* EL/IX API levels, services
@ 2000-01-12 16:21 Michael A. Olson
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From: Michael A. Olson @ 2000-01-12 16:21 UTC (permalink / raw)
As everyone emerges from their Y2K bunkers to survey the wreckage,
I thought I would reopen the API discussion.
My point of view is that of a database software vendor (Sleepycat
distributes Berkeley DB) who would like to port once, to EL/IX,
and not have to do ports for every embedded OS vendor.
The EL/IX spec characterizes the four levels of interfaces as:
Level 1: RTOS compatible layer.
Level 2: Linux single process only.
Level 3: Linux multiprocess for embedded applications.
Level 4: Full POSIX or Linux compliance.
When I think about deploying Berkeley DB on an embedded OS, I
characterize levels of service in an analogous way. These are
basically the levels of service that applications that embed
Berkeley DB are looking for:
Single-user, no transactions: Basically a dbm-alike.
Single-user with transactions: Multiple changes can be
grouped into single operations, system and app crashes
don't lose data.
Multi-user, no transactions: Multi-thread, maybe multi-
process. Dbm, except that multiple threads can read
and write the database concurrently without trouble.
Multi-user with transactions: Combine the last two.
Under the current draft, level 1 of the EL/IX spec is all
we need to deliver threaded, transaction-protected database
management. If the application wants to use multiple
processes, rather than multiple threads, to operate on
a single database, then we jump to level 4.
The only thing missing from level 3 of the spec that makes
it impossible to do multi-process database services is shared
memory. The shm_ interfaces show up in EL/IX level four.
Why is that? It seems to me that the level 3 goal (Linux
multi-process) would be furthered by the availability of
named shared memory. The high-end embedded vendors, like Wind
River and QNX, have good named shared memory abstractions
already. I don't know about the embedded Linux vendors.
Also, I'm a little curious about the interface names.
Berkeley DB uses shmat()/shmdt()/shmget() now. The
EL/IX spec calls for shm_open() and shm_unlink(). I
can imagine how those map to the calls we use now, but
I am not familiar with them.
My vote would be to move the named shared memory interfaces
into level 3, because my code would run on more devices that
way. I don't know how hard the implementation would be for
the OS vendors, though.
The good news is that the draft spec would permit us to
port to EL/IX quite easily. We've got customers on embedded
OSes who would be quite happy with the services we could
offer at level 2 of the current draft.
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