# Thrift Eio Flow transport - suggestions?
**URL:** https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385
**Category:** Ecosystem
**Tags:** eio
**Created:** [June 13, 2023, 3:10am UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385 "2023-06-13T03:10:39Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@yawaramin](https://discuss.ocaml.org/u/yawaramin)
#### Post date: [June 13, 2023, 3:10am UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/1 "2023-06-13T03:10:39Z")
I am exploring using Thrift with Eio. Currently, the Thrift support library for OCaml uses plain old Unix sockets. But Thrift is fairly flexible in how the low-level networking is done, and abstracts away the details behind a [‘transport’ interface](https://github.com/vbmithr/ocaml-thrift-lib/blob/1e5b7924636d8926114cbd34398a0848cd472e83/src/Thrift.ml#L42). The interface however does not use async I/O types, so before Eio it was not possible to adapt it to use Lwt:
```auto
class virtual t =
object (self)
method virtual isOpen : bool
method virtual opn : unit
method virtual close : unit
method virtual read : bytes -> int -> int -> int
method readAll (buf:bytes) off len = (* this is implemented *)
method virtual write : bytes -> int -> int -> unit
method virtual write_string : string -> int -> int -> unit
method virtual flush : unit
end
```
Thanks to Eio though, we can implement this virtual class to use a flow:
```auto
(* TFlowTransport.ml *)
open Thrift
module T = Transport
class t flow = object
val mutable opened = true
inherit Transport.t
method isOpen = opened
method opn = ()
method close =
Eio.Net.close flow;
opened <- false
method read buf off len =
let cstruct = Cstruct.create len in
try
Eio.Flow.read_exact flow cstruct;
Cstruct.blit_to_bytes cstruct 0 buf off len;
len
with
| End_of_file ->
raise (T.E (T.END_OF_FILE, (Printf.sprintf "TSocket: Could not read %d because: end of file" len)))
| _ -> raise (T.E (T.UNKNOWN, (Printf.sprintf "TSocket: Could not read %d" len)))
method write buf off len =
let cstruct = Cstruct.of_bytes ~off ~len buf in
Eio.Flow.write flow [cstruct]
method write_string buf off len =
let cstruct = Cstruct.of_string ~off ~len buf in
Eio.Flow.write flow [cstruct]
method flush = ()
end
```
But as you can see above, reading from/writing to the flow is a little clumsy because flows don’t work directly in terms of byte buffers with offsets and lengths, they work in terms of cstructs. Can anyone advise whether I am missing something or if allocating cstructs is really the way to implement these methods using Eio?
---
### Author:  [@patricoferris](https://discuss.ocaml.org/u/patricoferris)
#### Post date: [June 13, 2023, 10:37am UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/2 "2023-06-13T10:37:17Z")
I don’t think there’s much you can do at the moment, but see this issue for a discussion on Cstruct vs. Bytes [Decide on cstruct vs bytes · Issue #140 · ocaml-multicore/eio · GitHub](https://github.com/ocaml-multicore/eio/issues/140). I think there’s a desire to move to bytes if the GC-story works out nicely, but I’m not an expert on that.
---
### Author:  [@lambda\_foo](https://discuss.ocaml.org/u/lambda_foo)
#### Post date: [June 13, 2023, 1:33pm UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/3 "2023-06-13T13:33:54Z")
> [@yawaramin](#):
>
> I am exploring using Thrift with Eio.
Unrelated to the EIO usage, what is the reason for wanting to use Thrift compared to something else?
---
### Author:  [@yawaramin](https://discuss.ocaml.org/u/yawaramin)
#### Post date: [June 13, 2023, 4:14pm UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/4 "2023-06-13T16:14:44Z")
I have existing Thrift services running in production and I want to interact with them/extend them.
However I think Thrift compares pretty favourably to many other options–supports a large number of languages, solid story for schema evolution, elegant and decoupled design. Sure, it has some warts and performance-wise maybe not at the top. But with RPC systems the thing is that nobody is totally satisfied with any of them 🙂
---
### Author:  [@lambda\_foo](https://discuss.ocaml.org/u/lambda_foo)
#### Post date: [June 22, 2023, 6:09am UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/5 "2023-06-22T06:09:10Z")
Cool good to know. It wasn’t clear that project was active or being used by anyone.
I’ve had a PR open against it since Feb to fix the mutable string/bytes warnings. [Add builds for 4.13, 4.14 and 5.0 by tmcgilchrist · Pull Request #7 · vbmithr/ocaml-thrift-lib · GitHub](https://github.com/vbmithr/ocaml-thrift-lib/pull/7)
---
### Author:  [@jonsterling](https://discuss.ocaml.org/u/jonsterling)
#### Post date: [August 2, 2023, 11:32am UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/6 "2023-08-02T11:32:11Z")
I am also interested in this question. I have a lot of code that uses `formatter`, and I am doing something similar to @yawaramin in wrapping a flow in a formatter, but there is a mismatch between the models.
```auto
let formatter_of_flow flow =
let out buf off len =
let cstruct = Cstruct.of_string ~off ~len buf in
Eio.Flow.write flow [cstruct]
in
let flush () = () in
Format.make_formatter out flush
```
I also noticed that writing to one of these formatters constructed from a `flow` is significantly slower than writing to a formatter constructed from an `out_channel`. Are there any suggestions, or should I hold off on using Eio for this?
---
### Author:  [@talex5](https://discuss.ocaml.org/u/talex5)
#### Post date: [August 2, 2023, 12:41pm UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/7 "2023-08-02T12:41:22Z")
`out_channel` is buffered, whereas a flow typically isn’t. You could try something like this:
```ocaml
let formatter_of_writer w =
let out buf off len =
Eio.Buf_write.string w buf ~off ~len
in
let flush () = () in
Format.make_formatter out flush
```
```ocaml
Eio.Buf_write.with_flow flow @@ fun b ->
let f = formatter_of_writer b in
...
```
---
### Author:  [@jonsterling](https://discuss.ocaml.org/u/jonsterling)
#### Post date: [August 2, 2023, 1:06pm UTC](https://discuss.ocaml.org/t/thrift-eio-flow-transport-suggestions/12385/8 "2023-08-02T13:06:01Z")
@talex5 Thanks for this suggestion, that is a bit better performing.