Waiting For The Electrician, Or Someone Like Him

Introduction

Processes (remote clients, self-serving clients, servers, background i/o processes, etc.) that are using a multi-user Progress database operate on shared data. These data are either in the database, in the form of tables and indexes, or in the shared-memory data area in the form of various data structures and disk buffers. To ensure that two processes cannot change the same datum at exactly the same instant, access to these data are regulated with a system of locks and resource queues. The shared data structures are often called resources.

When a process needs to use a shared data structure, another process may already be using it. The process that encounters the conflict must wait for the prior user to finish. It does this by inserting itself into a wait queue for the resource and then waiting for a notification to continue. When the process that is using it is finished, it will remove the first waiting process from the wait queue and notify it that it can continue.

The Progress Monitor Utility will show the state of waiting processes and for which resource they are enqueued.

Resource Wait Queues

The first three resources listed below are sometimes held for long periods, perhaps as long as several minutes or more. The remaining resources are locked for much shorter periods, usually less than a millisecond (0.001 seconds)

When processes are waiting for access to one of these resources, the Progress Monitor will show a 4 character code in the "Wait" column of the User Control display (menu choice 1) and in oither displays that include process state information. The 4 character code indicates which resource the process is waiting for.

Here is a list of the 4 character codes shown in the Progress Monitor Utility, the purpose of the queue, and the meaning of the value in the next column of the User Control display:

Wait Column	Reason For Wait	Value of Next Column
--	Process is not waiting for a shared resource	Not meaningful
REC	Record lock conflict	Rowid to lock
SCH	Database schema lock	Lock type. 9 = share lock, 10 = exclusive
TRAN	Another transaction to end	Transaction identifier
RGET	Record get (fetch) consistency lock	Rowid
TXE	Transaction commit consistency lock	Lock type. 1 = share, 2 = exclusive
BKSH	Shared (read) access to a buffer in the database buffer pool	Block's dbkey
BKEX	Exclusive (update) access to a buffer in the database buffer pool	Block's dbkey
BKSI	Read with intent to update a buffer in the database buffer pool	Block's dbkey
DBRD	Database block being read from disk into buffer pool	Block's dbkey
DBWR	Database block to be written to disk from buffer pool	Block's dbkey
DBBK	Database block to be backed up before it is updated	Block's dbkey
BIRD	Before-image log data to be read from disk into bi buffer	Before-image log address
BIWR	Before-image log data to be written to disk from bi buffer	Before-image log address
AIRD	After-image log data to be read from disk into ai buffer	After-image log address
AIWR	After-image log data to be written to disk from ai buffer	After-image log address
IDLE	AIW or BIW unemployed: will write for food	Address of last data written

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