Detecting System Clock Discrepancies — Timewarp

A timewarp (clock skew) occurs when the discrepancy between the system clocks on a client machine and a server machine exceeds the allowable tolerance of 5 seconds. A timewarp can also occur in a replication environment, when the clocks on the master server and a replica server differ by more than 5 seconds. (See Replication of the AccuRev Repository.)

In this topic:

Overview

Timewarp problems typically occur during initial system setup and during time zone adjustments. For example, the change from Eastern Standard Time to Eastern Daylight Time can cause a timewarp on a machine that is not configured correctly to handle the time zone adjustment.

For most AccuRev operations, a timewarp check is performed when the client contacts the server, or when a replica server forwards an AccuRev command to the master server.

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AccuRev's Timewarp Detection Scheme

Each time a client program contacts the server program — or a replica AccuRev Server contacts the master AccuRev Server — AccuRev compares the system clocks on the two machines. If the discrepancy is less than or equal to 5 seconds, AccuRev proceeds to execute the user's command.

If there's a timewarp exceeding 5 seconds between a client machine and a server machine and the user's command specifies the -t option with a time specification (not a transaction number), AccuRev uses the (case-insensitive) value of variable AC_SYNC in the user's environment to determine how to proceed:

• If AC_SYNC is unset or has the value ERROR, an error occurs and a message like this appears:

	client_time:    2008/07/14 10:54:54 Eastern Standard Time (1216047294)
	server_time:    2008/07/14 10:49:56 EDT (1216046996)
	timewarp:       298 seconds
	The time on this machine is more than 5 seconds different than the
	time on the server. Please fix this and try again.
	You may have a problem with your system clock.
	You can force the time on your system to match the server time
	using the accurev synctime command.

• If AC_SYNC has the value WARN, no error occurs, but a warning that includes the timewarp details is displayed. Command execution proceeds.

• If AC_SYNC has the value IGNORE, no error occurs and no warning is displayed. Command execution proceeds.

Notes on timewarps when a client interacting with a replica server:

• A client command that "reads" information from the repository is handled by the replica AccuRev Server; only a timewarp between the client and replica server machines is relevant.

• A client command that "writes" information to the repository is handled by the master AccuRev Server; only a timewarp between the client and master server machines is relevant.

• If there's a timewarp exceeding 5 seconds between a replica server machine and the master server machine and a client of the replica server invokes the command accurev replica sync, a warning message is recorded in the AccuRev Server log on the replica server machine.

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