Entries in taq (2)

I'm in a hurry. Can multiple kdb processes write simultaneously to a splayed database?

Short answer: Yes, multiple processes can write to a database as long as the processes avoid concurrent write operations to the same table on the same partition. To see a significant speedup, you'll need to use segments.

With the exception of the ? operator, which can be used to provide concurrency protection for sym files, q processes do not coordinate file I/O operations in any way. Thus, kdb splayed table write operations require the same degree of care as ordinary file writes.

The mechanism to protect sym files, however, is used throughout the functions in the .Q namespace that pertain to writing splayed tables. So, you can use those functions to build a database faster by running multiple loader processes in parallel.

Example 1: Concurrent writes to the same table on the same partition. This means two processes are writing to the same (table column) files simultaneously, and that is not safe! The potential for corruption is high.

kdb_proc1: .Q.dpft[`:/vol/db; 2011.04.07; `sym; `t]
kdb_proc2: .Q.dpft[`:/vol/db; 2011.04.07; `sym; `t]

Example 2: Concurrent writes to two different tables on the same partition. This is will not cause corruption.

kdb_proc1: .Q.dpft[`:/vol/db; 2011.04.07; `sym; `t]
kdb_proc2: .Q.dpft[`:/vol/db; 2011.04.07; `sym; `q]

Example 3: Concurrent writes to the same table on different partitions. This is also safe, with no potential corruption.

kdb_proc1: .Q.dpft[`:/vol/db; 2011.04.07; `sym; `t]
kdb_proc2: .Q.dpft[`:/vol/db; 2011.04.08; `sym; `t]

If the two partitions are on different I/O channels using segments, this last approach can be much faster than performing the writes serially.

What does '\l .' or 'system “l ."' do?

Typically, it's used to reinitialize and alert an existing kdb process of a new partition. Note that command triggers the execution of any q scripts existing in the current working directory of the running kdb process.

This command allows kdb to continue running and servicing requests while a new partition is constructed. Running kdb remains unware of the new partition until the \l . command is issued.

See also: .Q.l