package mssql

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Async SQL Server client using FreeTDS


Dune Dependency






Mssql wraps FreeTDS in a nicer and safer interface, with support for parameterized queries, thread-based async IO, and a thread pool.

Published: 05 Mar 2021


Mssql is an Async OCaml SQL Server library, currently using FreeTDS.


  • Queries run in a background thread and play nicely with Async (assuming you're pinning freetds so it releases the runtime lock)

  • Supports single connections or connection pools

  • Supports automatic conversions for common data types

  • Supports parameterized queries (although internally it's not very nice; we parse the query for $ parameters and then insert quoted params)

  • We have workarounds for FreeTDS madness, like how there's no simple way to know what date format FreeTDS was configured with

  • Intellegently handles concurrent usage for both executing queries and with transactions

  • Heavily tested

Regarding concurrent usage, the following code is safe:

let%map res1 = Mssql.execute db "SELECT * FROM table_a"
and res2 = Mssql.execute db "SELECT * FROM table_b"

Since we don't support actual concurrency, this will run one query and then the other (order is not defined).

This is also safe and will never cause a duplicate primary key error, since we prevent concurrent usage of a connection outside of a transactio if a transaction is in progress on that connection:

Mssql.execute_unit "CREATE TABLE x (id INT PRIMARY KEY)"
>>= fun () ->
let%map () =
  Mssql.with_transaction db (fun db ->
    Mssql.execute_unit db "INSERT INTO x (id) VALUES (1)"
    >>= fun ()->
    Mssql.execute_unit db ~params:Mssql.Param.[Some (Int 1)]
      "DELETE FROM x WHERE id = $1")
and Mssql.execute db "INSERT INTO x (id) VALUES (1)"

Obviously this only works if we know about the transaction, so using Mssql.begin_transaction or Mssql.execute_unit "begin" won't have this feature.


This library is heavily optimized for our use-case, but we would love to see contributions to:

  • Support Lwt and blocking IO

  • Support parameterized queries in a better way

  • Switch to pure OCaml and not use FreeTDS

This is not an exhaustive list -- feel free to create an issue if you're considering making a new feature and want to know if we'll accept it, or just open a pull request.


You can find mssql on opam:

opam install mssql

Or you can pin this repo if you want:

opam pin add mssql


The tests are full of examples and you can find documentation here.


Mssql.with_conn ~host ~db ~user ~password (fun db ->
  Mssql.execute_unit db
    "CREATE TABLE example (id INT NOT NULL, value INT NOT NULL)"
  >>= fun () ->
  Mssql.execute_unit db "INSERT INTO example (id, value) VALUES (1, 2)"
  >>= fun () ->
  Mssql.execute db ~params:Mssql.Param.[Some (Int 1)]
    "SELECT id FROM example WHERE id = $1"
  >>| function
  | [ row ] ->
    let id = Mssql.Row.int_exn row "id"
    and value = Mssql.Row.int_exn row "value" in
    printf "Got row with id=%d value=%d" id value
  | _ -> assert false)

Dependencies (9)

  1. dune >= "1.11"
  2. freetds >= "0.7"
  3. text >= "0.8.0"
  4. logs
  5. ocaml >= "4.06.1"
  6. iter >= "1.2"
  7. ppx_jane < "v0.15"
  8. bignum < "v0.15"
  9. async_unix < "v0.15"

Dev Dependencies (4)

  1. bisect_ppx dev & >= "2.0.0"
  2. odoc with-doc
  3. alcotest-async with-test & >= "1.0.1"
  4. alcotest with-test & >= "1.0.1"

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