Introduction

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The SSDT executes ODDEX on more than one application server simultaneously. This provides load-balancing across the servers. On login, each user is randomly assigned to one of the available servers.  For the length of the user's session, the load-balancer ensures users will remain "connected" to connected to the server where their session resides.  

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At all times, at least one server is available for new sessions, giving the appearance that the system was never shut down.  As users log out of the previous version, they will be automatically connected to the new version.

Long Running Sessions

In practice, it often takes four to eight hours to complete a rolling upgrade.  This is because a few users keep an active session for the entire business day.  A single user can prevent the upgrade from proceeding by keeping a single session active. For most upgrades, this is not an issue so the sessions are allowed to expire normally.  However, for an important upgrade or bug fix, the SSDT may decide to speed up the process by forcibly terminating a few users.   Whenever possible, we only terminate sessions that have been inactive for at least 15 minutes but occasionally are forced to terminate more active sessionsusers. 

Therefore, if you are unexpectedly directed back to the login or expired page, it most likely means that an upgrade is in progress and you were the last user who was blocking the upgrade.  

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