Blocking occurs in SQL Server 2005 when one process has one or more records locked, while another process tries to acquire locks on the same set (or subset) of records. This causes the second process to wait until the first process is done so it can acquire the records; this can cause other processes to be blocked if they are waiting on the resources that the second connection has acquired. These processes are ultimately dependent upon the first process to complete its work before they can complete theirs. This can create a daisy-chain of processes waiting to complete their work.
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