[Gluster-devel] compound fop design first cut

[Shyam]

Hi,

Why not think along the lines of new FOPs like fop_compound(_cbk) where, 
the inargs to this FOP is a list of FOPs to execute (either in order or 
any order)?

With a scheme like the above we could,
  - compound any set of FOPs (of course, we need to take care here, but 
still the feasibility exists)
  - Each xlator can inspect the compound relation and chose to 
uncompound them. So if an xlator cannot perform FOPA+B as a single 
compound FOP, it can choose to send FOPA and then FOPB and chain up the 
responses back to the compound request sent to it. Also, the intention 
here would be to leverage existing FOP code in any xlator, to 
appropriately modify the inargs
  - The RPC payload is constructed based on existing FOP RPC 
definitions, but compounded based on the compound FOP RPC definition

Possibly on the brick graph as well, pass these down as compounded FOPs, 
till someone decides to break it open and do it in phases (ultimately 
POSIX xlator).

The intention would be to break a compound FOP in case an xlator in 
between cannot support it or, even expand a compound FOP request, say 
the fxattropAndWrite is an AFR compounding decision, but a compound 
request to AFR maybe WriteandClose, hence AFR needs to extend this 
compound request.

The above is just a off the cuff thought on the same.

The scheme below seems too specific to my eyes, and looks like we would 
be defining specific compound FOPs than the ability to have generic ones.

On 12/07/2015 04:08 AM, Pranith Kumar Karampuri wrote:
> hi,
>
> Draft of the design doc:
>
> Main motivation for the design of this feature is to reduce network
> round trips by sending more
> than one fop in a network operation, preferably without introducing new
> rpcs.
>
> There are new 2 new xlators compound-fop-sender, compound-fop-receiver.
> compound-fop-sender is going to be loaded on top of each client-xlator
> on the
> mount/client and compound-fop-receiver is going to be loaded below
> server-xlator on the bricks. On the mount/client side from the caller
> xlator
> till compund-fop-encoder xlator, the xlators can choose to implement
> this extra
> compound fop handling. Once it reaches "compound-fop-sender" it will try to
> choose a base fop on which it encodes the other fop in the base-fop's
> xdata,
> and winds the base fop to client xlator(). client xlator sends the base fop
> with encoded xdata to server xlator on the brick using rpc of the base fop.
> Once server xlator does resolve_and_resume() it will wind the base fop to
> compound-fop-receiver xlator. This fop will decode the extra fop from
> xdata of
> the base-fop. Based on the order encoded in the xdata it executes
> separate fops
> one after the other and stores the cbk response arguments of both the
> operations. It again encodes the response of the extra fop on to the
> base fop's
> response xdata and unwind the fop to server xlator. Sends the response
> using
> base-rpc's response structure. Client xlator will unwind the base fop to
> compound-fop-sender, which will decode the response to the compound fop's
> response arguments of the compound fop and unwind to the parent xlators.
>
> I will take an example of fxattrop+write operation that we want to
> implement in
> afr as an example to explain how things may look.
>
> compound_fop_sender_fxattrop_write(call_frame_t *frame, xlator_t *this,
> fd_t * fd,
>          gf_xattrop_flags_t flags,
>          dict_t * fxattrop_dict,
>          dict_t * fxattrop_xdata,
>          struct iovec * vector,
>          int32_t count,
>          off_t off,
>          uint32_t flags,
>          struct iobref * iobref,
>          dict_t * writev_xdata)
> ) {
>          0) Remember the compound-fop
>          take base-fop as write()
>          in wriev_xdata add the following key,value pairs
>          1) "xattrop-flags", flags
>          2) for-each-fxattrop_dict key -> "fxattrop-dict-<actual-key>",
> value
>          3) for-each-fxattrop_xdata key ->
> "fxattrop-xdata-<actual-key>", value
>          4) "order" -> "fxattrop, writev"
>          5) "compound-fops" -> "fxattrop"
>          6) Wind writev()
> }
>
> compound_fop_sender_fxattrop_write_cbk(...)
> {
>          /*decode the response args and call parent_fxattrop_write_cbk*/
> }
>
> <compound_fop_sender_parent>_fxattrop_write_cbk (call_frame_t *frame,
> void *cookie,
>                                          xlator_t *this, int32_t
> fxattrop_op_ret,
>                                          int32_t fxattrop_op_errno,
>                                          dict_t *fxattrop_dict,
>                                          dict_t *fxattrop_xdata,
>                                          int32_t writev_op_ret, int32_t
> writev_op_errno,
>                                          struct iatt *writev_prebuf,
>                                          struct iatt *writev_postbuf,
>                                          dict_t *writev_xdata)
> {
> /**/
> }
>
> compound_fop_receiver_writev(call_frame_t *frame, xlator_t *this, fd_t *
> fd,
>          struct iovec * vector,
>          int32_t count,
>          off_t off,
>          uint32_t flags,
>          struct iobref * iobref,
>          dict_t * writev_xdata)
> {
>          0) Check if writev_xdata has "compound-fop" else default_writev()
>          2) decode writev_xdata from above encoding -> flags,
> fxattrop_dict, fxattrop-xdata
>          3) get "order"
>          4) Store all the above in 'local'
>          5) wind fxattrop() with
> compound_receiver_fxattrop_cbk_writev_wind() as cbk
> }
>
> compound_receiver_fxattrop_cbk_writev_wind (call_frame_t *frame, void
> *cookie,
>                                              xlator_t *this, int32_t
> op_ret,
>                                              int32_t op_errno, dict_t
> *dict,
>                                              dict_t *xdata)
> {
>          0) store fxattrop cbk_args
>          1) Perform writev() with writev_params with
> compound_receiver_writev_cbk() as the 'cbk'
> }
>
> compound_writev_cbk (call_frame_t *frame, void *cookie, xlator_t *this,
>                       int32_t op_ret, int32_t op_errno, struct iatt
> *prebuf,
>                       struct iatt *postbuf, dict_t *xdata)
> {
>          0) store writev cbk_args
>          1) Encode fxattrop response to writev_xdata with similar
> encoding in the compound_fop_sender_fxattrop_write()
>          2) unwind writev()
> }
>
> This example is just to show how things may look, but the actual
> implementation
> may just have all base-fops calling common function to perform the
> operations
> in the order given in the receriver xl. Yet to think about that. It is
> probably better to Encode
> fop-number from glusterfs_fop_t rather than the fop-string in the
> dictionary.
>
> This is phase-1 of the change because we don't want to change RPCs
> in phase-2 we can implement the compound fops that are commonly used by
> lot of translators throughout the stack so that
> quota/bitrot/geo-rep/barrier etc handle them
> in phase-3 may be just in time for 4.0 we can convert them to on the
> wire RPCs
>
> Thanks to Raghavendra G, krutika, Ravi, Anuradha for the discussions
>
> Pranith
>
>
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