (wasteful). When we don't have hguid and hgsid is not passed forward via URL or posted form, settings are lost.
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