Entries in sym file (3)

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 should I know about the sym file, and why back it up?

In a partitioned database, the sym file is a string intern pool which grows every time a new distinct symbol is introduced and persisted to that database. The sym file cannot be regenerated (unless you know the exact order in which your data was loaded from the inception of your database) and does not automatically shrink when symbols are removed from the database.

See this solution to compacting a bloated sym file.

How do I choose whether to make a field a symbol or string?

A q string is a vector of characters. If you need to manipulate the string, or the set of possible values the string might take is unbounded, then go with string.

Use symbol when the set of values is restricted, e.g., exchange names, or tickers. Symbols are faster for comparison operations (and that means lookup as well).

See string interning.