10 Apr
2020
10 Apr
'20
2:01 a.m.
Hello ceph-devel,
I'm one of the members of the team currently maintaining, and hopefully
improving, go-ceph (ceph library bindings for Go) [1].
An issue I keep returning to is the nature of api calls such as:
- rados_mon_command
- rados_mgr_command
- ceph_mds_command
... etc
The argument named cmd for these functions is of type `const char **cmd` and
is followed by a `size_t cmdlen`. I'd like to better understand the rationale
for array-of-char* strings and how this is intended to be used.
When I first saw the function I initially thought it was meant to support
multiple "commands" getting issued at once but later started thinking that a
single command could be split across multiple strings, and experimentation
demonstrated that this does indeed work. I've attempted reading the sources
but nothing jumps out at me to explain this approach. Most callers of these
functions seem to only use a single "command string".
I also tried looking to see how the code makes use of the array (or vector
after the transition to C++) but I will admit I'm not very familiar with C++
and get a bit lost in some of the templates and overloading.
Could someone who is familiar with this better explain how this argument and
it's type is meant to be used?
I want to make sure we're making good use of the apis that ceph provides in
the wrapper library, or at the very least I want to make sure we're not mis-
using the apis. :-)
Thank you for your time.
1 - https://github.com/ceph/go-ceph
10 Apr
10 Apr
9:02 a.m.
On Fri, Apr 10, 2020 at 5:31 AM John Mulligan
<phlogistonjohn(a)asynchrono.us> wrote:
Hello ceph-devel,
hi John,
I'm one of the members of the team currently
maintaining, and hopefully
improving, go-ceph (ceph library bindings for Go) [1].
An issue I keep returning to is the nature of api calls such as:
- rados_mon_command
- rados_mgr_command
- ceph_mds_command
... etc
The argument named cmd for these functions is of type `const char **cmd` and
is followed by a `size_t cmdlen`. I'd like to better understand the rationale
for array-of-char* strings and how this is intended to be used.
When I first saw the function I initially thought it was meant to support
multiple "commands" getting issued at once but later started thinking that a
single command could be split across multiple strings, and experimentation
demonstrated that this does indeed work. I've attempted reading the sources
but nothing jumps out at me to explain this approach. Most callers of these
functions seem to only use a single "command string".
yes, see https://github.com/ceph/ceph/blob/master/src/test/librados/cmd.cc
and https://github.com/ceph/ceph/blob/master/src/test/librados/cmd_cxx.cc
. if you look these test cases closely, you might have two findings:
1. actually, one can pass multiple strings (not commands) to
*_command() methods.
2. the cmd array should compose a serialized JSON
I also tried looking to see how the code makes use of the array (or vector
after the transition to C++) but I will admit I'm not very familiar with C++
and get a bit lost in some of the templates and overloading.
see https://github.com/ceph/ceph/blob/master/src/common/cmdparse.cc#L292,
the elements in `cmd` vector will be concatenated into a single string
which is in turn fed to a JSON parser. and the JSON parser will parse
that string to a dictionary. in general, the value of "prefix" would
be the string identifying a command handled by one of the daemons
targeted by the *_command() method. for instance, MonCommands.h
defines the commands served by monitor.
Could someone who is familiar with this better explain how this argument and
it's type is meant to be used?
i am not sure why we need to do the boxing-unboxing dance to set the
JSON string to an array (vector) of strings, and then concatenate them
again. but probably, it's designed to ease the pain to set different
argument with a template. something like:
vector<string> config_set{
"{",
R"("prefix": "df",)",
R"("format": )",
"", // json, xml, plain ....
"}"
};
config_set[3] = quoted("json");
rados.mon_command(config_set, input_bufferlist, &output_bufferlist, nullptr);
one can do quite the same in C language:
char* config_set[] =
"{",
"\"prefix\": \"df\",",
"\"format\": ",
NULL, // json, xml, plain ....
"}"
};
config_set[3] = "\"json\""; // or it could be a variable or command
line argument
int r = rados_mon_command(config_set, sizeof(config_set) / sizeof(*config_set),
NULL, 0,
buf, &buf_len
out, &out_len);
HTH,
I want to make sure we're making good use of the
apis that ceph provides in
the wrapper library, or at the very least I want to make sure we're not mis-
using the apis. :-)
Thank you for your time.
1 - https://github.com/ceph/go-ceph
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--
Regards
Kefu Chai
6:26 p.m.
On Thu, 9 Apr 2020, John Mulligan wrote:
Hello ceph-devel,
I'm one of the members of the team currently maintaining, and hopefully
improving, go-ceph (ceph library bindings for Go) [1].
An issue I keep returning to is the nature of api calls such as:
- rados_mon_command
- rados_mgr_command
- ceph_mds_command
... etc
The argument named cmd for these functions is of type `const char **cmd` and
is followed by a `size_t cmdlen`. I'd like to better understand the rationale
for array-of-char* strings and how this is intended to be used.
It's purely historical. The command used to be an array of
strings/arguments directly from the CLI so that the parsing was done on
the mon. Way back when we changed it to JSON, but never cleaned up the
message strcture.
sage
>
> When I first saw the function I initially thought it was meant to support
> multiple "commands" getting issued at once but later started thinking that
a
> single command could be split across multiple strings, and experimentation
> demonstrated that this does indeed work. I've attempted reading the sources
> but nothing jumps out at me to explain this approach. Most callers of these
> functions seem to only use a single "command string".
>
> I also tried looking to see how the code makes use of the array (or vector
> after the transition to C++) but I will admit I'm not very familiar with C++
> and get a bit lost in some of the templates and overloading.
>
> Could someone who is familiar with this better explain how this argument and
> it's type is meant to be used?
> I want to make sure we're making good use of the apis that ceph provides in
> the wrapper library, or at the very least I want to make sure we're not mis-
> using the apis. :-)
>
> Thank you for your time.
>
> 1 - https://github.com/ceph/go-ceph
>
> _______________________________________________
> Dev mailing list -- dev(a)ceph.io
> To unsubscribe send an email to dev-leave(a)ceph.io
>
>
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2 comments
3 participants
participants (3)
-
John Mulligan -
kefu chai -
Sage Weil